Enhanced lightweighted thermal management devices

Abstract

A heat sink includes a body of an aluminum material defining at least one heat transfer surface, the body being configured to receive a fluid and to dispose the at least one heat transfer surface in direct contact with the fluid, in which the at least one heat transfer surface includes a plurality of microstructures disposed thereon, the plurality of microstructures being dimensionally non-uniform and characterized by different sizes that include length dimensions equal to or greater than 2 µm.

Description

ENHANCED LIGHTWEIGHTED THERMAL MANAGEMENT DEVICESRELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore patent application no. 10202402194P filed on July 23, 2024, the contents of which are hereby incorporated by reference in their entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure relates to thermal management devices and methods of making thereof, and more particularly to heat sinks.BACKGROUND

[0003] It is generally agreed that lightweighting vehicles (including aircrafts, ground vehicles, etc.) can improve fuel efficiency and reduce carbon emissions. Having lightweight and effective thermal management options for vehicles can make a significant improvement in lightweighting in view of the increasing amount of onboard electronics used, e.g., for navigation, communications, and safety-related applications. Cast aluminum can be a relatively economical replacement for heavier metals for the purpose of lightweighting. Unfortunately, the typical cast aluminum alloy has a thermal conductivity of only 100 W / m • K to 237 W / m • K and is not expected to perform as well as copper which has a thermal conductivity of 401 W / m - K. When it comes to thermal management of critical electronics and the prevention of overheating, high performance heat dissipation or heat transfer is essential and should not be compromised.SUMMARY

[0004] A heat sink includes a body of an aluminum material defining at least one heat transfer surface, the body being configured to receive a fluid and to dispose the at least one heat transfer surface in direct contact with the fluid, in which the at least one heat transfer surface includes a plurality of microstructures disposed thereon, the plurality of microstructures being dimensionally non-uniform andcharacterized by different sizes that include length dimensions equal to or greater than 2 m. The aluminum material may be additively formed aluminum.

[0005] The different sizes of the plurality of microstructures may include: a first plurality of first microstructures characterized by a first range of sizes, the first range of sizes; and a second plurality of second microstructures characterized by a second range of sizes, wherein the second range of sizes is different from the first range of sizes.

[0006] The different sizes of the plurality of microstructures may include length dimensions ranging from 3 pm to 8 pm, inclusive. The different sizes of the plurality of microstructures may include length dimensions ranging from 10 pm to 15 pm, inclusive. The heat sink may be characterized by a heat transfer coefficient greater than the heat transfer coefficient of a cast aluminum alloy in any one of pool boiling and flow boiling. The heat sink may be characterized by a critical heat flux greater than the critical heat flux of a cast aluminum alloy in any one of pool boiling and flow boiling.

[0007] A method of making the heat sink, including additively manufacturing an article from AISilOMg powder; subsequently, subjecting the article to a first heat treatment; and subsequently, chemical etching of the article. The method may further include, after the chemical etching, subjecting the article to a second heat treatment.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] To aid understanding, various embodiments of the present disclosure will be described with reference to the following figures.

[0009] FIG. 1A shows an exploded view of an isometric diagram of a test module for flow boiling.

[0010] FIG. 1 B is a top view of a schematic diagram of a heat sink or a specimen under testing.

[0011] FIG. 1C is a cross-sectional view of the heat sink of FIG. 1 B, showing minichannels and a channel cover spaced apart from the minichannels by a gap.

[0012] FIG. 1 D is another cross-sectional view of the heat sink of FIG. 1 B.

[0013] FIG. 2 show images of fabricated test specimen showing minichannels, plenum, O-ring groove, and mounting holes of (a) AI6061 , (b) AM AISil OMg, and (c) AM-E(total) with sub-figures at a scale of 30 mm, (II) SEM analysis of the micro / nanostructures at a scale of 15-25 pm, (III) SEM analysis at a scale of 2-3 pm, and FIB analysis at the scale of (IV) 5 pm.

[0014] FIG. 3 is a schematic flow chart of a method of making an embodiment of the heat sink.

[0015] FIG. 4A and FIG. 4B are cross-sectional schematic diagrams of AM-E(top) and AM-E(total), respectively.

[0016] Fig. 5A shows boiling curves of AI6061 , AM, AM-E(top), and AM-E(total) specimens with flow visualizations of the onset of nucleation boiling, vigorous flow boiling, and intermittent dryout of AI6061 at m - 0.005 kg / s.

[0017] Fig. 5B shows the average heat transfer coefficient of four cold plates / heat sinks as a function of effective heat flux at m - 0.005 kg / s.

[0018] FIG. 6A illustrates two-phase flow patterns in AI6061 at m = 0.005 depicting (a) bubbly flow, (b) intermittent bubbly / slug flow, and (c) intermittent dryout with images captured from high-speed visualization and reconstructed top view and cross-sectional view schematic diagrams.

[0019] FIG. 6B show exemplary time-lapse images obtained from high-speed visualization capturing onset of nucleation boiling of AI6061 at qeff = 7.7 kW / m2and m = 0.005 kg / s and schematics illustrating bubble dynamics, and corresponding cross-sectional view schematic diagrams. Fluid flows from left to right, showing (a) trajectories of bubbles nucleated within a channel from 0 s to 0.041 s (bubbles from nucleation boiling at channel bottom corners to condensation when flowing downstream) and (b) trajectories of bubbles nucleated on fin top from 0 s to 0.042 s (bubbles flowing from fin top to channel directly and moving by coalescence with other bubbles within channel).

[0020] FIG. 7 shows images from high-speed visualization capturing the onset of nucleation boiling in (a) AI6061 , (b) AM, and (c) corresponding top view and cross- sectional view schematics of bubble dynamics in AI6061 and AM, as well as images from high-speed visualization capturing the onset of nucleation boiling in (d) AM-E(top), (e) AM-E(total), and (f) corresponding top view and cross-sectional view schematics of bubble dynamics in AM-E(top) and AM-E(total). Flow direction is from left to right, and at qefr = 7.5 kW / m2, m = 0.005 kg / s.

[0021] FIG. 8 shows images from high-speed visualization capturing the two-phase flow pattern in (a) AI6061 , (b) AM, (d) AM-E(top), and (e) AM-E(total), with (c) and (f) corresponding top view and cross-sectional side view schematics of bubble dynamics, Flow direction is from left to right, and at qeff = 74 kW / m2, m - 0.005 kg / s.

[0022] FIG. 9 shows images from high-speed visualization capturing the two-phase flow pattern in (a) AI6061 , (b) AM, (c) AM-E(top) and (d) corresponding top view and cross-sectional side view schematics of bubble dynamics. Flow direction is from left to right, and at qetf = 113 kW / m2, m = 0.005 kg / s.

[0023] FIG. 10 shows images from high-speed visualization capturing the two- phase flow pattern in (a) AM-E(top), and (b) corresponding top view and cross- sectional side view schematics of bubble dynamics. Flow direction is from left to right, and at qeff = 113 kW / m2, m - 0.005 kg / s.

[0024] FIG. 11 shows images from High-speed visualization capturing intermittent dryout phenomenon on top of fins of AI6061 at qeff = 140.8 kW / m2, m - 0.005 kg / s, at timestamps of (I) 0 s, (II) 0.072 s, (III) 0.094 s, and (IV) 0.113 s, respectively, with schematic diagrams illustrating intermittent dryout phenomenon of AI6061 , AM, and AM-E(top). Flow direction is from left to right.

[0025] FIG. 12 shows images from high-speed visualization capturing intermittent dryout phenomenon on top of fins and within minichannels of AM-E(total) at qerr = 119.2 kW / m2, m = 0.005 kg / s, at timestamps of (I) 0 s, (II) 0.085 s, (111) 0.21 s, and (IV) 0.319 s, respectively. Flow direction is from left to right, with schematic diagrams illustrating intermittent dryout phenomenon of AM-E(total).

[0026] FIG. 13 are graphs showing the effect of mass flow rate on average heat transfer coefficient for (a) AI6061 , (b) AM, (c) AM-E(top) and (d) AM-E(total).

[0027] FIG. 14 shows the variation of pressure drop in AI6061 , AM, AM-E(top) and AM-E(total), respectively, at heat flux between 0 and 142 kW / m2, with (a) m = 0.005 kg / s, (b) m = 0.0075 kg / s and m = 0.01 kg / s.

[0028] FIG. 15 is a schematic flow chart illustrating a method of making a thermal management device according to another embodiment of the present disclosure.

[0029] FIG. 16A is a schematic diagram of a dielectric fluid pool boiling test facility.

[0030] FIG. 16B is a schematic diagram of a test specimen supported for testing in the test facility of FIG. 16A.

[0031] FIG. 16C is a schematic diagram of a water pool boiling test facility.

[0032] FIG. 16D is a schematic diagram of a test specimen supported for testing in the test facility of FIG. 16C.

[0033] FIG. 17 shows the one-dimensional heat flux validation of (a) 6061 -B and (b) AM-H(500)B.

[0034] FIG. 18 compares DI water pool boiling performance between 6061 -B and other reported performance of plain AI6061 surfaces.

[0035] FIG. 19 shows images from a timelapse recording of bubble ebullition during CHF on 6061 -B when (a) silicone gasket or (b) epoxy was used to seal the gaps adjacent to the test surface

[0036] FIG. 20 shows pool boiling performance of microstructured surfaces, plain AM and 6061 surfaces in HFE-7100 in the form of (a) plots of heat flux against wall superheat, and (b) plots of heat transfer coefficient against heat flux.

[0037] FIG. 21 are images of bubble nucleation on the plain surfaces (a) 6061 and (b) AM at a heat flux of about 4 W / cm2using HFE-7100 as coolant.

[0038] FIG. 22 shows SEM images (a) and (b) of potential cavity sites for bubble nucleation on the plain AM surface, and (c) a cross-sectional view of a cavity on the plain AM surface.

[0039] FIG. 23 is a schematic diagram of the CHF occurring due to excess favorable nucleation sites which causes the wall superheat to be too low, in which other nucleation sites are not activated to dissipate the heat.

[0040] FIG. 24 shows images from high-speed pool boiling visualizations of (a) 6061 , (b) AM, (c) AM-E(7.5)H(500), and (d) AM-H(400)E(5) in HFE-7100 at the last steady state heat flux prior to CHF.

[0041] FIG. 25 are SEM images of larger cavities ranging from 3 pm to 8 pm on (a) AM-E(7.5) and (b) AM-E(7.5)H(500) and (c) a cross-sectional view of the larger cavities on AM-E(7.5).

[0042] FIG. 26 are images of larger quantities of favorable cavities ranging from 3 to 8 pm on (a) AM-H(300)E(10) and (b) AM-H(300)E(10)H(500) as compared to (c) AM-E(7.5)H(500).

[0043] FIG. 27A compares hierarchically nanostructured surfaces against its microstructured counterparts in the form of plots (i) and (ii) of heat flux against wall superheat.

[0044] FIG. 27B compares hierarchically nanostructured surfaces against its microstructured counterparts in the form of plots (i) and (ii) of heat transfer coefficient against heat flux.

[0045] FIG. 28 are cross-sectional views of a cavity showing a reduction in size due to the additional layer of boehmite nanostructures.

[0046] FIG. 29 are images from high-speed time lapse recordings of isolated bubble departure on 6061 -B at 12.7 W / cm2using DI water as coolant.

[0047] FIG. 30 are images from high-speed time lapse recordings of constantcoalescence bubble departure on 6061 -B at 12.7 W / cm2using DI water as coolant.

[0048] FIG. 31 are images from high-speed time lapse recordings of bubble dynamics on AM-H(500)B at 59.5 W / cm2using DI water as coolant.

[0049] FIG. 32 are images from high-speed time lapse recordings of rapid vaporization and bubble formation on AM-H(500)B at 59.5 W / cm2using DI water as coolant.

[0050] FIG. 33 are images from high-speed time lapse recordings of large bubble pulsation dynamics on AM-H(500)B at 191.6 W / cm2using DI water as coolant.

[0051] FIG. 34 are images from high-speed time lapse recordings of all bubble simultaneously growing from nucleation sites on the surface after a necking bubble on AM-H(500)B at 191 .6 W / cm2using DI water as coolant.

[0052] FIG. 35 shows a plot of the average wicking front radius of water droplet over time on 6061 -B, AM-H(500)B, AM-E(7.5)H(500)B, AM-H(300)E(10)H(500)B, andAM-H(400)E(5)H(500)B. Images on the right show the wicking front of the various surfaces at 400 ms.

[0053] FIG. 36 compares pool boiling performance of AM-H(500)B and 6061 -B in DI water in terms of (a) plot of heat flux against wall superheat and (b) heat transfer coefficient against heat flux.

[0054] FIG. 37 are images showing bubble dynamics on (a) 6061 -B and (b) AM- H(500)B at a low heat flux of approximately 12 W / cm2with DI water as coolant.

[0055] FIG. 38 are images showing in (a) and (b) an abundance of cavity sizes up to 8 pm on 6061 -B, and in (c) and (d) the presence of cavity sizes 10 pm and larger on AM-H(500)B.

[0056] FIG. 39 compares pool boiling performance of micro / nanostructured surfaces in DI water in terms of (a) a plot of heat flux against wall superheat and (b) a plot of heat transfer coefficient against heat flux.

[0057] FIG. 40 are images showing in (a) the presence of cavity sizes of up to 8 pm on AM-E(7.5)H(500)B; (b) larger cavity sizes between 10 pm and 15 pm are present on AM-E(7.5)H(500)B; (c) and (d) the presence of more cavities with sizes between 10 pm and 15 pm on AM-H(300)E(10)H(500)B surface; (e) and (f) surfaces of AM- H(400)E(5)H(500)B showing a lack of distinct 10 pm to 15 pm cavity features on the surface.

[0058] FIG. 41 are images showing bubble dynamics on (a) AM-E(7.5)H(5000)B, (b) AM-H(300)E(10)H(500)B, and (c) AM-H(400)E(5)H(500)B at a low heat flux of approximately 12 W / cm2with DI water as coolant.

[0059] FIG. 42 is a schematic diagram illustrating hotspot activated nucleation sites leading to lower average wall temperatures at higher heat fluxes.DETAILED DESCRIPTION

[0060] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these otherembodiments. Additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0061] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise. In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0062] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0063] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0064] Terms such as “first”, “second”, "third", "other", "another", etc., are used in the description and claims only for the sake of brevity and clarity, and do not necessarily imply a priority or order, unless required by the context.

[0065] Some methods may be described in terms of steps, stages, phases, or the like, merely to aid understanding and / or for convenient reference. The delineation between one step and another step may be merely for convenient reference in the present disclosure. It will be understood that in actual implementation there may not be a clear division or transition from one step to another subsequent step. There may be a certain amount of overlap among the steps and / or more than one step may occur or be performed concurrently or simultaneously in time, etc., unless the context dictates otherwise. The terms "concurrently" and "simultaneously" may be used interchangeably in the present disclosure. Events described as concurrent or simultaneously overlap in time but are not limited to beginning and / or ending at the same time instants.

[0066] In the present disclosure, the term "length scale" will be understood by one skilled in the art to refer to a length dimension or generally to the size of the microstructures. In the context of the present disclosure, one skilled in the art would understand that the terms "microstructures" and "micro / nanostructues" are used interchangeably and refer to physical structures characterized by linear dimensions of about 1 pm or (if alternatively expressed) as a linear dimension within a range greater than 0.1 pm and smaller than 20 pm. In the present disclosure, examples of microstructures (also referred to herein as micro / nanostructures) include but are not limited to nanobumps, nanosteps, nanopores, microcells, microsteps, etc.

[0067] Various embodiments of the present disclosure provide a thermal management device 100 made of aluminum and configured to enable enhanced thermal performance. Prototypes and test specimens of the proposed enhanced lightweighted thermal management device demonstrated enhanced thermal performance (relative to a conventional cast aluminum part). The thermal management device of the present disclosure is therefore believed to be a viable option for thermal management applications where lightweighting is desired.

[0068] The heat sink 101 was chosen as a non-limiting example of the thermal management device 100. Prototypes of the proposed heat sink 101 were made and tested. FIG. 1A is schematic drawing of a test set-up to study flow boiling performance. The test set-up may include an insulated housing 701 (e.g., made of Teflon) provided to surround a specimen (e.g., in this example, a heat sink with minichannels). A copper heating block 702, serving as a heat source, may be disposed under and in thermal contact with the specimen. The copper heating block may in turn be heated up by a number of cartridge heaters 703.

[0069] A channel cover 704 (e g., made of polycarbonate) may be disposed over the heat sink to form "open channels" as will be described in greater detail below. A sealing O-ring 705 or gasket may be provided between the heat sink 101 and the channel cover 704. A top piece 706 may be fastened above the channel cover 704.

[0070] Prototypes or specimens were made in the form of a heat sink 101 . The heat sink 101 was configured with a plurality of open minichannels 200. In the present disclosure, the terms "channel", "microchannel", and "minichannel" are usedinterchangeably. The heat sink 101 includes a base 300 formed of with an array of fins 400 extending from the base, with adjacent fins forming a channel 200 or channel cavity therebetween.

[0071] The top surfaces of neighboring or adjacent fins 400 define an opening via which the environment is in fluid communication with a minichannel 200 (also referred to as the channel or channel cavity). The minichannel 200 is defined by a part of the base (bottom wall) and two opposing side walls of the fins 400. In some embodiments, at least one wall or surface includes proposed surface features (e.g., microstructures), as will be further elaborated below. In some embodiments, only the top surfaces of the fins are provided with the proposed surface features. In some other embodiments, both the top surfaces of the fins and the side walls of the fins are provided with the proposed surface features. In some embodiments, the bottom surfaces (e g., part of the base between adjacent fins) are provided with the proposed surface features.

[0072] FIG. 1 B shows a top view of an example of a heat sink 101 of the present disclosure. The heat sink 101 includes a plurality of minichannels 200 defined on a base. In use, a fluid (also referred to as a coolant fluid) may be provided at a coolant inlet 711 at one end of the heat sink. The coolant fluid may be pumped or otherwise enabled to traverse the heat sink 101 , and thereafter to exit the heat sink at a coolant outlet 712 at an opposite end of the heat sink.

[0073] FIG. 10 illustrates a cross-sectional view of the heat sink of FIG. 1B. The heat sink 101 (also referred to as a "cold plate") may be configured with a plurality of parallel minichannels 200. In one of the prototype heat sinks used in the experiments, there were 16 parallel open minichannels of width (Wmc) and depth (Hf) of 2 mm and 1 mm, respectively. Each of the fins 400 separating adjacent minichannels were solid additively manufactured aluminum with width (Wf) and height (Hf) of 2 mm and 1 mm, respectively.

[0074] FIG. 1 D shows another cross-sectional view of the heat sink of FIG. 1 B. The views of FIG. 1C and FIG. 1D are cut along orthogonal lines. A series of thermocouples may be disposed at 2 mm below the base of the minichannels for the purpose of measuring temperatures during the experiments. A plurality of T-type thermocouples (Reach Electrical, 1 mm probe diameter) were calibrated to an accuracy of ± 0.2°C and inserted into the base of the heat sink with thermal grease for determining the base temperature of the test specimen.

[0075] To form "open" channels, a cover channel may be disposed spaced apart from the top surfaces of the fins (as schematically illustrated in FIG. 1C). The incorporation of the gap (Hg) above the minichannels 200 forms what is referred to as an open minichannel or an open microchannel configuration. For the sake of brevity, the terms "open minichannel" and "open microchannel" are used interchangeably. For example, the channel cover 704 and the top surface of the fins 400 may be spaced apart by a gap (Hg) that is half of the channel height (Hf), forming a small manifold. The gap (Hg) is preferably no bigger than half of the channel height (Hf) to reduce the subcooling effects of the fluid in the minichannel 200 and to promote more bubble nucleation sites (even at low heat flux). In the experiments, the channel cover 704 was made of polycarbonate and disposed over the heat sink 101 at a gap (Hg) of 0.5 mm between the cover 704 and the heat sink 101. Preferably, the gap (Hg) is significantly larger than the surface roughness of the materials forming the heat sink 101 .

[0076] Open channels are conventionally not desirable. Owing to the increased hydraulic diameter caused by the introduction of the gap above the flow channels, conventional open channels typically suffer from a lower heat transfer coefficient as compared to closed channel designs. Contrary to conventional thinking, according to embodiments of the present disclosure, an open channel configuration is preferred.

[0077] Compared to a conventional closed channel, the open minichannels of the present disclosure was found capable of providing benefits that outweigh the setbacks of the conventional open channel. In the heat sink of the present disclosure, there is a greater wetted area available for heat transfer, e.g., all sides of the fins may contribute to the promotion of bubble nucleation. The open minichannel configuration was found capable of alleviating large pressure losses during flow boiling, helping to reduce flow instability at high heat fluxes by enabling bubbles to grow and depart into the manifold area while preserving liquid filledchannels to minimize the adverse effects of fluid back flow and to avoid early occurrence of critical heat flux (CHF).

[0078] In the present disclosure, reference is made to pool boiling and flow boiling as two heat transfer mechanisms. Flow boiling refers to a mechanism by which heat is removed by providing a flow of liquid over the heat source. In the case of flow boiling, a liquid delivery system (e.g., pumps) may be utilized to generate the required liquid flow. The heat removal mechanisms in both pool boiling and flow boiling can depend to a large extent on the nucleation of bubbles, the growth of bubbles, and the departure of bubbles from the interface between the liquid and the solid surface of the heat source. The heat sink was tested for its thermal performance under conditions where flow boiling was expected to be the dominant mechanism for heat dissipation.

[0079] In contrast, pool boiling refers to a mechanism by which heat is removed by immersing a heat source in a quiescent pool of liquid. Embodiments of the present disclosure may also be implemented in a thermal management device suitable for pool boiling (as will be described below making reference to FIG. 16A to FIG. 16D).

[0080] Referring to FIG. 2, a heat sink with similar dimensions was fabricated using conventional milling technique (also referred to as the AI6061 specimen) to serve as a baseline for comparing performance. Images of the AI6061 specimen are shown in column (a) of FIG. 2.

[0081] Prototypes or specimens of the proposed heat sink were formed according to embodiments of the present disclosure, by a method schematically shown in FIG. 3. The heat sink was made of aluminum by additively manufacturing and selected surfaces of the heat sink was surface engineered. Various aluminum powders could conceivably be used. As an example, for the making of the prototypes, aluminum alloy AISi Mg powder was used as the additive manufacturing material. For the purposes of the experiments, the entire test specimen, including the heat sink with the minichannels, plenums, O-ring groove, and two 60 mm long and 3 / 8" (or 9.5 mm) outer diameter inlet and outlet pipes, was fabricated as an integrated piece by selective laser melting (SLM) in one print job.

[0082] The additive manufacturing processes were conducted at a platform temperature of 170°C and a chamber pressure of 12 mbar. The chamber was filled with inert argon gas to keep the oxygen levels in the chamber below 0.2% and to prevent ignition of aluminum powder during the laser melting process. A laser source was used to melt and fuse base aluminum alloy powder layer-by-layer to form three-dimensional parts. As an example, in the making of the prototypes, a laser power of 350 W with an 80 pm laser spot diameter, 1150 mm / s scan speed, and 0.17 mm hatch spacing were employed. The boundaries of each layer were scanned with a 300 W laser at a speed of 600 mm / s. After a layer scan was completed, the platform was lower by a layer thickness of 50 pm and a new powder layer was deposited. Throughout the fabrication process, the platform was maintained at 170°C. Images of the as-fabricated or intermediate additively manufactured specimen (also referred to as the "AM AISilOMg" or "AM" specimen) are shown in column (b) of FIG. 2.

[0083] The as-fabricated AM specimens were then subjected to a surface engineering process, according to embodiments of the present disclosure.

[0084] In some examples, the as-fabricated AM specimen was subjected to a first heat treatment at 400°C for 24 hours in a furnace at atmospheric pressure.

[0085] The first heat treatment encouraged recrystallization and grain growth within the material to eliminate anisotropy caused by the directional nature of the additive manufacturing process. The first heat treatment also facilitated the precipitation and clustering of the Si-rich phase, resulting in the formation of micron size silicon particles surrounded by an Al-rich phase. Upon the completion of the first heat treatment, the specimen was removed from the furnace and left to cool in ambient air temperature of 25°C.

[0086] The specimen was then first cleaned by successive sonication in deionized water, ethanol, and isopropanol, for at least 15 minutes each.

[0087] One or more surfaces may be selected for further surface engineering. One or more surfaces may be isolated or covered up to prevent further surface engineering. In the experiments, surfaces such as the plenums and the regions of the base near the coolant inlet and coolant outlet (e.g., entrance / exit regions of thespecimen) were isolated from chemical treatment by using multiple layers of scotch tapes and / or nail polish.

[0088] Subsequently, the entire specimen was subjected to a chemical etching process, e.g., by being immersed in diluted hydrochloric (HCI) solution of concentration 2.4 M for 6 minutes. During this chemical immersion process, the hydrochloric acid reacted vigorously with the entire surface of the minichannels resulting in the removal of the Al-rich phase. To end the chemical etching process, the specimen was then immersed in a pool of de-ionized water.

[0089] Thereafter, the scotch tape was removed, and the specimen was thoroughly cleaned by successive sonication in deionized water, ethanol, and isopropanol, for at least 15 minutes each. The specimen was cleaned to remove any foreign materials, including the nail polish.

[0090] The resulting specimen (also referred to as the AM -E (total) specimen) had microstructures formed on all surfaces of the fins. Column (c) of FIG. 2 shows images of the AM-E(total) specimen. FIG. 4A is a schematic cross-section illustrating one of the microchannels, showing microstructures formed on all surfaces of the fins / minichannel, e.g., the top surfaces of the fins 400, the side surfaces of the fins 400, and the bottom surface of the minichannel 200.

[0091] In some other examples, the same method was carried out, except that the side walls and bottom wall of the microchannel 200 were isolated during the chemical etching process. The resulting specimen (also referred to as the AM- E(top) specimen or a specimen with partial channel wall microstructuring) had only the top surfaces of the minichannels endowed with microstructures, as schematically illustrated in FIG. 4B.

[0092] Referring again to FIG. 2, images in row II of FIG. 2 are scanning electron microscopy (SEM) images of top views at the scale of 15-25 pm, and images in row III of FIG. 2 are SEM images of top views at the scale of 2-3 pm. Images in row IV of FIG. 2 are focused ion beam (FIB) images of side views at a scale of 5 pm.

[0093] Images II and III of column (a) of FIG. 2 show the SEM images of the AI6061 specimen at the scales of 25 pm and 3 pm, respectively. It can be seen that numerous elongated structures, approximately 2 pm in width, were regularly alignedin one direction on the surface. This was caused by substantial plastic deformations during the milling process, which stretched the material in the milling direction. These continuous long structures, arising from the milling process, may be too large to trap vapor for bubble nucleation of low surface tension refrigerant.

[0094] Images II and III of column (b) of FIG. 2 show irregularly shaped grooves with approximately 20 pm diameter and numerous 3 pm spherical features on the surface of the AM specimen. These features were formed by unmelted or partially melted powder during the powder bed fusion process. Even though these structures may serve as additional nucleation sites, it was expected that there were too few of these structures to have a significant influence on the flow boiling performance of the heat sink.

[0095] Images II and III of column (c) of FIG. 2 reveal the presence of numerous cavities in the form of step-like structures with cavity sizes ranging from 3 pm to 8 pm. It would appear that the first heat treatment process, which eliminated the anisotropic and directional nature of the as-fabricated AM sub-grain structures, had resulted in the uniform generation of microstructures across all minichannels. This is despite the observation that the bottom and side walls of the channels, when initially fabricated, had columnar grains formed in different directions owing to the difference in thermal gradient during the SLM process.

[0096] In addition, the cavity sizes (ranging from 3 pm to 8 pm) obtained in the AM- E(total) specimen were significant as they fall within the optimal cavity size range (from 1 pm to 20 pm) for bubble nucleation of the refrigerant R134a.Table 1.

[0097] The surface roughness of the specimens was investigated and the results shown in Table 1 above, in which Ra is the average surface roughness, Wmc is the average minichannel with, Wf is the wall thickness, and Hf is the depth of the specimens.

[0098] Owing to the presence of unmelted and partially melted particles, the average surface roughness (Ra) of the AM surface is approximately ten times higher than that of the AI6061 specimens. These highly irregular sintered powders were removed during etching process, resulting in a smoother surface of approximately 16 pm measured for AM-E(total).

[0099] In the AM-E(total) specimens, it was observed the etching process resulted in an increase in the minichannel width from 2036.1 pm to 2412.7 pm, with thinning of the fin width from 1944.6 pm to 1505.6 pm. The fin height still remained at approximately 1000 pm, as the etching solution reacts with both fin top and minichannel bottom simultaneously. In the AM-E(top) specimens, an approximately 200 pm decrease in the fin height was also observed.

[0100] It was observed that the etching process would result in slight changes of the fin dimensions, leading to the increase of the cross-sectional free flow area within open minichannels by less than 15% as compared to the un-etched specimen. However, despite the slight increase in free flow volumes which resulted in a larger hydraulic diameter of the minichannels of AM-E(total) and AM-E(top), it was experimentally verified that these changes had a negligible impact on the flow boiling performance of the AM-E(total) and AM-E(top) specimens.

[0101] Experiments were conducted on the specimens to study the flow boiling performance in terms of the cooling efficiency and corresponding flow patten, given a mass flow rate of m = 0.005 kg / s.

[0102] Results

[0103] FIG. 5A shows the boiling curves and depict the relationship between average superheated temperature and heat flux for each type of the specimens, i.e. , the AI6061 , AM, AM-E(total), and AM-E(top) specimens.

[0104] Considering variation in temperature change behavior across the test specimens with increasing heat flux, for illustration purpose, the range of heat fluxtested can be divided into three regions, i.e., low heat flux region where qetf lies between 0 kW / m2and 7.6 kW / m2, medium heat flux region where qetr lies between 7.6 kW / m2and 93.3 kW fm2, and high heat flux region where qerr is above 93.3 kW / m2.

[0105] In the low heat flux region, the base temperature of all the specimens experienced significant increases with heat flux until qeff = 7.623 kW / m2. At this heat flux, the onset of nucleate boiling was observed at an axial distance of around 40 mm from the coolant inlet through high-speed visualization. Below the heat flux threshold of 7.6 kW / m2, heat dissipation was mainly governed by single-phase liquid forced convection with a relatively poor cooling efficiency. In the medium heat flux region, all specimens exhibited small temperature rise with increasing heat flux, resulting in a steep gradient of the boiling curve. For instance, the average superheated temperature of AM-E(top) and AI6061 was found to increase by only 0.9 °C and 3 °C, respectively, as heat flux increased by more than tenfold. These significantly improved cooling capabilities mainly resulted from the utilization of the large latent heat caused by large quantities of bubble generations during boiling process.

[0106] It can be observed from FIG. 5A that the AM-E(top) demonstrated the best cooling performance, characterized by its steepest boiling curve, followed by AM- E(total), AM, and AI6061 .

[0107] In the high heat flux region, the average superheated temperature of AM- E(total) interestingly remained relative constant and even exhibited small reductions as heat flux increased beyond 100 kW / m2. A consistent increase in the superheated temperatures were observed in all other specimens.

[0108] Further increase of heat flux to 119.2 kW / m2would trigger intermittent dryout in the channel downstream of AM-E(total), whereas a similar phenomenon was observed beyond heat fluxes of 139.4 kW / m2, 140.9 kW / m2, and 141.5 kW I m2for AM-E(top), AM, and AI6061 , respectively.

[0109] Heat transfer characteristics

[0110] FIG. 5B shows the variation of the average heat transfer coefficient with heat flux for the four specimens at a mass flow rate of m = 0.005 kg / s.

[0111] The general trend of increasing heat transfer coefficient, and the variant cooling efficiency from different quantities of nucleation sites on the wall surfaces, further substantiate the inference that nucleate boiling is the predominant cooling mechanism at work. In the low heat flux region, all specimens presented a similar single-phase liquid forced convection performance at qeff = 1.9 Ar IV / m , which implies the proposed etching method in this study does not enhance the surface area or change the liquid flow pattern effectively as compared to porous structures. Further increase in heat flux to 7.5 kW / m2resulted in the onset of nucleation boiling, with AM showing 60% higher average heat transfer coefficient than AI6061 .

[0112] For the sake of comparison, FIG. 6A illustrates three main flow patterns, i.e., bubbly flow, intermittent bubbly / slug flow and intermittent dryout at low, medium and high heat fluxes regions, respectively. Specifically, row (a) of FIG. 6A presents the bubbly flow at the onset of nucleate boiling, where bubbles simultaneously nucleate from active nucleation sites situated at channel bottom corners (the turning points between channel bottom and side walls) and top of fins. Distinct bubble trajectories were observed between these two locations.

[0113] Regarding bubbles nucleated from the former, they would experience a rapid condensation process when flowing into the middle and downstream part of channels as shown in row (a) of FIG. 6A. In contrast, bubbles originated from the top of the fins tend to grow larger and flow downstream as shown in row (a) of FIG. 6A. This could be due to the different effects of subcooling liquid within the specimens, whose gap (Hg = 0.5 mm) above fins is only half the size of channel height (Hf = 1 mm) while fin top surface area is as large as that of the channel bottom surface. As a result, even though the bottom surface of the channel is closer than its top surface to the heat source, the smaller volume of subcooled refrigerant that flows on the top of the fins, renders only a small amount of sensible heat to be removed before phase change occurs. This postulation can be further supported by the observations that both quantity and size of bubbles nucleated from top of the fins are larger than those nucleated from channel bottom corners, where bubbles are only observed near the side rather than middle parts of channel as shown in row (a) of FIG. 6B.

[0114] However, as shown in row (b) of FIG. 6B captured at channel downstream, bubbles generated on fin tops were observed to flow into the channel section directly or by coalescence with another bubble. One factor could be because the vigorous bubble coalescences on the top of the fins would accelerate bubble growth, which induce a predominance of buoyancy force over surface tension surface to facilitate their detachment from the solid walls. Another factor could be because the low surface tension of surrounding high temperature liquid refrigerant on the top of fins could further amplify the difference of the two forces. Yet another factor could be because the large bubble might constrain its surrounding liquid flow to generate a localized differential pressure, contributing to flow instability and bubble movement. Consequently, almost all bubbles observed in the middle of channels originated from the top of the fins rather than channel bottom walls. While this phenomenon was observed to vanish at higher heat flux when the liquid is in complete saturated state, it may be expected to remain prevalent in flow boiling of higher subcooling low surface tension refrigerant, which usually caused the activation of potential cavities difficult even in high heat fluxes. From this observation, it can be deduced that open minichannels, with its gap size much smaller than the channel parts, are able to provide a more suitable environment for bubble nucleation and growth as compared to conventional closed minichannels counterpart. Further increase of heat flux would result in a higher bubble growth rate and more vigorous bubble coalescence by activating numerous nucleation sites. Also, the interconnected manifold above the fins could facilitate unrestricted lateral growth and coalescence of bubbles, making them span across the top of both channels and fins as illustrated in row (b) of FIG. 6A. Below the elongated bubble, nucleation boiling at channel bottom corners still prevails then the generated bubbles float up by buoyance to coalesce with the former. This phenomenon is one of the most distinctive characteristics for open minichannels, which implies its cooling efficiency is usually determined by nucleation boiling instead of forced convection boiling across a wide range of heat fluxes. With further increase in heat flux, as shown in row (c) of FIG. 6A, intermittent dryout near the channel outlet is observed, especially at the top of fins.

[0115] In comparison, diagrams (a) and (b) of FIG. 7 show that both quantity and dimension of bubbles formed on the top of AM fins are significantly larger than that in AI6061 . This is because the higher surface roughness of AM would introduce more favorable nucleation sites for bubble generation. Both present a similar phenomenon that most large bubbles grow on the top of fins while small bubbles nucleating from channel bottom corners condense back to liquid state as illustrated in their corresponding schematic diagrams (c) of FIG. 7.

[0116] In contrast, a different two-phase flow behavior is found in AM-E(top) and AM-E(total) even though they show similar heat transfer coefficients as AM. From diagram (d) of FIG. 7 it can be observed that all bubbles observed in AM-E(top) are nucleated from the top of the fin while no potential cavities are activated within channels. This is probably because both smaller subcooling liquid effects and more nucleation sites at the top of the fins (introduced by the etching process) have provided a better environment for bubble nucleation. This resulted in most of the heat being dissipated at the top of the fins, which inhibited the onset of bubble nucleation within channels as illustrated in the schematic diagram (f) of FIG. 7.

[0117] This point could be further verified by diagram (e) of FIG. 7, where a similar phenomenon is also found in AM-E(total) even though its side and bottom channel walls also have numerous nucleation sites introduced by etching process.

[0118] Thus, it should be emphasized that all bubbles flowing within the AM-E(top) and AM-E(total) channels at low heat fluxes are originated from fin tops.

[0119] In medium heat flux range, four specimens in FIG. 5B clearly show the different cooling performance as heat flux increases from qerf = 15 kW / m2to qefr = 85 kW / m2, where AM, AM-E(total) and AM-E(top) exhibit average improvements in heat transfer coefficient of around 47%, 75%, and 107% as compared to AI6061 , respectively. As discussed previously, the different active nucleation site density could be the main contribution of these enhancements.

[0120] Images from corresponding high-speed videos captured in the middle of channel parts at qefr = 74 kW / m2and m = 0.005 kg / s are shown in FIG. 8 for illustration. It should be noted that the two-phase flow patterns outside the camera’s field of view are also very similar to the following images. This was verified bymoving high-speed camera in the direction of fluid flow during the experimental process. FIG. 8 clearly shows the top manifolds are mostly occupied by large elongated bubbles for all specimens, however, significantly different nucleation boiling phenomena below these elongated bubbles are found. Specifically, diagrams (a) and (b) of FIG. 8 present that numerous bubbles are continuously generated within the channels and then grow to coalesce with elongated bubbles above. However, the bubbles nucleated within the AM channels are of larger sizes than those within AI6061 channels. This is due to a larger number of potential cavities provided by the rough AM surface. Also, it should be emphasized that the nucleation sites observed within these two minichannels are mostly located at the channel bottom corner, i.e. , the turning point between bottom and side wall, rather than the middle part of bottom wall as shown in the schematic graphs (c) of FIG. 8.

[0121] In contrast, only few nucleation boiling is found at the gap between the top of the fins and elongated bubbles. This is probably because the liquid film in this gap is so thin that the boiling mechanism is governed by liquid film evaporation instead of nucleation boiling. Interestingly, distinctly different flow patterns are found in AM-E(top) and AM-E(total) as compared to AM and AI6061 , which could lead to their significantly higher heat transfer coefficients. Apart from continuous nucleation boiling within channels as AM and AI6061 , substantial nucleation boiling is also observed at top of the fins, as shown in diagrams (d) and (f) of FIG. 8. This phenomenon may be attributed to the numerous nucleation sites introduced on the top of fins by chemical etching process, which enhance the predominance of nucleation boiling there.

[0122] Furthermore, careful analysis shows that the bubbles nucleated on top of the fins of AM-E(top), with approximately 300 pm diameters are about twice the size of those at the same location of AM-E(total). Also, the average heat transfer coefficient of former is 30% higher at this test condition. It should be emphasized that both AM-E(top) and AM-E(total) experience the same etching process, resulting in similar number of nucleation sites on the top of the fins while the latter has more nucleation sites on the channel bottom and side walls. This intriguing finding indicates that the heat dissipation from the top of the fins, for openminichannels with the gap (Hg) smaller than the channel height (Hf), is more efficient than that from channel bottom and side walls. This is in alignment with the phenomena at the onset of nucleation boiling, where more nucleation sites are activated on the top of the fins than channel corners. In other words, the presence of additional nucleation sites within channels at medium heat flux regions may have minimal or even adverse effects on cooling efficiency, as it may reduce the number of active nucleation sites on the top of fins.

[0123] In the range of high heat flux regions, it can be observed from FIG. 5B that there is a steep increase of heat transfer coefficient of AM-E(total) from qetf = 93.3 kW / m2while those of other specimens increases at a relatively constant rate. At qeff = 99.8 kW / m2, the heat transfer coefficient of AM-E(total) exceeds that of AM-E(top) and maintains the best cooling performance before intermittent dryout is attained. This interesting phenomenon could be elucidated by images from the high-speed videos.

[0124] Diagrams (a) and (c) of FIG. 9 present that the flow patterns of AI6061 , AM and AM-E(top) at this high heat flux are quite similar to those at medium heat flux of qeff = 74 kW / m2as shown in FIG. 8. However, significantly vigorous nucleation boiling phenomenon under elongated bubbles is exclusively observed within the channels of AM-E(total), as shown in FIG. 10, which presents the existence of much larger bubbles (around 1 mm diameter) near channel side and bottom walls. Thus, these large number of nucleation sites within channels contribute to this exclusive and sudden increase on cooling performance of AM-E(total) while other specimens only show a steady increase with heat flux due to the lack of microengineering structures there. This finding also implies that a sequential order of nucleation site activation exists for open minichannels, where the potential cavities on the top of the fin would first be activated, followed by channel bottom corners, and lastly progressed to the channel bottom and side walls. However, it should be noted that this sequence may not be applicable for all open mini / microchannels and would be affected by other factors, such as inlet temperature of liquid, gap thickness and number of nucleation sites at different locations.

[0125] Further increase of heat flux would induce intermittent dryout near the outlet of the flow channels leading to periodic dryout phenomenon with relatively large temperature fluctuations. From FIG. 5B it can be observed that AM-E(total) exhibits the lowest intermittent dryout value of 119.2 kW / m2as compared to other specimens which have relatively similar intermittent dryout of 1 0 kW / m2, when the outlet vapor quality is approximately 0.73 for AM-E(total) and 0.85 for all other specimens. Interestingly, different intermittent dryout phenomena were also observed between AM-E(total) and the other. As similar intermittent dryout behavior is observed on AM, AM-E(top), and AI6061 , only the high-speed images of AI6061 are depicted in FIG. 11, due to its shiny base background for easier visualization. From diagram (a) of FIG. 11, it can be seen that most bubbles nucleating below the elongated bubbles are within the minichannels while some are found on the top of the fin. This phenomenon is similar to their flow pattern at high heat flux regions as depicted in FIG. 9. However, at t = 0.072 s, while the bubble nucleation on the top of the fins completely ceases, boiling is still observed to take place within the channels. This observation indicates only the liquid film between elongated bubbles and fin tops has dried out while that within channel still exists as shown in the schematic diagram (b) of FIG. 11. This phenomenon lasts for approximately 0.04 s until the upstream liquid slug flushes in for bubble nucleation to recommences. This process, then, repeats resulting in the fluctuation of local temperature of more than 0.5 °C in 5 mins. In contrast, for AM-E(total), intermittently partial dryout is observed not only on the top of the fin but also within its channels as depicted in FIG. 12, which shows that the bubbles are too large to be flushed away immediately by incoming liquid slug at this mass flow rate. This will lead to the occurrence of thermal crisis both on fin tops and within channel. However, it should be noted that, different from AM-E(total), the heat flux triggering intermittent dryout of AM-E(top) is similar to AM and AI6061 , which implies the vigorous bubble nucleation between the elongated bubbles and top of the fins do not result in early intermittent dryout. This is probably because these bubbles generated on fin tops tend to flow into channel rather than along the top of the fins. Thus, these differences of heat transfer coefficient and maximum allowable heat flux values between AM-E(top) and AM-E(total), clearly elucidate the underlying mechanisms of chemical etching to enhance cooling performance. In addition, it provides useful guidelines to further optimize the performance of open minichannels by appropriately incorporating microstructures at different locations of the channel walls.

[0126] It was noted that AM-E(total) was still to maintain significant increase in average heat transfer coefficient with increasing heat flux while other specimens maintain relatively constant cooling efficiency before intermittent dryout, as shown in FIG. 5B. This result may suggest that, at high heat flux region, a substantial number of potential nucleation sites are activated for AM-E(total), while nearly all nucleation sites may have been occupied for other specimens before. Considering the predominance of nucleate boiling in AM-E(total), it is highly possible that the performance improvement resulted from the proposed chemical etching on AM would become more significant, when larger heat power is applied at high mass flow rates.

[0127] Mass flow rate

[0128] Experiments were also performed at m = 0.0075 and 0.01 kg / s to assess its effects on the cooling performance of different specimens.

[0129] From the results shown in FIG. 13, it can be concluded that lower mass flow rate leads to a better cooling performance, especially at high heat fluxes when more potential cavities could be activated for nucleation boiling.

[0130] In other words, the adverse effects of subcooling liquid on nucleation sites activations decrease with mass flow rate, which lead to a better cooling capacity, especially when the boiling mechanism is governed by nucleation boiling instead of forced convection boiling for AM-E(top) with numerous potential nucleation sites, whose average heat transfer coefficient at m = 0.005 kg / s is more than 20% higher than that at m = 0.01 kg / s when heat fluxes are larger than 20 kW / m2as shown in diagram (c) of FIG. 13. However, for the same specimen, less than 5% difference in average heat transfer coefficient is found between m = 0.0075 kg / s and m = 0.01 kg / s. This is probably because some potential nucleation sites could only be activated at low mass flow rate of 0.005 kg / s and not at higher mass flow rate of 0.0075 and 0.001 kg / s. Specifically, the proposed method, including chemicaletching, would generate numerous cavities with very different diameters and depths, and some cavities could be easily filled by the subcooled low surface tension liquid, rendering them inactive at high mass flow rates due to higher liquid subcooling effect. This is different from those with uniformly sized cavities generated by photolithography etching techniques with extremely high precision. For AM-E(total), diagram (d) of FIG. 13 shows a similar 15% improvement at m = 0.005 kg / s as compared to m = 0.01 kg / s when heat fluxes are between 20 kW / m2to 100 kW / m2. However, this enhancement becomes much more obvious when heat flux increases, with a minimum 40% difference in heat transfer coefficient between m = 0.005 kg / s and m = 0.01 kg / s. This is because the dominance of nucleation boiling on AM-E(total) is more pronounced than on AM-E(top) at high heat fluxes, since all surfaces of the former have experienced etching process to increase the number of nucleation sites while only the top of the fins are etched for the latter. Besides, the value of heat flux, resulting in the significant increase of heat transfer coefficient on AM -E (total) at high heat flux region, would increase with mass flow rate.

[0131] Pressure Drop

[0132] FIG. 14 shows the pressure drop experiences a slight decrease with the increase of heat flux in single-phase flow region, i.e. , for heat flux between 0 and 8 kW / m2.

[0133] The pressure drop mostly increases with heat flux at all mass fluxes This is mainly because vapor velocity increases significantly with heat flux and vapor quality, which leads to high shear force on the fluid-channel wall and vapor-liquid interface.

[0134] When heat flux is larger than 40 kW / m2, it can be seen that the pressure drop of AM is considerably higher than AI6061 and this discrepancy becomes more pronounced as the heat flux increases beyond 40 kW / m2. Specifically, at qeff- 43 kW / m2, the pressure drop for AM is approximately 400 Pa higher that of AI6061 , and it increases to more than two times higher near qe^ - 100 kW / m2Even though AM have much high manufacturing flexibility and slightly better cooling performance than AI6061 , this significant penalty in pressure drop might constrain the extensiveapplications of AM. However, this adverse effect of high pressure drop in AM resulted from its highly rough surfaces can be significantly alleviated by the proposed chemical etching process, which results in smoother surface and the pressure drops of AM-E(total) and AM-E(top) are reduced approximately 30% as compared to AM. This comparison further exemplifies the advantage of microstructured minichannels, which could not only increase nucleation site density to enhance cooling efficiency, but also compensate the increased pressure drop (caused by the shear forces exerted on two-phase fluid by channel walls) by the reduction in surface roughness, the proposed AM etching methods still reasonably demonstrate it could not only enhance cooling performance significantly, but also reduce the pressure drop penalty of AM.

[0135] Based on the above investigation, the following conclusions can be drawn:

[0136] (i) The significantly larger flow boiling heat transfer coefficients of approximately 110% and 200% as compared to AI6061 were achieved by AM-E(top) and AM-E(total) at the moderate and high heat flux ranges, respectively. This is due to the numerous nucleation sites introduced by the AM microstructures. In addition, the continuously increasing trend before intermittent dryout of AM-E(total) implies that this preferential microstructuring strategy has large potential to further enhance the flow boiling efficiency of AM at higher heat fluxes and mass flow rates.

[0137] (ii) The comparison of heat transfer coefficients and corresponding flow patterns of all the specimens showed that AM-E(top) had the highest average heat transfer coefficient of 43.9 kW / m2K at moderate heat fluxes with more bubble nucleation between the top elongated bubbles and fin tops. In contrast, AM-E(total) exhibited the best cooling efficiency with the heat transfer coefficient of 72.5 kW / m2K in high heat flux region with vigorous bubble nucleation found within channels. This verified a sequential order of nucleation site activation with increasing heat flux for the test open minichannels, i.e., starting from the fin top surface followed by channel bottom corners, channel side and bottom walls, and could provide useful guidelines for future surface modifications to enhance flow boiling.

[0138] (iii) The effects of mass flow rates on cooling performance were only found to be significant on AM -E (total) and AM-E(top), which showed better cooling performances at lower mass flow rate. This not only demonstrated that the boiling mechanism of open minichannels was governed by nucleation boiling but also revealed the adverse effects of liquid subcooling to activate cavities for bubble nucleation, which could be effectively alleviated by the design of small gaps on the top of fins.

[0139] (iv) Even though AM showed slightly better cooling performance than AI6061 , with heat transfer coefficient approximately 40% higher in the test heat flux regimes, as a result of AM’s highly rough surfaces, its pressure drop penalty is approximately two times higher. However, our proposed microstructuring method, achieved by chemical etching process, has effectively remove the unmelted or partially melted powder particles to reduce the surface roughness of AM. As a result, the pressure drop across AM-E(top) and AM-E(total) are up to 30% lower than AM.

[0140] The foregoing described an example of a thermal management device with enhanced flow boiling performance The enhanced effectiveness was confirmed through an analysis of the two-phase flow behavior and bubble dynamics in the open minichannels with a small gap. It is further envisioned that the microstructured minichannels would show good cooling efficiency at higher heat fluxes.

[0141] According to embodiments of the present disclosure, a thermal management device is configured with a microstructural morphology at the additively manufactured aluminum surfaces so as to provide a distribution of cavities of a range of cavity sizes, i.e., a presence of a plurality of cavities of non- uniform cavity sizes, at the surfaces. Further, consistent with the microstructural morphology proposed above, the present disclosure further proposes improved thermal management devices, including but not limited to heat sinks, which enable enhanced thermal performance when used with a variety of coolant fluids.

[0142] Conventionally, heat removal systems are optimized for single-fluid systems. This usually does not pose an issue as the coolant fluid can be selectedaccording to the expected temperatures to be managed. In the conventional case, if a wide of range of temperatures have to be dealt with, the user can choose to use different types of heat removal systems, each optimized for a specific coolant fluid to deal with different thermal situations. Nevertheless, in the context of lightweighting, it would not be ideal to have multiple and various types of thermal management devices installed on the vehicle.

[0143] According to embodiments of the present disclosure, a thermal management device includes one or more heat transfer surface made of the same material composition. The one or more heat transfer surface may be disposed in a coolant fluid for heat flux removal from the heat transfer device, e.g., by a mechanism that includes vaporization of the coolant fluid.

[0014] The one or more heat transfer surface may be characterized by a plurality of surface cavities or cavities at the (heat transfer) surface. A cavity may be referred to as a pit, a hole, a sunken area, a depression, or a concave area. In the present disclosure, reference to a microstructure size may be understood in terms of an average diameter of the microstructure. For example, the microstructure size may be represented by a diameter of a cavity (also referred to as cavity size).

[0145] The heat transfer surface includes an additively manufactured aluminum surface with a plurality of microstructures disposed thereon, the plurality of microstructures being characterized by a range of cavity sizes or by non-uniform cavity sizes. The range of cavity sizes may be selected to include two or more respective optimal cavity sizes of different coolant fluids. Further, the one or more heat transfer surface may be disposed in a quiescent pool of liquid (e.g., to facilitate pool boiling) and / or in a flow of liquid (e.g., to facilitate flow boiling).

[0146] According to embodiments of the present disclosure, the proposed thermal management device includes a heat transfer surface characterized by a first plurality of a first microstructures and a second plurality of a second microstructures, in which the first microstructures and the second microstructures are characterized by different sizes falling within an operable range for facilitating bubble nucleation, bubble growth, and bubble departure. In some examples, the thermal management device includes at least one heat transfer surfacecharacterized by a range of microstructures, e.g., the plurality of icrostructures may be described as being dimensionally non-uniform and characterized by different sizes that include length scales or length dimensions equal to or greater than 2 pm.According to embodiments of the present disclosure, the thermal management device is formed of an additively manufactured substrate in which the tunable subgrain structures are surface engineered to generate a plethora or a plurality of three-dimensional microstructures of different length scales or cavity sizes. The different sizes of the plurality of microstructures may include: a first plurality of first microstructures characterized by a first range of sizes, the first range of sizes; and a second plurality of second microstructures characterized by a second range of sizes, wherein the second range of sizes is different from the first range of sizes.In some examples, the different sizes of the plurality of microstructures may include length dimensions ranging from 3 pm to 8 pm, inclusive. In some examples, the different sizes of the plurality of microstructures may include length dimensions ranging from 10 pm to 15 pm, inclusive.

[0147] In some examples, the microstructures may be single-tier structures of various morphologies (for example, including but not limited to nanobumps, nanosteps, nanopores, microcells, microsteps, etc.) characterized by length scales or length dimensions in a range from 3 pm to 15 pm,

[0148] In some examples, the microstructures may be dual-tier structures including nanostructures hierarchically formed on microstructures. For example, the microstructures may include but are not limited to a second tier of nanostructures hierarchically formed on a first tier of microstructures. Examples of the first-tier microstructures may include but are not limited to nanobumps, nanosteps, nanopores, microsteps, etc. Examples of the second-tier nanostructures may include boehmite nanostructures if the substrate is formed of aluminum.

[0149] According to various embodiments of the present disclosure, the heat transfer device includes an additively manufactured aluminum substrate with one or more heat transfer surfaces. The heat transfer surfaces may be formed in the following manner: (i) additively manufacturing to form an as-printed surface, and (ii)the as-printed surface being surface engineered to provide a surface engineered surface with micro / nanostructures. The resulting one or more heat transfer surfaces are characterized by two different ranges of length scales within an overall range, the overall range having a lower limit that is greater than or equal to 2 pm, the overall range having an upper limit that is less than or equal to 15 pm.

[0150] The body of the heat sink may be configured to receive a pool of liquid and to dispose the at least one heat transfer surface in the pool of liquid.

[0151] The heat sink may be characterized by a heat transfer coefficient of 37.3 W / cm2K if in use for pool boiling of deionized water.

[0152] The heat sink may be characterized by a critical heat flux greater than 190 W / cm2if in use for pool boiling of deionized water.

[0153] The body of the heat sink may be configured to receive a flow of liquid across the at least one heat transfer surface.

[0154] A method of making a heat transfer device according to another aspect of the present disclosure is schematically illustrated in FIG.15.

[0155] Selective laser melting (SLM) or powder bed fusion metal additive manufacturing may be used to fabricate pristine ALSiWMg substrates. The additively manufactured substrates may be made by melting successive layers of aluminum alloy powder using a laser power of 350 W (watts) with an 80 m laser spot diameter, 1150 mm / s scan speed, and 0.17 mm (millimeters) hatch spacing. Each layer of the built may be 50 pm. The laser parameters were configured to achieve an energy density of 35 J / mm3(Joules per cubic millimeter). The substrates obtained has > 99% density. The boundaries of each layer were scanned using a laser power of 300 W and a scan speed of 600 mm / s. The fabrication was conducted at a platform temperature of 170 °C with the build chamber maintained at an absolute pressure 12 mbar (millibar). Argon gas was delivered into the built chamber to maintain an oxygen level of less than 0.2% to prevent combustion of the aluminum power during the laser melting process.

[0156] In the present disclosure, the terms "pristine" or "as-built" may be used interchangeably and refers to the substrate after completion of the additive manufacturing process before or without post-processing treatments (such as shotpreening or sandblasting) and without laser surface remelting of the final layer of the built.

[0157] The pristine substrate was then subjected to a first heat treatment at a temperature of 400 °C for two hours.

[0158] The substrate or workpiece was then subjected to a chemical etching by diluted hydrochloric acid (2.4 M HCI) for five minutes.

[0159] The substrate or workpiece was then subjected to a second heat treatment (also referred to as a post heat treatment) at a temperature of 500 °C for 24 hours.

[0160] Optionally, the substrate or workpiece may be subjected to a boehmitization process by immersion in deionized water of 95 °C for one hour.

[0161] To evaluate thermal performance under conditions where pool boiling was expected to dominate, the specimen was disposed in a container which could hold a quiescent pool of fluid. The container may be made with transparent sides for recording high speed videos to study bubble nucleation, growth, and departure.

[0162] FIG. 16A and FIG. 16C schematically illustrate the two experimental set ups used for evaluating the specimens for pooling boiling. Two different experimental set ups were required as the two coolant fluids investigated have different optimal operating conditions. FIG. 16B and FIG. 16D show magnified views of the specimen under test in the test facility of FIG. 16A and FIG. 16C, respectively.

[0163] In general, the experiments demonstrated that the boiling performance of the proposed heat sink surpassed that of a conventionally manufactured aluminum article. As compared to the conventional pristine aluminum alloy, the proposed heat transfer surface demonstrated enhancement or improvement to the heat transfer coefficient by up to 103.8% for the dielectric fluid HFE-7100, up to 200% for the refrigerant R134a, and up to 259.9% for the deionized water.

[0164] Table 2 below summarizes the pool boiling studies considered. The specimen name ends with "B" if the specimen was boehmitized for one hour.Table 2.

[0165] Table 3 summarizes the roughness measurements of some of the specimens. Table 4 shows thermophysical fluid properties of HFE-7100 and water.Table 3.Table 4.

[0166] Validation of new water pool boiling facility

[0167] FIG. 17 shows the plots of temperature measured by thermocouples T1 , T2, and T3 (see, e.g., FIG. 16B and FIG. 16D), against their vertical distance from the top boiling surface for plain 6061 -B and AM-H(500)B at three different heat fluxes. It can be seen that the coefficient of determination (R2) of each linear fittingline is very close to 1 , confirming that the heat conduction in the specimen is onedimensional. To further validate the integrity of the data collected from the water boiling setup, the pool boiling results of 6061 -B were compared against the pool boiling results of AI6061 , as shown in FIG. 18. It can be seen that the CHF of 6061 - B of 160.3 W / cm2falls within the range of CHF values of 130 to 200 W / cm2.

[0168] In addition, it can be seen from FIG. 18 that the 6061 -B investigated exhibited lower wall superheat (AT) as compared to all other boehmite surfaces reported by others. This is because unlike the surfaces reported in the literature, which were polished to the surface roughness of 0.1 pm - 0.4 pm, 6061 -B surface was as-fabricated through a milling process with an average Ra value of 3.6 pm, as shown in Table 3.

[0169] The increased surface roughness of 6061 -B may have introduced additional nucleation sites to improve boiling, thus resulting in lower AT as compared to other surfaces. Additionally, it is interesting to note that, from the present study, it was determined that the specimen CHF values were affected by the type of sealants adjacent to the test specimen that was used to prevent leakages. In this study, two sealants were studied, i.e., silicone gasket (Permatex 81160 High-Temp Red RTV) and epoxy (Araldite Rapid). When silicone gasket was used, it was observed that minor contact between the silicone gasket and hydrophilic boehmite surface, would cause the surface to lose its hydrophilicity and wicking properties. On the other hand, no change in wetting properties were observed when epoxy was used as the sealant. As a result, it was found that when silicone gasket was used, the DI water boiling experiments with 6061 -B showed a lower CHF value of 135.9 W / cm2.

[0170] It was also observed that when silicone gasket was used, the CHF phenomena observed were very different as compared to the case when the epoxy was utilized. As shown in FIG. 19, it was observed that when the epoxy was used, a thin vapor layer was formed across the surface during CHF which regularly pulsated. On the other hand, when the surface was surrounded by a silicone gasket, a much thicker parabolic-shaped vapor layer was formed, and the bubble formation from the vapor layer slowly grew and detached from the vapor layer. The thick vaporlayer (image (a) of FIG. 19) likely formed due to the hydrophobic characteristic of the surface, and the surrounding sealant prevented the surrounding bulk liquid from approaching the surface to vaporize. This phenomenon reduced the frequency of bubble departure. In fact, during CHF, the bubble departure frequency was approximately 23.5 Hz and 8.6 Hz when epoxy and silicone gasket were used as the sealant, respectively. As silicon gasket has significant influence on the surface wetting characteristics, epoxy was selected as the sealant.

[0171] Pool boiling of HFE-7100

[0172] The pool boiling performance of HFE-7100 on the microstructured AM specimens and comparisons against the plain AM and 6061 specimens are presented in FIG. 20.

[0173] The pool boiling performance of the plain 6061 , plain AM, microcell with 1 pm cavity (AM-E(7.5)), and microstep with 5 pm cavity (AM-H(400)E(5)) characterized in the inventors' earlier work are included in FIG. 20 for comparison. The filled marker at the end of each pool boiling curve denotes the last steady state heat flux attained before the occurrence of CHF. The boiling curves of q " against AT, and h against q" are shown in graphs (a) and (b) of FIG. 20, respectively. For the boiling curves presented here, the q " error bars are omitted for visual clarity as they are similar to the marker sizes. Some error bars for AT and h are also omitted for visual clarity, although it should be noted that the error bars for AT and h are similar across the different surfaces.

[0174] It can be seen from graph (a) of FIG. 20 that the boiling performance of all AM surfaces is better than 6061 , with the AM surfaces dissipating higher heat fluxes than 6061 at the same wall superheat. It was observed from image (a) of FIG. 21 that even for the plain AM surface, at the heat flux of 4 W / cm2, the quantity of bubbles generated on the AM surface was higher than 6061 . This resulted in higher h values of AM, as shown in graph (b) of FIG. 20.

[0175] The increase in bubble nucleation even on the plain AM surface arises from the increased number of nucleation sites that exist on the plain AM surface due to the SLM fabrication process, whereby the rapid melting and solidification of the melt pool generates random cavities, as shown in FIG. 22. The sizes of theserandom cavities are typically larger than 3 pm. However, interestingly, it can be seen from FIG. 20 that maximum stable heat fluxes prior to CHF are lower for the microstructured surfaces than the plain AM surface. The lower CHF values of the microstructured surfaces are due to the presence of many favorable nucleation sites on the microstructured surface. This is because, as the surface temperature increases, a larger range of nucleation sites are activated. Hence, once the surface attains a critical threshold temperature whereby further temperature increments results in excess nucleation site activation, the deduction in the distance between adjacent bubbles would promote undesirable bubble coalescence to form a continuous vapor layer, triggering CHF. In terms of surface roughness, despite the higher roughness of AM surfaces, it is not the predominant factor leading to the increased nucleation site density. This can be verified from the RMS values of plain AM and AM-E(7.5) recorded in Table 3. Even though AM-E(7.5) has higher surface roughness than the plain AM surface, both specimens exhibit similar pool boiling performance.

[0176] From FIG. 20, it can be seen that 6061 , attained CHF of 27.2 W / cm2and hmax of 1.42 W / cm2K. On the other hand, the plain AM surface attained CHF of 30.9 W / cm2and hmax of 1 .88 W / cm2- K. For the new nanobump structures, i. e. , AM- E(7.5)H(500), CHF of 26.0 W / cm2and hmax of 2.15 W / cm2K were attained. Hence, a CHF reduction of 4.3% and a hmax enhancement of 51.4% were achieved as compared to the plain 6061 surface. It can be seen that the boiling performance of AM-E(7.5)H(500) is similar to AM-E(7.5), and the trend of their boiling curves is similar to that of the plain AM surface. This is despite the AM-E(7.5) and AM- H(7.5)H(500) having vastly different microstructure topologies and slightly different length scales of 1 pm microcell structures and 300 nm - 700 nm nanobumps, respectively. Hence, this comparison shows that the topology of the microstructures of 0.3 pm - 1 pm in length scale does not significantly affect the pool boiling performance.

[0177] As compared to plain AM surface, the CHF of AM-H(7.5)H(500) was reduced by 15.8%, whereas its hmax was increased by 14.3%. The lower CHF value of AM-H(7.5)H(500) is again due to its higher active nucleation site densityas compared to plain AM. Hence, as the heat flux increases further, the surface temperature does not rise significantly enough to activate new bigger or smaller cavities. Therefore, the bubbles from the current active cavities have to increase in their departure size and frequency to dissipate the increased heat flux. As illustrated schematically in FIG. 23, the increase in bubble departure size and frequency increases the tendency in which the neighboring growing bubbles attached to the surface coalesce to form a continuous vapor layer leading to CHF. Such a phenomenon could be observed from the high-speed visualization of boiling on AM- E(7.5)H(500), as depicted in image (c) of FIG. 24. It was observed that a layer of small individual bubbles immediately above the surface of AM-E(7.5)H(500) exist before they coalesce to form a layer of large bubbles above. Such a bubble morphology was not observed on the plain 6061 and AM surfaces (see images (a) and (b) of FIG. 24). Hence, the individual bubble layer on AM-E(7.5)H(500) prior to CHF suggests that due to the low wall superheat temperatures, only a limited range of cavity sizes are activated and dissipating the heat. Thus, the bubble nucleating from those activated nucleation sites had to aggressively increase its departure size and frequency when the heat flux increases, leading to CHF.

[0178] Additionally, with the abundance of active nucleation sites on its surface, the h curve of AM-E(7.5)H(500) in graph (b) of FIG. 20 is observed to not plateau and dip, unlike the plain surfaces. For the plain surfaces (6061 and AM), the h values plateau and dip near its CHF due to the aggressive large and frequent bubble nucleation across the surface to dissipate the high heat flux. Hence, with large bubbles growing and expanding on the surface, the incoming replenishing fluid is disrupted, which is required to aid in heat removal from the surface. This leads to a larger increase in surface temperatures, which lowers the h values at heat fluxes near its CHF. However, the h values at the higher heat fluxes near CHF do not dip for AM-E(7.5)H(500) as there were ample liquid replenishment pathways on the boiling surface, similar to the h curve of AM-H(400)E(5). Such reduction in CHF phenomena is more prominent in microstructured surfaces with larger quantities of favorable cavity sizes such as AM-H(400)E(5).

[0179] In terms of h values, AM-E(7.5)H(500) is observed to perform slightly better, with a higher hmax enhancement of 51.4% as compared to 6061. On the other hand, AM-E(7.5) only recorded a hmax enhancement of 40.5% as compared to 6061. This minor enhancement of AM-E(7.5)H(500) could be due to the post heat treatment process applied on AM-E(7.5)H(500), where some surface structures weakly attached to the bulk surface from the etching process disjoints due to the Si particle agglomeration from the post heat treatment process (second heat treatment) and were observed to be removed from the surface during sonication. Hence, with the removal of additional loose particles from the surface, the surface roughness of AM-E(7.5)H(500) slightly increases as compared to AM-E(7.5) (see Table 3). This leads to a slight increase in surface area to increase the quantity of cavity, thus increasing h. It is also possible that this increase in cavity density on the surface as compared to AM-E(7.5) has led to the phenomenon described in FIG. 23, which results in lower CHF performance. Even though the microstructure morphology and length scale on AM-E(7.5) and AM-E(7.5)H(500) are different, it can be seen from graph (b) of FIG. 20 that they do not result in significant differences in their h values. Hence, it can be deduced from this comparison that the cavities with sizes ranging from 0.3 to 1 pm are not optimal for bubble nucleation. On the other hand, the cavities on AM-E(7.5) and AM-E(7.5)H(500) that are responsible for bubble nucleation are shown in FIG. 25. These larger cavities ranging from 3 pm to 8 pm on AM-E(7.5) and AM-E(7.5)H(500) are either likely due to internal porous defects from the SLM printing process that were revealed during the etching process, or larger silicon particles that were formed during the printing process and were removed during the etching process.

[0180] The AM microstructures formed through the heat treatment temperature of 300°C, i.e., AM-H(300)E(10) and AM-H(300)E(10)H(500), their pool boiling curves are similarly shown in FIG. 20. For the surface with nanostep features, AM- H(300)E(10), CHF of 25.6 W / cm2and hmax of 2.45 W / cm2- K were attained. This corresponds to a CHF reduction of -5.8% and hmax enhancement of 73.1 %, respectively, as compared to 6061. For the surface with nanopore features, AM- H(300)E(10)H(500), CHF of 24.6 W / cm2and hmax of 2.6 W / cm2- K were attained.This corresponds to a CHF reduction of -9.6% and hmax enhancement of 83.4%, respectively, as compared to 6061. In addition, AM-H(300)E(10) and AM- H(300)E(10)H(500) also exhibited 17.2% to 20.5% lower CHF as compared to the plain AM surface, which is again because both AM-H(300)E(10) and AM- H(300)E(10)H(500) have a larger quantity of favorable cavity cavities as compared to plain AM surface. It should also be noted that despite AM-H(300)E(10) and AM- H(300)E(10)H(500) having significantly lower surface roughness than AM- E(7.5)H(500), their CHF values are similar. This comparison further verified that the CHF reduction phenomenon is not dependent on surface roughness AM- H(300)E(10) and AM-H(300)E(10)H(500) having microstructure length scale of 0.5 pm - 2.0 pm and similar to the microstructure length scale of AM-E(7.5)H(500), the h values of AM-H(300)E(10) and AM-H(300)E(10)H(500) are noticeably higher than AM-E(7.5)H(500). This is because, there are more cavities of 3 pm - 8 pm in size on AM-H(300E(10) and AM-H(300)E(10)H(500) as compared to AM-E(7.5)H(500), and this is evidenced by comparing the SEM images of all three specimens as shown in FIG. 26. Based on our investigation, where AM-H(400)E(5) with microstructure of cavity size of 5 pm demonstrated the highest h, we postulate that it is the larger quantity of 3 pm - 8 pm cavities (such are ideal sites for bubble nucleation) on AM-H(300E(10) and AM-H(300)E(10)H(500) that are responsible for the higher h as compared to AM-E(7.5)H(500). The larger quantities of cavity sizes between 3 pm - 8 pm exist on the surface of AM-H(300)E(10) and AM- H(300)E(10)H(500) is likely due to the precipitation and agglomeration of the Si- phase forming large local clumps of Si particles of 3 pm - 8 pm during the heat treatment process at 300 °C. Hence, when these large clumps of Si particles were removed during the etching process, cavities of 3 pm - 8 pm were formed. However, it should be noted that the formation of larger clumps of Si particles could also be dependent on the local defects during the SLM fabrication process, and this resulted in the formation of a range of sizes of Si particles (3 pm - 8 pm). The presence of a range of cavity sizes of 3 pm - 8 pm on AM-H(300)E(10) and AM- H(300)E(10)H(500) is also evident from the characteristics of the boiling curves depicted in FIG. 20. It can be seen from this figure that the gradients of the boilingcurves of AM-H(300)E(10) and AM-H(300)E(10)H(500) are not as steep as AM- H(400)E(5). The steeper boiling curve of AM-H(400)E(5) is due to the presence of cavities that are uniform in size (5 pm). Hence, as the heat flux supplied to AM- H(400)E(5) increases, the wall superheat only needs to increase slightly to activate significantly more nucleation sites due to the size uniformity of the microstructure cavities.

[0181] However, unlike AM-H(400)E(5), a wider range of wall superheats are required to active the cavities on AM-H(300)E(10) and AM-H(300)E(10)H(500) due to their size difference, resulting in a gentler boiling curve gradient. However, also due to the large variation of cavity sizes on the microstructured 300°C heat treated specimens, an even larger and more optimal cavity size of approximately 8 pm exists on the surface, which enhances the h values at low heat fluxes. Hence, for AM-H(300)E(10) and AM-H(300)E(10)H(500) at a heat flux of 4 W / cm2, relatively high h values of 1 .29 W / cm2- K and 1.11 W / cm2K were recorded, respectively.

[0182] The pool boiling performance of the hierarchical micro / nanostructured surface (AM-E(7.5)H(500)B), its single-tier counterpart (AM-E(7.5)H(500)), and the plain surfaces (6061 and AM) are shown in graphs (i) and (ii) of FIG. 27A. For ease of comparison, the hierarchical micro / nanostructured surfaces (AM-H(300)E(10)B and AM-H(300)E(10)H(500)B), their single-tier counterparts (AM-H(300)E(10) and AM-H(300)E(10)H(500)), and the plain surfaces (6061 and AM) are shown in graphs (i) and (ii) of FIG. 27B. Generally, it can be seen that the hierarchical addition of the 300 nm boehmite reduces the h values and has no significant impact on the CHF performance as compared to the single-tier microstructured counterparts. In particular, the CHF values of AM-E(7.5)H(500)B, AM-H(300)E(10)B, and AM- H(300)E(10)H(500)B were 28.2 W / cm2, 27.5 W / cm2, and 26.4 W / cm2, respectively. These results correspond to CHF enhancements of 3.5%, 1.2%, and -3.1 % as compared to 6061 . These results suggest that the addition of nanostructures does not aid in the replenishment of the coolant fluid towards the boiling surface to prevent drying out for low surface tension fluids. On the other hand, the hmax values of AM-E(7.5)H(500)B, AM-H(300)E(10)B, and AM-H(300)E(10)H(500)B were 1.98 W / cm2K, 1.96 W / cm2K, and 2.2 W / cm2K, respectively. These results correspondto hmax enhancements of 39.4%, 38.2%, and 55.3% as compared to 6061. The h values of the hierarchical micro / nanostructured surfaces reduce with the addition of a boehmite layer. Due to the addition of boehmite nanostructures, the layer of boehmite uniformly reduces the favorable cavity sizes on the surface, as exemplarily shown in FIG. 28. With the reduction in cavity sizes, some of the original cavities that were in the optimal size range for bubble nucleation no longer serve as nucleation sites.

[0183] This resulted in a rightward parallel shift in the boiling curves for the micro / nanostructured surfaces as compared to their single-tier microstructured surfaces, as seen in graph (i) of FIG. 27A and in graph (i) of FIG. 27B. On the other hand, it can be seen that the post heat treatment process slightly enhances the h values through the additional roughness, giving rise to additional nucleation sites. AM-E(7.5)H(500), AM-E(7.5)H(500)B, AM-H(300)E(10)H(500), and AM-H(300)E(10)H(500)B exhibited hmax enhancements of 7.8%, 27.7%, 6.0%, and 12.4% as compared to their non-post heat treated counterparts, respectively, as also seen in graph (ii) of FIG. 27A and in graph (ii) of FIG. 27B. It was noted that the reduction and downward shift of the h curves become more significant at the higher heat flux and larger wall superheats for AM-H(300)E(10)B and AM- H(300)E(10)H(500)B. It can be seen from graph (ii) of FIG. 27B that the single-tier microstructured surfaces (AM-H(300)E(10) and AM-H(300)E(10)H(500)) have similar h values as the plain AM at 4 W / cm2but deviates significantly from the plain AM surface at higher heat fluxes. On the other hand, the micro / nanostructured surfaces also have similar h values of approximately 1.15 W / cm2- K as the plain AM surface and their single-tier microstructured surface counterparts at the low heat flux region of 4 W / cm2.

[0184] However, the pool boiling curves of AM-H(300)E(10)B and AM- H(300)E(10)H(500)B are less steep than the single-tier microstructured surfaces, as seen in Figs. 20(c). For example, at higher heat fluxes of approximately 25 W / cm2, the h values of AM-H(300)E(10)B and AM-H(300)E(10)H(500)B are 20.1 % and 14.3% lower than the single-tier microstructured surfaces, respectively. This further shows that there are not many cavity sizes which are optimal for bubblenucleation on AM-H(300)E(10) and AM-H(300)E(10)H(500). Hence, the addition of boehmite, which reduces cavity sizes, reduces the number boiling performance between the hierarchical structured surfaces and the plain AM surface.

[0185] Although it has been shown that the use of AM can enhance pool boiling performance as compared to conventional metal surfaces due to the increase in nucleation site density, enhancing the h values further is only significant if heat treatment is performed prior to etching to generate the microstructures. Through the process of heat treatment, the formation of larger Si particles, which are removed during the etching process, creates larger and more favorable cavity sizes between 3 pm and 8 pm for bubble nucleation and enhances the h values. Additionally, by subjecting the specimens with microstructures through a post heat treatment process of 500°C, it is observed that due to the increased surface roughness, which increases the nucleation site density, a slight enhancement in the h values can be achieved. The increase in surface roughness appears to be only beneficial to enhance h if there is a lack of cavities that are close to the optimal cavity sizes of 5 pm - 8 pm. This can be seen from the best performing microstructured surface, AM-H(400)E(5), where despite the significantly lower surface roughness than the other microstructured surfaces (see Table 3), the abundance of 5 pm cavities on the surface dominates the pool boiling performance.

[0186] Owing to the excess formation of favorable nucleation sites on the surface through microstructuring, the CHF performance tends to reduce as wall superheats are low and insensitive to the change in heat flux. This may prevent a larger range of cavity sizes from being activated. As the heat flux increases further, the active nucleation sites on the surface would have to increase its bubble departure size and frequency to dissipate the additional heat. This, in turn, increases the likelihood of the formation of a thick continuous vapor layer.

[0187] For the hierarchical addition of 300 nm grass-like boehmite nanostructures, the additional layer of nanostructures reduces the cavity sizes on the surface by approximately 600 nm. Hence, as the cavity sizes on the microstructured AM surfaces are not exceedingly large, in which a cavity size reduction would then be favorable for bubble nucleation, the h values decrease withthe addition of nanostructures. Overall, different microstructure morphologies and length scales ranging from 0.3 pm to 2 pm, such as microcell (AM-E(7.5)), nanobump (AM-E(7.5)H(500)), nanostep (AM-H(300)E(10)), nanopore (AM- H(300)E(10)H(500)), and grass-like boehmite structures were investigated.

[0188] For the first time, a new 300°C heat treatment process was implemented, which enabled cavities of 3 pm to 8 pm size to be generated. Through the pool boiling investigation of these structures, it was shown that microcavities2 pm do not affect the pool boiling performance of low surface tension fluid, whereas cavities of 3 pm to 8 pm are favorable nucleation sites.

[0189] Pool boiling of deionized (DI) water

[0190] High-speed images of the bubble dynamics in the pool boiling of DI water at various heat fluxes were recorded. In the pool boiling of HFE-7100 with structured surfaces, small bubbles form and grow from the nucleation sites, and once the buoyancy forces are larger than the surface adhesion force, the bubble departs. At higher heat fluxes, individual small bubbles still form immediately above the surface. However, these smaller bubbles coalesced to form a larger bubble above them (see images (c) and (d) of FIG. 17. However, for the pool boiling of DI water, the bubble sizes are significantly larger, and this resulted in significantly different bubble dynamics as compared to the boiling of HFE-7100. In total three different bubble dynamics were observed in DI water boiling. At very low heat fluxes, i.e.,40W / cm2, isolated bubbles from different nucleation sites grew and departed from the surface, as shown in FIG. 29 and FIG. 30. Some of these bubbles exhibited the typical bubble ebullition cycle where the bubbles grew and departed, and subsequently, a short waiting time was observed prior to the beginning of the next bubble growth, as shown in FIG. 29. On the other hand, FIG. 30 shows that some bubbles exhibited a constant-coalescence behavior whereby no waiting time was observed between a bubble departure and the next bubble growth from the same nucleation site. At slightly higher heat fluxes, i.e.,60 W / cm2, isolated bubbles were no longer present, but rather, the bubbles growing from the nucleation sites on the entire surface constantly coalesced with adjacent bubbles during their growth period to form a larger bubble over the surface before departure, as shown in FIG.31. Interestingly, it was observed that some of these bubbles constantly remain relatively large despite coalescence with a larger bubble on the surface, due to rapid liquid vaporization on the surface, as evident in FIG. 32. It is likely that small bubbles perpetually remain on the surface due to the high vaporization rates to dissipate the heat. This phenomenon, coupled with DI water’s high surface tension, resulted in the surrounding water being unable to completely penetrate the base of the bubble to facilitate bubble departure. At higher heat fluxes, i.e. , $= 120 W / cm2, a very large bubble forms on the surface that spans up to three times the dimension of the boiling surface. After its departure, a smaller bubble forms and grows on the surface that necks to the bigger bubble, which creates a mushroom-shaped vapor bubble and hence, in a bubble pulsation cycle, two bubbles of different sizes grow and depart, as shown in FIG. 33. The formation of the large bubble was observed due to the synchronized bubble growth from all the nucleation sites on the surface as evident in FIG. 34. It is likely that rapid vaporization of the fluid is due to a temperature spike on the surface after the necking bubble cycle as not much liquid was vaporized during this stage. Then, after the rapid growth of the large bubble, it is expected that the surface temperature drastically dipped, which caused the liquid vaporization rate to decrease drastically. Hence, this leads to the smaller necking bubble whereby the bubbles forming from the nucleation sites constantly coalesce to form a larger bubble, similar to the phenomenon observed in FIG. 31.

[0191] Surface wickability has been widely reported to affect CHF of water pool boiling. This is because, with an increase in the surface capillary suction, additional cool liquid is drawn from the bulk fluid due to capillary action to replenish the nucleation sites on the surface. Hence, the replenishing fluid can increase liquid vaporization, vapor detachment, and delay surface dry-out. However, it is believed that surface wickability is not the sole significant parameter in delaying the onset of CHF. Rather, other parameters such as the bubble dynamics (such as the bubble departure size, frequency, and quantity) or material thermal diffusivity are suspected to affect the CHF. Due to such knowledge gaps in existing literature, the surface wickability of the micro / nanostructures generated and investigated in thewater pool boiling experiments were characterized and now described in the following.

[0192] The method of temporal propagation front of the liquid spreading across the surface was used to determine the wickability of superhydrophilic surfaces. In the wicking setup, a camera is positioned vertically above and perpendicular to the surface, and used to visualize the droplet spreading and wicking across the specimen surfaces. A syringe needle of diameter 0.3 mm was attached to the outlet of flexible tube. The tube was in turn connected to the outlet of an automatic dispensing system (Rame-hart Instrument Co.) to dispense a droplet volume of 4.5 pL ± 0.2 pL. The syringe needle tip was positioned 3 mm above the surface to minimize the effect of droplet gravitational acceleration and impingement on the surface, and a slow droplet dispense rate of 2.5 pL / s was used to reduce the water droplet’s dispense inertia. Micro / nanostructured flat plate specimens of 25.4 x 25.4 x 2 mm in dimension were used for the wicking characterization. The specimen size used for wicking test is larger than the dimensions of boiling surfaces (10 x 1 o mm) because the highly wicking micro / nanostructures generated in this study would result in which the dispensed water droplet to spread beyond 10 x 1 o mm. All of the water droplets were dispensed at a room temperature of 23 °C ± 2 °C. The droplet wicking phenomena were recorded using a high-speed camera (iX Camera i- SPEED 508) at 2000 fps(1920 x 1080 pixels), and the specimens were positioned as close to the camera as possible which results in a spatial resolution of 24.9 pm / pixel. It was observed that the droplet wicking front was not a uniform circular shape due to the irregular surface roughness of the specimens causing an asymmetric droplet spreading front. Hence, the average water wicking front radius were determined by measuring the area of the water spreading on the surface and equating to a perfect circle and calculating its corresponding radius.

[0193] At least two wicking tests were performed on each surface to ensure repeatability of results and the wicking radius results were averaged. The average and maximum wicking radius deviation from the average radius are 2.8% and 6.6%, respectively. FIG. 35 plots the wicking performances of 6061 -B, AM-H(500)B, AM- E(7.5)H(500)B, AM-H(300)E(10)H(500)B, and AM-H(400)E(5)H(500)B. It can beseen that despite the highly wetting boehmite nanostructures generated on all surfaces, the wicking performances are significantly different across the surfaces due to the difference in morphology of the first-tier microstructures and the surface microroughness. The average wicked radius at 400 ms for 6061 -B and AM-H(500)B were 2.97 mm and 5 mm, respectively. A 68.1 % higher wicked radius of AM- H(500)B as compared to 6061 -B is likely due to the higher surface roughness and microfeatures present on the as-printed AM surface, which induces a larger capillary force on the water. For AM-E(7.5)H(500)B, AM-H(300)E(10)H(500)B, and AM-H(400)E(5)H(500)B, the average wicked radius at 400 ms were 7.32 mm, 6.75 mm, and 6.47 mm, respectively. These results corresponds to a 146%, 127%, and 117% increase in average wicked radius as compared to 6061 -B. Significantly larger wicking radius were observed for the micro / nanostructured surfaces as compared to the single-tier nanostructured surfaces (6061 -B and AM-H(500)B) were due to the larger capillary force induced by the microstructures. It should be noted that slightly smaller average wicked radius were observed for the porous micro / nanostructured surfaces, such as AM-H(300)E(10)H(500)B and AM- H(400)E(5)H(500)B, due to liquid imbibition in the porous structure morphology. Hence, the non- porous micro / nanostructure, AM-E(7.5)H(500)B, had the largest average wicked radius observed.

[0194] The pool boiling performance of DI water with 6061 -B and AM-H(500)B is shown in FIG. 36. The error bars similar to the marker sizes are omitted for visual clarity. The nanostructured conventional aluminum surface, 6061 -B, attained a CHF of 160.3 W / cm2and hmax of 10.4 W / cm2K, whereas AM-H(500)B attained a CHF of 191.6 W / cm2and hmax of 18.7 W / cm2K. As compared to 6061 -B, the CHF and hmax of AM-H(500)B are 19.5% and 80.2% higher. As boehmite nanostructures are present on both AM-H(500)B and 6061 -B, the higher CHF attained for AM- H(500)B could be due to the higher surface roughness of the pristine AM surface (see Table 3). Hence, leading to improved water-wicking surface performance (see FIG. 35) to delay the onset of CHF. Also, the higher CHF performance of the AM- H(500)B surface could be due to the higher thermal diffusivity of the heat treated AISi Mg as compared to AI6061. This is because it has been reported that in thepool boiling of water, large temperature fluctuations exist on the surface, especially at higher heat fluxes.

[0195] As the specimen material's thermal diffusivity increases, a smaller span of temperature fluctuations exist on the surface, and this has been shown to be favorable in delaying CHF.

[0196] With regard to the h values, the higher h values of AM-H(500)B as compared to 6061 -B is due to the former containing a larger number of large cavities on its surface, which are favorable nucleation sites for DI water. Images (a) (b) of FIG. 37 show the high-speed images of bubble nucleating on AM-H(500)B and 6061 -B at low heat flux of 12 W / cm2. It can be clearly seen that more bubbles are nucleated on AM-H(500)B as compared to 6061 -B. However, given that an abundance of cavity sizes up to 8 pm were observed on 6061 -B as seen in images (a) and (b) of FIG. 38, it shows that the cavity size required for bubble nucleation is larger. Additionally, it was observed that only a few cavities were activated on the plain AM surface in water as compared to HFE-7100, as seen in images (b) of FIG. 38 and image (b) of FIG. 21. It further shows that the cavity size required for bubble nucleation is larger, such as those shown in images (c) and (d) of FIG. 38. An abundance of cavity quantities of up to 8 pm on the 6061 -B surface was likely present due to the milling fabrication process as the tool cuts the material. On the other hand, larger cavities and features likely exist on the surface of AM-H(500)B due to the SLM fabrication process, where rapid solidification of the melt pool occurs after the laser melts the layer of powder during the fabrication process, forming large cavities and features as the melt pool rapidly cools.

[0197] The pool boiling performance of the hierarchical micro / nanostructured surfaces, i.e., AM-E(7.5)H(500)B, AM-H(300)E(10)H(500)B, and AM-H(400)E(5)H(500)B, and nanostructured surfaces, 6061 -B and AM-H(500)B, are shown in FIG. 39. AM-E(7.5)H(500)B, AM-H(300)E(10)H(500)B, and AM- H(400)E(5)H(500)B have attained CHFs of 195.5 W / cm2, 191.3 W / cm2, and 203 W / cm2, which corresponds to enhancements of 22%, 19.3%, and 26.6% as compared to 6061 -B, respectively. In addition, hmax of these hierarchical surfaces, AM-E(7.5)H(500)B, AM-H(300)E(10)H(500)B, and AM-H(400)E(5)H(500)B, are11.6 W / cm2K, 37.3 W / cm2K, and 18.2 W / cm2K, which are 11.6%, 259.9%, and 75.5% higher than 6061 -B, respectively. It can be seen that the hierarchical surfaces have similar CHF as compared to AM-H(500)B of 191.6 W / cm2even though the hierarchical structures have completely different structure morphology from the boehmite grass-like nanostructures on AM-H(500)B. Additionally, even though the micro / nanostructuring processes have resulted in different surface roughness on different hierarchical surfaces, the surface roughness difference seems to have an insignificant effect on the CHF. For instance, even though AM- E(7.5)H(500)B and AM-H(400)E(5)H(500)B have a significantly different average surface roughness of 42.0 and 29.2 pm, respectively, as shown in Table 3, their CHF values are almost similar. Although surface roughness has been reported by others to affect the CHF performance, it is likely that once sufficient surface roughness is present, the wettability or wickability of the surface plays a more dominant role in the CHF performance as it influences the fluid replenishment to the surface, which delays the surface from drying out. However, no significant differences in CHF were observed between AM-H(500)B and the hierarchical micro / nanostructured surfaces despite a significant difference in surface wickability as observed in FIG. 35. Hence, this suggests that once sufficient surface wickability is present, its influence in delaying the onset of CHF diminishes.

[0198] It may be deduced from the experimental results that the nucleation site density does not appear to have a significant effect on the CHF, given that the CHFs of the hierarchical micro / nanostructured (which are expected to have higher nucleation site density) are similar to AM-H(500)B. This analysis further shows that the difference in the CHF performance may be attributed to the specimen's thermal diffusivity once sufficient surface roughness and wickability are present on the surface. The specimen's thermal diffusivity influences the CHF performance. The high thermal diffusivity of copper might also explain the micro / nanostructured copper surface achieving a higher CHF of 300 W / cm2.

[0199] With regards to h values obtained from DI water boiling, it can be seen from FIG. 39 that the h values of AM-E(7.5)H(500)B and 6061 -B are similar. This suggests that both specimens have similar quantities of cavities for bubblenucleation. This is an interesting observation because AM- E(7.5)H(500)B and 6061 -B have completely different micro / nanostructure morphologies and length scales. This also suggests that the micro / nanostructures on AM-E(7.5)H(500)B and 6061 -B are not responsible for the bubble nucleation of DI water.

[0200] FIG. 40 shows the low-magnification SEM images of AM-E(7.5)H(500)B where large cavities of approximately 8 pm and 10 pm - 15 pm are found, as shown in images (a) and (b) of FIG. 40, respectively. These cavities could be sites that were previously occupied by the un-melted Si particles as a result of the AM fabrication process and were subsequently removed during the etching process to generate the microstructures. However, it can be seen from image (a) of FIG. 40 that most of these cavities on AM-E(7.5)H(500)B are in the 8 pm range with limited cavities that are 10 pm - 15 pm. Here, it should be noted that large cavities of up to 8 pm were also found on 6061 -B, which probably caused the h values of 6061 -B and AM-E(7.5)H(500)B to be similar. On the contrary, as shown from the SEM analysis of Figs. 33(c) and 33(d), AM-H(300)E(10)H(500)B consists of a larger number of cavities that are between the size of 10 pm - 15 pm. Similar to AM- E(7.5)H(500)B, these large cavities on AM-H(300)E(10)H(500)B could be sites that were previously occupied by Si particles.

[0201] Unlike AM-E(7.5)H(500)B, H(300)E(10)H(500)B had undergone a 300°C heat treatment process prior to etching, and the heat treatment process has resulted in the agglomeration and growth of the Si particles, thus increasing the size of the Si particles prior to their removal by etching and leaving behind significant quantities of cavities that are 10 pm - 15 pm in size. It is evident from the boiling pool results of FIG. 39 that these cavity sizes of 10 pm - 15 pm are the most optimal sites for bubble nucleation as AM-H(300)E(10)H(500)B has the highest h values among all the surfaces. The excellent boiling performance of H(300)E(10)H(500)B can also be clearly observed from the high-speed images, e g., image (b) of FIG. 41, where a significantly larger number of bubbles were nucleated at the low heat flux of 12 W / cm2as compared to other micro / nanostructured surfaces and the nanostructured surfaces in FIG. 37 and FIG. 41. With more nucleation sites activated, more heat can be removed from the increased bubble ebullition from thesurface during the bubble growth period, and a lower average surface temperature and a higher h value can be achieved.

[0202] It was noted that the period of the mushroom-shaped pulsation cycle on the AM-E(7.5)H(500)B is similar to AM-H(300)E(10)H(500)B with an average period difference of only 2 ms (two bubble pulsation cycles), at a heat flux of approximately 193 W / cm2. The same bubble pulsation frequency was observed even though the h value of AM-H(300)E(10)H(500)B at 193 W / cm2is 3.2 times of AM- E(7.5)H(500)B. With a similar bubble departure rate, it is postulated that it is likely that more vaporization occurred on AM-H(300)E(10)H(500) with the increased nucleation site density to lower the average surface temperatures than on AM- E(7.5)H(500)B.

[0203] The DI water pool boiling results of AM-H(400)E(5)H(500)B further verified our hypothesis that the optimal cavity size for bubble nucleation is between 10 pm and 15 pm. As shown in FIG. 39, AM-H(400)E(5)H(500)B exhibited poorer h values as compared to AM-H(300)E(10)H(500) and is the lowest amongst all the structured surfaces. An analysis of the surface structure morphology of AM- H(400)E(5)H(500)B reveals that it only consists of 5 pm size cavities (see images (e) and (f) of FIG. 40). Even though such cavity size is ideal for boiling HFE-7100, they are unfavorable for bubble nucleation of DI water. Larger nucleation sites are likely required for boiling incipience in DI water due to the significantly higher surface tension, saturation temperature, and lower vapor density as compared to HFE-7100. This is also evident from the few nucleation sites activated on the surface of AM-H(400)E(5)H(500)B at a heat flux of approximately 12 W / cm2, as shown in image (c) of FIG. 41. However, interestingly, the wall superheat was observed to decrease as the heat flux is increased beyond 50 W / cm2, similar to AM- H(300)E(10)H(500)B, as seen in graph (a) of FIG. 39. This is likely due to the hotspots that exist on the surface during pool boiling. These hotspots led to an increase in nucleation site activation on the surface, which lowers the surface average temperatures.

[0204] Such a phenomenon of the hotspot-activated nucleation sites is shown schematically in FIG. 42. More nucleation sites could be activated on AM-H(300)E(10)H(500)B and AM-H(400)E(5)H(500)B due to the microstructured surface being relatively rough and uneven, which depending on the local microstructure morphology, could still act as effective nucleation sites for liquid vaporization to take place although a higher wall superheat is required to initiate bubble ebullition. It should be noted that the reduction in wall superheats at higher heat fluxes in water pool boiling was also observed in other microstructured surface studies, and such a phenomenon has been termed the secondary boiling effect. It has been postulated by others that additional nucleation sites were activated at the tips of the microstructures as the heat flux increased due to the large temperature drop across the microstructures, and hence, such a phenomenon was only observed in materials with low thermal conductivity. However, a reduction in the wall superheat at higher heat fluxes was also observed in microstructured materials such as copper.

[0205] The proposed thermal management device may be configured to harness the synergistic effect of micro / nanostructures length scale and fluid thermophysical properties on pool boiling performance, In general, the AM surface, with its higher surface roughness and larger quantities of nucleation sites on the as-printed surface, is shown to be favorable to enhance pool boiling performance for both HFE-7100 and DI water. Also, the variance in the micro / nanostructure morphologies of the AM surface has no significant effect on the pool boiling performance for both fluids, whereas the structure length scale, which influences the cavity size, has a predominant effect on the boiling performance of different fluids. Hence, due to the differences in thermophysical properties of different engineering fluids, different micro / nanostructuring strategies of the AM are required to generate optimal structure length to boiling.

[0206] For example, AM-H(400)E(5) gave the largest h values for HFE-7100 with its abundance of 5 pm cavities on the surface, while AM-H(300)E(10)H(500)B achieved the highest h values in DI water with its increased quantities of cavity feature sizes between 10 pm and 15 pm. With regard to the CHF values attained, it was found that similar AM micro / nanostructures may result in different performance as compared to the baseline specimen for different fluids. For HFE-7100, the abundance of cavities on the micro / nanostructured surface was reported to reduce the CHF performance. On the other hand, for DI water, the different micro / nanostructured surfaces do not influence CHF significantly due to sufficient surface roughness and wickability, and the CHF performance is then dependent on the specimen material's thermal diffusivity. A summary of CHF and hmax values of all the specimens investigated, as well as their magnitude of enhancements, are provided in Table 5.Table 5.

[0207] For HFE-7100 pool boiling, plain AI6061 and AM were used as the baseline for comparison against single-tier microstructures with differentmorphologies (e g., nanobump, nanostep, nanopore, microcell and microstep, etc.) and with length scales ranging from 0.3 pm - 2 pm. Additionally, boehmite nanostructures of lengths 300 nm were hierarchically generated on the microstructures. For DI water pool boiling, AI6061 and AM with boehmite nanostructures were used as the baseline for comparison against hierarchical micro / nanostructured surfaces with nanobump, nanostep, nanopore, and microstep as the first-tier structures, and with boehmite as the second-tier structures.

[0208] The pool boiling performance of the various surfaces was analyzed by comparing their pool boiling curves, high-speed bubble dynamics analysis, surface characteristics, and wickability. Some of the findings are summarized below.

[0209] (i) In HFE-7100, owing to the increased presence of favorable cavity sizes between 3 pm - 8 pm, AM-H(300)E(10)H(500) exhibited the highest hmax enhancement of 83.4% as compared to the plain 6061 amongst the new specimens tested. Owing to an abundance of favorable nucleation sites, a CHF reduction of -9.6% was observed in AM-H(300)E(10)H(500) as compared to the plain 6061 .

[0210] (ii) In water, owing to the larger quantities of cavity sizes greater than 10 pm and likely the increased surface roughness of the as-printed AM surface, AM-H(500)B performed better than 6061 -B with an enhancement of CHF and hmax by 19.5% and 80.2%, respectively.

[0211] (iii) In water, owing to the optimal agglomeration of Si particles followed by their removal through chemical etching, AM-H(300)E(10)H(500)B had the most quantities of favorable cavity sizes larger than 10 pm. This resulted in AM-H(300)E(10)H(500)B exhibiting the highest enhancements in hmax by 259.9%, along with a CHF enhancement of 19.3%, as compared to nanostructured 6061 -B.

[0212] (iv) Through the systematic tunning of microstructure length scale from 0.3 - 2 pm, it was found that microstructures with length scale smaller than 2 pm do not play a significant role in altering pool boiling performance of all fluids tested. On the other hand, cavity sizes of 3 pm - 8 pm and cavitysizes of 10 m - 15 m would be optimal for enhancing the pool boiling heat transfer coefficient of HFE-7100 and DI water, respectively.

[0213] The present disclosure describes various embodiments of a thermal management device and a method of making thereof. The thermal management device includes additively manufactured aluminum configured with a plethora of three-dimensional micro / nanostructures of different length scales, e g., cavity sizes.

[0214] The micro / nanostructures of one thermal management device may include a first plurality of micro / nanostructures characterized by a first range of a first length scale (e.g., first cavity size) and a second plurality of the micro / nanostructures characterized by a second range of a second length scale (e g., second cavity size).

[0215] The first length scale range and the second length scale range may be selected such that the single thermal management device enables enhanced pool boiling of two or more coolant fluids of different thermophysical properties.

[0216] The enhancements (with respect to AI6061 ) were experimentally verified. Prototypes or specimens were additively manufactured from AIS i 10Mg powder and then surface engineered to provide microstructures characterized by non-uniform length scales or different length scales. The prototypes or specimens were made and tested with a dielectric fluid and with deionized water as examples of coolant fluids with different thermophysical properties. The results demonstrated that the relatively lightweight aluminum can be enhanced such that the aluminum thermal management device could acquire thermophysical properties comparable to a device made of copper. The enhancement was found to be significant and can qualify the aluminum-based thermal management device for use one a vehicle. This means that there is now a viable lightweight alternative to heavy cooper thermal management devices.

[0217] It is envisioned that other materials may be used to make the additively manufactured intermediate product, and to surface engineer the additively manufactured intermediate product, taking advantage of the tunable subgrain structures that had resulted from the additive manufacturing to produce adistribution of non-uniform microstructures on one or more surfaces intended for heat transfer or heat dissipation.

[0218] In one aspect, the present disclosure presents the first surface micro / nanoengineering method to significantly enhance pool and flow boiling for additively manufactured metals and for different coolants that can be utilized for thermal management applications.

[0219] In another aspect, the present disclosure presents the first surface micro / nanoengineering method for additively manufactured metals that is able to tune the micro / nanostructure length scale to enhance pool and flow boiling performance.

[0220] In yet another aspect, the present disclosure presents the first demonstration of combining micro / nanoengineering surfaces and structures with macro-geometry for additively manufactured metals to enhance two-phase pool and flow boiling.

[0221] The present disclosure further presents the first method to preferentially generate micro / nanoengineered surfaces on additively manufactured structures with macro-geometry to optimize pool and flow boiling heat transfer performance.

[0222] The present disclosure shows enhanced boiling performances as compared to micro / nanoengineered surfaces formed on non-additively manufactured metals.

[0223] The present disclosure shows reductions in pressure drop penalty and pumping power reduction as compared to pristine additively manufactured metals.

[0224] The present disclosure synergizes the advantages of (1 ) the wide design space of additive manufacturing to produce structures with macro-geometry and (2) tunability of micro / nanostructures morphology and length scale to optimize boiling heat transfer performance and pressure drop reduction.

[0225] Practical applications and other benefits

[0226] The commercial applications of the newly methods to enhance cooling include thermal management of electronics, high-power semiconductor devices such as motor control devices, motor housing, consumer electronics, avionics, power electronics, fuel cell or electric vehicle battery thermal management. It canalso be used for immersion cooling of high-performance servers in data centers and indirect cooling of consumer desktops.

[0227] The experiments described herein involved three coolant fluids of different thermophysical properties, namely, a dielectric fluid HFE-7100 (e.g., hydrofluoroether available from the 3M company), a refrigerant R134a (e.g., a hydrofluorocarbon available from the Chemours company), and deionized water (DI water).

[0228] Further experiments

[0229] Further experiments conducted on flow boiling verified the enhanced flow boiling performance observed above using heat sinks (also referred to as cold plates) made of non-additively manufactured aluminum material. Aluminum alloy AI6061 was used to fabricate specimens with minichannels. For the sake of brevity, the as-fabricated AI6061 test specimens with open minichannels and closed minichannels were referrred to as "Plain AI(O)" and "Plain AI(C)", respectively.

[0230] In addition, an additional specimen, with a larger fin dimension of 2 mm width (Wr) to account for the material loss during microstructuring, was also fabricated for the chemical etching process to generate microstructures on the surfaces. The microstructuring of the specimen was achieved by first subjecting the as-fabricated aluminum 6061 specimen to a heat treatment process at 530 °C for 24 hours in a furnace at atmospheric pressure. Upon the completion of the heat treatment process, the specimen was removed from the furnace and left to cool in ambient air temperature of 25 °C. Prior to the chemical etching process, the test specimen was cleaned by successively sonication in deionized water, ethanol, and isopropanol for at least 15 min each. For the purpose of the present investigation, surfaces other than the minichannels, including the plenums and entrance / exit regions of the specimen, were isolated from chemical treatment process by using multiple layers of tape and nail polish. Subsequently, the entire test specimen was immersed in diluted hydrochloric (HCI) solution of 2.4 M concentration for about 30 minutes.

[0231] During this chemical immersion process, HCI reacted vigorously with the entire surface of the minichannels, resulting in the removal of the aluminummaterial. To stop the chemical reaction after HCI immersion, the specimen was then immersed in a pool of de-ionized water. Thereafter, the tape and nail polish were removed, and the specimen was thoroughly cleaned by sonicating in deionized water. The AI6061 specimen, fabricated using this microstructuring process, is termed Etched AI(O) and Etched AI(C) with the thinner and thicker polycarbonate covers (0.4 mm height difference), respectively.

[0232] Accurate estimations of the surface surface topology and surface roughness measurements were obtained. The measured roughness parameters are summarized in Table 6, which shows a comparable surface roughness value between the etched and un-etched sample due to the uniform etching reaction.Table 6

[0233] To evaluate the surface morphology of the plain AI6061 and etched AI6061 , scanning electron microscopy (SEM) analysis was performed, with imaging voltages of 5 kV ~ 10 kV. It could be seen that numerous elongated structures, approximately 2 pm in width, were regularly aligned in one direction on the surface of the plain specimen. This was caused by substantial plastic deformations during the milling process, which stretched the material in the milling direction. These continuous long structures, arising from the milling process, are believed to be too large to trap vapor for bubble nucleation of low surface tension refrigerant. In contrast, the SEM analyses of etched AI6061 revealed the existence of numerous cavities in the form of step-like structures with cavity sizes ranging from 3 pm to 8 pm. These cavity sizes are believed to possess significant potential to increase thesurface nucleation site density.

[0234] The boiling curves of open (Plain AI(O)) and closed (Plain AI(C)) plain minichannels were obtained The average heat transfer coefficient could also be deteremined as a function of heat flux of Plain AI(C) and Plain AI(O). The boiling curves depicted the relationship between the average wall superheat temperature (ATsat,ave) and heat flux (qeff) at the refrigerant mass flow rates (m) of 0.005 kg / s and 0.009 kg / s.

[0235] It was noted that, at mass flow rate m = 0.005 kg / s, the wall temperatures of Plain AI(C) and Plain AI(O) experienced a significant increase by about 5 °C, when heat flux was increased from qetf = 2.9 kW / m2to qeff = 15.3 kW / m2, where the onset of nucleate boiling (ONB) was observed. Below this heat flux threshold, heat dissipation was mainly governed by single-phase liquid forced convection with relatively poor cooling efficiency. In contrast, a much steeper gradient of the boiling curve was found when heat flux was increased above qeff = 15.3 kW / m2Specifically, the average wall superheat temperature was found to increase by only about 4 °C, as heat flux was increased by eightfold, i.e., to approximately 119 kW / m2. The significantly improved cooling performance was mainly due to the utilization of large latent heat, caused by the large number of bubbles generated during boiling process.

[0236] The results of flow boiling in open and closed plain minichannels, and in open and closed microstructured minichannels, were compared. It could be seen that the presence of microstructures significantly improved the cooling performance, with Etched AI(O) and Etched AI(C) exhibiting 1 to 3 times higher average heat transfer (have) than the plain minichannels. The enhanced cooling performance was mainly due to the presence of larger number of nucleation sites in Etched AI(O) and Etched AI(C), which were generated by the process described above. The experimental results also suggested that nucleate boiling was the predominant cooling mechanism in all specimens, as compared to convective cooling.

[0237] For practical implementation of the minichannels for thermal management applications, it is also relevant to consider the associated pressure drop penaltiesacross the cold plates. The experimental results chained further substantiated the hypothesis that the flow boiling instabilities are the predominant factors that caused the large pressure drop across the cold plates. To enhance the thermohydraulic performance of the microstructured minichannels, it would be desirable to eliminate the adverse effect of lower have in open minichannels while preserving their advantage of reducing two-phase flow pressure drop penalties. In the experiments conducted, it was demonstrated that this is achieveable by boiling hysteresis.

[0238] Experiments investigating flow boiling of R134a in various minichannels, i.e. , Plain AI(O), Plain AI(C), Etched AI(O), and Etched AI(C), at Psat of 7.27 bar, m between 0.005 kg / s and 0.009 kg / s (corresponding to mass fluxes of 60.5 kg / m2-s to 187.4 kg / m2-s), and qeff ranging from 2.1 kW / m2to 151 kW / m2were conducted. The minichannels in Plain AI(O) and Plain AI(C) were fabricated by the conventional milling method, while the ones in Etched AI(O) and Etched AI(C) were further incorporated with microstructures on their flow channel walls by the highly scalable chemical etching process proposed herein. The closed minichannels, i.e., Plain AI(C) and Etched AI(C), were developed through full contact between the channel cover and fin tips. To integrate a manifold above the minichannels, the thickness of the channel cover in Plain AI(C) and Etched AI(C) was reduced, leading to Plain AI(O) and Etched AI(O), respectively.

[0239] The mechanism and phenomenon of boiling hysteresis in various specimens were investigated, and the resulting flow boiling performance and flow behavior were analyzed. To understand the effect of surface morphology and flow patterns on boiling hysteresis, the flow boiling characteristics of Etched AI(O) were independently compared with Plain AI(O) and Etched AI(C). The effects of inlet fluid subcooling, maximum heat flux in thermal history, and refrigerant mass flow rates on boiling hysteresis were also examined. Based on the investigations, the following conclusions may be drawn:

[0240] 1 . A significant reduction in flow instability could be achieved by integrating a manifold above the minichannels. The resulting open minichannels exhibited approximately a 150 % reduction in pressure drops as compared to the closed minichannel. This is due to the effective suppression of vapor backflow and thereduction in flow velocity at the same mass flow rate with the addition of the manifold. Furthermore, the presence of microstructures on the minichannels, generated through the highly scalable chemical etching process proposed in the present disclosure, was found to enhance the average heat transfer coefficient have by 1~3 times as compared to plain minichannels.

[0241] 2. Comparison of boiling hysteresis across all the specimens showed that Etched AI(O) exhibited the most dominant boiling hysteresis effect, resulting in the largest have enhancement of 120 %, and occurring over the widest range of heat flux (qeff) from 7.5 kW / m2to 119 kW / m2. In contrast, boiling hysteresis on other specimens, i.e., Plain AI(O), Plain AI(C), and Etched AI(C), could only be observed near the onset of nucleate boiling (ONB) with up to a 30 % enhancement. These results validated that the boiling hysteresis effects can be significant when both manifold and microstructures are present.

[0242] 3. The comparison of cooling performance, at different vapor qualities and heat fluxes, between Plain AI(O) and Etched AI(O) demonstrated that boiling hysteresis was due to the sequential order of nucleation site activation with increasing heat flux, i.e., starting from the top of fin surface followed by channel bottom corners, channel side and bottom walls, when there were numerous nucleation sites on the surfaces. However, this phenomenon would be significantly diminished by the early occurrence of flow instability in the closed minichannels, where the explosive bubble growth in the confined flow channels reduced the flooding effect of low surface tension fluid, causing nearly all nucleation sites to be activated at low heat fluxes. This proposed boiling hysteresis mechanism could also explain the different flow boiling hysteresis phenomena reported in the literature. In addition, even though boiling hysteresis resulted in a significant enhancement of have in Etched AI(O), it did not result in an increase in pressure drop, thus exhibiting an enhancement factor of up to 2.5 once all nucleation sites were activated, as compared to Etched AI(C).

[0243] 4. A similar hysteresis phenomenon was found at different degrees of inlet fluid subcooling of Tsub = 2 °C and Tsub = 7 °C. In contrast, the hysteresis effect significantly increased with the maximum heat flux value in thermal history, whilea negligible effect of minimum heat flux during thermal history was found above qeff = 2.9 kW / m2. In addition, the hysteresis of the boiling curve became more pronounced at higher mass flow rates, but this could be avoided by the early activation of nucleation sites at a relatively smaller mass flow rate before increasing the fluid velocity.

[0244] In one aspect, according to various embodiments of the present disclosure, a heat sink includes a body of an aluminum material defining at least one heat transfer surface, the body being configured to receive a fluid and to dispose the at least one heat transfer surface in direct contact with the fluid, in which the at least one heat transfer surface includes a plurality of microstructures disposed thereon, the plurality of microstructures being dimensionally non-uniform and characterized by different sizes that include length dimensions equal to or greater than 2 pm. The aluminum material may be additively formed aluminium material.

[0245] The different sizes of the plurality of microstructures may include: a first plurality of first microstructures characterized by a first range of sizes, the first range of sizes; and a second plurality of second microstructures characterized by a second range of sizes, wherein the second range of sizes is different from the first range of sizes.

[0246] The different sizes of the plurality of microstructures may include length dimensions ranging from 3 pm to 8 pm, inclusive.

[0247] The different sizes of the plurality of microstructures may include length dimensions ranging from 10 pm to 15 pm, inclusive.

[0248] The heat sink may be made of an aluminum material that is additively formed and characterized by a heat transfer coefficient greater than the heat transfer coefficient of a cast aluminum alloy in any one of pool boiling and flow boiling.

[0249] The heat sink may be made of an aluminum material that is additively formed and characterized by a critical heat flux greater than the critical heat flux of a cast aluminum alloy in any one of pool boiling and flow boiling.

[0250] The body may be configured to receive a pool of liquid and to dispose the at least one heat transfer surface in the pool of liquid.

[0251] The heat sink may be characterized by a heat transfer coefficient of 37.3 W / cm2K if in use for pool boiling of deionized water.

[0252] The heat sink may be characterized by a critical heat flux greater than 190 W / cm2if in use for pool boiling of deionized water.

[0253] The body may be configured to receive a flow of liquid across the at least one heat transfer surface.

[0254] The body may include: a base; a plurality of fins extending from the body, adjacent ones of the plurality of fins defining minichannels therebetween; and a channel cover, the channel cover being spaced apart from and disposed opposing to the fins to define an open channel, in which the at least one heat transfer surface includes respective top surfaces of the plurality of fins.

[0255] The at least one heat transfer surface may include respective side surfaces of the fins.

[0256] The at least one heat transfer surface includes respective bottom surfaces between adjacent ones of the plurality of fins.

[0257] The fluid may be any one fluid selected from the group consisting of water, dielectric fluid, and refrigerant.

[0258] In another aspect, according to various embodiments of the present disclosure, a method of making the heat sink, including additively manufacturing an article from AISilOMg powder; subsequently, subjecting the article to a first heat treatment; and subsequently, chemical etching of the article.

[0259] The method may further include, after the chemical etching, subjecting the article to a second heat treatment.

[0260] The method may further include forming boehmite on the article.

[0261] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the claimed invention.

Claims

CLAIMS1 . A heat sink comprising: a body of an aluminum material defining at least one heat transfer surface, the body being configured to receive a fluid and to dispose the at least one heat transfer surface in direct contact with the fluid, wherein the at least one heat transfer surface includes a plurality of microstructures disposed thereon, the plurality of microstructures being dimensionally non-uniform and characterized by different sizes that include length dimensions equal to or greater than 2 micrometers (pm).

2. The heat sink as recited in claim 1 wherein the different sizes of the plurality of microstructures comprises: a first plurality of first microstructures characterized by a first range of sizes, the first range of sizes; and a second plurality of second microstructures characterized by a second range of sizes, wherein the second range of sizes is different from the first range of sizes.

3. The heat sink as recited in claim 1 or claim 2, wherein the different sizes of the plurality of microstructures comprises length dimensions ranging from 3 pm to 8 pm, inclusive.

4. The heat sink as recited in any one of claims 1 to claim 3, wherein the different sizes of the plurality of microstructures comprises length dimensions ranging from 10 pm to 15 pm, inclusive.

5. The heat sink as recited in any one of claims 1 to 4, wherein the aluminum material is additively formed and characterized by a heat transfer coefficient greater than the heat transfer coefficient of a cast aluminum alloy in any one of pool boiling and flow boiling.

6. The heat sink as recited in any one of claims 1 to 4, wherein the aluminum material is additively formed and characterized by a critical heat flux greater than the critical heat flux of a cast aluminum alloy in any one of pool boiling and flow boiling.

7. The heat sink as recited in any one of claims 1 to 4, wherein the body is configured to receive a pool of liquid and to dispose the at least one heat transfer surface in the pool of liquid.

8. The heat sink as recited in any one of claims 1 to 7, characterized by a heat transfer coefficient of 37.3 W / cm2K (Watts per square centimeter Kelvin) if in use for pool boiling of deionized water.

9. The heat sink as recited in any one of claims 1 to 7, characterized by a critical heat flux greater than 190 W / cm2(Watts per square centimeter) if in use for pool boiling of deionized water.

10. The heat sink as recited in any one of claims 1 to 4, wherein the body is configured to receive a flow of liquid across the at least one heat transfer surface.11 . The heat sink as recited in claim 10, wherein the body comprises: a base; a plurality of fins extending from the body, adjacent ones of the plurality of fins defining minichannels therebetween; and a channel cover, the channel cover being spaced apart from and disposed opposing to the fins to define an open channel, wherein the at least one heat transfer surface includes respective top surfaces of the plurality of fins.

12. The heat sink as recited in claim 11 , wherein the at least one heat transfer surface comprises respective side surfaces of the fins.

13. The heat sink as recited in claim 11 , wherein the at least one heat transfer surface comprises respective bottom surfaces between adjacent ones of the plurality of fins.1 . The heat sink as recited in any one of claims 1 to 14, wherein the fluid is any one fluid selected from the group consisting of water, dielectric fluid, and refrigerant.

15. A method of making the heat sink of any one of claims 1 to 14, comprising: additively manufacturing an article from AISilOMg powder; subsequently, subjecting the article to a first heat treatment; and subsequently, chemical etching of the article.

16. The method as recited in claim 15, further comprising: after the chemical etching, subjecting the article to a second heat treatment.

17. The method as recited in claim 15 or claim 16, further comprising: forming boehmite on the article.

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