Increasing boiling heat transfer using low thermal conductivity materials

Inactive Publication Date: 2018-01-18
DREXEL UNIV
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Benefits of technology

The patent text describes a way to enhance the heat transfer between a surface and a liquid by using different materials with different thermal conductivities arranged in a specific way. This arrangement creates a regular variation in surface temperature and increases the critical heat flux, which is the amount of heat transfer needed to boil a liquid. This technique can improve the efficiency of heat transfer and reduce energy waste.

Problems solved by technology

CHF, also referred to as the boiling crisis, occurs when the production of vapor cannot be adequately balanced by the amount of liquid returning to the heated surface.
This leads to an immediate, uncontrollable, and drastic increase in surface temperature with dangerous and potentially catastrophic consequences, such as the destruction of electronic components or the meltdown of a nuclear reactor.
All structured surfaces are inherently susceptible to mechanical failure (breaking of the micro / nano-structures), as well as fouling and clogging over time.
During boiling, any and all contaminants within the fluid will inevitably be drawn into the structures.
This will lead to clogging and filling of the small micro / nano-scale voids, and thus a loss of enhancement.
Similarly, the robustness of extremely thin nanostructured coatings (≦1 μm) has not yet been demonstrated, leaving the potential for the coatings to be destroyed or altered over time via chemical reactions.
The use of low-surface-energy materials to promote nucleation at small superheats has been successfully demonstrated as well, however these biphilic surfaces are prone to degradation of the thin non-wetting films.
Additionally, they are not effective with all working fluids, in particular highly wetting fluids like FC-72.
While doubly reentrant surfaces have been demonstrated to repel these highly wetting fluids, their use in biphilic designs for boiling enhancement has not yet been demonstrated.
These enhancement techniques rely on surface properties and interfacial phenomena, which are inherently susceptible to degradation.

Method used

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  • Increasing boiling heat transfer using low thermal conductivity materials
  • Increasing boiling heat transfer using low thermal conductivity materials
  • Increasing boiling heat transfer using low thermal conductivity materials

Examples

Experimental program
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Effect test

example 1

Bi-Conductive Surface Fabrication

[0107]Bi-conductive surfaces were fabricated by embedding lines of a low-conductivity epoxy into a copper substrate. Copper sheets (1 mm thickness) were cut to size and grooves were machined into them using Wire Electrical Discharge Machining (EDM). The EDM wire thickness was 0.254 mm with a reported minimum spark gap of 0.381 mm±0.127 mm. The copper was then treated with an alkaline solution to produce an oxide layer with nanoscale surface roughness to promote adhesion between the copper and epoxy. The surfaces were then coated with a non-conductive high-temperature two-part epoxy (Aremco™ 526N) filling all of the grooves. The epoxy was cured at 93° C. for 2 hours, followed by 163° C. for 12 hours to achieve a maximum strength bond. After curing, the surfaces were manually sanded with 200 grit sandpaper until the bare copper between each epoxy division was exposed. The bare copper surfaces (with no epoxy divisions) were also sanded using the same me...

example 2

[0109]Evaluating equation S8 using the experimentally measured heat transfer coefficients during nucleate boiling, h=50-210 kW / m2K (FIG. 3), results in qE / qT=0.2%-2% for the highest performing surfaces (P=3.7-1.8 mm). For the lower performing surfaces (P=1.39-0.96 mm) this increases to qE / qT=0.8%-5%. Similarly, by evaluating equation S10 it can be shown that the ratio of the superheats between the epoxy and fluid to the copper and fluid is ΔTsat,E / ΔTsat,C=1.9%-6.7%, over the same range of h.

[0110]Prior to the onset of nucleate boiling, the measured heat transfer coefficients reach values of h˜5 kW / m2K. During this convection-dominated region, the lower heat transfer coefficients result in higher convective resistances that become comparable to the conductive resistances in the epoxy. Because of this, the percentage of heat transfer over the epoxy increases to qE / qT=5%-11% for the highest performing surfaces (P=3.7-1.8 mm), and qE / qT=15%-24% for the lower performing surfaces (P=1.39-...

example 3

Pool Boiling Characterization and Imaging

[0111]Surfaces were characterized using a custom-built test set-up as previously reported by Rahman et al. See, Rahman, M. M., Ölçero{hacek over (g)}lu, E. & McCarthy, M. Role of Wickability on the Critical Heat Flux of Structured Superhydrophilic Surfaces. Langmuir 30, 11225-11234 (2014) and Rahman, M. M., Ölçero{hacek over (g)}lu, E. & McCarthy, M. Scalable Nanomanufacturing of Virus-templated Coatings for Enhanced Boiling. Advanced Materials Interfaces 1, 1300107 (2014). The setup consists of a copper heater block with PTFE insulation embedded with two cartridge heaters allowing for a maximum power of 1,000 W. Five T-type thermocouples were inserted into the copper block equally spaced 6 mm apart with the topmost thermocouple located directly beneath the sample.

[0112]The temperature measurements were recorded using NI DAQ system, where the average heat flux in the copper block was calculated using Fourier's conduction law. The sample surfa...

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PUM

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Abstract

An apparatus having a heat transfer surface for transfer of heat from the apparatus to a liquid. The heat transfer surface includes at least two different materials and at least two of said materials have different thermal conductivities. A method of boiling at least one liquid which provides increased heat transfer from a heat transfer surface of an apparatus to a liquid comprising a step of boiling said liquid in contact with the apparatus, is also disclosed. Also described is a method of tuning a heat transfer surface by forming the heat transfer surface using at least two different materials having different thermal conductivities arranged in a predetermined spatial relationship including a spacing between the portions of one said material that is from about 0.1λC to about 5λC, wherein λC is a capillary length of a bubble of a predetermined liquid to be boiled using said heat transfer surface.

Description

STATEMENT OF GOVERNMENT INTEREST[0001]This invention was made with government support under Grant No. 1454407 awarded by the National Science Foundation. The Government has certain rights in this invention.BACKGROUNDField of the Invention[0002]The present invention is directed to increasing heat transfer to a liquid during boiling by providing a heat transfer surface including a combination of a high thermal conductivity material and a low thermal conductivity material to form the heating surface. More specifically, the invention provides a heat transfer surface with a spatial variation in the temperature profile of the surface to aid bubble dynamics during boiling.Description of the Related Technology[0003]Boiling is a well-studied mode of heat transfer and is common in many industrial applications. The large amount of energy in the form of latent heat makes phase change phenomena, like boiling, of critical importance to the development of next-generation thermal and energy systems...

Claims

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Application Information

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IPC IPC(8): F28F13/18F28F13/14
CPCF28F13/14F28F13/187F28D15/02F28D15/046H01L23/427
Inventor MCCARTHY, MATTHEWRAHMAN, MAHAMUDURPOLLACK, JORDAN FORREST
Owner DREXEL UNIV
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