Micro-nano structure for strengthening low-temperature droplet evaporation heat exchange, preparation method and application

Abstract

The invention belongs to the technical field of micro-nano manufacturing and low-temperature heat exchange, and relates to a micro-nano structure for strengthening low-temperature liquid drop evaporation heat exchange, a preparation method and application, the micro-nano structure takes an upward raised micro-column array as a substrate, and a multi-layer nanoflower micro-nano composite structure is prepared on the substrate by using a water bath heating method. The nano flower is in a nano flower cluster shape formed by growing a plurality of fusiform nanowires along the radiation direction; the side length or diameter of the microcolumn is 5-90 microns, the height of the microcolumn is 5-100 microns, and the center distance of the microcolumn is 10-200 microns; the diameter of the fusiform nanowire is 50-2400 nm, and the length of the fusiform nanowire is 1-20 [mu] m; the pore diameter between the nano flower clusters is 5-100 microns, and the thickness of an accumulation layer formed by the multi-layer nano flower micro-nano composite structure above the micro column is 5-150 microns; the preparation process of the micro-nano structure is simple, parameters are controllable, and evaporation heat exchange of liquid drops of low-temperature working media such as liquid nitrogen can be effectively enhanced.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of micro-nano manufacturing and low-temperature heat exchange, and particularly relates to a micro-nano structure for enhancing evaporation heat exchange of low-temperature droplets, a preparation method and application thereof. BACKGROUND

[0002] Low-temperature droplet heat exchange has a wide range of applications in aerospace propellant combustion, electronic device cooling, cutting processing, and cryotherapy. In liquid hydrogen / liquid oxygen rocket engines, the vaporization rate of fuel droplets directly affects the mixing efficiency and thrust stability in the combustion chamber. In titanium alloy cutting processing, the evaporation rate of liquid nitrogen droplets affects the cooling degree of the workpiece surface, thereby affecting the cutting quality. In cryotherapy, low-temperature spray cooling can be used to pre-cool the epidermis during laser skin treatment, reducing postoperative complications. Low-temperature droplet spray cooling is also used for heat dissipation of high-power, high-heat flux electronic devices, ensuring stable operation of electronic devices.

[0003] Micro-nano structure modified surfaces have been widely used to enhance the evaporation heat exchange of different droplets, such as water, ethanol, and fuel droplets. Microstructures can increase the specific surface area, and proper size design can promote droplet spreading and penetration, thereby increasing the heat exchange area. In addition, compared with smooth surfaces, microstructures of a certain height can pierce the vapor film below the droplet, thereby promoting intermittent contact between the solid and liquid, which delays the formation of a continuous stable vapor film. Nanostructures not only increase the specific surface area, but also have capillary action that can promote droplet penetration and transport on the surface, reducing liquid film thermal resistance. Finally, the combination of micro and nano structures can have a synergistic effect on enhancing evaporation. Microstructures can regulate droplet spreading area, provide stable flow paths for liquid, and reduce flow resistance, while nanostructures can drive liquid film transport quickly through capillary force, reducing local thermal resistance, which can further enhance droplet evaporation heat exchange.

[0004] However, the thermophysical properties of low-temperature working fluids are different from those of normal-temperature working fluids, with low boiling point (easy to vaporize), low viscosity, and small surface tension. In particular, extremely low-temperature working fluids such as liquid nitrogen, liquid helium, and liquid hydrogen have completely different molecular polarity and wettability than normal-temperature working fluids such as water, and the mechanism of micro-nano structure enhancement of their heat transfer is more complex. Therefore, in the prior art, only super-hydrophilic square micro-pillar copper surfaces and hollow layered nanowire surfaces have been reported to enhance the effect of liquid nitrogen spray cooling. Compared with smooth surfaces, the former has a maximum critical heat flux density of only 2.9 times, and the latter has a maximum critical heat flux density of only 1.8 times, which still cannot meet the needs of low-temperature working fluid evaporation heat exchange enhancement.

[0005] In summary, due to the different boiling mechanisms of low-temperature fluids and normal-temperature fluids, the existing surface structures still have limited ability to enhance the evaporation heat exchange of low-temperature droplets, and it is urgent to develop more excellent micro-nano structures based on the physical properties of low-temperature working fluids to further enhance the evaporation heat exchange effect of low-temperature droplets. Summary of the Invention

[0006] This invention provides the following technical solution: a micro / nano structure for enhancing heat transfer during low-temperature droplet evaporation. The micro / nano structure is a multilayer nanoflower micro / nano composite structure fabricated on a substrate using a water bath heating method, with an upwardly convex micropillar array as the substrate. The nanoflowers are clusters of nanoflowers formed by multiple spindle-shaped nanowires growing along a radial direction. Multiple layers of nanoflowers are stacked on top of the micropillar array structure, forming staggered micron-sized pores. The micropillar array structure provides excellent structural support for the distribution of the nanoflowers and also acts as a channel for vapor escape.

[0007] Preferably, the protruding micropillars have a side length or diameter of 5–90 μm, a height of 5–100 μm, and a center-to-center distance of 10–20 μm; the spindle-shaped nanowires have a diameter of 50–2400 nm and a length of 1–20 μm; the pore diameter between the nanoflower clusters is 5–100 μm; and the thickness of the stacked layer formed by the multilayer nanoflower micro-nano composite structure above the micropillars is 5–150 μm.

[0008] Preferably, the nanoflowers are made of zinc oxide (ZnO), and the spindle-shaped nanowires have a hexagonal wurtzite structure.

[0009] Preferably, the substrate material includes: metal, glass, ceramic, and plastic.

[0010] This invention also discloses a method for preparing micro / nano structures that enhance heat transfer during cryogenic droplet evaporation. This method is used to prepare the aforementioned micro / nano structures for enhancing heat transfer during cryogenic droplet evaporation. The method includes the following steps:

[0011] Step 1: Use laser or photolithography to process a micropillar array on the heat exchange surface as a substrate for the micron structure;

[0012] Step 2: On the substrate prepared in Step 1, multilayer nanoflowers with nanostructures are grown by water bath heating to obtain a multilayer nanoflower micro-nano composite structure.

[0013] Preferably, step 2 includes the following specific steps:

[0014] Step 2-1: Pre-treat the substrate with micron-sized structure prepared in Step 1. The pre-treatment includes cutting and cleaning.

[0015] Step 2-2: Prepare the seed solution using ethanol as the solvent;

[0016] Steps 2-3: Take the seed solution and spread it on the substrate with micron structure. Deposit the seed crystals onto the substrate by spin coating or natural evaporation. Repeat the seed crystal deposition operation more than five times, and then bake at high temperature.

[0017] Steps 2-4: Prepare the growth solution and fill it into the inner liner of the reactor;

[0018] Steps 2-5: Place the substrate with the micron structure into the inner liner of the reactor, and then place the reactor in an oven to allow it to react fully;

[0019] Steps 2-6: After the reaction is complete, remove the substrate, gently rinse the substrate with micron-sized structures with deionized water and dry it with nitrogen gas.

[0020] This invention also discloses an application of a micro / nano structure to enhance heat transfer during cryogenic droplet evaporation, wherein the method for enhancing heat transfer during cryogenic droplet evaporation includes:

[0021] Using a cryogenic droplet nozzle, cryogenic droplets are continuously dropped onto the surface of a micro / nano structure. The droplet height is 0.5–50 cm above the surface of the micro / nano structure. The droplet size is adjusted by changing the nozzle diameter, and the droplet interval is adjusted by controlling the pressure inside the nozzle.

[0022] Alternatively, a cryogenic spray nozzle can be used to spray cryogenic droplets onto the surface of the micro / nano structure. The nozzle height is 5–50 cm above the surface of the micro / nano structure, the spray speed ranges from 1 to 30 m / s, and the angle between the nozzle centerline and the surface of the micro / nano structure is 10°–170°. The droplet size is adjusted by changing the nozzle diameter, and the droplet interval is adjusted by controlling the pressure inside the nozzle.

[0023] Even better, ultrasonic vibration is applied to the droplet while using a cryogenic droplet nozzle, with the ultrasonic vibration frequency being 1MHz to 10MHz and the amplitude being 0.1 to 100μm.

[0024] The beneficial effects of this invention are:

[0025] 1. The method of the present invention is simple and safe to operate, and the structural parameters are easy to control; the preparation process of nanoflower structure is simple, the reaction is completed in one step in a liquid phase environment, the cycle is short, and no complicated post-processing is required.

[0026] 2. The substrate used in the preparation of micro-nano composite structures in this invention has a high degree of freedom in material selection. The materials that can be used as substrates include metals, glass, ceramics, plastics, etc., as well as substrates with microstructure lattice made from the above materials.

[0027] 3. In the micro-nano composite structure of the present invention, the micron lattice structure can be adjusted in photolithography by changing the diameter and center-to-center distance of the light-blocking points on the mask, and the depth of the micron lattice can be controlled by changing the number of etching cycles. In laser processing, the diameter and center-to-center distance of the micron lattice can be adjusted by changing the processing pattern, and the depth of the micron lattice can be controlled by changing the laser power, pulse number, and processing cycle number.

[0028] 4. The multilayer nanoflower micro-nano composite structure of the present invention can regulate the diameter of the nanospindle by changing the distribution of seed layer particles, concentration of growth solution, heating time, heating temperature, etc., and regulate the length of the nanospindle by adjusting the reaction growth time, concentration of growth solution, heating temperature, etc., thereby regulating the morphology of the nanoflower.

[0029] 5. The micro-nano composite structure of the present invention has excellent heat transfer performance. Even at extremely low temperatures, it can significantly shorten the evaporation time of cryogenic droplets (compared with a smooth surface, the heat flux density at -174℃ is increased by up to 62.5 times, and the evaporation time is reduced to 1.6% of that of a smooth surface), which significantly improves the evaporation heat transfer performance of droplets. Attached Figure Description

[0030] Fig. 1 This is a flowchart illustrating the preparation process of the multilayer nanoflower micro / nano composite structure for enhancing low-temperature droplet evaporation heat transfer in this invention.

[0031] Fig. 2 These are high-speed photographs (25°C) of the liquid nitrogen droplets evaporating on a smooth surface and a multilayer nanoflower micro-nano composite structure according to the present invention; wherein, (a) is a smooth surface and (b) is a multilayer nanoflower micro-nano composite structure.

[0032] Fig. 3 This is a heat flux density graph showing the application of a multilayer nanoflower micro-nano composite structure to enhance heat transfer in liquid nitrogen droplets in this invention. The white hollow dots represent smooth surfaces, and the black solid dots represent the multilayer nanoflower micro-nano composite structure. Detailed Implementation

[0033] The related technologies of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figs. 1-3 As shown, this embodiment addresses the limitation in improving the heat transfer of low-temperature working fluid evaporation by individual micron or nanostructures in existing research. It proposes using a hydrothermal synthesis method to prepare micro / nano composite structures, and uses these structures to significantly shorten the evaporation time of low-temperature droplets, thereby greatly increasing the average heat flux density of droplet evaporation heat transfer. This embodiment employs the following scheme:

[0035] This embodiment utilizes a composite structure composed of multilayer nanoflower structures and micropillar lattice structures to enhance the evaporative heat transfer of cryogenic droplets. The multilayer nanoflower micro-nano composite structure is prepared using a water bath heating method, while the micropillar lattice structure can be obtained through photolithography or laser processing. The main advantages of this method are: (1) The micro-nano composite structure integrates the different functions of micron and nanostructures in enhancing heat transfer, possessing the ability to synergistically enhance the heat transfer of cryogenic droplets; (2) The substrate used for fabrication offers high freedom of choice, with materials including metals, glass, ceramics, and plastics; (3) Micron structures are easy to fabricate and can be rapidly fabricated through photolithography or laser processing; (4) The nanoflower structure fabrication process is simple, low-cost, and the experimental operation is simple and safe; (5) This method enhances the evaporative heat transfer efficiency of cryogenic droplets (the heat flux density of approximately 6 μL liquid nitrogen droplets is increased by a maximum of 62.5 times), has a wide applicable temperature range, and can withstand extremely low temperatures (-190℃~25℃).

[0036] In this embodiment, the micron structure is prepared by laser or photolithography, while the nanostructure is prepared by chemical synthesis.

[0037] Methods and procedures for fabricating micron-sized structures:

[0038] The micron structure is prepared by laser or photolithography. The laser processing method and process are as follows: (1) Fix the substrate in the processing area of ​​the three-axis displacement platform; (2) Select the laser type, set the number of laser pulses, power, etc., and then turn on the laser; (3) Open the laser processing sketch drawn in CAD software and connect it with the laser control software so as to control the laser processing path, processing times, etc.; (4) Move the three-axis displacement platform to make the sample appear in the camera field of view, and adjust the z-axis position to make the laser point focus on the sample surface; (5) Set the processing cycle number and start the processing.

[0039] The photolithography process is as follows: (1) Substrate pretreatment, including cleaning and drying. (2) Adsorb the substrate onto the spin coater tray and set the spin coater speed and corresponding duration. (3) Take an appropriate amount of adsorption agent and spread it on the substrate. Start the spin coater. After the adsorption agent is coated, bake the substrate at high temperature. (4) Adsorb the substrate onto the spin coater tray again and set the spin coater speed and corresponding duration. (5) Take an appropriate amount of photoresist and spread it on the substrate. Start the spin coater. After the photoresist is coated, bake the substrate at high temperature. (6) Fix the substrate in the UV exposure machine to be exposed and set the exposure time for exposure. (7) Prepare the developing solution and develop the exposed substrate. (8) After successful development, bake the substrate at high temperature. (9) Etch the photolithographic substrate after hardening.

[0040] The nanoflower structure in this embodiment is a zinc oxide nanoflower structure grown on the above-mentioned micron lattice structure using a chemical synthesis method.

[0041] Nanostructure preparation methods and processes:

[0042] (1) Pretreatment of substrate with micron structure, including cutting and cleaning; (2) Preparation of seed solution, the solvent is ethanol; (3) Apply seed solution to the substrate, and deposit seed crystals onto the substrate by spin coating or natural evaporation. Repeat the seed crystal deposition operation more than five times, and then bake at high temperature; (4) Prepare growth solution and put it into the inner liner of the reactor; (5) Put the substrate into the inner liner of the reactor, and then put the reactor into the oven to allow it to react fully; (6) After the reaction is completed, take out the substrate after the reaction, rinse the substrate gently with deionized water and blow it dry with nitrogen.

[0043] The surface structure obtained after the above preparation is as follows: Zinc oxide multilayer nanoflower clusters are grown on a micropillar array substrate. The diameter of the cylinders ranges from 5 μm to 90 μm, the depth from 5 μm to 100 μm, and the center-to-center distance is fixed at 10–200 μm. Each nanoflower is composed of multiple spindle-shaped nanowires (hexagonal wurtzite structure) with different growth orientations. The diameter of the spindle-shaped nanowires constituting the nanoflowers ranges from 50 nm to 2400 nm, and the length ranges from 1 μm to 20 μm. The nanoflowers are stacked in multiple layers, forming staggered micron-sized pores with a diameter of 5 μm to 100 μm. The thickness of the stacked layer above the micropillars is between 5 μm and 150 μm.

[0044] The specific method for enhancing low-temperature droplet evaporation heat transfer using micro / nano structures in this embodiment is as follows:

[0045] Using a cryogenic droplet nozzle, cryogenic droplets are continuously dropped onto the surface of a micro-nano composite structure with multiple nanoflowers. The droplet height is 0.5–50 cm above the surface. The droplet size can be adjusted by changing the nozzle diameter, and the droplet interval can be adjusted by controlling the pressure inside the nozzle.

[0046] Alternatively, a cryogenic spray nozzle can be used to spray cryogenic droplets onto a surface with a multi-layered nano-flower micro-nano composite structure. The nozzle height is 5–50 cm above the surface, the spray velocity ranges from 1 to 30 m / s, and the angle between the nozzle centerline and the surface is 10°–170°. The droplet size can be adjusted by changing the nozzle diameter, and the droplet interval can be adjusted by controlling the pressure inside the nozzle.

[0047] A cryogenic nozzle can also be used to continuously drop cryogenic droplets onto a surface with a multilayered nanoflower micro-nano composite structure while simultaneously applying ultrasonic vibrations to the droplets. The droplet height above the surface is 0.5–50 cm, the ultrasonic vibration frequency is 1 MHz–10 MHz, and the amplitude is 0.1–100 μm. The droplet size can be adjusted by changing the nozzle diameter, and the droplet interval can be adjusted by controlling the pressure inside the nozzle. Applying ultrasonic vibrations to the droplets weakens the thermal boundary layer on the droplet surface due to mechanical disturbances, and the high-frequency vibrations induce microscale turbulence, increasing the thermal diffusivity, all of which are beneficial for enhancing the evaporative heat transfer of the droplets.

[0048] The principle behind using a multilayer nanoflower micro-nano composite structure to enhance evaporative heat transfer in cryogenic droplets is as follows: the cryogenic working fluid has low latent heat (easily vaporized), and the micrometer-scale cavities formed between the nanoflowers and the micropillar array below them can act as channels for vapor dispersion, enabling timely discharge of vapor from the solid-liquid interface; the nanospindle rods constituting the nanoflowers have nanometer-scale diameters but micrometer-scale lengths, allowing them to pierce the vapor film and thus promote solid-liquid contact. The cryogenic working fluid has low surface tension and low viscosity, and the capillary action of the nanostructure more easily promotes droplet penetration and wetting to the surface, ensuring that even with high superheat, the droplets can still maintain local contact and boiling with the surface, effectively delaying the formation of a continuous and stable vapor film, inhibiting film boiling of the droplets, and enhancing evaporative heat transfer in cryogenic droplets.

[0049] The embodiments of the present invention use liquid nitrogen droplets as a representative of low-temperature working fluids, and specifically prepare multilayer nanoflower micro-nano composite structures to enhance the evaporation heat transfer of liquid nitrogen droplets.

[0050] In this embodiment, the substrate is a silicon wafer with a micropillar array (hereinafter referred to as silicon wafer) prepared by photolithography. The micropillars have a diameter of 50 μm, a height of 55 μm, and a center-to-center distance of 100 μm. A ZnO multilayer nanoflower micro-nano composite structure is grown on the substrate by hydrothermal synthesis.

[0051] Substrate pretreatment: Using a 500μm thick silicon wafer as the substrate, the wafer was made into a size of 2cm×2cm. It was ultrasonically cleaned with acetone and ethanol for 5 minutes in succession, rinsed with deionized water, and then dried with nitrogen. The treated silicon wafer was then adsorbed onto the spin coater tray with the side with the micropillar lattice facing upwards to prepare for the deposition of the seed layer.

[0052] Seed solution preparation and ZnO seed layer deposition: Measure 100 mL of anhydrous ethanol into a beaker using a graduated cylinder. Weigh a certain amount of zinc acetate hexahydrate and mix it with the ethanol. Sonicate the mixture for at least 15 minutes until the solution becomes clear to obtain the seed solution. Use a pipette to transfer a certain amount of the seed solution onto the silicon wafer, ensuring the seed solution covers the entire surface of the wafer. Start the spin coater. When rainbow diffraction fringes appear on the silicon wafer surface, it indicates that the seed solution is about to evaporate. Transfer the same amount of anhydrous ethanol onto the rotating silicon wafer to redistribute the precipitated crystals evenly. Repeat the process of transferring the seed solution and anhydrous ethanol at least 5 times. Then stop the rotation and turn off the spin coater.

[0053] Seed layer annealing: Remove the silicon wafer and place it on a 350℃ baking rack for more than 30 minutes (seed layer deposition side facing up).

[0054] Preparation of growth solution: Measure 95 mL of deionized water into a beaker using a graduated cylinder, weigh out a certain proportion of zinc nitrate, hexamethylenetetramine, and ethylenediamine-terminated polyethyleneimine and add them to the beaker. Finally, measure out 5 mL of ammonia water and pour it into the beaker. Stir magnetically for more than 10 minutes until the solution is clear.

[0055] Reactive growth: After the annealed silicon wafer has cooled, it is placed in the inner liner (100mL) of the reactor with the seed layer deposition side facing upwards. The prepared growth solution is poured in, the inner liner is sealed, and then transferred to the reactor. The reactor is then placed in a 95℃ oven and heated for at least 5 hours to ensure a complete reaction. After the reaction is complete, the oven is turned off, and the reactor is allowed to cool to room temperature. The silicon wafer is then removed from the inner liner, gently rinsed with deionized water, and dried with nitrogen gas.

[0056] This embodiment employs a chemical hydrothermal synthesis method to reactively grow a ZnO spindle-shaped nanoflower micro / nano composite structure on a silicon wafer with a micropillar lattice. The structural diagram is attached. Fig. 1 As shown in the figure, the pore diameter between the nanoflower clusters ranges from 5 μm to 100 μm, the length of the nanospindle ranges from 1 μm to 20 μm, and the diameter ranges from 50 nm to 2400 nm.

[0057] The ZnO multilayer nanoflower micro-nano composite structure, reactively grown on a silicon wafer surface with a micropillar lattice, benefits from the low surface tension and viscosity of the cryogenic fluid. The capillary action of the nanostructure facilitates the penetration and wetting of cryogenic droplets onto the surface. The micrometer-scale cavities formed between the nanoflowers and the underlying micropillar lattice act as channels for vapor dispersion, allowing for the timely removal of vapor from the solid-liquid interface. The nanospindle rods constituting the nanoflowers, with lengths in the micrometer range, can pierce the vapor film, promoting solid-liquid contact. This ensures that even with significant superheat, the droplets maintain local contact and boiling with the surface, effectively delaying the formation of a continuous and stable vapor film, suppressing film boiling, and significantly improving the droplet's evaporative heat transfer performance.

[0058] As attached Fig. 2 As shown, the composite structure composed of the ZnO multilayer nanoflower structure and the micropillar lattice in this embodiment can maintain local contact boiling between the liquid nitrogen droplet and the surface even at high superheat, thereby suppressing film boiling and promoting evaporative heat transfer. Taking liquid nitrogen droplets as a representative of low-temperature working fluid droplets, and using a silicon wafer with a micropillar array as a substrate, a multilayer nanoflower micro-nano composite structure with micron-sized pores was fabricated as an example to illustrate the main methods used in this structure and its enhanced heat transfer capability.

[0059] When the surface temperature reaches the test temperature, liquid nitrogen droplets are dropped onto the surface with the micro-nano composite structure using a cryogenic droplet nozzle. The droplets fall approximately 10 mm above the sample surface. A stopwatch records the evaporation time of each liquid nitrogen droplet on the sample surface and the surface temperature T at the moment the first droplet falls within that temperature range. w1 and the surface temperature T at the end of the evaporation of the last droplet w2 A high-speed camera was used to record the interaction between droplets and the surface during evaporation.

[0060] Use a stopwatch to record the temperature T. w1 and T w2 The evaporation time of the five droplets between them is denoted as t. i It is temperature T w1 and T w2 Evaporation time of the i-th droplet (in seconds). Average heat flux of the droplet (kW / m³). 2 It can be derived from the formula q. i =Q / (A·t) i The calculation is performed, where Q (kJ) is the heat required for the complete vaporization of the liquid nitrogen droplet (ignoring sensible heat change). The heat of vaporization required for droplet evaporation can be given by Q = m·L, where m (kg) is the mass of the liquid nitrogen droplet, and L (kJ / kg) is the latent heat of vaporization of the liquid nitrogen droplet. A (m 2 ) is the contact area between the liquid nitrogen droplet and the surface (according to D) 2 The law approximates the contact area as the contact area at the midpoint of the droplet's evaporation. Where D0(m) is the initial diameter of the liquid nitrogen droplet. The mean of the average heat flux density within this temperature range. The standard deviation of the average heat flux density in this temperature range is:

[0061] The data on the average heat flux density of droplets evaporating on the surface of a composite structure composed of multilayer nanoflower structures and micropillar lattices as a function of surface temperature are provided by [the data]. Fig. 3The results show that, compared to a smooth silicon wafer surface, the heat flux density of liquid nitrogen droplets evaporating on the composite structure surface (temperature approximately -174°C) composed of multilayer nanoflower structures and micropillar lattices is increased the most, reaching 62.5 times that of a smooth surface, while the evaporation time is reduced to 1.6% of that of a smooth surface. Therefore, the multilayer nanoflower micro-nano composite structure of this invention can significantly improve the evaporation rate of liquid nitrogen droplets and the heat flux density exchanged with the surface.

[0062] In summary, the preparation process of this invention is clear, the usage process is simple, and it is easy to implement. The structure of this invention has superior characteristics in enhancing the evaporative heat transfer of cryogenic droplets, maintaining high-efficiency heat transfer performance even under low-temperature conditions. Therefore, this invention has broad application prospects in the field of enhancing the evaporative heat transfer of cryogenic droplets, and is expected to be used in the future for rocket engine heat exchange, titanium alloy machining heat exchange, cryotherapy spray cooling, and temperature control of electronic equipment, etc.

[0063] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any modifications to the above embodiments based on the technical essence of the present invention are prohibited.

[0064] Any simple modifications, equivalent changes, or alterations made shall still fall within the scope of the technical solution of this invention.

[0065] Within the enclosure.

Claims

1. A micro / nano structure for enhancing heat transfer during cryogenic droplet evaporation, characterized in that, The micro-nano structure is a multilayer nanoflower micro-nano composite structure prepared on the substrate using a water bath heating method with an array of upwardly convex micropillars as the substrate. The nanoflower is a cluster of nanoflowers formed by multiple spindle-shaped nanowires growing along the radial direction.

2. The micro / nano structure for enhancing cryogenic droplet evaporation heat transfer according to claim 1, characterized in that, The diameter or side length of the raised micropillar array is 5-90 μm, the height is 5-100 μm, and the center distance is 10-200 μm; The spindle-shaped nanowires have a diameter of 50–2400 nm and a length of 1–20 μm; the pore diameter between the nanoflower clusters is 5–100 μm; and the thickness of the stacked layer formed by the multilayer nanoflower micro-nano composite structure on the micropillar array substrate is 5–150 μm.

3. The micro / nano structure for enhancing cryogenic droplet evaporation heat transfer according to claim 1, characterized in that, The nanoflowers are made of zinc oxide, and the spindle-shaped nanowires have a hexagonal wurtzite structure.

4. The micro / nano structure for enhancing cryogenic droplet evaporation heat transfer according to claim 1, characterized in that, The substrate material includes: metal, glass, ceramic, and plastic.

5. A method for preparing micro / nano structures for enhanced low-temperature droplet evaporation heat transfer, characterized in that, The preparation method is used to prepare the micro / nano structure for enhanced low-temperature droplet evaporation heat transfer as described in any one of claims 1 to 4, and the preparation method includes the following steps: Step 1: Use laser or photolithography to process a micropillar array on the heat exchange surface as a substrate for the micron structure; Step 2: On the substrate prepared in Step 1, multilayer nanoflowers with nanostructures are grown by water bath heating to obtain a multilayer nanoflower micro-nano composite structure.

6. The method for preparing a micro / nano structure for enhanced low-temperature droplet evaporation heat transfer according to claim 5, characterized in that, Step 2 specifically includes the following steps: Step 2-1: Pre-treat the substrate with micron-sized structure prepared in Step 1. The pre-treatment includes cutting and cleaning. Step 2-2: Prepare the seed solution using ethanol as the solvent; Steps 2-3: Take the seed solution and spread it on the substrate with micron structure. Deposit the seed crystals onto the substrate by spin coating or natural evaporation. Repeat the seed crystal deposition operation more than five times, and then bake at high temperature. Steps 2-4: Prepare the growth solution and fill it into the inner liner of the reactor; Steps 2-5: Place the substrate with the micron structure into the inner liner of the reactor, and then place the reactor in an oven to allow it to react fully; Steps 2-6: After the reaction is complete, remove the substrate after the reaction, gently rinse the substrate with deionized water and dry it with nitrogen gas.

7. An application of the micro / nano structure for enhancing cryogenic droplet evaporation heat transfer as described in any one of claims 1 to 4, characterized in that, The application method for enhancing low-temperature droplet evaporation heat transfer includes: A cryogenic droplet nozzle is used to continuously drop cryogenic droplets onto the surface of the micro / nano structure, thereby evaporating and absorbing heat to cool the surface. The droplet height from the surface of the micro / nano structure is 0.5–50 cm, the droplet size is adjusted by changing the nozzle diameter, and the droplet interval is adjusted by controlling the pressure inside the nozzle. Alternatively, a cryogenic spray nozzle can be used to spray cryogenic droplets onto the surface of the micro / nano structure. The nozzle height is 5–50 cm above the surface of the micro / nano structure, the spray speed is 1–30 m / s, and the angle between the nozzle centerline and the surface of the micro / nano structure is 10°–170°. The droplet size is adjusted by changing the nozzle diameter, and the droplet interval is adjusted by controlling the pressure inside the nozzle.

8. The application of the micro / nano structure for enhancing cryogenic droplet evaporation heat transfer according to claim 7, characterized in that, The use of a cryogenic droplet nozzle allows for the application of ultrasonic vibrations to the droplets, thereby enhancing heat transfer. The frequency of the ultrasonic waves is 1 MHz to 10 MHz, and the amplitude is 0.1 to 100 μm.

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