Non-uniform fractal micro-pit structure and super-hydrophilic composite surface for enhancing boiling heat transfer

Abstract

The invention belongs to the technical field of enhanced boiling heat exchange, and designs a non-uniform fractal micro-pit structure and super-hydrophilic composite surface, which comprises a micro-pit structure, a micro-pit structure area and a super-hydrophilic channel, the micro-pit structure and the micro-pit structure area can be obtained through a nanosecond laser micromachining technology, a non-uniform fractal micro-pit structure is formed on the surface of the copper base, and the super-hydrophilic channel can be obtained in a chemical oxidation mode. According to the non-uniform fractal micro-pit structure and super-hydrophilic composite surface, the number of vaporization cores can be increased under the low heat flux, the directional bubble convergence process can be achieved under the medium heat flux, liquid backflow can be enhanced under the high heat flux, bubble film forming can be delayed, and the critical heat flux can be increased; therefore, the enhanced boiling heat exchange effect under different heat flux densities is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of enhanced boiling heat exchange, and designs a non-uniform fractal microporous structure and a super-hydrophilic composite surface which can increase the number of bubble cores and delay bubble convergence. Background Art

[0002] As industrial equipment and electronic devices develop towards high performance, integration and miniaturization, the heat flux density generated increases dramatically, which puts higher requirements on the thermal management of devices, especially in supercomputers, hybrid electric vehicles, X-ray medical equipment, satellites and spacecraft equipment. The nucleate boiling heat transfer method uses the latent heat of phase change of the working fluid for heat exchange. It has the characteristics of small temperature difference, large heat transfer and high reliability, and is widely used in the cooling of high heat flux equipment. Therefore, strengthening the boiling heat transfer effect of the surface to meet the increasing heat dissipation requirements is of great practical significance for the safe operation of high heat flux equipment.

[0003] With the advancement of surface processing technology, researchers have proposed that composite structure surfaces be used to enhance boiling heat transfer, that is, combining surface structure and surface wettability to enhance boiling heat transfer characteristics. For example, in the literature XZ Sun, Q. Li, WX Li, et al. Enhanced pool boiling on microstructured surfaces with spatially-controlled mixed wettability [J]. International Journal of Heat and Mass Transfer, 2022, 183: 122164. On the surface of the microcolumn structure prepared by online cutting, a polytetrafluoroethylene coating (hydrophobic material) is structured on the top by magnetron sputtering to form a microcolumn structure and hydrophobic composite surface. The mechanism of enhanced boiling heat transfer is that the hydrophobic material reduces the initial boiling superheat of the composite surface, thereby increasing the number of vaporization cores and enhancing the boiling heat transfer effect under low heat flux density, and the capillary force generated by the microcolumn structure enhances the liquid reflux, thereby delaying the formation of the steam column and increasing the critical heat flux density. However, the small spacing between adjacent micropillars makes it easy for bubbles nucleated at the top of the micropillar to gather together, and bubbles are also generated at the bottom of the micropillar in the high heat flux area, which makes the bubbles converge seriously and limits the increase of critical thermal density. The patent number ZL109295431A designs a non-uniform wetting composite surface with fractal characteristics, in which the hydrophobic region structure is distributed according to the fractal characteristics of the Sierpinski carpet curve, and the hydrophilic channels are interspersed with the hydrophobic lattice region. Its enhanced boiling heat transfer mechanism is that the bubbles are nucleated in the hydrophobic region, thereby planning the bubble nucleation point, and the hydrophilic region can strengthen the liquid supply, delay the bubble film formation, and increase the critical heat flux density. However, the structure is complex, and the hydrophobic coating used is extremely unstable and easy to fail in practical applications. Therefore, it is particularly important to design a simple, efficient and stable microstructure and wettability composite surface to meet the requirements of reducing the initial boiling superheat and increasing the number of vaporization cores at low heat flux density, and delaying the bubble film formation and increasing the critical heat flux density at high heat flux density. Summary of the invention

[0004] The purpose of the present invention is to propose a non-uniform fractal micro-pit structure and super-hydrophilic composite surface for efficiently enhancing boiling heat transfer.

[0005] The technical solution of the present invention: A non-uniform fractal micro-pit structure and super-hydrophilic composite surface comprises a micro-pit structure, a micro-pit structure region and a super-hydrophilic channel; the micro-pit structure region is composed of 9 micro-pit structures, and the micro-pit structure region is fractally distributed; the super-hydrophilic channels are interlaced in the micro-pit structure region to form a non-uniform fractal micro-pit structure and super-hydrophilic composite surface.

[0006] The diameter D of the micro-pit structure is 100µm, the depth H is 100µm, and the adjacent spacing P1 is 200µm; the length L and width W of the micro-pit structure area are both 500µm, and the spacing P2 between adjacent micro-pit structure areas is 1000µm; the contact angle of the super-hydrophilic channel is less than 12º.

[0007] A further improvement of the present invention is that the diameter D and the adjacent spacing P1 of the micro-pit structure can be reduced, thereby increasing the number of micro-pit structures.

[0008] A further improvement of the present invention is that the length L and width W of the micro-pit structure region, as well as the spacing P2 between adjacent micro-pit structure regions can be increased, thereby increasing the spacing between adjacent steam columns.

[0009] A further improvement of the present invention is that the static water contact angle of the super-hydrophilic channel is less than 10°, thereby enhancing liquid reflux.

[0010] A further improvement of the present invention is that the distribution of the micro-pit structure area can also match the heat flux density distribution of the heating surface. For example, in an electronic chip, the heat flux density is concentrated at the center of the chip, and the corresponding micro-pit structure area can be concentrated at the center of the surface, while in other places with smaller heat flux density, smaller micro-pit structure areas can be arranged.

[0011] A specific preparation method of a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface is as follows: first, a copper-based surface is polished in a polishing machine to reduce the surface roughness; then, the copper-based surface after polishing is processed by nanosecond laser micromachining, so that a fractal-distributed micro-pit structure area is formed on the surface; then, paraffin is used as a filling material to fill the prepared micro-pit structure, so that only the smooth area is processed in the subsequent super-hydrophilic treatment process; then, the micro-pit structure surface filled with paraffin is placed in a chemical solution composed of NaClO2, NaOH, Na3PO4 and deionized water, with a corresponding mass ratio of 3.75:5:10:100, and a super-hydrophilic treatment is performed; finally, the micro-pit structure surface after chemical oxidation is placed in an acetone solution to remove the filled paraffin material, so as to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

[0012] Compared with the prior art, the present invention has the following beneficial effects: In the present invention, each micropit structure is used as a vaporization core point to generate bubbles, and the bubbles nucleated in the micropit structure can absorb heat from the micropit wall, thereby obtaining a larger local heat flux density, accelerating the growth of the bubbles and reducing the bubble growth time. At the same time, multiple micropit structures in a single micropit structure area can increase the number of vaporization cores, thereby enhancing the boiling heat transfer effect of the surface under low heat flux density. The larger micropit area spacing limits the large bubbles gathered in the adjacent micropit structure areas from further developing into steam columns, thereby maintaining stable nucleate boiling and enhancing the boiling heat transfer effect under medium heat flux density. In addition, the super-hydrophilic channel can enhance the liquid replenishment effect, accelerate the detachment of the converged large bubbles, delay the bubble film formation and increase the critical heat flux density.

[0013] In the present invention, only nanosecond laser micromachining technology and chemical oxidation methods are needed to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface, which has the advantages of easy operation and stable structure. The non-uniform fractal micro-pit structure proposed in the present invention can not only reduce the initial superheat of boiling and increase the number of vaporization cores, but also plan the bubble nucleation points to limit the development of the converged bubbles into steam columns. At the same time, super-hydrophilic treatment in the smooth area can enhance the liquid replenishment effect, delay bubble film formation and increase the critical heat flux density, and the fractal distribution of the micro-pit structure area and the super-hydrophilic channel can reduce the countercurrent between the liquid inflow and the steam outflow, which is beneficial to the separation of suspended bubbles from the surface and the replenishment of liquid, thereby delaying the occurrence of critical heat flux density and improving the boiling heat transfer performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of non-uniform fractal micro-pit structure and super-hydrophilic composite surface. (a) is a schematic diagram of a classic non-uniform fractal micro-pit structure and super-hydrophilic composite surface, (b) is a schematic diagram of an improved non-uniform fractal micro-pit structure and super-hydrophilic composite surface; Figure 2 are electron microscope scanning photos of the surface, where (a) is an electron microscope scanning photo of the surface of the non-uniform fractal micro-pit structure, and (b) is an electron microscope scanning photo of the surface of the super-hydrophilic structure; Figure 3 The bubble growth process on the composite surface of non-uniform fractal micro-pit structure and super-hydrophilicity, where (a) is the low heat flux density area, (b) is the medium heat flux density area, and (c) is the high heat flux density area. DETAILED DESCRIPTION

[0015] In order to make the objectives, technical solutions and advantages of the present invention more clear, the specific implementation methods of the present invention are further described below in conjunction with the accompanying drawings and technical solutions.

[0016] like Figure 1As shown, a non-uniform fractal micro-pit structure and super-hydrophilic composite surface includes a micro-pit structure, a micro-pit structure region and a super-hydrophilic channel with super-strong wettability, wherein the micro-pit structure region and the super-hydrophilic channel are staggered.

[0017] The square-distributed micro-pit structures form a single micro-pit structure area, and the micro-pit structure areas are evenly distributed on the surface with equal axial and longitudinal spacing. The micro-pit structure spacing is twice the diameter of the micro-pit structure, and the spacing of the micro-pit structure area is several times the micro-pit structure area. There is a super-hydrophilic channel between the micro-pit structure and the micro-pit structure area, thereby forming a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

[0018] In different heat flux density zones, the mechanisms of enhancing boiling heat transfer of micropit structures, micropit structure regions and super-hydrophilic channels are different. In the low heat flux density zone, bubbles are nucleated in the micropit structure, and the smaller micropit structure spacing can increase the number of vaporization cores, thereby enhancing the boiling heat transfer effect. In the medium heat flux density zone, the bubble growth diameter increases significantly, and the bubbles in a single micropit structure region converge, while the larger micropit structure region spacing limits the further merging of the converged bubbles, thereby realizing the directional convergence process of the bubbles and accelerating the detachment of the bubbles, thereby enhancing the boiling heat transfer. In the high heat flux density zone, the bubbles converged in a single micropit structure region further merge with the nearby converged bubbles into a steam column, and the super-hydrophilic channel can enhance the liquid reflux and delay the bubbles covering the heating surface, thereby increasing the critical heat flux density.

[0019] The specific working process of the present invention is as follows: Details are as attached Figure 3As shown in the figure, in the low heat flux area, the non-condensable gas retained in the micro-pit structure is used as the initial nucleation point for bubble nucleation, so that bubbles are generated in the micro-pit structure. At the same time, the nucleated bubbles can also absorb heat from the micro-pit wall, thereby obtaining a larger local heat flux, so that the nucleated bubbles will not be annihilated by the condensation of the liquid. Therefore, each micro-pit structure is used as a vaporization core point for bubble growth. In the low heat flux area, the appearance of the micro-pit structure can increase the number of vaporization cores, and the smaller micro-pit structure spacing can further increase the number of vaporization cores, thereby enhancing the boiling heat transfer effect. With the increase of heat flux, the growth diameter of the bubbles nucleated in the micro-pit structure increases significantly, so that the bubbles nucleated in a single micro-pit structure area converge into a large bubble, and the larger micro-pit structure area hinders the further development of the converged bubbles in the adjacent micro-pit structure area into a steam column, so that the converged bubbles are separated from the nucleated micro-pit structure area. At the same time, the super-hydrophilic channels in the micro-pit structure area accelerate the liquid reflux, prompting the gathered bubbles to quickly detach from the surface, thereby enhancing the boiling heat transfer effect in the medium heat flux area. When the heat flux increases further, boiling will enter the fully developed area, and the bubbles gathered in the micro-pit structure area will continue to grow and merge with the bubbles gathered in other areas to form a steam column. The super-hydrophilic channels between the micro-pit structure areas can quickly expand the replenished liquid, which can reduce the backflow between the liquid inflow and the steam outflow, and is conducive to the separation of the suspended steam column and the heating surface. At the same time, the presence of wider super-hydrophilic channels limits the expansion of the dry spot at the bottom of the steam column, restricts the rapid film formation of steam, thereby delaying the steam film covering the heating surface and increasing the critical heat flux. The purpose of enhancing the boiling heat transfer effect under different heat flux densities is achieved.

[0020] Example 1, for a heating surface with uniform heat flux density A copper-based material is selected as the heat exchange surface substrate. First, it is ground and polished until the surface is mirror-like to remove irregular surface microstructures.

[0021] Using nanosecond laser micromachining technology and the classic orthogonal line-by-line etching method, a non-uniform fractal micro-pit structure was manufactured on the copper-based surface. The diameter D of a single micro-pit structure is 100µm, the depth H is 100µm, and the adjacent spacing P1 is 200µm. The length L and width W of the micro-pit structure area are both 500µm, and the spacing P2 between adjacent micro-pit structure areas is 1000µm.

[0022] The micro-pit structure is filled with molten paraffin as the filling material. NaClO2, NaOH, Na3PO4 and deionized water are prepared into a chemical solution at a mass ratio of 3.75:5:10:100, and then the surface of the non-uniform fractal micro-pit structure with the filling material is placed in the chemical solution for a constant temperature (96±2℃) chemical oxidation process to form a super-hydrophilic channel. After the chemical oxidation treatment, the contact angle of the channel will be less than 12º, showing super-hydrophilicity.

[0023] The non-uniform fractal micro-pit structure surface after chemical oxidation is immersed in an acetone solution, and a certain amount of fatty amide is added, and an ultrasonic device is used to accelerate the dissolution of paraffin in the acetone solution. Then, the structure surface after immersion in the acetone solution is immersed in anhydrous ethanol to remove the residual paraffin and acetone. Subsequently, it is immersed in deionized water to dissolve the anhydrous ethanol on the surface. Finally, it is placed in a drying oven for room temperature drying to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

[0024] Example 2, for a heating surface with heat flux concentrated at the center (e.g., an electronic chip) A copper-based material is selected as the heat exchange substrate. First, it is ground and polished until the surface is mirror-like to remove irregular surface microstructures.

[0025] Using nanosecond laser micromachining technology, the classic orthogonal line-by-line etching method was selected to manufacture a non-uniform fractal micro-pit structure on the copper-based surface, where the diameter D of a single micro-pit structure is 100µm, the depth H is 100µm, and the adjacent spacing P1 is 200µm; the L1 and width W1 of the micro-pit structure area at the center are both 900µm, that is, it contains 25 micro-pit structures, and the spacing P3 between the micro-pit structure area at the center and the surrounding micro-pit structure areas is 1500µm. The length L and width W of other micro-pit structure areas are both 500µm, and the spacing P2 between adjacent micro-pit structure areas is 1000µm.

[0026] The micro-pit structure is filled with molten paraffin as the filling material. NaClO2, NaOH, Na3PO4 and deionized water are prepared into a chemical solution at a mass ratio of 3.75:5:10:100, and then the surface of the non-uniform fractal micro-pit structure with the filling material is placed in the chemical solution for a constant temperature (96±2℃) chemical oxidation process to form a super-hydrophilic channel. After the chemical oxidation treatment, the contact angle of the channel will be less than 12º, showing super-hydrophilicity.

[0027] The non-uniform fractal micro-pit structure surface after chemical oxidation is immersed in an acetone solution, and a certain amount of fatty amide is added, and an ultrasonic device is used to accelerate the dissolution of paraffin in the acetone solution. Then, the structure surface after immersion in the acetone solution is immersed in anhydrous ethanol to remove the residual paraffin and acetone. Subsequently, it is immersed in deionized water to dissolve the anhydrous ethanol on the surface. Finally, it is placed in a drying oven for room temperature drying to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

[0028] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural change made to the above embodiment based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A composite surface of a non-uniform fractal micro-pit structure and super-hydrophilicity for enhancing boiling heat transfer, characterized in that: It includes a micro-pit structure, a micro-pit structure area and a super-hydrophilic channel; the micro-pit structure area is composed of 9 micro-pit structures, and the micro-pit structure area is fractal distributed; the super-hydrophilic channel is staggered and interspersed in the micro-pit structure area to form a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

2. According to claim 1, the diameter D of the micro-pit structure is 100µm, the depth H is 100µm, and the adjacent spacing P1 is 200µm; the length L and width W of the micro-pit structure area are both 500µm, and the spacing P2 between adjacent micro-pit structure areas is 1000µm; the contact angle of the super-hydrophilic channel is less than 12°.

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