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

Abstract

The invention belongs to the technical field of enhanced flow boiling heat exchange, and designs a non-uniform fractal micro-pit structure and super-hydrophilic composite surface, which comprises a super-hydrophilic channel, a micro-pit structure area, a micro-pit structure block and a micro-pit structure, the super-hydrophilic channel can be obtained by performing chemical oxidation on the surface of the copper base, and the micro-pit structure area, the micro-pit structure block and the micro-pit structure can be obtained through a nanosecond laser micromachining technology. According to the non-uniform fractal micro-pit structure and super-hydrophilic composite surface, the number of vaporization cores can be increased at the surface inlet section, disturbance of liquid can be enhanced, directional convergence of steam bubbles can be achieved at the surface middle section, formation of steam columns can be limited, spreading of local dry points can be limited at the surface outlet section, and steam bubble film forming can be delayed; therefore, the boiling heat exchange effect can be enhanced in different areas of the heating surface.

Description

Technical Field

[0001] The invention belongs to the technical field of enhanced flow boiling heat exchange, and designs a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface which can enhance the bubble nucleation process, increase flow disturbance and delay bubble film formation. Background Art

[0002] As a common two-phase cooling method, flow boiling heat transfer is widely used in refrigeration systems, electronic equipment cooling, and chemical and petrochemical engineering. Improving the flow boiling heat transfer effect is the key to ensuring the safety, efficiency and economy of heat energy transfer, conversion and management systems. There are two main ways to improve flow boiling heat transfer: one is surface modification (surface structure and surface wettability); the other is to add surfactants or nanoparticles. Among them, surface modification is recognized as an effective method and has attracted the attention of many scholars. Therefore, using surface modification () to enhance the flow boiling heat transfer effect is of great practical significance to ensure the safe operation of equipment.

[0003] Related studies have found that composite surfaces (combining surface structure and wettability) can combine the characteristics of both to enhance flow boiling heat transfer, that is, they can significantly increase the number of vaporization cores and enhance liquid reflux, thereby further enhancing the flow boiling heat transfer effect. For example, the literature Q. Zhao, JC Qiu, JH Zhou, et al. Visualization study of flowboiling characteristics in open microchannels with different wettability [J]. International Journal of Heat and Mass Transfer, 2021, 180: 121808. First, a microgroove structure is formed on the copper-based surface using wire cutting technology, and then the prepared microgroove structure is laser ablated using a fiber laser engraving method to form an irregular microstructure on the microgroove structure. Finally, the ablated microgroove structure is treated by chemical oxidation to form a microgroove-superhydrophilic composite surface. The strengthening mechanism is that the superhydrophilic structure can limit the formation of local drying points, thereby increasing the critical heat flux density. However, the superhydrophilic structure also increases the initial boiling point, making the boiling heat transfer effect of the microgroove-superhydrophilic composite surface poor at low heat flux density. The patent application number is 202410285324.8, which designs a surface that synergistically enhances flow boiling heat transfer by using streamlined microcolumns and circular hole partitions. The streamlined microcolumns and circular holes are arranged in partitions to increase the surface nucleation density while increasing the disturbance to the gas-liquid working fluid and reducing bubble blockage, thereby achieving an improvement in the surface heat transfer performance. However, the structure is complex, and the increased number of vaporization cores easily causes the convergence of bubbles, making the boiling effect not obvious in high heat flux areas. Therefore, it is particularly important to design a simple, efficient and stable microstructure and wettability composite surface that can delay bubble convergence and slow down the diffusion of dry points while increasing the number of surface vaporization cores, thereby significantly enhancing boiling heat transfer. Summary of the invention

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

[0005] The technical solution of the present invention: A non-uniform fractal micro-pit structure and super-hydrophilic composite surface for enhancing flow boiling heat transfer comprises a super-hydrophilic channel 1, a micro-pit structure region 2, a micro-pit structure block 3 and a micro-pit structure 4; the super-hydrophilic channel 1 is distributed throughout the entire heating surface; the super-micro-pit structure region 2, the micro-pit structure block 3 and the micro-pit structure 4 are respectively arranged at the inlet end, the middle section and the outlet section of the heating surface, and a plurality of rows of micro-pit structures, the micro-pit structure blocks and the micro-pit structures are distributed in a cross-row.

[0006] The contact angle of the super-hydrophilic channel is less than 15°, the diameter of a single micro-pit structure is 100µm, and the depth is 100µm, wherein, in the micro-pit structure area 2, the spacing between adjacent micro-pit structures is 300µm; a single micro-pit structure block is composed of 4 micro-pit structures, the axial and longitudinal spacing between adjacent micro-pit structures in the micro-pit structure block is 300µm, and the spacing between adjacent micro-pit structure blocks is 1500µm; the adjacent spacing between the micro-pit structures distributed at the surface outlet is 1500µm.

[0007] A further improvement of the present invention is that the area ratio of the micro-pit structure region, the micro-pit structure block and the micro-pit structure can be adjusted according to the heat flux density of the heating surface. For example, on a heating surface with a smaller heat flux density, the area of ​​the micro-pit structure region can be significantly increased, while the area of ​​the micro-pit structure can be significantly reduced; and on a heating surface with a higher heat flux density, the area of ​​the micro-pit structure region can be reduced, while the area of ​​the micro-pit structure can be significantly increased.

[0008] A specific preparation method of a non-uniform fractal microporous structure and a super-hydrophilic composite surface is as follows: first, a copper-based surface is pre-treated by a water-polishing method to remove the irregular microstructure on the surface; then, a nanosecond laser micromachining method is used on the copper-based surface to prepare a non-uniform fractal microporous structure; then, a chemical oxidation method is used to perform a super-hydrophilic treatment on the surface of a micro-pit structure with a filling material to obtain a super-hydrophilic surface; finally, an acetone solution is used to remove the filling material in the structure surface to obtain a non-uniform fractal microporous structure and a super-hydrophilic composite surface.

[0009] Compared with the prior art, the present invention has the following beneficial effects: The present invention utilizes the characteristic that bubbles are easily nucleated in the micro-pit structure, thereby reducing the superheat of the initial boiling heat exchange, and a smaller structure spacing can increase the number of micro-pit structures, thereby increasing the number of vaporization cores. At the same time, in the micro-pit structure area, the cross-row distribution of the micro-pit structure can increase the disturbance of the flow, prompting the bubbles to detach from the surface, thereby strengthening the boiling heat exchange effect at the surface inlet end. In the middle section of the heating surface, the bubble growth size increases significantly, and the bubbles nucleated in the micro-pit structure blocks begin to converge, and the distance of the larger micro-pit structure blocks can limit the further convergence of the converged bubbles. At the same time, the super-hydrophilic channels around the micro-pit structure blocks prompt the converged bubbles to detach from the surface. When the liquid enters the outlet section of the heating surface, the bubbles nucleated in the micro-pit structure quickly converge into a steam column, and the super-hydrophilic channels around the micro-pit structure will limit the spread of the dry spots of the converged bubbles. In addition, the larger microporous structure spacing delays the bubble film formation, thereby increasing the critical heat flux density.

[0010] In the present invention, only nanosecond laser micromachining technology and chemical oxidation are needed to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface, which has the characteristics of easy operation and stable structure. The non-uniform fractal micro-pit structure and the super-hydrophilic composite surface proposed by the present invention can significantly increase the number of bubbles by using the micro-pit structure, and can also limit the further convergence of bubbles and delay the formation of bubbles by increasing the spacing between adjacent micro-pit structure blocks and micro-pit structures. At the same time, the super-hydrophilic channel can quickly spread the liquid, thereby prompting the bubbles to quickly detach and delay the critical heat flux density. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Schematic diagram of a composite surface of non-uniform fractal micro-pit structure and super-hydrophilicity for enhancing flow boiling heat transfer; Figure 2 Schematic diagram of the non-uniform fractal micro-pit structure and super-hydrophilic channel arrangement; Schematic diagram of the arrangement of superhydrophilic and superhydrophobic structures; Figure 3 The bubble growth process on the composite surface of non-uniform fractal micro-pit structure and super-hydrophilicity; In the figure: 1 super-hydrophilic channel; 2 micro-pit structure area; 3 micro-pit structure block; 4 micro-pit structure. DETAILED DESCRIPTION

[0012] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0013] like Figure 1~2 As shown, a non-uniform fractal micro-pit structure and super-hydrophilic composite surface for enhancing flow boiling heat transfer includes a super-hydrophilic channel 1, a micro-pit structure area 2, a micro-pit structure block 3 and a micro-pit structure 4. Among them, several rows of micro-pit structures, micro-pit structure blocks and micro-pit structures are distributed in a cross-row.

[0014] The micro-pit structure area is arranged at the surface entrance section, and the distance between adjacent micro-pit structures is 3 times the diameter of the micro-pit structure; the micro-pit structure block is arranged in the middle section of the surface, and the distance between adjacent micro-pit structure blocks is 3 times the width of the micro-pit structure block; the micro-pit structure is arranged at the surface outlet end, and the distance between adjacent micro-pit structures is 15 times the diameter of the micro-pit structure; the rest of the surface is a super-hydrophilic channel.

[0015] In the inlet section of flow boiling, bubbles are nucleated in the micro-pit structure in the micro-pit structure area, and the dense micro-pit structure can significantly increase the number of vaporization cores, thereby enhancing boiling heat transfer. The cross-distributed micro-pit structure can increase the disturbance of the liquid, thereby prompting the bubbles to detach from the surface, further enhancing boiling heat transfer. In the middle section of the surface, bubbles are nucleated and converged in the micro-pit structure blocks, and the larger spacing between the micro-pit structure blocks will limit the further development of the converged bubbles, thereby enhancing the flow boiling heat transfer effect. In the surface outlet section, bubbles rapidly nucleate and grow in the micro-pit structure, and quickly converge with the bubbles in the surrounding micro-pit structures. The super-hydrophilic channel can limit the expansion of the dry spots formed outside the micro-pit structure, thereby delaying the bubble film formation and increasing the critical heat flux density.

[0016] The specific working process of the present invention is as follows: Details are as attached Figure 3 As shown, in the micro-pit structure area arranged at the surface inlet section, the non-condensable gas retained in the micro-pit structure is used as the nucleation point of initial boiling for bubble nucleation, so that the nucleated bubbles grow in the micro-pit structure. At the same time, the smaller micro-pit structure spacing can significantly increase the number of vaporization cores, which can enhance the flow boiling heat transfer. The bubbles nucleated in the cross-row distributed micro-pit structure can increase the disturbance of the fluid, which is conducive to the detachment of the bubbles. In the micro-pit structure block arranged in the middle section of the surface, the bubbles quickly nucleate in the micro-pit structure and converge with the bubbles in the micro-pit structure block. The micro-pit structure blocks with larger spacing can limit the further convergence of the converged bubbles with the surrounding converged bubbles. And the super-hydrophilic channels around the micro-pit structure blocks can promote the detachment of bubbles, thereby achieving directional convergence and rapid detachment of bubbles, thereby enhancing the flow boiling heat transfer effect. In the micro-pit structure arranged at the surface outlet section, the bubbles quickly nucleate from it, grow and converge with the periodic bubbles into a steam column. Local dry spots will form at the bottom of the converging steam column (around the micro-pit structure), and the continuous infiltration of liquid in the super-hydrophilic channel can limit the spread of the local dry spots, thereby delaying the formation of bubble films. Therefore, the boiling dry phenomenon can be effectively delayed and the critical heat flux density can be increased.

[0017] In the present invention, the non-uniform fractal micro-pit structure and super-hydrophilic composite surface for enhancing flow boiling can be obtained by using nanosecond laser micro-machining technology and chemical oxidation. In the processing process, first, in the software controlling the nanosecond laser micro-machining technology, the micro-pit structure parameters and area size to be processed are drawn, and then the pulse energy, scanning spacing and scanning speed are set. Then, the processed micro-pit structure is filled with a filling material. Subsequently, the copper-based surface after filling is immersed in a chemical solution composed of NaClO2, NaOH, Na3PO4 and deionized water, and chemical oxidation treatment is performed to obtain a super-hydrophilic structure. Finally, the filling material is removed to obtain a non-uniform fractal micro-pit structure and a super-hydrophilic composite surface.

Claims

1. A composite surface of non-uniform fractal micro-pit structure and super-hydrophilicity for enhancing flow boiling heat transfer, characterized in that: It includes a super-hydrophilic channel 1, a micro-pit structure area 2, a micro-pit structure block 3 and a micro-pit structure 4; the super-hydrophilic channel 1 is spread over the entire heating surface; the super-micro-pit structure area 2, the micro-pit structure block 3 and the micro-pit structure 4 are respectively arranged at the inlet end, the middle section and the outlet section of the heating surface, and several rows of micro-pit structures, micro-pit structure blocks and micro-pit structures are distributed in a cross-row.

2. According to claim 1, the contact angle of the super-hydrophilic channel is less than 15°, the diameter of a single micro-pit structure is 100µm, and the depth is 100µm, wherein: In the micro-pit structure area 2, the spacing between adjacent micro-pit structures is 300µm; a single micro-pit structure block is composed of 4 micro-pit structures, the axial and longitudinal spacing between adjacent micro-pit structures in the micro-pit structure block is 300µm, and the spacing between adjacent micro-pit structure blocks is 1500µm; The adjacent spacing of the micro-pit structures distributed at the outlet end of the surface is 1500µm.

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