Method for constructing nucleation gradient multi-element structure spliced surface based on femtosecond laser direct writing technology

Abstract

The invention provides a construction method of a nucleation gradient multi-element structure spliced surface based on a femtosecond laser direct writing technology, and relates to the technical field of boiling heat transfer enhanced surface engineering. Femtosecond laser is adopted to conduct cross scanning on a metal substrate to prepare a dot matrix cavity with the automatic liquid supply capacity, and the opening size of the cavity and the bubble production behavior are controlled by adjusting the number of cross lines; preparing a laser-induced periodic structure LIPSS with different roughness on the external surface of the cavity so as to strengthen hydrophilicity; capillary grooves with low nucleation activity are processed in the center of a lattice grid to serve as gap channels, and gas-liquid channel separation and ordered bubble separation are achieved. The surface can cooperatively regulate and control the nucleation density, wettability and gas-liquid separation path, and meanwhile, the boiling heat transfer coefficient and the critical heat flux density are improved.

Description

Technical Field

[0001] This invention relates to the field of surface engineering technology with enhanced boiling heat transfer, and in particular to a method for constructing a multi-structure spliced ​​surface based on nucleation gradient technology using femtosecond laser direct writing. Background Technology

[0002] A key characteristic of boiling heat transfer, and a significant advantage over forced convection heat transfer, is the growth and detachment of bubbles on the heat exchange surface. This means it utilizes the latent heat of phase change, which has a higher heat transfer efficiency, to achieve more efficient heat exchange. For example, water has a latent heat of vaporization of 2260 kJ / kg, but its specific heat capacity is only 4.18 kJ / (kg·°C). Therefore, boiling heat transfer is applied in fields such as thermal / nuclear power generation, concentrated photovoltaics, data centers, fast-charging batteries, and electronic products to achieve thermal management under high heat flux. The heat transfer efficiency of boiling heat transfer is typically expressed by the heat transfer coefficient HTC (W / m³). 2 The heat transfer coefficient (HTC) is measured by the ratio of heat flux density (q) to wall superheat (ΔTW), which represents the amount of heat transferred per unit area of ​​heat exchange surface per unit time under a unit temperature difference. It is defined as the slope of the boiling curve, where wall superheat refers to the temperature difference between the heat exchange surface and the saturated liquid. Increasing the HTC is beneficial for energy conservation and cost savings. For example, in the electronics industry, immersion-in-liquid boiling cooling technology can improve power efficiency by 58%. Increasing HTC allows water to be heated into steam faster, which allows for smaller equipment dimensions and lower initial costs. This also improves the power generation efficiency of thermal / nuclear power plants that use steam to generate electricity, thereby reducing coal consumption. Increasing the density of nucleation points on the heat exchange surface to facilitate sufficient surface phase change is the core strategy for improving heat transfer temperature (HTC). This is because increasing the nucleation point density allows for more frequent bubble generation, enabling more frequent replenishment of the heat exchange surface by the cold liquid. It also increases the evaporation area of ​​the micro-liquid layer trapped by bubbles and the length of the three-phase contact line between bubbles and the heat exchange surface. Furthermore, more frequent bubble movement enhances liquid turbulence. In other words, increasing the nucleation point density fully utilizes the four important heat transfer mechanisms of pool boiling heat exchange: transient heat conduction, micro-liquid layer evaporation, contact line heat transfer, and enhanced convection. However, in practical research and production applications, it has been found that the density of nucleation points has an upper limit. Excessively high nucleation point density provides a "short-lived and dangerous" enhancement to HTC. Its improvement in HTC is limited to low heat flux conditions. With increased heat flux density, high-density nucleated bubbles easily form a heat-resistant gas film, causing HTC to drop rapidly. The surface may reach heat flux density (CHF) prematurely, facing the risk of burn-out. CHF is another key parameter for measuring boiling performance and represents the safety margin for the boiling heat exchange system. This shows that there is a balance between the two key parameters of boiling heat transfer, HTC and CHF.

[0003] Current technical solutions to this trade-off often involve enhancing hydrophilicity or increasing the spacing between nucleation sites, sacrificing hydrothermal tract (HTC) to improve thermal flux (CHF). However, this strategy relies on single-parameter control, optimizing only one path: liquid wettability or nucleation site spacing. This leads to decreased surface phase transition efficiency and limited HTC improvement. Furthermore, because it fails to consider both nucleation site density and liquid replenishment capacity, it remains difficult to achieve simultaneous increases in HTC and CHF under high heat flux conditions. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for constructing a multi-structure spliced ​​surface based on femtosecond laser direct writing technology with nucleation gradient. This method can achieve coordinated control of nucleation density, wettability, and gas-liquid separation path through structural splicing, thereby simultaneously improving HTC and CHF and breaking through the performance constraints of traditional surface strengthening.

[0005] To achieve the above objectives, the present invention provides the following solution: A method for constructing a multi-structured spliced ​​surface based on femtosecond laser direct writing technology with nucleation gradient is disclosed. This method utilizes femtosecond laser processing to prepare various micro / nanostructures on the surface of a metal material and achieves the splicing of these structures in spatial arrangement. The metal material can be stainless steel, copper, or nickel. The construction method includes: A high nucleation density lattice cavity structure surface is prepared on the surface of a metal material using femtosecond laser processing technology. The lattice cavity structure surface includes multiple cavities. The cavities have autonomous liquid supply capabilities, and the lattice cavity structure surface is prepared by a cross-scanning processing method. The cross-scanning processing method involves the femtosecond laser scanning the surface of the metal material along the scanning path of the intersecting lines. By adjusting the laser processing parameters to adjust the geometry of the cavity, and by reducing the number of intersecting lines to reduce the opening diameter of the cavity, the dynamic process of bubble generation in the cavity can be adjusted. Based on the optimization of the self-supplying liquid cavity, a laser-induced periodic structure (LIPSS) was prepared on the external interface of the lattice cavity structure to achieve overall enhancement of the hydrophilicity of the lattice cavity structure surface. Laser-induced periodic structures (LIPSS) with different roughnesses were prepared at the cavity interface by adjusting the laser processing parameters, so as to further regulate the dynamic process of bubble generation in the cavity. Capillary groove structures with the lowest nucleation activity are prepared at the center point of the grid on the surface of the lattice cavity structure to serve as gap channels. These channels are used to manage the dynamic behavior of bubbles on the heat exchange surface and guide the efficient detachment of bubbles, thereby forming a multi-structure spliced ​​surface with nucleation gradient.

[0006] Preferably, the cross-scan processing method is as follows: A femtosecond laser with a power of 14W was used. The surface of the metal material is scanned and processed using a laser scanning speed of 12 m / s along the scanning path of the intersecting lines; A high nucleation density lattice cavity structure surface is formed by using laser power and laser scanning speed. The diameter of the cavity is 0.6 mm; the spacing between adjacent cavities is 1 mm. This distribution method forms a high nucleation density heat transfer surface system.

[0007] Preferably, reducing the opening diameter of the cavity by reducing the number of intersecting lines includes: Set the number of intersecting lines to any value of 15, 13, 11, or 9; Scanning processing is performed on the surface of metal materials using a cross-scanning method with a number of intersecting lines; The opening diameter of the cavity is reduced by decreasing the number of intersecting lines.

[0008] Preferably, adjusting the laser processing parameters to adjust the geometry of the cavity includes: The cavity is cone-shaped as a whole; The center of the cavity is concave at the tip; The cavity's interior and sidewalls are designed with columnar protrusions. The outer periphery of the cavity is made into a grooved structure.

[0009] Preferably, laser-induced periodic structures (LIPSS) with different roughnesses are prepared at the external cavity interface by adjusting the laser processing parameters, including: The laser processing power of the laser-induced periodic structure LIPSS is set to any value of 12W, 10W, 8W, 6W or 4W. Laser-induced periodic structures (LIPSS) were fabricated at the extracavity interface using laser processing power. Laser-induced periodic structures (LIPSS) with different roughness were obtained based on different laser processing powers.

[0010] Preferably, laser-induced periodic structures (LIPSS) with different roughnesses are prepared at the external cavity interface by adjusting the laser processing parameters, including: Fabrication of contact wire pinned laser-induced periodic structures (LIPSS); Laser-induced periodic structures (LIPSS) with contact line spreading were fabricated. Among them, the LIPSS with contact line pinning had circular and band-like gaps between its stripe structures, while the LIPSS with contact line spreading had a denser stripe structure.

[0011] Preferably, capillary groove structures with the lowest nuclear activity are prepared at the center points of the grid on the surface of the lattice cavity structure as interstitial channels, including: A capillary groove structure was fabricated at the center point of the grid using a laser processing power of 2W. The gap channel is formed by the capillary groove structure.

[0012] Preferably, the gap channel is formed by a capillary groove structure, including: The separation of gas and liquid channels is achieved by using capillary groove structures as isolation channels; The capillary groove structure is made to have heterogeneous wettability in the direction parallel to the groove and in the direction perpendicular to the groove, so as to guide the orderly detachment of bubbles.

[0013] Preferably, the laser-induced periodic structure (LIPSS) is fabricated at the external interface of the lattice cavity structure surface, including: The external interface is defined as the outer periphery of the cavity. Laser-induced periodic structures (LIPSS) are fabricated on the outer interface of the cavity.

[0014] The present invention discloses the following technical effects: This invention uses femtosecond laser processing technology to prepare a high nucleation density lattice cavity structure surface on the surface of a metal material. The cavity has an autonomous liquid supply capability. Compared with the existing strategy of directly increasing the nucleation point density, it can enhance the local liquid replenishment capability while maintaining high nucleation activity, thereby avoiding the risk of surface drying and film boiling caused by excessively high nucleation point density. This helps to improve the boiling heat transfer coefficient (HTC) and delay the arrival of the critical heat flux density (CHF).

[0015] This invention prepares multiple cavities through a cross-scanning process and adjusts the cavity opening diameter by reducing the number of cross lines, thus enabling controllable nucleation point density. Regarding the nucleation mechanism, this structure enables more frequent nucleation events while avoiding bubble merging, coverage expansion, and gas film formation after high-density nucleation. Therefore, it overcomes the traditional trade-off of "high-density nucleation strengthening HTC but sacrificing CHF."

[0016] This invention prepares a laser-induced periodic structure (LIPSS) on the external interface of a lattice cavity structure and forms different roughnesses by adjusting the laser processing parameters, thereby improving surface wettability and extending the three-phase contact line. This is beneficial for enhancing two key boiling heat transfer mechanisms: micro-liquid layer evaporation and contact line heat transfer, thereby further improving the boiling heat transfer coefficient (HTC) and reducing the boiling initiation superheat.

[0017] This invention prepares capillary groove structures with the lowest nucleation activity at the center point of the grid on the surface of the lattice cavity structure to serve as gap channels, thereby achieving structural decoupling of the gas-liquid path. This allows the liquid to be supplied along the grooves while the gas is discharged along the gaps, thus suppressing heat-resistant processes such as bubble stagnation and bubble coverage. It also transforms disordered bubble aggregation into ordered aggregation and timely detachment, significantly reducing the probability of forming a continuous gas film and improving the stability of the heat exchange surface in maintaining nucleation boiling.

[0018] This invention achieves coordinated control of nucleation density, wettability, and phase separation pathways by spatially splicing a lattice cavity structure surface, a laser-induced periodic structure (LIPSS), and a capillary groove structure. It no longer relies on traditional single enhancement methods, enabling the simultaneous improvement of two key performance indicators of boiling heat transfer, HTC and CHF, thus breaking through the trade-off between "enhancing HTC leading to premature drying and increasing CHF leading to a decrease in nucleation efficiency" in existing technologies. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart of the method provided in an embodiment of the present invention; Figure 2 A schematic diagram of the femtosecond laser processing flow and scanning path strategy for the nucleation gradient multi-element structure splicing surface provided in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating the comparison of the microstructural features of three self-supplying liquid cavities and the dynamic process of their bubble production, provided for embodiments of the present invention. Figure 4 A schematic diagram of the final boiling test results provided in an embodiment of the present invention; Figure 5 A comparative diagram showing the wettability differences of two external cavity interface LIPSS structures provided in this embodiment of the invention and their impact on bubble dynamics and boiling heat transfer performance of the ENC surface; Figure 6 Provided for embodiments of the present invention Detailed Implementation The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The purpose of this invention is to provide a method for constructing a multi-structure spliced ​​surface based on nucleation gradient technology using femtosecond laser direct writing technology. This method can balance high nucleation activity and high liquid replenishment capacity in a single surface structure, thereby achieving an overall improvement in boiling heat transfer performance rather than a performance swap.

[0022] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 The method flowchart provided in the embodiments of the present invention is as follows: Figure 1 As shown, this invention provides a method for constructing a multi-structure spliced ​​surface based on femtosecond laser direct writing technology with nucleation gradient. The method uses femtosecond laser processing technology to prepare various micro / nano structures on the surface of a metal material and achieves the splicing of these structures in the surface space. The metal material can be stainless steel, copper, or nickel. The construction method includes: Step 100: A high nucleation density lattice cavity structure surface is prepared on the surface of a metal material using femtosecond laser processing technology; the cavity has autonomous liquid supply capability, and the lattice cavity structure surface is prepared by cross-scanning processing; the cross-scanning processing method is that the femtosecond laser scans the surface of the metal material along the scanning path of the cross lines; the lattice cavity structure surface includes multiple cavities; Step 200: Adjust the geometry of the cavity by adjusting the laser processing parameters, and reduce the opening diameter of the cavity by reducing the number of intersecting lines, thereby adjusting the dynamic process of bubble generation in the cavity; Step 300: Based on the optimization of the self-supplying liquid cavity, laser-induced periodic structure LIPSS is prepared on the external interface of the lattice cavity structure to achieve overall enhancement of the hydrophilicity of the lattice cavity structure surface. Step 400: Adjust the laser processing parameters to prepare laser-induced periodic structures (LIPSS) with different roughness at the cavity interface, so as to further regulate the dynamic process of bubble generation in the cavity; Step 500: At the center point of the grid on the surface of the lattice cavity structure, a capillary groove structure with the lowest nucleation activity is prepared as an interstitial channel to manage the dynamic behavior of bubbles on the heat exchange surface and guide the efficient detachment of bubbles, thereby forming a nucleation gradient multi-structure spliced ​​surface.

[0024] In this embodiment, the raw material is a metal material, preferably stainless steel, copper, or nickel sheet. All of these metal materials can be obtained through commercially available metal sheets, ensuring a stable source and facilitating mass processing and application.

[0025] The specific preparation process is as follows: (1) First, a high nucleation density lattice cavity structure surface is prepared on the surface of the metal material using femtosecond laser processing technology. By rationally designing the structural parameters of the lattice cavity, the resulting cavity has the ability to supply liquid autonomously. In this embodiment, a cross-scanning processing method is used to process the surface of the metal material, that is, the processing path of the femtosecond laser is set to the form of intersecting lines, and scanning processing is performed under the conditions of laser power of 14W and laser scanning speed of 12m / s, thereby obtaining a cavity structure surface with a regular lattice distribution.

[0026] (2) Based on the initial lattice cavity structure, the geometry of the cavity is further adjusted by adjusting the laser processing parameters. Specifically, the cavity opening diameter is reduced by decreasing the number of intersecting lines, thereby achieving fine control over the dynamic process of bubble generation in the cavity. In this embodiment, the number of intersecting lines is selected as 15, 13, 11 and 9 in sequence. Different numbers of intersecting lines correspond to different cavity opening sizes and nucleation behaviors, thereby achieving graded adjustment of bubble growth and detachment characteristics.

[0027] (3) After optimizing the lattice cavity structure with self-supply capability, the external interface of the lattice structure surface is subjected to secondary laser processing to prepare a laser-induced periodic structure (LIPSS). By forming a regular periodic stripe structure in the external interface region, the overall hydrophilicity of the lattice structure surface can be significantly improved, enhancing the wetting and replenishment capability of the liquid on the heat exchange surface, thereby providing more favorable interface conditions for the subsequent boiling heat exchange process.

[0028] (4) During the preparation of LIPSS at the cavity interface, by further adjusting the laser processing parameters, LIPSS structures with different roughness levels are obtained on the outer surface of the cavity, so as to achieve more precise secondary adjustment of the dynamic process of bubble generation in the cavity. In this embodiment, the laser processing power of the LIPSS structure is set to 12W, 10W, 8W, 6W and 4W respectively. By the differences in surface roughness and micromorphology formed under different power conditions, the behavior of the three-phase contact line and the evaporation characteristics of the micro-liquid layer are effectively controlled, thereby further affecting the nucleation, growth and detachment process of bubbles.

[0029] (5) Finally, a capillary trench structure with the lowest nucleation activity is prepared in the center region of the lattice structure, which is then used as a gap channel for gas-liquid separation. By introducing a trench structure with capillary effect in this region, the generation, aggregation, and detachment of bubbles on the heat exchange surface can be managed in an orderly manner, guiding the bubbles to detach from the surface efficiently along a preset path. In this embodiment, the capillary trench structure is prepared using a laser processing power of 2W. While ensuring the integrity and continuity of the trench morphology, it effectively constrains and guides the bubble movement trajectory, thereby significantly improving the boiling stability and overall heat exchange performance of the heat exchange surface.

[0030] like Figure 2 As shown, Figure 2 The overall presentation illustrates the processing equipment and scanning strategy for the "nucleation gradient multi-structure spliced ​​surface" described in this invention, the step-by-step surface structure construction process, and the corresponding microstructure characterization results. The surface construction process sequentially includes: STEP I processing lattice cavities, STEP II processing laser-induced periodic structures (LIPSS), and STEP III processing capillary grooves. These three types of structures are spliced ​​together in the surface space to synergistically regulate bubble growth and detachment behavior.

[0031] Figure 2 Part A shows a schematic diagram of the processing head and optical path of the femtosecond laser processing system. A galvanometer is used to achieve rapid deflection and scanning of the laser beam, which is incident on the surface of the metal material to form processing points. This diagram illustrates that the present invention mainly relies on the scanning path planning controlled by the galvanometer and the spatial positioning of the laser spot on the surface during the fabrication of micro-nano structures, thereby completing the fabrication of micro-nano structures with different functional areas on the same metal material surface.

[0032] Figure 2 Part B shows the scanning strategy corresponding to the three-step processing flow of the present invention, where the top legend clearly shows: the blue dashed line is the zone divider, the red hollow circle is the laser spot, and the black arrow is the laser processing path. Figure 2 Part B1 corresponds to STEPICrossscanning, which means using a cross-scanning strategy to process the target area in circles (shown in the figure as concentric regions and directional arrows). Through multi-directional cross-scanning, a high nucleation density lattice cavity structure (i.e., a self-supplying liquid cavity structure) is formed at the cavity position. Figure 2 Part B2 corresponds to STEPII Parallelscanning, which uses a parallel scanning path for in-plane striping. The figure shows the parallel scanning line spacing 1 = 10 μm and the laser spot radius is marked. ≈40μm, used to form fine periodic stripe morphology in the external interface region; Figure 2 Section B3 corresponds to STEPIII Parallelscanning, which also uses a parallel scan path, but the spacing between the parallel scan lines is adjusted to D2=60μm, and is also labeled accordingly. ≈40μm, used to process and form groove-shaped structures at specified locations, thereby constructing subsequent capillary groove gap channels.

[0033] Figure 2 Part A1 to Figure 2 Section A3 provides a planar schematic of the step-by-step construction of the surface structure. Figure 2 Part A1 is STEPI-Machiningcavities, which form a cavity structure with a lattice distribution on the surface of a metal material. The figure shows the lattice period P=1mm and the cavity feature size D=0.5mm, which are used to characterize the array layout and scale of the cavity. Figure 2 Part A2 is STEPII-MachiningLIPSS. After the lattice cavity structure is completed, LIPSS is formed in the external interface region of the cavity (the processing area / representative area is marked by a rectangle "LIPSS" in the figure) to enhance the hydrophilicity of the external interface and introduce different roughness levels. Figure 2 Part A3 is STEPIII-Machiningcapillarygrooves, which are strip-shaped capillary grooves (illustrated as vertical striped rectangles and labeled "Capillarygrooves") formed in the central area of ​​the grid between the dot matrix cavity array. These capillary grooves act as interstitial channels to participate in the organization of gas-liquid channels and guide the detachment of bubbles.

[0034] Figure 2 Part C presents the morphological characterization results of the above structure at the microscale. Figure 2 Part C shows the microscopic morphology of a typical area of ​​the surface. Three characteristic areas are marked with color boxes: C1 is a magnified local area of ​​a single lattice cavity, C2 is a magnified local area of ​​LIPSS, and C3 is a magnified local area of ​​capillary grooves. The scale bar of C is 200 μm. Figure 2 The middle C1 section is a magnified view of the lattice cavity (scale bar 100μm), which can be used to show the radial / petal-shaped microstructure features of the cavity in the plane; Figure 2 The C2 section shows the magnified morphology of LIPSS (scale bar 5 μm), which reveals a dense, periodic striped texture. Figure 2 Part C3 shows a magnified view of the capillary grooves (scale bar 50 μm), revealing multiple grooves distributed parallel to each other in the same direction, forming a gap channel structure for guiding bubble movement and detachment. These sub-figures collectively illustrate that this invention can sequentially construct three types of structures—lattice cavities, LIPSS, and capillary grooves—on the same metallic surface, and through spatial splicing, form a multi-dimensional structural surface with nucleation gradient and channel organization capabilities.

[0035] In this embodiment, as Figure 3 As shown, Figure 3 This is used to compare and illustrate the microstructural characteristics of three self-supplying liquid cavities and the dynamic process of their bubble production. Among them, Figure 3 Part a Figure 3 Part c shows the SEM structural characterization of the three self-supplying liquid chambers; Figure 3 part d~ Figure 3 The f-parts are sequence diagrams of the dynamic process of bubble production corresponding to the three self-supplying liquid chambers. The dynamic process is captured by high-speed CCD with a frame rate of 4000 frames / second, which is used to characterize the time evolution characteristics of bubble growth, detachment and interaction.

[0036] like Figure 3 Part a Figure 3 As shown in section c, the three self-supplying liquid chambers share a consistent basic structural configuration: the chamber is generally conical with a concave center; the interior and sidewalls extending outward from the center are columnar protrusions; and the periphery is a grooved structure. This composite morphology of "conical chamber—concave center—columnar protrusions on the interior and sidewalls—peripheral grooves" constitutes the structural basis of the self-supplying liquid chamber, serving to form and maintain the vaporization nucleus during boiling, while simultaneously organizing the liquid supply path through the peripheral grooves and the chamber structure.

[0037] Furthermore, with Figure 3 Compared to part a, Figure 3 part b and Figure 3 The self-supplying liquid cavity corresponding to section c uses fewer intersecting lines during processing, causing the cavity opening diameter to decrease as the number of lines decreases; at the same time, the proportion (length proportion) of the outer groove structure of the cavity increases as the number of lines decreases. That is, as the number of intersecting lines decreases step by step, the cavity exhibits a geometric evolution trend of "smaller opening and longer outer groove (larger proportion)," thus providing more favorable structural conditions for subsequent continuous bubble nucleation and high-frequency detachment.

[0038] like Figure 3 As shown in section d, the self-supplying liquid chamber with the largest opening diameter exhibits a discontinuous bubble production characteristic, with a significant waiting time between adjacent nucleated bubbles. This is because when the opening diameter of the conical chamber is too large, the chamber's ability to maintain the vaporization nucleus is insufficient, and the detached bubble carries the vaporization nucleus along with it when leaving the chamber (in...). Figure 3 In part d, small bubbles / residual vaporization nuclei carried by the detached bubble tail can be observed, causing the vaporization nuclei inside the cavity to need to be re-established, resulting in a waiting time and discontinuous nucleation. Based on this characteristic, this embodiment names this type of cavity with "largest opening diameter and discontinuous bubble production" as Unstable Nucleation Cavity (UNC).

[0039] like Figure 3part e and Figure 3 As shown in part f, the other two self-supplying liquid cavities with smaller opening diameters have achieved continuous nucleation, allowing bubbles to be continuously produced from inside the cavity, and the dynamic process no longer exhibits the characteristics of... Figure 3 The waiting time is shown in part d; furthermore, as the cavity opening diameter further decreases, the bubble detachment diameter decreases and the detachment frequency increases, exhibiting "high-frequency production of small bubbles". Based on this differentiated nucleation behavior, this embodiment names the cavity with "medium-sized opening diameter and continuous bubble production" as the Stable Nucleation Cavity (SNC), corresponding to... Figure 3 Part e; the cavity with the smallest opening diameter and the highest bubble production frequency is named the High-Efficiency Nucleation Cavity (ENC), corresponding to... Figure 3 Part f. Through the above... Figure 3 Part a Figure 3 The correspondence of part f can clearly characterize the influence of the change in the proportion of cavity opening and groove caused by the change in the number of intersecting lines on the continuity of bubble nucleation, detachment diameter and detachment frequency.

[0040] The aforementioned cavities were subjected to a lattice treatment, with a cavity diameter of approximately D≈0.6mm. The spacing P between the cavities was set to 1mm, and the ratio of the cavity spacing to its diameter, P / D≈1.67<<10. In the initial boiling stage, bubbles generated in adjacent cavities will coalesce laterally, meaning the heat exchange surface is already in a state of extremely efficient boiling as early as the initial boiling stage. The final boiling test results are as follows... Figure 2 The higher the frequency and the smaller the diameter of the bubbles produced by the cavity, the higher the corresponding HTC on the lattice structure surface, and this applies throughout the entire heat transfer process. The CHF also gradually increases, meaning the ENC lattice structure surface exhibits the best heat transfer characteristics, raising the CHF to 147.5 W / cm². 2 At the critical state, the hCHF (HTC at the critical state) increases to 6.0 W / cm². 2 The temperature was increased by 61.4% and 160.9% respectively compared to the polished surface. The improvement in HTC may be attributed to the fact that the high-frequency bubble production shortens the bubble coverage time on the heat exchange surface to a certain extent and intensifies the disturbance of the thermal boundary layer (the scouring of the hot surface by the cold liquid). The improvement in CHF is mainly due to the self-supply capability of the cavity. The liquid supply is not blocked due to the higher frequency of bubble production, thus forming a hot dry point and reaching modal boiling earlier.

[0041] In this embodiment, Figure 5 The meanings of the sub-diagrams shown can be summarized as follows: Figure 5 Part a is a schematic diagram of the LIPSS structure for enhanced water replenishment prepared on the outer surface of the cavity; Figure 5 Part b presents the SEM structural characterization and wetting characterization of two LIPSS structures, in which... Figure 5The b1 and b2 parts of the b portion correspond to two different LIPSS structural morphologies, with static contact angles of 20.3° and 0°, respectively. Figure 5 Part c is a comparison of the bubble behavior of the polished original surface and the bubbles produced by the two LIPSS structures. Figure 5 Part d is a schematic diagram showing the contact line spreading between the bubble and the outer interface. Figure 5 The e part is a comparison of the frequency and diameter of bubbles produced by the cavity after combining with two LIPSS structures; Figure 5 The f-section represents the boiling curve; Figure 5 The g-part represents the heat transfer coefficient curve. The above content supports the present invention's ability to achieve zoned control of bubble dynamics and boiling heat transfer performance through "differences in LIPSS structure types at the external interface."

[0042] To enhance the hydrophilicity of the heat exchange surface and improve the water replenishment capacity of the cavity's outer interface, a laser-induced periodic structure (LIPSS) was fabricated on the outer interface of the self-supplying liquid cavity using a femtosecond laser. Figure 5 Part a shows a schematic diagram of preparing LIPSS structures on the outer surface of the cavity to enhance water replenishment. Specifically, LIPSS stripe micro / nano structures are introduced into the outer interface region of the lattice cavity structure surface, transforming the outer interface from a relatively smooth interface into a hydrophilic interface with periodic stripe texture, thereby providing more favorable interface conditions for the liquid replenishment and bubble detachment process around the cavity.

[0043] like Figure 5 As shown in section b, two LIPSS structures with different morphologies and wetting properties were prepared by adjusting the femtosecond laser processing parameters, and their structures and wetting properties were characterized by SEM. Figure 5 The LIPSS structure shown in part b1 is relatively rougher, with circular and banded gaps between its stripe structure; this structure has stronger hydrophilicity than the original polished surface, with a static contact angle of 20.3°, while the static contact angle of the polished surface is 85°. Figure 5 The LIPSS structure shown in part b2 exhibits a denser striped morphology. The top-view SEM image reveals a more compact and continuous striped structure, with a static contact angle reaching 0°, forming a superhydrophilic interface. For ease of subsequent mechanism analysis and comparison, this embodiment names the two LIPSS based on the dynamic behavior of the bubble contact lines at the interface: Figure 5 The rough and gapped structure shown in part b1 is named contact wire pinned LIPSS (CLPLIPSS). Figure 5 The dense superhydrophilic structure shown in part b2 is named contact line spreading type LIPSS (CLSLIPSS).

[0044] like Figure 5 part c and Figure 5As shown in section d, although both types of LIPSS significantly enhance hydrophilicity compared to polished surfaces, their effects on bubble contact line spreading behavior are opposite due to differences in micro / nano structures. CLPLIPSS, with its rough protrusions and gap structure, inhibits and pins the bubble contact line, limiting bubble spreading at the interface. Consequently, the base diameter of the generated bubbles is smaller than that of bubbles generated on polished surfaces; correspondingly, the overall bubble size is smaller and detaches faster. Conversely, CLSLIPSS enhances bubble contact line spreading on smooth and superhydrophilic interfaces, resulting in a larger bubble base diameter, leading to larger generated bubble diameters and a slower bubble detachment process. Figure 5 Part d further illustrates the process of bubble contact with the external interface and the occurrence of "contact and spreading" to explain the differentiated dynamic behavior of contact lines exhibiting pinning or spreading on different LIPSS interfaces.

[0045] After combining the two LIPSS structures mentioned above with the ENC lattice structure surface, this embodiment further compares the frequency and diameter of bubbles produced by the cavity, such as... Figure 5 As shown in section e, the introduction of the LIPSS structure alters the wettability and water replenishment conditions at the external interface, leading to significant differences in bubble substrate spreading ability, detachment diameter, and detachment frequency. Specifically, contact line pinning LIPSS (CLPLIPSS) tends to form bubbles with smaller substrate diameters and faster detachment, while contact line spreading LIPSS (CLSLIPSS) tends to form bubbles with larger substrate diameters and relatively slower detachment. This difference provides a bubble kinetics-based explanation for the enhanced characteristics at different stages in the subsequent boiling curve and heat transfer coefficient curve.

[0046] like Figure 5 Part f shows the comparison results of boiling curves. After combining the two LIPSS structures with the ENC lattice surface, although the surface nucleation site density was further increased, thanks to the enhanced hydrophilicity, both LIPSS structures achieved a further increase in CHF compared to the ENC lattice surface (ENC-Polished surface). Among them, CLSLIPSS showed the largest increase, raising the CHF to 232.0 W / cm³. 2 CLPLIPSS increased CHF to 171.8 W / cm². 2 Compared to polished surfaces, the CHF of the two surfaces increased by 153.8% and 88.0%, respectively. This result indicates that under high heat flux density conditions, introducing a hydrophilic LIPSS structure at the cavity interface can effectively delay film formation and improve the system's safe operating margin.

[0047] like Figure 5As shown in section g, the comparison results of the heat transfer coefficient curves (HTC curves) further reveal that the enhancement of HTC by the two LIPSS structures has a "stage-based rather than full-process" characteristic, and the enhancement stages differ. Compared to the ENC-Polished surface, the ENC-CLPLIPSS surface corresponding to CLPLIPSS exhibits better performance at heat flux densities less than 120 W / cm². 2 (In the early to mid-boiling stage) HTC was further improved; however, as the heat flux density continued to increase, its HTC was actually lower than that of the ENC-Polished surface. The ENC-CLSLIPSS surface corresponding to CLSLIPSS exhibited the following behavior: during the nucleus boiling stage of ENC-Polished, its overall HTC was lower than that of the ENC-Polished surface; when the heat flux density increased to 160 W / cm²... 2 During the mid-to-late boiling stage, its heat transfer coefficient only begins to exceed that of the ENC-Polished surface. Therefore, this embodiment clearly demonstrates that different LIPSS structures, by altering the bubble contact line spreading and detachment behavior, result in different advantageous regions for HTC enhancement, while all can achieve further improvement in CHF.

[0048] In this embodiment, Figure 6 The meanings of each sub-diagram can be summarized as follows: Figure 6 Part a is a schematic diagram of preparing a groove structure at the center point of the grid to guide the detachment of bubbles; Figure 6 Part b is a comparison of the dynamic behavior of surface bubbles before and after the preparation of the trench structure; Figure 6 Part c is the boiling curve; Figure 6 The d-part represents the heat transfer coefficient curve. Figure 6 Part e shows a comparison of the detachment time of large bubbles; Figure 6 The f-section compares the surface state near the critical heat flux density state and the critical heat flux state. The above illustrations and data jointly support the conclusion that by introducing a groove structure with lower nucleation activity and heterotropic wettability at the grid center point, orderly bubble coalescence and liquid supply channels can be achieved, thereby synergistically enhancing HTC and CHF. Figure 6As shown, to further alleviate the heat-blocking effect caused by excessive bubble aggregation during the high heat flux density stage, and to achieve organized separation of gas-liquid channels while maintaining a high nucleation density, this embodiment prepares groove structures to inhibit nucleation at the grid center positions on the surface of the ENC-CLSLIPSS and ENC-CLPLIPSS lattice structures. These groove structures have lower nucleation activity and serve as isolation channels to achieve gas-liquid channel separation and guide the orderly detachment of bubbles. In addition to the isolation effect, the anisotropic wettability exhibited by the groove structure in the parallel and perpendicular directions is also a key factor in its ability to guide bubble detachment, resulting in a spatially path-based distribution of liquid replenishment and bubble discharge, thereby improving the stability and safety margin of the heat exchange surface under high heat flux conditions.

[0049] like Figure 6 Part a shows a schematic diagram of the mechanism by which groove structures are prepared at the center of the grid to guide bubble detachment. This schematic diagram visually represents the transformation from "disordered coalescence" to "ordered coalescence": without the introduction of groove structures, bubbles easily coalesce randomly on the surface and form covering gas masses; after the introduction of groove structures, the groove structures act as isolation channels to separate adjacent nucleation regions, while forming organized paths for liquid and vapor channels, so that bubbles detach along a predetermined path after coalescence in local areas, thereby reducing the probability of gas film formation and improving the sustainability of the boiling process.

[0050] like Figure 6 Part b shows a comparison of the dynamic behavior of bubbles on the ENC-CLSLIPSS heat exchange surface before and after the preparation of the groove structure. Before the preparation of the groove structure, the nucleation bubbles on the ENC-CLSLIPSS surface exhibit a disordered aggregation state. Bubbles produced at each nucleation point continuously coalesce and eventually form clumps large enough to cover the entire heat exchange surface. The horizontal diameter of the bubbles detaching from the heat exchange surface is comparable to the length of the heat exchange surface, appearing as large "cloud-like" bubbles. These bubbles have a long residence time and are prone to forming heat-resistant gas films. After the preparation of the groove structure, under the isolation effect of the groove structure, the aggregation of bubbles on the ENC-CLSLIPSS-Grooves surface exhibits significant orderliness: adjacent bubbles in small areas coalesce and then directly detach from the heat exchange surface. Compared to the case without the preparation of the groove structure, the bubbles detaching from the surface are dispersed and have a smaller diameter. The bubble detachment follows a path, thereby reducing the probability of the formation of covering gas clumps and increasing the detachment and renewal rate.

[0051] This embodiment further conducts boiling performance testing, such as... Figure 6 part c and Figure 6 As shown in section d, the introduction of the groove structure further enhances the heat transfer performance. Specifically, the CHF of ENC-CLSLIPSS-Grooves is further increased to 268.1 W / cm². 2Compared to polished surfaces, this represents a 193.3% improvement. More importantly, the ENC-CLSLIPSS surface resolves the HTC drop issue caused by excessive bubble coalescence in the early to mid-boiling stages. Compared to the ENC-Polished heat exchange surface, the ENC-CLSLIPSS-Grooves achieves a full-stage HTC improvement. Meanwhile, Figure 6 In part c, the boiling curve of ENC-CLSLIPSS-Grooves shows a hook-back phenomenon, that is, the wall superheat of the heat exchange surface decreases significantly near the film boiling stage. Figure 6 In the d-part curve, the heat transfer coefficient of ENC-CLSLIPSS-Grooves increases significantly in the later stage of boiling, and the hCHF of ENC-CLSLIPSS-Grooves further increases to 14.8 W / cm². 2 The temperature was increased by 543.5% compared to the polished surface. This phenomenon indicates that the groove structure not only improves the heat flux (CHF) but also significantly improves the heat transfer capacity and thermal safety margin of the heat exchange surface in the later stage of high heat flux.

[0052] To explain the aforementioned performance improvement mechanism, this embodiment further analyzes the large bubble detachment time and the surface state near the critical heat flux density. For example... Figure 6 As shown in part e, the fabrication of the trench structure significantly shortens the residence time of coalesced bubbles, indicating that "ordered bubble coalescence" can improve the bubble detachment rate, thereby reducing the hindering effect of bubble coverage on the heat transfer surface and improving heat transfer efficiency. Figure 6 As shown in part f, near the critical CHF, the heat flux density is 245.3 W / cm². 2 During the heat transfer phase, the water channels, which are acted by the groove structure, can still be clearly observed; the water channels only disappear completely when the surface reaches CHF. It can be inferred that the reason for the decrease in wall superheat in the ENC-CLSLIPSS-Grooves at the critical CHF stage is that the late nucleation properties of the groove structure and capillary action can maintain liquid supply for a longer period during the high heat flux phase. Furthermore, near CHF, the groove structure may be activated to generate bubbles, leading to the activation of numerous potential nucleation sites, resulting in a decrease in wall temperature and a rebound in the boiling curve.

[0053] Furthermore, the enhancement effect of the groove structure on the ENC-CLPLIPSS heat transfer surface is basically the same as that of the ENC-CLSLIPSS heat transfer surface described above: the CHF is further enhanced after the introduction of the groove structure, the boiling curve shifts to the left overall, and HTC is improved throughout the entire process. Specifically, ENC-CLPLIPSS-Grooves increases CHF to 208.6 W / cm². 2 hCHF increased to 16.9 W / cm 2The temperature was increased by 128.2% and 634.8% respectively compared to the polished surface. These results indicate that introducing groove isolation channels that inhibit nucleation on the outer surface of different LIPSS types can simultaneously enhance heat transfer performance and stability through gas-liquid channel separation and bubble decoupling pathways.

[0054] The beneficial effects of this invention are as follows: Compared to existing methods for enhancing heat transfer, such as increasing surface hydrophilicity, mixing and wetting surfaces, structural modifications, and increasing surface roughness, this invention allows for an extremely high density of nucleation sites on the surface and maintains stable boiling heat transfer at a high heat flux density. In other words, the pool boiling heat transfer of the structurally spliced ​​surface can be maintained at an extremely efficient state for a long time. The key to the excellent heat transfer performance of the structurally spliced ​​surface lies in the preparation of the self-supplying liquid cavity and the enhanced bubble production frequency achieved through its structural optimization, the high liquid spreadability of the LIPSS structure, and the guiding role of the groove structure for bubble detachment.

[0055] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0056] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology, characterized in that, Multiple micro / nanostructures are fabricated on the surface of a metal material using femtosecond laser processing technology, and the various micro / nanostructures are then spatially assembled on the surface. The metal material is stainless steel, copper, or nickel. The construction method includes: A high nucleation density lattice cavity structure surface is prepared on the surface of a metal material using femtosecond laser processing technology; the lattice cavity structure surface includes multiple cavities; the cavities have autonomous liquid supply capability, and the lattice cavity structure surface is prepared by a cross-scanning processing method; the cross-scanning processing method involves the femtosecond laser scanning the surface of the metal material along the scanning path of the intersecting lines; The dynamic process of bubble production in the cavity is adjusted by regulating the laser processing parameters to adjust the geometry of the cavity and by reducing the number of intersecting lines to reduce the opening diameter of the cavity. Based on the optimization of the self-supplying liquid cavity, a laser-induced periodic structure (LIPSS) is prepared on the external interface of the lattice cavity structure surface to achieve overall enhancement of the hydrophilicity of the lattice cavity structure surface. By adjusting the laser processing parameters, laser-induced periodic structures (LIPSS) with different roughnesses are prepared at the external interface of the cavity, so as to further regulate the dynamic process of bubble generation in the cavity. Capillary groove structures with the lowest nucleation activity are prepared at the center point of the grid on the surface of the lattice cavity structure as gap channels to manage the dynamic behavior of bubbles on the heat exchange surface and guide the efficient detachment of bubbles, thereby forming the nucleation gradient multi-structure spliced ​​surface.

2. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, The cross-scan processing method is as follows: The femtosecond laser with a power of 14W was used; The surface of the metal material is scanned and processed using a laser at a scanning speed of 12 m / s along the scanning path of the intersecting lines; The high nucleation density lattice cavity structure surface is formed using the laser power and the laser scanning speed.

3. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Reducing the opening diameter of the cavity by decreasing the number of intersecting lines includes: Set the number of intersecting lines to any value of 15, 13, 11 or 9; The surface of the metal material is scanned using the cross-scanning method with the specified number of cross lines; The opening diameter of the cavity is reduced by decreasing the number of intersecting lines.

4. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Adjusting the geometry of the cavity by regulating the laser processing parameters includes: The cavity is made to be cone-shaped as a whole; The center of the cavity is a pointed concave shape; The cavity interior and sidewalls are made into columnar protrusions. The periphery of the cavity is made into a groove structure.

5. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Also includes: The cavity is arranged in a matrix on the surface of the metal material; The diameter of the cavity is set to 0.6 mm; The spacing between adjacent cavities is set to 1 mm.

6. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Laser-induced periodic structures (LIPSS) with different roughnesses are prepared at the external cavity interface by adjusting laser processing parameters, including: The laser processing power of the laser-induced periodic structure LIPSS is set to any value of 12W, 10W, 8W, 6W or 4W. The laser-induced periodic structure (LIPSS) is fabricated at the external cavity interface using the laser processing power described above. Laser-induced periodic structures (LIPSS) with different roughnesses are obtained based on different laser processing powers.

7. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Laser-induced periodic structures (LIPSS) with different roughnesses are prepared at the external cavity interface by adjusting laser processing parameters, including: Fabrication of contact wire pinned laser-induced periodic structures (LIPSS); A contact line spreading laser-induced periodic structure (LIPSS) is prepared; wherein the stripe structure of the contact line pinning laser-induced periodic structure LIPSS has circular and band-shaped gaps, and the stripe structure of the contact line spreading laser-induced periodic structure LIPSS is more dense.

8. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Capillary groove structures with the lowest nuclear activity are fabricated at the center points of the grid on the surface of the lattice cavity structure as gap channels, including: The capillary groove structure was prepared at the center point of the grid using a laser processing power of 2W. The gap channel is formed by the capillary groove structure.

9. The method for constructing a multi-structure spliced ​​surface based on femtosecond laser direct writing technology according to claim 8, characterized in that, The gap channel is formed by the capillary groove structure, including: The capillary groove structure is used as an isolation channel to achieve the separation of gas and liquid channels; The capillary groove structure is made to have opposite wettability in the direction parallel to the groove and in the direction perpendicular to the groove, so as to guide the orderly detachment of bubbles.

10. The method for constructing a multi-structure spliced ​​surface based on nucleation gradient using femtosecond laser direct writing technology according to claim 1, characterized in that, Laser-induced periodic structures (LIPSS) are fabricated on the external interface of the lattice cavity structure surface, including: The external interface of the cavity is defined as the peripheral interface of the cavity. The laser-induced periodic structure (LIPSS) is fabricated on the outer interface of the cavity.

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