| With the rapid development of science and technology,photoelectric equipment gradually presents the characteristics of high throughput,high integration and high frequency,which poses a challenge to efficient thermal management technology.The porous structure has good heat and mass transport performance due to its large specific surface area and strong capillary force,which has been widely used in the field of aerospace photovoltaic power generation and seawater desalination.The uniform porous structure of single scale can not meet the requirement of high heat dissipation performance of photoelectric equipment,how to improve the heat and mass transport performance of composite porous structure through effective control of wettability and surface structure has become the key to ensure the efficient operation of equipment under high power.In this paper,based on the sintered porous metal structure as the research benchmark,combined with theoretical analysis,numerical simulation and experimental observation,the heat and mass transfer characteristics of multi-scale composite porous structures during capillary evaporation and boiling heat transfer are systematically studied,and the physical mechanism of the enhancement of bubble transport and capillary performance by porous structures based on wettability and surface structure regulation is clarified;Combined with the regulation mechanism of multi-scale synergistic effect,the functional porous structure is designed and fabricated,and the internal relationship between microstructure and surface heat transfer performance is studied,and the control strategy of capillary evaporation and boiling heat transfer is clarified to realize heat transfer enhancement.In addition,based on the effective regulation of composite porous structure in capillary evaporation and boiling heat transfer process,a capillary seawater desalination system integrated with high concentration photovoltaic technology is proposed.The control factors and regulation mechanism of capillary driven seawater desalination are clarified,and the effective regulation strategy is given.The main research contents and conclusions are as follows:Taking sintered copper particles as a model,the optimum clearance of liquid inflo w is given through theoretical analysis.The microcosmic influence mechanism of wettability of porous structure on bubble transport process is revealed,compared with the hydrophilic structure,the hydrophobic structure shows stronger adhesion to the bubbles during their growth,leading to the bubbles mainly expanding horizontally and merging easily with neighboring bubbles.The microscopic mechanism of bubble transport enhancement by surface microcavity structure is clarified,for the micro-cavity nucleated bubbles,many vortices are formed at the bottom of the bubble,and the disordered flow of fluid significantly increases the disturbance of the gas-liquid interface,leading to an increase in velocity gradient,which promotes the departure of the bubble,and the liquid film in the microcavity provides the rehydration path.Because the microcavity structure significantly increases the vortex microclusters and increases the diffusion of fluid,the microclusters formed by fluid constantly shear with the bubble boundary,which significantly improves the growth behavior of bubbles.Combined with the control mechanism of multi-scale synergistic effect,the grooved porous structure covered microcavities is designed,experimental results show that the wettability and capillary performance of the proposed grooved porous structure covered microcavities are significantly improved.The dynamic behavior of bubbles in boiling heat transfer process and the rewetting and rehydration performance under high heat flux are effectively optimized.The boiling heat transfer characteristics of the grooved porous structure covered microcavities are tested under atmospheric pressure.The results show that the grooved porous structure covered microcavities can increase the site density of bubble nucleation,decrease the bubble departure diameter and increase the bubble departure frequency compared with the grooved porous structure.Compared with pure copper flat surface,the critical heat flux of the grooved porous structure covered microcavities increased by 125%,the heat transfer coefficient increased by 150%,and the initiation temperature of nuclear boiling decreased by85%.Due to the formation of a large number of microcavities,more nucleation sites are activated,and the small nucleation bubbles in the microcavity can absorb the heat from the wall around the microcavity,thus accelerating the growth rate of bubbles and the instability of the vapor liquid interface in the microcavity accelerates the departure of bubbles.Due to the limitation of the size,the bubbles in the microgroove grow close to the wall of the microgroove,and the liquid film between the microgroove and the bubbles accelerates the growth of the bubbles.The linear relationship between the wicking velocity and CHF of different porous structures is obtained.By transpiration of the trees and water transport process,combined with the synergistic effect of micro-cavity structure and gradient porosity,we construct a gradient porous structure covered microcavities to enhance capillary evaporation process,the microstructural synergistic effect significantly improves the bubble transport process,leading to more disorderly fluid flow.There are nonlinear forces between vortices of different scales,which makes the flow state more complex.Moreover,the disturbance at the vapor-liquid interface significantly increases,which greatly promotes the bubble movement.When the superheat is5 K,compared with the gradient porous structure,the average heat transfer coefficient of the gradient porous structure covered microcavities is increased by 47%.At the same time,this structure couples the high permeability of gradient pores and the high capillary pressure of micro-cavity structure,which effectively optimizes the problem of mismatch between liquid suction and rapid steam discharge.The capillary evaporation characteristics of gradient porous structure covered microcavities under atmospheric pressure are investigated systematically.The study shows that compared with other porous structure,under the same heat flux,the gradient porous structure covered microcavities significantly reduces the superheat,the heat transfer performance is improved significantly.Due to the curvature of the meniscus interface,fluid is continuously added from the vapor-liquid interface to the burn out position,limiting the liquid burn out in the microcavity,and the synergistic effect of the microscale structure leads to a significant increase in the bubble nucleation site density,bubble departure frequency and area of bubble nucleation.Compared with the gradient porous structure,the gradient porous structure covered microcavities does not reach CHF,and the heat flux and heat transfer coefficient increase by 32.5%and 220%respectively.A novel capillary seawater desalination system integrated with high concentration photovoltaic technology has been studied.It has been shown that the U-shaped porous structure covered microcavities can achieve a high concentration ratio(CR)at the allowable temperature of the cell surface.At the maximum CR,the freshwater collected by the U-shaped porous structure covered microcavities can reach up to 18 kg/(min·m~2).Compared with the ordinary U-shaped porous structure,the U-shaped porous structure covered microcavities has no salt adhesion at CR=867,showing better salt resistance.Further exploration reveals that the U-shaped porous structure covered microcavities can ensure more than 50%thermal efficiency for seawater desalination.When the CR>1000,the U-shaped porous structure covered microcavities can continue to work,keep the efficiency of the cell above 36.4%,and maximum equivalent power output per square meter is 418 k W. |
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