Capillary Flow And Heat Transfer Characteristics Of Thin Liquid Film On Micro-nano Composite Structure

Thin liquid film has a wide range of applications in the enhancement of heat transfer processes such as evaporation and boiling due to its low thickness and low thermal resistance.Thin liquid film phase change can achieve efficient heat dissipation under the condition of low liquid holding capacity,and has received widespread attention in the field of miniaturized heat dissipation device design.Realizing the rapid spreading of thin liquid film and maintaining stability on the hot wall are the basis for the enhancement of thin liquid film phase change heat transfer.Constructing micro-nano composite structure on the surface can effectively improve the surface capillary performance,realize rapid spread of the liquid film on the surface,and maintain a stable phase change process on the hot wall.In this paper,surface with a micro-nano composite structure（MNCS）was prepared,and the capillary characteristics of the surface with different structural feature sizes were measured.The micro-scale flow characteristics was analyzed by the Micro-PIV system,which further optimized the surface preparation method.We came up with the mechanism of nanocomposite structure improving capillary performance,and the theoretical model of vertical capillary suction process is improved to predict the surface wicking coefficient of micro-nanocomposite structure.Through the liquid film phase change experiment,the spreading and wetting characteristics of the liquid film on the hot wall surface under different heating conditions were explored,and the spreading speed and the limit spreading length of the liquid film were explored.The heat transfer characteristics of the liquid film in the process of spreading-boiling-retracting were studied.The nano-scale structure on the MNCS can generate high capillary pressure as the driving force of the suction flow,and the micro-scale structures provided internal transport channels which effectively reduces the suction flow resistance.The micro-nano composite structure effectively improves the surface capillary suction performance.The surface wicking coefficient is 16.11mm·s^-0.5,which is higher than the surface of the nanowire(6.45 mm·s^-0.5)and the surface of the nanograss(1.83 mm·s^-0.5).Through the Micro-PIV system,the micro scale flow was visually observed,which directly proved the existence of the internal transport channel;the thickness of the liquid film of the suction flow in the vertical direction was partitioned;the climbing speed of the three-phase line at the suction height position,the fluid flow speed of the working fluid,and the lag time of fluorescent particles were measured.The vertical suction mathematical model is used to predict the surface suction coefficient.When the characteristic structure size is large,the predicted value is in good agreement with the experimental results.The spreading and wetting characteristics of the liquid film on the surface were studied under different heating conditions.The influence of the surface temperature on the phase change rate,as well as the spreading speed and the limit spreading length of the liquid film under the competitive relationship between the phase change rate and the liquid supply rate were discussed.The different spreading and wetting forms of the liquid film are partitioned.Under evaporation:（1）completely wetting（2）wetting and stable（3）wetting-retracting;under boiling:（4）spreading-boiling-retracting（5）spreading-boiling.The morphology and heat transfer laws of the liquid film at different stages among the process of spreading,boiling and drying-out are studied.It is found that the liquid film boiling have good adaptability,which can be used under different starting operating conditions.It can effectively suppress the surface temperature rise.The heat flux（10.0W/cm²）in the process of liquid film boiling is higher than the evaporation heat transfer（<6W/cm²）.