Three-dimensional Numerical Simulation Of Nucleate Boiling On Square Micropillar Structured Surfaces

The energy problem is the primary issue that needs to be solved for the further development of human civilization.Breakthroughs in methods and technologies to improve energy efficiency and heat transfer efficiency are particularly important.Nucleate boiling is an efficient heat transfer method,is widely used in many industrial fields with high heat flux.Various methods and technologies of enhance nucleate boiling have been studied in in great quantities.According to many experimental results,micropillar structured surfaces can enhance the boiling heat transfer coefficient and increase the critical heat flux.However,the nucleate boiling on micropillar structured surfaces is a problem coupling vapor,liquid and solid three phases with some randomness,there are limits in experimental methods to measure the physical details of nucleate boiling.Therefore,it is necessary to study the nucleate boiling on micropillar structured surfaces by numerical simulation ways.In this paper,the numerical simulation study of nucleate boiling on square micropillar structured surfaces is carried out based on computational fluid dynamics.The capture of vapor-liquid interface,the mechanism of vapor-liquid evaporation,the mechanism of microlayer evaporation,the reasonable setting of nucleation site and the reasonable description of the flow on sidewalls of computational domain are all considered in numerical simulations.Based on numerical simulations,the differences of nucleation site positions on micropillar structured surfaces and the influences of micropillar height and density on single bubble nucleate boiling are studied.For the two different nucleation site positions of corner nucleation and center nucleation,in the growth stage,there is difference in the contact pattern between bubble and surface,and the temperature distribution of surface also has some differences,but there are no significant differences in the evaporation rates of vapor-liquid interface and the bubble growth rate;in the departure stage,the difference of bubble dynamics increases,the bubble with corner nucleation depart from the surface quickly due to asymmetric forces.The departure processes on two sides of bubble with corner nucleation are also asymmetric,whereas the departure processes on two sides of bubble with center nucleation are always keep asymmetric.The slower departure of the bubble with center nucleation delay the rise of surface temperature in departure stage,and have larger microlayer and vapor-liquid interface evaporation rates at the same time.The limitation of bubble growth in micropillar gap is enhanced by the increase of micropillar height,the area of bubble bottom shows a decrease trend and then an increase trend after a certain time in the growth stage,the bubble appears"mushroom-like" shape with large head and small root at the end of the growth stage.As a result,the time of departure stage is significantly shortened,while the time of growth stage increases or remains unchanged with the increase of micropillar height,so the ratio of the time of growth stage to the total time increases continuously.Moreover,when the micropillar is higher,the evaporation rates of vapor-liquid interface are higher,and the bubble growth rate is faster,the bubble depart more quickly with more total heat transfer.Therefore,the effect of enhanced boiling heat transfer increased significantly with the height of micropillar.The increase of micropillars density reduces the micropillar gap,enhances the capillary force in the micropillar gap flow channel,and significantly accelerates bubble departure.Different from the height variation,the increase of micropillars density makes the bubble enters the departure stage earlier and leaves the entire surface earlier.The ratio of growth stage time to total time decreases significantly when the micropillars density increases to a certain extent.Increasing the density of micropillars has almost no effect on the evaporation rate of vapor-liquid interface,but significantly reduces the evaporation rate of microlayer.Although the bubble departure time is shortened,the time-averaged heat exchange rate is reduced,the effect of enhanced boiling is weakened.