Experimental Study Of Flash Spray Cooling On Different Wettability Surfaces During Depressurization

The experimental study of flashing spray cooling different wettability surfaces has practical significance for electronic chip cooling,and aerospace device cooling.In this paper,the copper-based surfaces with hydrophilicity and hydrophobicity were prepared by means of chemical oxidation.A visible test system was established for droplet evaporation on different wettability surfaces during depressurization.At the same time,the experimental phenomenon and heat transfer characteristics of flash spray cooling different wettability surfaces were studied.In this paper,the working fluid is deionized water,and the experimental phenomenon of vaporization on the hydrophilic surface and the hydrophobic surface during the depressurization process were studied.The effects of ambient pressure and initial surface temperature on the morphological changes of droplet evaporation process were summarized.The experimental results show that the droplet vaporization process in the rapid depressurization environment can be divided into four stages: the initial stage,the rapid vaporization stage,the constant contact line evaporation stage and the rapid shrinkage stage.The vaporization time of the droplets on the hydrophobic surface is significantly longer than that of the hydrophilic surface,and the rapid shrinkage phase of the droplets on the hydrophobic surface takes up less time than the hydrophilic surface.For hydrophilic surface,the lower the ambient pressure,the higher the surface temperature,the droplet vaporization time is shorter,and the proportion of constant contact line vaporization stage in the whole vaporization process is larger.The variation of the spray angle with the ambient pressure and the typical phenomenon of spray cooling different wettability surfaces were studied.The results show that there is a maximum value of the spray angle during the spraying process.And the maximum angle increases as the ambient pressure decreases and the working temperature rises.The Visible experiments of spray-cooled different wettability surfaces under low-pressure environment show that the vaporization process from spray to hydrophilic surface and smooth copper surface can be divided into four stages: the liquid film formation stage,the liquid film vaporization stage,the vaporization stage of the constant liquid film contact line,and the liquid film interface shrinkage stage.The spreading speed of the liquid droplet formed on the hydrophilic surface is higher than that of the smooth copper surface.And bubbles burst on both surfaces.After the bubble burst,the smooth copper surface will have a partial dry phenomenon.For a hydrophobic surface with poor wettability,no liquid film spreading occurs.Then,the heat transfer characteristics of spray-cooled different wettability surfaces under low pressure environment were studied.During the spray phase,the heat flux density rapidly rises to a maximum and then gradually decreases.Under the same working conditions,the spray-cooled hydrophilic surface has a higher heat flux density and a faster surface temperature drop.In the vaporization stage,the heat flux density decreases more quickly due to the lower hydrophilic surface temperature.The hydrophobic surface mainly cools during the spraying stage,the heat flux density fluctuates greatly,and the surface temperature drops slightly.The effects of ambient pressure,working temperature and initial surface temperature on heat transfer characteristics were analyzed.The results show that on the hydrophilic surface and the smooth copper surface,the maximum heat flux density during the spray stage does not increase with the decrease of the ambient pressure.This is because the influence of environmental pressure on heat transfer characteristics has two aspects: 1)When the environmental pressure is reduced,the vaporization is more intensive,and the surface temperature drops more quickly,which helps to improve the heat transfer performance;2)While,when the ambient pressure is lower,the atomization effect is better,the impact of the working fluid on the surface is weaker,therefore,the heat transfer performance is lower.During the vaporization stage after spray,as the ambient pressure decreases,the vaporization is more intensive,and the surface temperature drops faster.When the working temperature decreases and the initial surface temperature increases,the heat flux in the spray stage increases,but it has little effect on the heat flux during the vaporization t.For the spray cooling hydrophobic surface,since the liquid film is difficult to spread,the heat transfer only exists during the spray stage.The liquid impact has a significant effect on the heat transfer performance.When the working temperature is lower and the initial surface temperature is higher,the heat flux during the spray stage is greater.