Study Of Flash Spray Characteristics On Non-flat Surface

The flash evaporation of the working fluid under low pressure environment will result in the intensive vaporization and take away a lot of heat.The non smooth surface can expands the heat transfer area and increases the vaporization core by micro machining on the surface,which helps to enhance the heat transfer effect.This paper combines the technologies in order to enhance surface heat transfer performance of the surface.In this paper,a visualized experimental system for flash spray in low pressure environment was set up.The deionized water was used as the working fluid,and the spray pattern from the swirl atomizer and the straight nozzle was studied.The results show that when the swirl atomizer is used,the working fluid inside the nozzle has been partially atomized,and the liquid has been atomized instantaneously when the valve is opened.When the straight nozzle is used,the spray shape changes with time,the spray morphology experiences the process of liquid column,fragmentation,and complete atomization.As the ambient pressure decreases,the initial temperature of the working fluid increases and the spray cone angle increases.The effects of atmospheric pressure spray and low pressure spray on the surface temperature of structural surfaces with time were studied experimentally.The effects of surface structure,environmental pressure,working fluid temperature and working fluid flow on the temperature change were obtained.The results show that during the normal pressure spray and low pressure spray process,the surface temperature changes experience three stages: 1)spray stage:the surface temperature decreases rapidly;2)vaporization stage: after the end of the spray,the liquid remaining on the surface continues to vaporize and the temperature continues to decreases;3)the temperature rising phase: due to the continuous heating at the bottom,when the surface temperature drops to the lowest point,the heatcarried by the liquid vaporization is less than the heat continuously provided,and the surface temperature gradually rises.Three kinds of surface structures were fabricated on copper substrate,including cylindrical structure,square column structure and sintered surface.By analyzing the change of surface temperature during the spraying process,it can be known that the heat transfer performance of the three nonflat surfaces is better than that of the smooth copper surface,whether it is a normal pressure spray or a low pressure spray process.Among them: the cylindrical surface is the best,and the square column surface is the second.In the normal pressure spraying process,the larger the working fluid flow rate and the lower the initial working temperature,the faster the surface temperature drops during the spray stage;but it has little effect on the minimum surface temperature during the vaporization stage.During the low pressure spray,the lower the ambient pressure,the faster the surface temperature drops during the spray stage,and the minimum surface temperature is lower.The initial temperature of the working fluid has little effect on the surface temperature change.The temperature of different positions of spray at the outlet axis of the nozzle was measured,and a mathematic model was established for the temperature change of the single moving droplet evaporation process under low pressure environment.The variation of droplet temperature withinjection distance was calculated by using Matlab software.Comparing the calculated results with the experimental data,the two are relatively close.Then,the effects of environmental pressure,initial temperature of working fluid on droplet temperature and size change were analyzed.The results show that if the ambient pressure is lower,the droplet evaporatesfaster,the droplet diameter decreases faster,and the droplet temperature dropsfaster.With a higher initial droplet temperature,the droplet evaporates faster at the nozzle exit,the droplet diameter decreases faster,and the temperature drops faster.Aftera certain distance of injection,the influence of initial droplet temperature is gradually weakened.