Investigation On The Heat Transfer Characteristics Of Vacuum Flash Evaporation Spray Cooling

Develops rapidly in space technology has resulted in an increase of the heating power in space instrumentation.There are a number of problems such as high heat flux dissipation and aerodynamic heat protection in the reentry stage and the orbit stage of the return cabin.How to effectively achieve the high heat flux dissipation of space equipment and ensure related equipment in good working condition has become the bottleneck of the development of space thermal control technology.The vacuum flash evaporation spray cooling technology(VFESC)is considered as an alternative effective method for dissipating high heat flux in the field of space thermal control by NASA.Because the VFESC has lots of advantages such as large heat flux dissipation,good temperature control effect,high cooling efficiency and small flow rate.At the same time,with the development of electronic integration and laser technology,the VFESC can also be used as a high heat flux density dissipation method in these fields,and it has been given widely attention.However,the detailed reporting of VFESC has not been found in the open literature as a new type of thermal control method.The heat transfer characteristics,heat transfer mechanism and heat transfer enhancement method of VFESC were studied in this paper.Firstly,in view of the vacuum condition of space thermal control,the experimental system of the VFESC was established,and the heat transfer characteristics of the VFESC were studied by the experimental method,and the influence law of the key parameters on the heat transfer of the VFESC was obtained.The results showed that an optimal spray height was obtained at which the maximum value of heat flux could be achieved.Compared with the conventional spray cooling,the flash evaporation could make the optimum use of the latent heat of working fluids,and the heat flux was removed efficiently in VFESC system with a very small flow rate,which was only one third of that of conventional spray cooling.However,differing from conventional spray cooling,an optimal flow rate existed in VSFEC at which the heat transfer approaches to a highest value.This study provided an experimental basis for the engineering application of vacuum flash evaporation spray cooling,especially for the strict control of weight of space thermal control.In order to explore the heat transfer enhancement method of VFESC,the influence of nanofluid on the heat transfer characteristics of VFESC was studied.Nanofluid was prepared by mixing 10nm,20nm and 30nm Al2O3 nanoparticles at different concentrations with water.It was found that when the cooled surface temperature was less than 80?,the nanoparticles of three particle sizes had little influence on the heat transfer effect of VFESC,but when the surface temperature was greater than 90?,the 10nm particle size of Al2O3-nanoparticles had a certain enhancement effect on the VFESC.However,the 20nm and 30nm particle size of Al2O3-nanoparticles played the hindrance role on the heat transfer.The reason might be that the nanoparticles had adsorption and sedimentation effects,which the nanoparticles of large size more was easier to adsorb and formed larger micelle structures which influenced the effect of droplet flash evaporation and the liquid film scoured the surface,making heat transfer deterioration.In practice,different dispersants added to nanofluid to reduce the adsorption between nanoparticles and maintain the characteristics of suspensions of nanofluid.However,the heat transfer effect of the dispersants to the nanofluid increased uncertainty and interfered with the analysis of the heat transfer of nanofluid,because it could not be explained whether the reason of the heat transfer enhancement or deteriorated only by the nanoparticles.Therefore,an experimental system of spray cooling was built in this paper.In the experiment,PVP was used as a dispersant.There were three types of working fluids:dispersant water solution,nanoparticles water solution and dispersing agent in nanoscale aqueous solution,respectively,to study their heat transfer effect on spray cooling,and compared with pure water to determine their influence on the heat transfer process.The results were as follows:when the nanoparticles-water solution was used as the working fluid,the heat transfer effect was worse than that of pure water.When dispersant-water solution was used as the working fluid,the heat transfer effect was better than that of pure water.When dispersing agent in nanoscale aqueous solution was used as the working fluid,the heat transfer effect was better than that of pure water-nanoparticles water solution.These conclusions indicated that when nanofluids with PVP as dispersant were used as working fluids for spray cooling,the effect of enhancing heat transfer was not nano particles themselves,but was PVP dispersants.Finally,in order to study the cooling performance of the VFESC for the local high temperature heat source and analyze the influence of surface strengthening structure,a flash evaporation spray cooling experiment system was established.In this system,the radio-frequency resistance was used to simulate the heating surface of high temperature heat source with an area of 1.2 cm2,and the smooth surface,and the straight fins surface and the cubic pin fins surface were used as the thermal expansion surface of the heating surface.The influence of flow rate and surface structure on the heat transfer characteristics of the VFESC were studied,and the transient cooling characteristics of the heated surface was studied.The experimental results showed that the heat transfer effect of VFESC was better than that of normal spray cooling under the same condition.The enhanced surface could improve the heat flux of the common spray cooling and VFESC,but the enhancement effect on the VSFEC was not as good as that of the commom spray cooling.The increase of the spray flow could reduce the surface temperature of common spray cooling and VFESC,but the cooling effect on common spray cooling was better.In the experimental range,the heat transfer effect of the straight fins surface was the best,when the flow increased from 1.6L/h to 3.1L/h,the temperature drop of the ordinary spray cooling was about 24%,while the temperature drop of the flash spray cooling was only 9%.The instantaneous cooling effect of flash spray cooling was better than that of common spray cooling,and had the advantages of low temperature difference and fast cooling.