Study On Heat Transfer Mechanism In Spray Cooling And Terrestrial Simulation Of Heat Transfer In Space

Spray cooling is an efficient method for high heat flux removal.With the advantages of strongly heat removing ability,low mass rate of working fluid, compact structure,and good adaptability for different environments,spray cooling can be used in almost any environments for high heat flux removal.In recent years, the power of electronic equipments used in aircrafts increases rapidly,spray cooling may be introduced into microgravity in the near future.This paper focuses on the study of the heat transfer mechanisms and the heat transfer ability in the spray cooling,and of the terrestrial simulation method for the space heat transfer.Heat transfer mechanisms in spray cooling were analyzed.A mathematical model of spray cooling was presented based on fundamentals of dynamics and heat transfer, which described the heat transfer mechanisms and physical phenomena in spray cooling,including:characterstics of the spray,dynamics of the film,wall nucleation bubbles,second nucleation bubbles,and bubbles broken by liquid drops etc.With this model,the following variables were predicted:thickness and velocity of film, heat flux associated with heat transfer mechanisms,surface temperature distribution, spray cooling curve,and heat transfer coefficient of spray cooling.When two phase heat transfer is introduced in spray cooling,the department of wall nucleation bubbles from the heating surface is important for the heat transfer performance.The effect of gravity direction on the departure of wall nucleation bubbles were analyzed by changing the angle between the heating surface and gravity.The heat transfer characteristics in spray cooling were studied both experimentally and theoretically under normal gravity and normal pressure condition.The surface temperature distribution,the spraying cooling curve,and the heat transfer coefficient of spray cooling were obtained.The prediction of the model accords with the experimental results well,with a deviation in range of 10%. Non-uniformity of surface temperature distribution was observed,which can be higher than 15℃.With the increase of surface heating power,the non-uniformity of surface temperature distribution increased under single phase heat transfer condition, however decerased under two phase heat transfer condition.The heat transfer ability of spray cooling was evaluated with pray cooling curve,and heat transfer coefficient of spray cooling.The effect of spray height,spray pressure,and nozzle angle on the spray cooling heat removal ability was obtained.The ratios of heat flux associated with various heat transfer mechanisms were obtained.The heat transfered by bubbles increased with the increase of the surface overheat.However,the ratio of heat transferred by bubbles kept stable when the heating power was beyond a certain value.Further investigation of heat transfer in spray cooling and its advanced method were presented:the heat removal ability of spray cooling under normal gravity and vaccum condition both experimentally and theoretically.The spray cooling curve and heat transfer coefficient were obtained.Results showed:two phase heat transfer were observed more obviously than under normal gravity and normal pressure conditions because of the lower saturation temperature.Therefore,the heat transfer ablity of spray cooling is higher under vacuum condition than under normal pressure condition.Results showed:non-uniformity of surface temperature was observed. The non-uniformity of surface temperature under vacuum condition was less compared under normal condition.With the increase of surface heating power,the non-uniformity of surface temperature distribution increased.With the comparison of heat transfer coefficient under normal pressure condition and under vacuum condition,the heat transfer enhancement ratio of spray cooling was obtained,which increased with the increase of the surface heating power.It is hard to study the spray cooling under microgravity condition by experimental methods.Therefore,the terrestrial simulation under normal gravity condition is important for the study of spray cooling under microgravity condition.A mathematical model of spray cooling under microgravity condition was presented. Based on the analysis of heat transfer in spray cooling,two terrestrial simulation methods for spray cooling under microgravity were presented,and the correlation of these two methods were obtained.The last part of this paper is the analysis of the lowering pressure method used in study on heat transfer in the space aircrafts.Effects of natural convection on the heat transfer characteristics in typical cabins of aircrafts were studied theoretically.Both the critical pressure ratio and the critical Gr/Re~2 Number were then obtained.With the use of critical pressure ratio,terrestrial simulation method for studying the heat transfer in cabins of a manned aircraft was presented and then analyzed.