Experimental Study On Heat Transfer Enhancement For Spray Cooling

With the continuous improvement of technology,aerial electrical system becomes more and more integrated and modularized.Therefore,how to efficiently and reliably solve the high heat flux density of airborne electronic equipment has great significance.The spray cooling technology has the advantages of low heat transfer temperature,good uniformity of surface temperature,less quantity of work and better heat exchange performance,so the application of this technology will effectively solve the problem of high efficiency heat exchange.In this thesis,the influence of design parameters and location of groove surfaces,low pressure environment and additive on the heat transfer performance of the spray cooling system are studied systematically which provides experimental and theoretical support for the practical application of spray cooling system.The specific work are as follows:?1?The influence of the design parameters of the groove surface are studied.Through analysis the optimization structural parameters such as width and depth of the micro-channel surface,the corresponding relationship between the spray flow rate and the optimal heat transfer surface is obtained.Based on the analysis of the force of fog drops on the surface of micro-channel and the flow condition of liquid film,the specific reasons of the optimal heat transfer on the surface of the channel are revealed.On this basis,the effect of the relative position of the surface of the groove on the heat transfer is also studied.?2?The effect of the low pressure environment for spray cooling are studied.Airborne environment is a typical low pressure environment,affected by low pressure,the boiling point of working medium will change accordingly.In order to better study the airborne environment features,the heat transfer performance under the ambient pressure of the spray cavity was studied under 25kPa,35kPa,45kPa,55kPa,65kPa and 75kPa,respectively.The results show that,the saturated temperature of working medium decreases in the low-pressure environment,spray cooling is easier to enter the two-phase zone,and the effect of boiling heat transfer is prominent,which increases the overall heat transfer capacity.?3?The influence of the additives for spray cooling are studied.The additives can affect the viscosity,surface tension,boiling point and other physical properties of the working substance,and then affect the spray cooling properties.Therefore,Nacl salt solution,ethanol solution and Al₂O₃ nano-fluid are used as working materials,and experimental study on spray characteristics under different solution concentrations is carried out to obtain spray properties unde different additives types and concentrations.The results show that,for salt solution,when the heating power is 700W,follow with the concentration of salt solution increases,heat transfer performance of spray cooling is improved first and then reduced.The heat transfer mechanism is analyzed by law of change for Re number,Pr number and We number of salt solution.While when the heating power reached 1000W,the heat transfer coefficient always decrease with the concentration increases.For ethanol solution,heat transfer performance of spray cooling first strengthened and then deteriorated.The reason of this phenomenon is that the boiling point decreases and change of We number.For alumina nanofluids,the heat transfer performance reaches the optimal when the concentration was0.04%,and then deteriorated when the concentration exceeded 0.1%.The reason is that the thermal conductivity of alumina is much larger than that of water,and the nanoparticles are affected by Brownian force.?4?non-dimensional correlation analysis of spray coolingOn the basis of summarizing previous work experience and combining experimental conclusions,the dimensionless correlation formula of Nu on smooth surface with the accuracy less than 10%is obtained through programming calculation.The accuracy of correlation prediction is mainly affected by four dimensionless Numbers of Re,Pr,We and evapotranspiration?.The structure size of the groove surface is taken as the characteristic parameter by introducing the groove depth and width ratio?,and the dimensionless correlation formula of spray cooling heat transfer on the surface of the micro-channel with an error range of less than 9%is obtained.Compared with the existing correlation formula,it is more suitable to reflect the heat transfer characteristics of spray cooling including the surface of micro groove.?5?Based on VOF model,the surface model of droplet impact solid surface and liquid film are established.In order to better understand and reveal the heat transfer mechanism of spray cooling,the VOF module in FLUENT software was used to conduct modeling and numerical analysis of droplet/wall/thin liquid film interactions during spray cooling.The results showed that the droplet velocity and droplet diameter had significant influence on heat transfer.With the increase of droplet velocity and decrease of droplet diameter,the surface heat transfer coefficient increased significantly.