| Modern electronic devices are lightweight,precise and highly integrated,and their local heat flux increases dramatically,posing severe challenges to traditional cooling technologies.The two-phase flow spray cooling technology has significant advantages such as high cooling efficiency and low demand for working fluids,and also has been applied to the cooling of space electronic equipments in the aerospace field.The research focus of the two-phase flow spray cooling technology mainly concentrated on the surface cooling mechanism of single nozzle and small-sized heat source.In order to solve the key problems of heat dissipation and cooling of electronic equipments with high thermal load,the paper established the design criteria and theoretical calculation model of dry ice spray array nozzles for large-sized heat source.Constructed an array nozzle dry ice sublimation spray cooling test bench and proposed a dry ice spray cooling test method,set up a three-dimensional CFD numerical simulation model of dry ice spray cooling,and systematically study the spray mass flow,spray height,dry ice volume ratio,nozzle inlet velocity,and simulated heat source heating heat flow Influence of factors such asheat flux on the heat transfer characteristics of array nozzle dry ice sublimation spray cooling.Dry ice array nozzles design and development and spray cooling test bench construction.In view of the complex characteristics of dry ice phase changed spray cooling mechanism,a mathematical model for the design of dry ice spray cooling array nozzles was proposed.According to the design criteria of array nozzles,two types of linear assembleable and disassembled array nozzles were innovatively designed and developed,namely convergent array nozzles and Laval array nozzles to solve the problems of enhanced heat transfer by dry ice spray cooling in aerospace thermal control applications with large flow and size heat sources.Based on the Joule-Thomson principle of carbon dioxide,an array nozzle dry ice sublimation spray cooling test bench was designed and constructed,the key components required for the dry ice spray cooling test were selected and optimized,and the operation guidelines for the array nozzle dry ice sublimation spray cooling test were proposed to reduce the errors of test data acquisition and processing,to ensure the accuracy of the influence law of heat transfer characteristics in the research of dry ice sublimation spray cooling test.Two types of array nozzles were tested in cold state,and the test results showed that the spray flow pattern of the dry ice array nozzles was uniform,and the spray area could completely cover the surface of the heat source.Array nozzle dry ice sublimation spray cooling test.According to the main influencing factors of dry ice spray cooling,the effects of spray mass flow and spray height on the heat transfer characteristics of dry ice sublimation spray cooling with two types of array nozzles were experimentally studied.The test results showed that there was a big difference in the spray cooling performance of the two types of array nozzles.The heat transfer characteristics of the convergent array nozzles were relatively simple.With the increasing of spray quality and the decreasing of spray height,the critical heat flux and convective heat transfer coefficient continue Raising and lowering the spray height could significantly enhance the cooling performance of dry ice spray cooling.The optimum spray height is h=1.5 cm,the mass flow rate is 36.29 g/s,and the heat flux and spray cooling convection heat transfer coefficient reached the highest values of 446.57 W/cm~2 and 39481.12.W/(m~2·℃),the spray cooling performance of the convergent array nozzle was greatly affected by the Reynolds number,and the higher the inlet spray mass flow rate,the better the spray cooling performance.For Laval array nozzles,the influence law of spray mass flow on its cooling characteristics is roughly the same as that of convergent array nozzles.The difference is that when the maximum spray mass flow rate is36.29 g/s,the optimal spray height corresponding to the spray cooling performance of the Laval array nozzle is h=2.5 cm,instead of the minimum spray height h=1.5 cm,and the two types under the same spray conditions,the spray cooling capacity of the Laval array nozzles was slightly better than that of the convergent array nozzles,which showed that adding a suitable length of the tapered section at the outlet of the array nozzle could effectively improve the actual effective dry ice sprayed to the surface of the heat source.The amount of particles increased the heat transfer capacity and spray cooling effect of dry ice spray cooling.In addition,adding a dry ice sublimation chamber device to the array nozzle dry ice sublimation spray cooling test system could reduce the forced convection heat transfer between the low temperature dry ice fluid and the surrounding air during the spraying of the low temperature dry ice fluid to the surface of the heat source,reduced the nozzle outlet temperature,and significantly improved the critical heat flux of dry ice spray cooling.Numerical simulation of dry ice sublimation spray cooling.In order to deeply study the influencing factors of the heat transfer characteristics of dry ice sublimation spray cooling of array nozzles,deduced the mathematical model of the sublimation phase transition process of dry ice particles,embed UDF to compile,and carried out numerical simulation research of dry ice sublimation spray cooling.Grid-independent verification of the numerical model for sublimation of dry ice particles and comparison with experimental results to ensure the accuracy of the numerical model.Aiming at the influencing factors of heat transfer characteristics that can not be explored in the experiment,numerical simulation method was used to study the effects of two types of array nozzles on the cooling performance of dry ice spray under different factors such as different inlet velocity,dry ice volume ratio,and heat source heating heat flux.The results showed that for the two types of array nozzles,the higher the dry ice inlet velocity and the dry ice volume ratio,the better the cooling performance.The convection heat transfer coefficients of the convergent array and Laval array nozzles both reached the highest values,which were 16839.74 and 13647.21 W/(m~2·℃)respectively when inlet velocity and dry ice voluem ratio were 50 m/s and 0.4.In the coupling exploration of the two influencing factors,the spray height and the surface heating heat flux of the heat source,the convergent array nozzles showed that the influence of the spray height on the cooling performance of the convergent array nozzle was almost linear,and the spray height h=3 mm was the optimal spray.At this time,the heating heat flux of the heat source surface was 1.3×106 W/m~2,the average temperature of the heat source surface reached the lowest value of 2.76℃,and the surface temperature uniformity achieved the best effect.At the same time,the heat source heating heat flux had little effect on the heat transfer coefficient of the convergent array nozzles.Increasing the heat source surface heat flux could only slightly increased the dry ice spray convective heat transfer coefficient. |
