Cooling Experimental Study On Spray Heat Transfer Of Liquid Nitrogen

With the continuous improvement of the detection ability of the infrared imaging system,the aerodynamic heating of the aircraft skin can be detected between 200K and350K.While using coating materials to reduce the surface incidence and achieve infrared stealth,thermal environment control measures are adopted to cool the surface,so as to reduce the infrared radiation intensity of the surface.Liquid nitrogen spray heat exchange,as a new rapid cooling technology,has the advantages of fast cooling speed and short response time,and is one of the effective ways to solve the rapid large-scale cooling in low temperature environment.In order to improve the performance of liquid nitrogen spray heat transfer,it is necessary to analyze the influencing factors of spray heat transfer with large surface temperature difference.In this paper,an experimental device for liquid nitrogen spray heat exchange is designed.The feasibility of the experiment and the effect of gravity on liquid nitrogen spray heat transfer are simulated by numerical simulation.Then the data processing methods involved in the experiment are introduced.Finally,the error analysis of the temperature data and the calculation results of heat transfer coefficient in the experiment is carried out according to the accuracy of the measuring instrument.In this paper,in the process of setting up the spray heat exchange experimental system,the cooling surface sample and nozzle selection,pipeline design,heat preservation and insulation measures and thermocouple layout measures are introduced.This paper describes the content and steps of the experiment.In the experiment,the thin aluminum plate made of 7075 aluminum was used as the hot part of the skin,and the electric heating sheet was applied to the surface of the aluminum plate to simulate the thermal environment of the skin subjected to pneumatic heating.After the spray heat transfer experiment,the experimental results are analyzed in this paper.In this paper,the feasibility of surface cooling and the uniformity of temperature are explained.Then,the heat transfer effect of several selected surface structures(including rough surface,surface with microchannels and three kinds of foams with different porosity)is analyzed.The experimental results of this paper show that the enhanced heat transfer with rough surface is the best.And the low temperature control function can still be realized under the large heating heat flux of 10k W/m~2.Therefore,in the subsequent experimental study,the rough surface was used as the surface of the cooled sample.In order to analyze the temperature control effect of spray heat transfer under different environments,the experiment simulated the surface dynamic spray heat transfer under different heating conditions.The experimental results show that the surface cooling can be realized rapidly under different heating heat flux and heating temperature zone.However,with the increase of heat flux,the fluctuation of surface temperature becomes more serious,and the difficulty of achieving uniform temperature on the surface becomes more difficult.When the aluminum plate is tilted or placed vertically,the liquid film will be subjected to gravity and flow downward.The flow of the liquid film will affect the phase transfer heat flow on the surface,and then affect the heat transfer effect.Therefore,the effect of gravity on spray heat transfer is studied in this paper.The experimental results show that the different spray direction can cause the change of heat transfer coefficient about±20%.In the process of spray heat transfer,spray pressure will affect spray flow rate and droplet velocity.Therefore,spray pressure will affect the cooling rate and surface temperature uniformity of spray heat transfer.When the spray pressure is too high,the strength and tightness of the pipeline will be challenged.Therefore,it is of great significance to study the effect of spray pressure on spray heat transfer.The effect of heat transfer under three different spray pressures was studied experimentally.The experimental results show that the surface heat transfer coefficient increases linearly with the increase of pressure.Finally,a significant temperature difference between the cooled surface and the room temperature environment was considered.This paper analyzes the thermal insulation effect of the thermal insulation structure and the influence degree of heat leakage of the thermal insulation structure on the experimental results.