| With the development of deep space exploration,the number of space missions is increasing,more and more spacecraft are sent into space,and the requirement for the life of spacecraft is becoming more strict and highly complicated.At present,electric propulsion technology and in-orbit storage technology have become important means to effectively extend the life span of spacecraft.According to the investigation,krypton gas has become one of the ideal working fluids for electric propulsion with the advantages of large atomic weight,small ionization energy and high specific impulse.Krypton is usually stored in liquid state,and the shortcomings of low boiling point and easy evaporation make it difficult to store and apply in complex space environments.It is of great significance for the development of in-orbit storage technology of liquid krypton cryogenic propellant to investigate the distribution of fluid in the tank and its influencing factors in microgravity environment and to analyze the heat transfer and pressurization mechanism of cryogenic propellant tank.The pressure change of propellant tank is a complex physical process,which is affected by many factors,Therefore,the heat transfer process of spacecraft in space thermal environment is studied in this paper,and provides guidance for the pressure control design of liquid krypton tank for large orbital transfer vehicle.The main work and related conclusions are as follows:（1）In this paper,a heat transfer model for liquid Krypton tank for a large orbit transfer vehicle in microgravity was established.The effectiveness and reliability of the model are verified by the experiment of liquid nitrogen self-pressurization.In the process of 230 s numerical calculation,the maximum deviation between the simulated value and the experimental value is less than 1.17%,indicating that the numerical model constructed in this paper can be used for the simulation study of self-pressurization of liquid krypton tank.（2）The ullage configuration in microgravity environment and the fusion characteristics of bubbles with different sizes in the tank were simulated.The influence of gravity level and contact Angle on the air pillow configuration was studied.The results show that under the condition of microgravity(10-4g),the Krypton bubbles of different sizes rise slowly,collide and fuse under the comprehensive action of multiple force fields,and form a stable ellipsoidal rotation at the top the tank.The research results prove that the hypothesis of lumped vapor model is feasible.Under the influence of gravity level and contact Angle,in the process of liquid climbing,climbing,the wettability of the liquid to the solid decreases with the increase of contact angle and gravity level.When the gravity level is less than 10-3g and the contact angle is less than 10°,the liquid wettability area accounts for the largest proportion which is more advantageous to store the liquid propellant in orbit for a long time.（3）The effects of gravity level,initial krypton temperature and initial filling rate on pressure and temperature stratification of tank were studied.The research results show that the pressure and stratification degree of the tank under microgravity are lower than that on the ground,and the pressure rise rate of g under the ground condition is 1.843 times,1.98 times and 2.04 times of 10-4g,10-5g and 10-6g,respectively.The temperature stratification degree in microgravity is 2～3 K,far lower than 90 K in ground condition,and the radial distribution of temperature stratification is different from the axial distribution in ground condition.The tank pressure decreases first and then increases with time.At the same time,the tank pressure decreases with the decrease of the initial liquid krypton temperature.The pressure drop rate at 133.5 K is1.38 times and 1.57 times of that at 134 and 134.5 K,respectively.With the continuous liquid evaporation of heat leakage,the tank pressure began to rise,and the pressure rise rate at 134.5K was 1.31 times and 3.78 times of that at 134,133.5 K,respectively.The tank has a critical initial filling rate ofФ=70%,when the initial filling rate ofФ<70%,liquid evaporation plays a leading role in the pressure rise of the container,the lower the initial filling rate,the faster the evaporation,the faster the pressure rise.When the initial filling rate isФ>70%,liquid phase thermal expansion plays a leading role in tank pressure rise.The larger the initial filling rate is,the more significant the influence of liquid krypton thermal expansion on tank pressure rise is.The results of this paper provides a certain numerical calculation basis for the future propellant tank pressure change and on-orbit pressure control strategy research. |
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