Research On The Self-Pressurization Phenomenon And Liquid Mass Gauge Of Cryogenic Propellant Storage

Economic Propellant with high performance is required for the on-orbit refueling and deep space exploration missions. Cryogenic liquids like liquid hydrogen and liquid oxygen will play an integral role in aerospace missions due to their high efficient thrust and non-pollution waste. It has been defined as the superior propellant for aerospace mission and widely concerned in the world. The cryogenic storage tanks are equipped with high quality insulation. However, heat leakage still occurs for the conduction of the support structure and the pipelines that connect the tank to the other devices, which in a closed tank will result in pressure rise and temperature stratification. The safe storage of cryogenic propellant is significant in its future aerospace application.In this paper, some fundamental issues in the two-phase tank system are studied, including the self-pressurization phenomenon, temperature stratification and liquid mass gauge. Based on the research on the liquid flow and heat transfer, a pressure control method is proposed and the performance is analyzed in detail. For liquid mass gauge, the ground test system of compression liquid mass gauging is established and the measurement error is verified. The main research activities of the thesis are:(1) The self-pressurization is theoretically studied. A lumped gas model coupled with an effective liquid conduction is used to predict pressure rise in the cryogenic tank. This coupled model is validated by experimental data and the predictive capability is assessed to be good. Based on the coupled thermal diffusion model, the effect of heat load, fill level, heat leakage distribution, tank size and non-condensable gas on the self-pressurization is investigated.(2) The effect of natural convection on self-pressurization is numerically simulated. A computational fluid dynamics(CFD) model is established by combining a volume of fluid(VOF) method and the Hertz-Knudsen-Schrage phase change model to simulate the storage in the cryogenic tank with heat leakage. Fluid flow and pressure rise tendency are obtained as well as the temperature stratification phenomenon. Results show the factors of heat load, heat leakage distribution and fill level play the dominant role in the self-pressurization in the ventless cryogenic tank. Subcooled boiling tends to occur as gravity acceleration decreases due to absence of natural convection and counter flow close to the tank wall.(3) The influence of subcooled boiling on cryogen storage self-pressurization is experimentally studied. The ground test system is established and experiments are conducted to investigate the effect of the initial pressure, fill level and heat load. Boil bubble in the closure tank is observed and recorded by high speed camera. By the measuring the bubble sizes in different heat cases, a bubble growth model is proposed.(4) The pressure/temperature control method of the cryogenic storage tank is analyzed. A method using the ribs on the tank wall is proposed. The mathematical model of the ribbed cryogenic tank is introduced. Comparison between normal tank and ribbed tank is carried out and results indicate tank ribs can drag the pressure rise passively. Enhancement of heat transfer can be accomplished with ribs mounted on the wall which makes the mixing between the heated liquid and the bulk liquid more fully before the warm liquid flows up to the interface for evaporation. Therefore, the degree of thermal stratification reduces and the pressure rise is dragged. The ratio of space to height of ribs, rib material and morphology contribute great to the pressure rise decrease. Meanwhile, the venting process for depressurization is studied. The Hertz-KnudsenSchrage phase change model is employed to simulate the liquid temperature variation. The prediction results agree well with the experimental data. The superheat and fill level play important role in the temperature change.(5) The liquid mass measurement in the cryogenic tank is experimentally studied. A method named Compression Mass Gauge(CMG) is studied thoroughly by tests in different fill levels and compression frequencies. The ground test system is developed with detail introduction of every subsystem. Test results indicate the system performance well with high reliability and the measurement errors are within ±1% of the total tank volume. During the implement of CMG, the harmonic phenomenon is observed, especially for low fill fractions. This phenomenon affects the design of filter when processing experimental data. On basis of the normal tests, the non-tank-contact CMG gauge is proposed. The compression apparatus is installed on a compression vessel which connected to the cryogenic tank. Tests in different fill levels and compression frequencies are carried out and low compression frequency is suggested to be good for less measurement error.(6) Liquid mass gauging under attitude disturbance and heat leakage is investigated. Simulation of the cryogenic tank sloshing with initial temperature stratification is conducted. Pressure is observed to decrease rapidly at the beginning of tank sloshing. Then it increases slowly after it reaches the minimum value. Temperature stratification in the liquid is destroyed during the later sloshing. Therefore, the thermodynamic parameters are not suitable to be used in the CMG measurement. The sloshing ground test system is established and liquid mass measurements under tank sloshing are carried out. The tank sloshing influence on the liquid mass gauge for tank with uniform temperature distribution can be ignored. Heat leakage through the tank wall results in pressure rise and temperature stratification which affect the liquid mass gauging error for cryogens. Heater is employed during ground tests and measurement accuracy is obtained. The influence of heat leakage depends on the change of propellant properties during the test.In conclusion, the presented work takes synthetic methods including theoretical analyses, numerical simulations and experimental measurements to investigate the self-pressurization phenomenon and liquid mass gauge method in the cryogenic tank. The research results would provide reference and guidance for cryogenic propellant allocation in our future space missions theoretically and practically.