Simulation Study of Fluid Sloshing and Pressure Transient Variation Inside Liquid Oxygen Tank under Variable Accelerations

After the space shutdown of the launch vehicle engine, the axial acceleration force on the liquid oxygen tank rapidly decreases, and the probable acceleration force variation along the radial direction could lead to significant propellant sloshing and pressure decrease inside the tank. A three-dimensional simulation model for cryogenic propellant sloshing under the acceleration force variations in three directions was established in this paper, and the heat and mass transfers between the tank ullage, liquid phase and tank wall were taken into account. A comparative analysis was conducted to assess the effects of the anti-sloshing structure, the initial temperature distribution, and the gas composition on the initial ullage on fluid movement and pressure decrease within the tank. It was found that when the axial acceleration force was reduced remarkably, the radial acceleration force could yield an apparent liquid oxygen sloshing inside the tank. Compared to the tanks without anti-sloshing structure, the combination of horizontal and cross-sloshing plates can significantly reduce the propellant sloshing amplitude. Specifically, the pressure drop of the cross-slosing plates tank and the full structure tank was respectively 0.141 MPa and 0.137 MPa, which was reduced by 23.8% and 25.9%. In addition, the tank pressure decrease during the sloshing process was affected by the decrease of the ullage average temperature and the gas condensation effect in the ullage region. Furthermore, when helium component existed in the ullage, the gas condensation effect was greatly reduced, resulting in a slower pressure reduction during sloshing process. Under the conditions where the gas ullage contained pure helium or 75% helium mixture, the pressure amplitude in the tank was approximately 0.047 MPa, which was lower than that under pure oxygen conditions. Generally, the introduced CFD model could be used for the designs of cryogenic propellant management techniques in space and for the reusable rocket.