Numerical investigation on full thermodynamic venting process of liquid hydrogen in an on-orbit storage tank

Abstract

Liquid hydrogen has been identified as a promising propellant for long-duration space missions, benefiting from its high specific impulse when paired with liquid oxygen. A full procedural computational fluid dynamic model was proposed for the jetting, mixing, throttling, and venting processes of thermodynamic venting systems (TVS) for the purpose of minimizing liquid hydrogen boil-off on orbit. In addition to the mixing and jetting components as well as the fluid domain inside the storage tank, the external fluid circulation loop and the throttling device were also coupled into the solver in the form of user-defined code, which enabled simulation of the continuous complete thermodynamic cycles. The evaporation/condensation intensity factors for the liquid-vapor phase change were confined in a smaller range than those used in other studies through theoretical analysis. To enhance the numerical stability, the well-known Lee model for liquid-vapor phase change was modified by introducing a smoothing function. Simulations with a variety of initial temperatures, mass flow-rates and vent ratios were conducted, which revealed that the condensation of superheated vapor contributes to the depressurization processes noticeably greater than the disturbance of the temperature stratification. Depressurization efficiency was introduced for evaluating the TVS performance during the jetting process. Significant performance enhancement was observed when the temperature difference between the liquid and the ullage before jetting was enlarged. The self-pressurization period was found lasting longer time under on-orbit condition than that on the ground, which is attributable to the floating of the bulk liquid and higher uniformity of the temperature distribution after the jetting process in the absence of gravity.