CFD investigation on thermodynamic characteristics in liquid hydrogen tank during successive varied-gravity conditions

Abstract

In the present paper, a computational fluid dynamics (CFD) model is developed to simulate the successive varied-gravity operations for a liquid hydrogen (LH2) tank, and pressure evolution, physical field distributions and phase change amounts are obtained and analyzed. The results show that owing to the existence of helium accumulation in ullage, excessive pressure decline could be restrained, and instead a relatively stable pressure is maintained during the most time of the mission. The diffusion process of helium towards liquid surface is probably beneficial to the occurrence of mass transfer from liquid to vapor. However, the H2-He diffusion is nearly terminated within a relatively short time, so that a uniform H2-He mixture could be assumed in the researches on propellant performance in space. Moreover, acceleration level plays the dominant role in liquid-gas distribution. When an acceleration change from 3g0 to 10−3g0 occurs, no apparent interface deformation is observed. The liquid-gas interface maintains approximately horizontal distribution and only a slight liquid bending upwards is observed in near-wall region. When a zero-gravity environment is encountered, however, a complete coverage of ullage by liquid phase could be reached. Specially, it should be noted that a superheated ullage could exist in the tank center region for a long time even after its coverage by liquid phase. To meet secondary ignition requirement, orbit control thrust engines, producing axial acceleration of 10−3g0 and lasting over 100 s, could realize reliable liquid-gas separation. In addition, along with pressure reduction in the beginning stage of launching, a remarkable vapor condensation occurs. Coverage of ullage by liquid phase also yields a significant vapor condensation, while under space thrust effect, liquid evaporation mostly happens. Generally, the present study gives clear exhibitions on the pressure evolution and thermodynamic behaviors of cryogenic propellant tank, which is beneficial for the design and optimization of fuel tank system.