Numerical Study of Effect of Pressurant Gas Species on Thermal Behavior in Cryogenic Tank

Abstract

This study numerically reveals effect of pressurant gas species on thermal behavior in a cryogenic propellant tank for space propulsion systems. The simulation was conducted for a ground experiment using liquid nitrogen in which gaseous helium and the same species as the liquid, gaseous nitrogen, were used as prepressurants. The tank was sealed after prepressurization to observe self-pressurization (pressure rise with evaporation) and thermal stratification (high-temperature liquid near the surface). A higher evaporation rate and a thicker thermal layer were observed for helium prepressurization in the experiment. To clarify the underlying physics, a simulation considering phase change and conjugate heat transfer was developed. The simulated pressure and temperature closely matched the experimental results. Conjugate heat transfer played an important role; that is, the heat flow through the ullage part of the tank skin into the cryogenic liquid was a dominant heat transfer process, causing evaporation to occur mainly at the contact point of the wall and the gas–liquid interface. Afterward, nitrogen vapor rose along the tank wall due to buoyancy under nitrogen prepressurization. However, buoyancy in the ullage is lower under helium prepressurization since light helium stayed in the upper part of the ullage, and a radial vapor flow is produced from the contact point, leading to a higher heat flux from the tank wall to the liquid nitrogen. This mechanism shows that evaporation rate and heat flow into the liquid are higher for helium prepressurization than for nitrogen prepressurization.