Pressurization Behavior of a Cryogenic Propellant Tank in Microgravity

Kurzfassung

This thesis presents a numerical investigation of the pressurization behavior of cryogenic propellant tanks in microgravity. This is done in the context of the FROST (Future-oriented Research platform for Orbital cryogenic Storage Technologies) project at the German Aerospace Center, which aims to study the long-term storage of cryogenic propellants in space. The goal is to provide experimental data on the storage and handling of cryogenic fluids in microgravity. In this work, the open-source package OpenFOAM was used to conduct computational fluid dynamic (CFD) simulations on the active pressurization process of nitrogen, which includes the modeling of the geometry and material properties, the execution of a mesh convergence study and the post-processing of the generated data. In the parameter study conducted, the effects of the diffuser design, the pressurant gas temperature, the Bond number, the pressurant gas and the fill level were analyzed. In addition, the influence of the free surface reconstruction scheme on the results was studied. A method based on an enthalpy balance around the vapor phase was devised, which enabled the evaluation of the effects different mechanisms have on the pressurization efficiency. During the mesh convergence study, the modeling of the tank wall in aluminum introduced large discretization errors, which could only be reduced by unreasonable refinement levels. This was due to the number of interface cells contacting the tank wall and the large thermal conductivity and thermal mass of aluminum. By modeling the tank wall in polycarbonate, which better matched the FROST geometry, the discretization error could be reduced. The pressurization using the conical diffuser design resulted in a ~ 21% increase in final pressure over the radial diffuser, due to a reduction of heat lost to the walls and better distribution of pressurant gas in the ullage. With increasing gravity, the influence of the diffuser design decreased. The data evaluation revealed the heat flow at the tank wall to be the largest loss during the pressurization process, followed by the enthalpy flow due to condensation. The variation of the pressurant gas temperature showed only a small impact on the phase change behavior. For larger Bond numbers, the pressurization efficiency reduced, due to the increased influence of buoyancy forces, leading to increased losses at the tank walls. The usage of helium as pressurant resulted in an increased pressurization rate. Due to the limitations of the saturation conditions modeling, however, the phase change could not be evaluated. Increasing the fill height, thereby reducing the ullage volume, revealed the influence of evaporation due to the hot pressurant gas impacting the liquid surface.