Testing and comparison of a thermodynamic vent system operating in different modes in a liquid nitrogen tank

Abstract Thermodynamic vent system (TVS) has been considered as a promising method for pressure control in cryogenic propellant tanks in microgravity environment. TVS could be operated in different modes, namely, the mixing-venting (MV) mode, the mixing-only (MO) mode, the mixing-only followed by mixing-venting (MOMV) mode, and the passive venting (PV) mode. An experimental investigation on the effect of operation mode on the TVS performance was conducted by using liquid nitrogen as the liquid oxygen simulant on a testing facility named Multi-functional Cryogenic Test System. Results of those four operating modes tests were compared regarding the ullage cooling effect, liquid temperature, pressure control capability, and mass loss. It was found that with the help of spraying, the MV mode and the MO mode cooled the ullage more effectively than the PV mode did. However, the PV mode is more effective in suppressing the increase of the liquid temperature. A non-dimensional parameter η has been proposed for comparison between the ullage heating and the liquid evaporation contribution to the pressurization in cryogenic fluid storage tanks. For the mixing-venting mode, the ullage heating is the main force that pressurizes the tank when the liquid is subcooled, while when the liquid is saturated, the evaporation takes over the role. Moreover, the MOMV mode performs the best in reducing mass loss within 0–1500 min. The results will be useful to make a control strategy for the operation of pressure control in on-orbit cryogenic tanks through thermodynamic venting method.