Dynamics of Condensation and Evaporation of Liquid Nitrogen in a Closed Vessel Pressurized with Helium, Nitrogen, and Their Mixture

Abstract

Pressurization systems for propellant tanks of launch vehicles (LVs) with a liquid rocket engine are complex LV systems to provide cavitation-free operation of pumps and constant pressure in the gas cushion of the fuel tanks at constant consumption of liquid components of rocket propellant. There were already studies on the influence of the pressurant type on the heat and mass transfer in the LV propellant tank. The simulation was performed for a ground experiment with liquid nitrogen with helium gas and nitrogen gas used as the pressurant. In this work, experimental studies were performed on the dynamics of evaporation and condensation, changes in the pressure in the gas cushion, as well as changes in the mass of liquid nitrogen with the vessel pressurized with helium, nitrogen, and their mixtures with molar content of nitrogen of 23.2 mol. % and 52.2 mol. % and filled with liquid nitrogen to 30–70%. The experiments were conducted in a cylindrical vessel with height of 650 mm and internal diameter of 213 mm, pressurized to pressure of 0.3 MPa. The experiments resulted in dependences of variation of the pressure in the vessel and the mass of liquid nitrogen in the vessel at the stage of pressurization with a nitrogen-helium mixture of various concentrations at different levels of initial filling of the vessel with liquid nitrogen. The dependences of temperature changes in the liquid and vapor phases at eight different levels in the vessel height (50–600 mm) were obtained. During condensation, at the pressurization stage, and immediately after the end of pressurization, the surface layer of the liquid (about 10 mm) was observed to warm to a temperature close to the saturation point corresponding to the pressure in the vessel. The formation of this layer leads to cessation of condensation, cessation of pressure drop in the vessel, and beginning of evaporation due to external heat inflows. When the vessel is half filled with the liquid, loss of stability of the heated near-surface layer of liquid is observed because of the formation of large-scale convective flows. The destruction of the heated surface layer and its cooling to the temperature of the core of liquid, which is significantly subcooled, results in abrupt intensification of the condensation of the nitrogen vapor and significant decrease in the pressure in the vessel to a pressure equilibrium with the temperature of the liquid core.