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The submerged helium injection process results in the heat and mass transfer between the helium bubble and the cryogenic liquid. The objective of this paper is to analyze the dynamics of the heat and mass transfer process. It is observed that during the helium injection process the dynamics of mass transfer is dominant and the transient heat transfer is negligible. The helium bubble shape and rising patterns are observed with a visualization device that helps to discern the dominant process between heat transfer and mass transfer. The clustering patterns such as coalescence of helium bubbles are observed with the visualization device. The visualization results indicate that, it is very difficult to determine the representative size of bubbles due to the irregular shape of the helium bubbles. The shape and size of the helium bubbles are important parameters for evaluating the overall mass transfer coefficient ( k G A ) which is the essential parameter for calculating the evaporation rate of the bulk liquid into the helium bubbles. In this paper, the simplified lumped model is considered to fairly approximate the evaporation rate of the cryogenic liquid into the bubbles and the cooling rate of helium injection. The empirical correlation for the average concentration ( C ‾ A ) of evaporated cryogenic liquid into the helium bubbles is presented and the overall mass transfer coefficients ( k G A ) are calculated as the result of the lumped model. The proposed model and empirical correlations are compared with the experimental results, and the comparison result shows good agreement with differences that are less than ±0.4 K. [ABSTRACT FROM AUTHOR]

• Cryogenic propellant is pressurized with hot helium in typical launch vehicle. • Depressurization of ullage was observed with cold helium input to the LN 2 tank. • Cryogenic liquid droplet propagation observed during the pressurization. • The ullage temperature and pressure decreased due to the ullage cooling effect. • The ullage depressurization is possible during the helium gas with liquid droplets. In liquid propellant propulsion systems, helium pressurization is required to transfer liquid propellant from the tank to the engine. When a cryogenic liquid propellant is used, the tank becomes pressurized by relatively high temperature helium. Typically, the ullage pressure increases with the helium inflow. However, in one particular case, the ullage pressure reduced while cold helium was supplied through direct gas injection. High-velocity gaseous helium strikes the cryogenic liquid surface that generates the liquid splash in the ullage. The cryogenic liquid splash in the ullage then transfers heat between the gas and the liquid. Therefore, the ullage pressure decreases because of the decrease in the ullage temperature. The pressure behavior of the cryogenic tank during the gas inflow is experimentally examined, and the depressurization during the pressurization process is analyzed. The results indicate that the gas–liquid interface disturbance decreases the ullage pressure. The observed gas–liquid interface disturbance effect can be employed in pressure decrease applications in cryogenic liquid storage tanks. [ABSTRACT FROM AUTHOR]