Nitrous Oxide Gas Blowdown Modeling

Nitrous oxide has been used in the space industry as an oxidizer and monopropellant mainly for its self pressurizing properties. This self pressurization, coupled with the multiphase nature of the problem, makes modeling the tank emptying process challenging. This study presents a model for simulating nitrous oxide gas blowdown, the gas phase inside the tank is at or near saturation, which results in a compressibility factor that deviates significantly from that of an ideal gas, therefore, conventional compressible flow equations do not accurately capture the gas behavior. In addition, during discharge the gas flows through the piping and transitions to a multiphase mixture. The proposed model combines detailed thermodynamic properties with a homogeneous equilibrium model to simulate the real gas and multiphase flow. The model uses conservation of mass and conservation of energy as governing equations, that accounts for the coupled mass and energy transfer between the liquid and gas phases, as well as heat transfer between the fluids and the inner tank walls, the outer tank walls and the ambient air. Comparison with experimental blowdown data from a < 5 N class thruster oxidizer tank shows good agreement in predicting the evolution of tank pressure and total mass. The model also accurately reproduces the system behavior during the transition to single phase gas flow following complete liquid depletion. These results demonstrate the potential of the model for use in system design, predictive analysis, and the interpretation of mass flow meter measurements for nitrous oxide.