Parametric Visualization Study of Self-Pressurizing Propellant Tank Dynamics

Nitrous oxide is frequently used as a self-pressurizing propellant in hybrid rocket propulsion systems. To study the behavior of these self-pressurized propellant tanks, two different pressure vessels with optical access have been developed that can be filled with nitrous oxide and drained in a way that replicates the conditions of a hybrid rocket motor firing. Carbon dioxide was used as a simulant fluid for nitrous oxide to mitigate explosion and environmental hazards. High speed cameras are used to visualize the internal flow and identify boiling, condensation, and liquid level motion while pressure and temperature sensors are used to determine the thermodynamic state within the tank. Tests have identified two separate temporal regimes that are common to all tests, a transient and steady state, each described by distinct features. The transient regime is characterized by a rapid pressure drop and recovery, with homogeneous condensation of the ullage and heterogeneous nucleation and growth of bubbles in the liquid. The steady state regime begins when a large population of bubbles has been formed in the liquid and a balance is established between nucleation of new bubbles and death of bubbles as they reach the free surface and transfer their mass to the ullage. During this steady state regime the liquid and vapor are both homogeneous two-phase mixtures and the pressure drops in a linear fashion as the liquid drains from the tank. Also presented in this paper are the results of tests of parameter variations, including flow rate, fill level, temperature, initial bubble population, and scaling effects. The same basic features are present in each of these tests.