Two-phase flow characteristics of cryogenic propellant in filling the head cavity of liquid rocket engine

The filling of the head cavity downstream of the valve with liquid oxygen in a cryogenic rocket engine is a crucial stage in ignition. The work presented here intends to study the two-phase flow characteristics in the head cavity through visualization experiments and simulations, which are carried out using liquid nitrogen as the substitution fluid. The geometry used comes from the scaling of the actual head cavity in the engine. The temperature of liquid nitrogen (T_LN2,s) supplied for the head cavity is higher than the saturation temperature in the head cavity (T_sat,c) with room pressure as the initial state, which is proved in the pro-cooling pipeline upstream of the valve before the filling process. Flash boiling is inevitable due to the sudden drop in pressure after the valve is opened. Then, the pressure in the head cavity rises in two steps. The production of gaseous nitrogen leads to the first step, and the mist flow is visualized because liquid nitrogen is broken up and carried by gaseous nitrogen in the form of droplets. The increase in the content of liquid nitrogen in the head cavity causes the second step, and the bubbly flow occurs as the gaseous nitrogen is dispersed as small bubbles in the continuous liquid nitrogen. The transition from mist flow to bubbly flow is led by the decrease in the superheat of liquid nitrogen (ΔT = T_LN2,s- T_sat,c). The transition occurs at ΔT = 4 K ∼ 5 K and the minimum achievable superheat is 1.3 K when the mass flow rate of liquid nitrogen supplied is 5.4 kg/s ∼ 6.9 kg/s. Furthermore, the gas nitrogen content in the head cavity is mainly determined in the first step of the pressure rise. Reducing the temperature of liquid nitrogen supplied can weaken the flash boiling, and increase the content of liquid nitrogen in the head cavity.