Simulation and analysis of low-frequency instability in hot-fire test of a cryogenic hydrogen-oxygen preburner

Low-frequency chamber pressure oscillation is observed in the hot-fire test of a subscale cryogenic hydrogen-oxygen preburner. A lumped parameter simulation model with bipropellant injector is established by solving differential equations directly. Under the condition of constant inlet pressure, the frequency of oscillation of combustion chamber pressure increases with the decrease of combustion delay, while the variation trend of amplitude of oscillation of chamber pressure is opposite. As the combustion delay decreases to 1.76 ms, the pressure oscillation gradually converges and stabilizes, but its initial oscillation frequency is still about 2.7 Hz lower than that of the test. In addition, the influence of the volume ratio of liquid propellant in the combustion chamber is analyzed. The combustion time delay is set to 1.76 ms, and the frequency and amplitude of the pressure oscillation increase with the increase of the liquid volume ratio. With the liquid volume ratio set at 2.8%, the frequency and amplitude obtained by simulation are in good agreement with the test. In order to reproduce the continuous oscillation of the combustion chamber pressure during the 30 s process in the test, the pre-injection pressure test curve varying with time is used as the inlet condition. The liquid volume ratio and combustion delay are empirically corrected in real time using propellant flow rate and mixture ratio, respectively. The continuous oscillation of the simulated pressure during the 30 s test was obtained, and its frequency value and trend are completely consistent with the test data. It can be inferred that low-frequency unstable combustion related to combustion delay may have occurred in the hot-fire test of the preburner.