Aeroacoustic modeling of a LOX/CH4 rocket engine for high-frequency combustion instability prediction analyses

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High-frequency combustion instabilities represent the main technical risk faced by design engineers when developing new chemical rocket propulsion systems. Such instabilities are driven by the coupling between the flames’ heat release rate and the combustion chamber’s acoustic field. As such, they can only be assessed during detailed design phases, making design iterations costly. Despite constant efforts being invested in developing numerical tools for their analysis, their reliable prediction remains a major issue. Furthermore, with the advent of the New Space industry, a need to cost-effectively analyze the stability of Hydrocarbon-based systems is stressed.

As a result, this work contributes towards the development of a reliable numerical framework to predict the onset of high-frequency combustion instabilities in liquid rocket engines by ameliorating their acoustic modeling. More specifically, the performance of a novel Linearized Navier-Stokes (LNS) solver in COMSOL is benchmarked against its established Helmholtz solver and validated against the data of two hot-fires of DLR’s LOX/CH4 LUMEN engine. Following a step-wise increase in modeling complexity and by investigating how the background flow description influences the results, sub-5% eigenfrequency errors were obtained. The results also showed that the moving background flow in rocket engines significantly reduces the eigenfrequencies, and that its influence can be accounted for by prescribing varying flow properties to the acoustic solvers. Besides achieving the most accurate eigenfrequency results for a LOX/CH4 system, a more representative velocity disturbance distribution is also attained with the LNS simulation, allowing future studies to better account for the influence of velocity fluctuations on the heat release rate driving the instabilities.