Modelling thermoacoustic stability of cryogenic rocket engines with novel acoustic network model elements

Reliable and computationally efficient prediction capabilities for combustion instabilities in liquid propellant rocket combustion chambers are still rare. This study uses a low-order tool based on the well-known acoustic network model principle to map the stability limits of two different research rocket combustors with different cryogenic propellant combinations. New elements were derived capable of describing any isentropic background flow field in a contoured chamber, resolving acoustic chamber modes in three dimensions, and supporting distributed flame response models. Furthermore, an improved boundary condition element for the sonic throat of a rocket nozzle was implemented. The new network elements were benchmarked against two different research rocket combustion chambers, one single injector experiment using liquid oxygen and natural gas, exhibiting longitudinal mode instabilities and one multi-element thrust chamber with transverse mode instabilities. The acoustic resonant frequencies are predicted with an average absolute error of less than 5% for both cases. The tool is capable of predicting stability based on classical time lag and gain parameters applied to the new distributed flame model. The resulting stability maps are consistent with the benchmark cases for flame response time lags which are close to those reported in literature from computational fluid dynamics simulations and experiments.