Simulating Acoustic-Flame Interactions in Rocket Engines

Combustion instabilities are strong acoustic disturbances that often form spontaneously inside rocket combustion chambers and may lead to catastrophic failure of the launch vehicle. Although all physical processes that may contribute to the onset of such instabilities are well understood, the exact mechanisms of how these processes interact are still largely unknown. It is, however, widely accepted that the interaction between acoustic waves and the flame inside the rocket engine is very important to the formation of combustion instabilities. Even though it is very difficult to investigate the flow field in rocket combustion chambers experimentally due to the harsh flow environment, DLR researchers successfully designed a subscale combustion chamber for specifically studying the interaction between acoustics and the flame. Combustion chamber H consists of five primary shear coaxial injectors that are placed in the middle of the combustion chamber through which oxygen and hydrogen are fed into the reaction zone. Besides its large main nozzle in the axial direction, BKH contains a secondary nozzle perpendicular to the main flow direction which can be opened and closed periodically by a siren exciter wheel. By controlling the rotation frequency of the exciter wheel, different acoustic oscillations, so­called eigenmodes, can be excited inside the combustion chamber. BKH is also equipped with two windows on its side walls allowing to observe the flame dynamics using high­speed cameras and optical measurement techniques. Even though BKH experiments provided a lot of insight into the nature of acoustic­flame interactions, many other aspects can only be observed by detailed numerical simulations. The purpose of this SuperMUC project is therefore to numerically reproduce BKH experiments for an operating condition that is vulnerable to combustion instabilities in real flight engines. Recent experiments on a different subscale rocket engine [2] showed that combustion instabilities occur when an eigenmode of the oxygen injector is in resonance with the main oscillation mode of the combustion chamber, i.e. when both modes have the same frequency. In this project we investigate for the first time such a mode coupling scenario by carefully controlling the eigenfrequency of the oxygen injector and tuning it to the combustion chamber eigenfrequency.