Combustion Dynamics of a Liquid Rocket Engine in subcritical Injection Conditions

In throttlable liquid rocket engines using methane and oxygen as propellants, the combustion chamber pressure is expected to vary from subcritical to supercritical with respect to the thermophysical state of the propellants during operation of the engine. The aim of the present work is to investigate the influence that thermodynamic injection conditions could have on combustion stability. A numerical framework based on a multi-fluid approach suitable to simulate both sub- and supercritical pressure flows is used. The research combustor BKN from DLR Institute of Space Propulsion is selected as the experimental test case of the current study. In particular, one unstable supercritical load point and two subcritical load points - one stable and one unstable - are selected. The two unstable cases are obtained through instability triggering. The simulated stable case shows good agreement with the experimental flame structure and the chamber pressure. The unstable cases reproduce the non-linear nature of the experimentally observed instabilities, however they slightly overestimate chamber pressure and oscillation amplitudes. Good agreement is found with the first longitudinal modes frequencies in all cases. The present work demonstrates the capability of investigating combustion instabilities in liquid rocket engines both for two-phase flow and for supercritical injection conditions, opening the path to a series of investigations on the associated flow physics.