Simulation of Multi-Injector H2-O2 Rocket Combustion Instability

An efficient large-eddy simulation code has been developed to simulate combustion instabilities in a hydrogen–oxygen ([Formula: see text]) rocket engine combustion chamber with multiple injectors. The code uses multiblock structured hexahedral grids with a patched-grid interface between the combustion chamber and the injection ports, which allowed improved accuracy with a relatively small number of total grid points compared to the use of unstructured tetrahedral or prism grids. Both the flow in the fuel/oxygen injection system and the combustion chamber are simulated and directly coupled through the patched interface method. A half model of an experimental rocket combustor chamber with 42 hydrogen–oxygen injection ports is sufficient. A one-step finite-rate chemistry is employed to capture the dynamics of chemical reaction. Modal analysis methods based on fast Fourier transform and dynamic mode decomposition are used to analyze the computed unsteady pressure field. Primary acoustic waves, including three longitudinal modes, two standing tangential modes, and one radial mode in the combustion chamber, are identified without any triggering. Also, an injection-coupled chugging frequency is clearly identified. The first tangential mode is identified as the dominant self-excited instability mode, in agreement with the experimental data for the combustor. The spatial distribution of the mode shapes in the combustion chamber is explained, accounting for sound speed variations in the axial direction. Analysis of the interaction between the longitudinal wave system within the injection ports and those in the combustion chamber reveals an important feature of the combustion instability. When a natural frequency of the combustion chamber occurs as a harmonic of that of the injection ports, waves from the chamber travel into the injection ports, inducing resonant oscillations within the ports, which in turn causes oscillations of the heat release rate in the reaction zone downstream of the injection plate. Since this fluctuation of heat release is in sync with the chamber pressure oscillations, it can heighten the Rayleigh index, strengthening the combustion instability. This coupling mechanism has not been previously analyzed mathematically, but it is found to agree with existing experimental and computational data and may provide guidance in practical design to avoid rocket engine combustion instability.