Experimental and numerical study of nonlinear growth process of self-excited combustion instability in a model rocket combustor

An experimental and numerical investigation of self-excited transverse combustion instabilities in a rectangular multi-injector, gaseous methane-oxygen rocket model combustor is presented. Numerical simulations based on flamelet-generated manifold and stress-blended eddy simulation are performed to reveal the dynamic features of the instabilities. Good agreement is achieved between numerical results and experimental data in the aspect of mean chamber pressure, characteristic frequency, and pressure waveforms. The nonlinear growth process (stable-rapid development-limit cycle) is clearly captured during the experiment, which is successfully predicted by numerical simulation. Nonlinear characteristics are analyzed via the phase space reconstruction method, and instantaneous flow fields are estimated to aid the interpretation of the combustion instability mechanisms. Results suggest that the linear characteristics dominate the initial growth stage of pressure oscillations. The nonlinear nature is significantly strengthened during the rapid exponential development process, indicated by the generation of harmonics. Both numerical and experimental phase space attractors vary from clutter to “Reuleaux triangle shaped” and ultimately to stretched trefoil-knot like structure, corresponding to the nonlinear growth process of combustion instabilities. The trefoil-knot like attractor is caused by the steep pressure wavefront (shock-like). Flow field analysis demonstrates that the gaseous propellants are highly disturbed by the transversely moving pressure wave. Large coherent structures of flame interactions and propellant gas stream collisions between adjacent injectors are generated, resulting in high deflection of propellants and heat release regions to both sides of the chamber. Coupling behavior of pressure oscillations between the main chamber and the oxidizer post (oxidizer manifold) is captured. A combustion instability mechanism associated with flame deflection and pulsated mass flow rate supply is proposed