NUMERICAL SIMULATIONS OF UNSTEADY REACTIVE FLOWS IN A COMBUSTION-CHAMBER

A potentially important source of large-pressure oscillations in combustors is an instability induced by the interactions between large-scale vortex structures, acoustic waves, and chemical energy release. To study these interactions, we have performed time-dependent, compressible numerical simulations of the flow field in an idealized ramjet consisting of an axisymmetric inlet and combustor and a choked nozzle. Both reactive and nonreactive flows have been simulated. The nonreactive flow calculations show complex interactions among the natural instability frequency of the shear layer at the inlet-combustor junction and the acoustics of both the inlet and the combustor. Vortex shedding occurs at the natural instability frequency of the shear layer but vortex mergings are affected by the acoustic frequencies of the system. The entire flow oscillates at a low frequency that corresponds to that of a quarter-wave mode in the inlet. For the particular reactive flow case studied, energy release alters the flow field substantially. In the first cycle after ignition, fluid expansion due to energy release quickly destroys the pattern of vortex mergings observed in the cold flow and a new pattern emerges that is dominated by a large vortex. In subsequent cycles, most of the energy release occurs after vortex mergings have produced this large vortex. Energy release in this large vortex is in phase with the pressure oscillation over a substantial region of the combustor between the axial stations 2.5 to about 5 D (where D is the diameter of the inlet). This results in the observed amplification of the low-frequency oscillations.