LOW-FREQUENCY COMBUSTION OSCILLATIONS IN A MODEL AFTERBURNER

Low-frequency combustion oscillations, involving the interaction between longitudinal acoustic waves and unsteady combustion, are investigated for a model afterburner. An experimental rig. in which a confined flame is stabilized in the wake of a conical gutter, is run with inlet conditions representative of an engine afterburner. Results are presented for inlet Mach numbers in the range of 0.15-0.27, with inlet temperatures up to 630 K. Comparison is made between theory and experiment. Although the theory was developed from low Mach number data, it is found to apply equally well at these faster flow rates. The theory is able to predict the frequency of the instability and the mode shape, accurately reproducing the changes due to variations in the inlet Mach number and temperature. The effect of altering the downstream boundary condition by replacing the open end by a choked nozzle is also investigated. Such a change is found to be highly destabilizing, both experimentally and theoretically. Again, predictions from the theory are in good agreement with the observations.