Low-frequency pressure oscillations in a model ramjet combustor

Low-frequency combustion instabilities are studied in a model ramjet combustor facility. The facility is two-dimensional, and is comprised of a long inlet duct, a dump combustor cavity with variable size capability, and an exhaust nozzle. The flame is observed to be unstable over a wide range of operating conditions. Acoustic pressure and velocity measurements are made at various locations in the system. They show that the inlet duct acts as a long-wavelength acoustic resonator. However, the instability frequency does not lock to any particular value. This result suggests that the instability mechanism is not purely acoustic in nature. Schlieren imaging reveals that the instability is associated with large-scale flame-front motions which are driven by periodic vortex shedding at the instability frequency. Vortices are generated at the dump in phase with the acoustic velocity fluctuations in the inlet duct. The unsteady heat addition process closely follows the vortex history: the vortices form, grow in size, convect through the combustor cavity, impinge on the exhaust nozzle, break down to small scales and burn. C2 and CH radical spectroscopy is used to determine the phase relation between heat release and pressure in the reaction zone. Rayleigh's criterion is thereby shown to be satisfied. Next, the crucial question of how the oscillation frequency is determined is addressed. Inlet velocity and combustor length are systematically varied to assess the role of vortices by modification of their characteristic lifetime. The influence of the acoustic feedback time is also studied by shortening the inlet duct. The results show that the instability frequency is controlled by both vortex kinetics in the combustor and acoustic response of the inlet section. Therefore, the instability may be considered as a mixed acoustic-convective mode. Finally, combining Rayleigh's criterion with a global feedback loop equation, it is found that the resonant frequencies are selected according to the restriction \[ \frac{1}{4N-1} < \frac{\tau_{\rm v}}{\tau_{\rm f}} < \frac{3}{4N-3}, \] where N is the mode of oscillation and τv is the time for vortices to be convected from inlet to exhaust with τf being the feedback time taken for a pressure disturbance to travel up the inlet system and back.