Low-frequency pressure oscillations in a model ramjet combustor

Abstract

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.

Shear flow-driven combustion instability: Evidence, simulation, and modeling

“…In [6], some peaks whose frequency scaled linearly with the flow velocity, almost independent of the acoustic conditions, were measured; these were typically low frequency modes. In [7], a double expansion experiment was used, and peaks with a characteristic time = acoustic time + vortex "lifetime", were identified. Low frequency modes scaled with the flow velocity to St = 0.08 -0.12.…”

Mixed acoustic–entropy combustion instabilities in gas turbines

“…They found that the time lag characteristic of temperature spots detached from the flame and convected to the nozzle could add a new family of eigenmodes affecting the overall stability of the system. However, as recalled by Yu et al (1991), there was a lack of experimental evidence on the contribution of a mixed, convective mode to the flame instability. Despite the fact that this latter work focused on the interaction between vortex and nozzle, the results can be extrapolated to the interaction between entropy and acoustic modes and constitute a first step to outline that the instability mechanism relies on the combination of acoustic and convective time lags.…”

Impact of the Flame-Holder Heat-Transfer Characteristics on the Onset of Combustion Instability

“…When the unsteady heat release rate fluctuations associated with large-scale vortical structures couple positively with the acoustic field, self-sustained acoustic oscillations are observed (Poinsot et al, 1987;Ghoniem et al, 2005). The mechanism of flame-vortex interactions has been examined in a number of studies (Poinsot et al, 1987;Ghoniem et al, 2005;Yu et al, 1991;Matveev and Culick, 2003;Altay et al, 2009), where the authors emphasize the role of vortex kinematics (formation, separation and convection) and its interactions with acoustics and the flame in driving the observed instabilities. This paper presents an experimental investigation into combustion instabilities observed in a laboratory scale backward-facing step combustor, where the dynamics are driven by flame-vortex interactions.…”