Instability mechanisms in a premixed prevaporized combustor

Instabilities in a premixed prevaporized 150-kW model scale combustor are investigated experimentally. The injector fed with liquid heptane and preheated air features two sets of swirling blades that induce flow rotation in the same direction (corotative) or in the opposite direction (counter-rotative). The flame is stabilized with swirl behind a dump. Instabilities occur in the low-frequency range around 400 Hz corresponding to a quarter-wave mode acoustic coupling of the system. Simultaneous measurements of pressure and heat-release oscillations and phase-locked CH chemiluminescence images are used to characterize the combustion dynamics. In both corotative (COS) and counter-rotative (CNS) cases, the reaction region moves closer to the injector when the flame becomes unstable by about one-third of the stabilization distance under normal operation. Experiments indicate that the two swirl configurations have distinct domains of instability. The instability boundary separating stable and unstable regions can be defined in terms of a critical velocity v,, which depends on the equivalence ratio 15, air injection temperature T-inj, and swirl geometry. In the coswirl configuration, instabilities occur when the injection velocity is lower than the critical velocity [u < v(c) (Phi, T-inj; COS)], while they appear for velocities higher than the critical velocity in the counterswirl geometry [u > v(c) (Phi, T-inj; CNS)]. In a range of conditions corresponding to low injection velocities, reduced eqivalence ratio, and for the coswirl configuration, an unsteady flashback takes place in which the flame moves periodically in and out of the fuel premixer. This mechanism is related to the existence of a low-velocity region near the injector exit plane. Observations of the space-time development of the heat release under unstable operation indicate that the oscillations are significantly influenced by the swirl geometry and are caused by different mechanisms. The coswirl configuration features a central recirculation, which gives rise to periodic vortex roll-up, convection, and sudden release of heat when the vortices impinge on the lateral walls. In the counterswirl geometry there are no identifiable How structure, but the heat-release pattern is convected periodically in the chamber. Estimates of the delay times associated with the two mechanisms support the view that coswirl instabilities are driven by vortex roll-up, whereas counterswirl instabilities are probably sustained by equivalence-ratio inhomogeneities.