Effects of Damköhler number on vortex–flame interaction

Oscillating heat release associated with large-scale vortex structures in a dump combustor was investigated experimentally. The objectives were to understand vortex–heat release interactions that may lead to combustion instability and to characterize spatial patterns in fluctuating heat release for future control purposes. The inlet jet, which consisted of a turbulent premixed ethylene–air flow, was acoustically forced to control shedding of periodic vortices. The reacting flowfield was characterized using a phase-locked schlieren technique, while the corresponding heat release pattern was investigated using CH* chemiluminescence. Phase-resolved data taken at various flow conditions were compared to identify important physical parameters. In the context of vortex-driven combustion instability, the results showed that the Damköhler number was a critical parameter for vortex–heat release interaction. When the Damköhler number was relatively high, the leading edge of the vortices coincided with the high point in the local heat release cycle and the trailing edge with the low point. However, a clear shift in the heat release pattern was observed at Damköhler numbers lower than four. Under these conditions, the high point in local heat release was observed either in the vortex core or at the trailing edge of the vortex. Accordingly, the effect of the Damköhler number must be taken into consideration when designing an active control strategy based on secondary fuel injection.