Experimental Investigation of High Frequency Flame Response on Injector Coupling in a Perfectly Premixed Multi-Jet Combustor

High frequency injector-coupled thermoacoustic instabilities are a major threat to multi-jet combustors in rocket and gas turbine engines. The complex three-dimensional acoustic coupling between the combustion chamber and injector acoustics cause local fluctuations in heat release. In turn, multiple thermoacoustic feedback mechanisms close the thermoacoustic loop and serve as a source of the thermoacoustic instability. Except for the flame deformation and flame displacement mechanism, the underlying feedback mechanisms for high frequency instabilities are to a large extent unknown. The paper at hand gives new insights into the injector-coupled convective driving mechanisms that are present in multi-jet combustors at perfectly premixed conditions. The forced flame response to the first transverse combustor mode is investigated for two distinct injector tube lengths: one with an axial acoustic velocity node and one with a velocity antinode coupling at the injector–combustor interface. Phase locked OH$*$ images reveal convectively transported coherent vortex structures as the main source of the flame response. The origin of the flame response can be linked to the axial acoustic velocity at the injector–combustor interface using numerical simulations. Both configurations show a clear oscillation of the heat release fluctuations in-phase with the acoustic pressure fluctuations. In similarity to time delay models in low frequency thermoacoustics, a wave number model is proposed to estimate the local flame response due to feed flow modulations and validated with the experimental results.