Large eddy simulation of flame response to transverse acoustic excitation in a model reheat combustor

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by autoignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model autoignition and heat release in compressible turbulent flow, a progress variable/ stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analyzed in detail with a filter-based postprocessing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.