Effect of wall heat transfer on the flame response to acoustic perturbation in a turbulent swirling combustor

This article presents a numerical study of the forced flame response to acoustic perturbation in a longitudinal turbulent swirling combustor taken from an annular rig built at Cambridge University. Incompressible large eddy simulations (LES) are performed via the open source CFD-toolbox OpenFOAM, applying the partially-stirred reactor (PaSR) combustion model and a one-step global ethylene (C 2 H 4 )-air reaction scheme. A lean premixed ethylene-air flame stabilised by a bluff-body is studied in the combustor, which has a long longitudinal extension and attaches to the combustor side walls. The unforced flame is firstly simulated using uniform inflow, for which the adiabatic and non-adiabatic wall conditions are respectively applied, showing that the existence of wall heat transfer will increase the flame's longitudinal length due to heat exchange on the side walls and locally quench the flame root in the outer shear layer of the bluff-body due to the heat losses on the flame holder wall. The unforced flame is then submitted to an upstream longitudinal harmonic velocity perturbation at combustor inlet, which has a forcing frequency varying from 500 Hz to 1,000 Hz at a forcing amplitude of 10% of the mean inflow velocity. The response of flame heat release rate is then computed over time for each frequency, eventually leading to the construction of flame transfer function (FTF) that relates the linear flame response to the upstream perturbation. The simulated FTFs are compared between adiabatic and non-adiabatic wall conditions, showing that the wall heat transfer largely decreases the FTF-gain, possibly due to the oscillation of flame surface area being reduced by the heat losses on the side walls. In contrast, the heat losses will increase the absolute FTF-phase, which is because the time delay between upstream perturbation and flame response becomes larger due to the extended flame length. This study highlights the importance of correctly accounting for the wall heat transfer in order to accurately predict the flame response to acoustic perturbations in a turbulent swirling combustor.