Large eddy simulation-based study of the influence of thermal boundary condition and combustor confinement on premix flame transfer functions

The influence of the thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large eddy simulations are carried out using the dynamically thickened flame combustion model. Then system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with a 90 mm × 90 mm combustor cross section and nonadiabatic walls: (1) a combustor cross section similar to the reference case with adiabatic combustor walls, and (2) a different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in the flame shape and flow field. In particular, the FTF computed with an adiabatic wall boundary condition which produces a flame with a significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in the flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in the FTF on stability limits is analyzed with a low-order thermoacoustic model.