Thermoacoustic finite element method modeling of partly autoignition and propagation-stabilized flames

This paper describes a methodology to thermoacoustically account for different flame types in a single Finite-Element-computation. To do so, the flame segmentation mechanism presented in previous studies is used to characterize the different flame zones. Differentiation is done between propagation-stabilized shear-layer flames and autoignition flames that respond to acoustic perturbations very differently. While autoignition flames mainly respond to acoustic pressure and temperature fluctuations, propagation-stabilized flames respond to acoustic velocity perturbations. Thus, flame transfer functions specific to each flame type are analytically implemented within the frequency domain Finite-Element-computation. Using the novel framework, it is shown that the global flame transfer function obtained from Computational Fluid Dynamics (CFD) simulations of a backward facing step reheat combustor can be reproduced accurately in the low-frequency regime for an operating point where the flame is forced in a planar manner. The investigated operating point operates on hydrogen fully premixed at lean and autoignitive conditions. The autoignition framework is validated by comparison to one-dimensional direct numerical simulation. The time-averaged CFD heat release rate result is validated with large eddy simulation data.