The effect of stretch and heat loss on the anchoring of a bluff body stabilized lean premixed flame during acoustic forcing

This paper presents analysis of a laminar lean premixed flame stabilized on a heat conducting bluff body. Harmonic velocity fluctuations are used to simulate the flame response to acoustic oscillations, which are of primary importance in the study of thermoacoustic instabilities. We use a fully resolved unsteady two-dimensional code with detailed chemistry and species transport, with no artificial flame anchoring boundary conditions. Calculations were conducted with different bluff body materials and different forcing frequencies and amplitudes. Results reveal significant displacement of the flame leading edge during the forcing, as well as large fluctuations in the flame stretch. We have found that a flame that stabilizes on a holder with lower thermal conductivity can sustain higher stretch due to the lower heat losses, and thus propagate further upstream and undergo relatively smaller displacement during the forcing. Additionally, we show that a flame which anchors further upstream leads to higher vorticity damping at the bluff body downstream side, which leads to shedding of weaker vortices and thus weaker heat release fluctuations associated with flame-vortex interaction. This results in a strong dependence of the overall flame heat release fluctuations during harmonic forcing on the bluff body thermal properties. The physical mechanism revealed in this study can explain recent experimental measurements in a backward facing step combustor, which showed that a ceramic step increased the stable operating window in comparison to steel because of experimentally observed weaker flame vortex interaction.