On the mechanics of oscillating flames

Abstract

The major drive to advance the understanding of flames comes from industry as many technical devices propelled by flames are in constant need of design revisions and efficiency improvements. Lean-premixed combustion technology was one of such areas, which saw major advancements. Unfortunately, combustion devices operating in premixed mode often exhibit undesired flame oscillations, which may lead to damage or even failure of the combustor. The present work is dedicated to better understanding and predicting flame oscillation. A modelling framework based on high-order accurate Discontinuous Galerkin methods is presented to investigate the interaction of steadily propagating flames with pressure waves. The analysis of the results show that the time scale associated with the pressure wave incident upon the flame must be of the same order as the time scales in the primary reaction zone to lead to partial wave reflection and wave transmission at the flame. Hence providing a mechanism for initiating flame oscillation. The predictive capabilities of the Large Eddy Simulation technique in conjunction with the Probability Density Function model of the unresolved turbulence-chemistry interaction are demonstrated in the simulation of flame oscillation in industrially relevant combustors. Two test cases with increasing complexity are considered: a forced oscillating flame in a bluff-body stabilised combustor, and a self-excited flame oscillation in a swirl combustor with complex geometry. Good agreement between the measurements and the simulations is obtained. The flame dynamics are well captured by both simulations. The results obtained in the study of the bluff-body flame are used to identify suitable chemical markers which correlate well with the total heat release rate. The product of molecular oxygen and the ketenyl radical is found to correlate better with the total heat release rate than the commonly used formaldehyde-based markers in the investigated premixed fuel-lean ethylene-air flame. In the case of the swirl burner the predictions lead to the suggestion of a self-excited flame oscillation in the lateral direction as the result of an interaction of the flame with a vortex ring. The difference between the predicted and experimentally determined frequency of oscillation is 11 %.