Dynamic Response of Turbulent Low Emission Flames at Different Vortex Breakdown Conditions

The classic Flame Transfer Function (FTF) used in the thermoacoustic stability analysis of lean premixed combustors is linked, in a mathematically strict way, to the flow coherent structures using Large Eddy Simulation. This is based on a methodology which combines the Wiener-Hopf system identification filter—separating any field variables into a dynamic contribution driven by external forcing plus a noise contribution given by turbulent fluctuations—with the extended formulation of the Proper Orthogonal Decomposition (POD). The method is applied to partially premixed flames stabilized at two different types of Central Recirculation Zone (CRZ) due to the mechanism of vortex breakdown of the flow through a swirl burner: type A, where the CRZ appears rather narrow in the radial direction with apex located close to the burner exit and type B, where the CRZ is entirely located in the combustor and appears more flat at its apex than what observed in case of the type A vortex flow. Rather different properties are observed for the FTF. Flames stabilized at the narrow CRZ (type A), respond to inflow forcing with a time delay which depends much more on the bulk equivalence ratio than flames stabilized at the thick CRZ (type B). On the other hand the amplitude of the FTF in the case of the narrow CRZ is in general lower than in case of the thick and flat CRZ where amplification factors of the order of 4–5 are reached. By allowing a reasonable explanation of the observed trends, the methodology developed here can give an important contribution to the development of gas turbine burners.