Tracking blowoff dynamics of flames utilizing proper orthogonal decomposition

The dynamics of bluff body stabilized flames near blowoff are examined for propane-air flames comparing the effects of non-uniform fueling about the bluff body, presented previously, to those of thermoacoustic coupling with longitudinal modes in the combustor. High speed chemiluminescence imaging is used to visualize the flame as blowoff occurs for each condition. Rather than qualitatively tracking the flame's physical response to blow off, proper orthogonal decomposition (POD) was used to quantitatively track the flame's behavior. POD provides spatial information, from a set of bases or modes, and temporal information from mode coefficients, regarding flame emission during the blowoff process. The bases were obtained from the data of many flames with different parameters simultaneously. Bases were also obtained separately for each flame. For all cases, the method of minimum mutual information was used to optimize the number of images used in the POD algorithm to find the bases. The time scale associated with the minimum mutual information was found to be half of a recirculation zone flow time. By examining the dynamics of mode constants, time scales for physical processes were studied. A blowoff time scale and vortex propagation time scale was determined. In addition, a quantitative description of phenomena observed near blowoff such as recirculation zone burning, shear layer pinching, and acoustically driven oscillations is described.