Blowoff dynamics of acoustically coupled bluff body stabilized flames using proper orthogonal decomposition

Partially premixed bluff-body stabilized flames are commonly found in combustion applications. These combustors can encounter instabilities and naturally occurring thermoacoustic interactions that may lead to flame blowoff. Under strong thermoacoustic coupling, induced vortices can cause oscillating aerodynamic stretch along the flame sheet. These vortices become stronger with increased thermoacoustic coupling and can cause flame blowoff. The blowoff dynamics of a bluff-body stabilized flame in an enclosed two dimensional combustor was studied under various degrees of thermoacoustic coupling. Simultaneous phase-resolved high-speed chemiluminescence imaging and pressure measurements were made of a turbulent partially premixed propane-air bluff-body stabilized flame throughout the blowoff process. A minimum mutual information algorithm was used to determine an optimum number of images to be included in proper orthogonal decomposition (POD) analysis to obtain significant modes associated with blowoff. Flames with strong thermoacoustic coupling required inclusion of images every ∼1.5 ms, while flames with low coupling required images every ∼0.67 ms. Flames with little or no acoustic coupling show burning in the recirculation zone right before final blowoff, as has been reported in previous work. Highly acoustically coupled flames do not demonstrate this recirculation zone burning. A spectral analysis of the time constants found from POD and the simultaneous pressure measurements are used to qualitatively understand the effects of thermoacoustics on flame blowoff.