Experimental characterization of response of lean premixed low-swirl flames to acoustic excitations

Acoustically excited lean premixed low-swirl flames were experimentally investigated to gain a better understanding of detrimental thermoacoustic couplings which can occur in applications like low-NOx gas turbines. Propane-air flames were imaged and analyzed at equivalence ratios of 0.6 to 0.8, mean flow velocities of 3.5 to 5.5 m/s and excitation frequencies of 135 Hz to 555 Hz. It was observed that with increasing excitation frequency, mean flame shape gradually became wider up to a Strouhal number of about 2.5 and then slowly reverted back to the unexcited flame shape. Such large changes in mean flame shape and possibly flow field under acoustic excitations can significantly affect flame dynamics. Increased heat release fluctuations were observed in the flame shear layer at Strouhal numbers from 1 to 2. Rolling up of the flame by vortices was suggested as a driving mechanism for these fluctuations which may lead to combustion instabilities.