Experimental analysis of the effects of the bulk flow velocity on the flame transfer function of premixed swirl flames

A study on how the bulk flow velocity ( ) affects the flame transfer function (FTF) of premixed swirl flames is carried out. The experimental apparatus comprises a swirl-stabilized burner and a loudspeaker system to acoustically force the flame. The diagnostics include a hot wire probe to measure the velocity, a photomultiplier tube to collect OH* chemiluminescence of the flame, and a camera to acquire direct images of the flame. Two fuels, methane and propane, and four values of for each fuel are analyzed. The gain of the FTF for the two fuels shows the characteristic trend for swirl flames: a series of local maxima and minima. On the other hand, the FTF phase increases with increasing forcing frequency. Increasing , the frequency of the second local maximum of the gain increases while its magnitude remains almost constant for both fuels. Moreover, the phase increases when is increased. The stabilization distance, or the flame base angle, is constant when is increased, resulting in a constant value of the gain at the frequency around the second local maximum. On the other hand, the flame height is observed to control the trend of the phase with increasing.