Experimental analysis of the effect of fuels on the flame describing function of a swirl-stabilized premixed flame

Abstract

A study of the fuel-dependence of the Flame Describing Function (FDF) of a swirl-stabilized premixed flame is carried out. A swirl-stabilized burner, a hot wire to measure the velocity and a loudspeaker system to generate acoustic perturbations compose the experimental apparatus. The optical diagnostics include a photomultiplier tube to collect CH* chemiluminescence and an ICCD camera to acquire phase-locked images of OH* chemiluminescence to analyze the flame dynamics. A propane/air mixture with an equivalence ratio (Φ) equal to 0.7 and a methane/air mixture with an equivalence ratio equal to 0.67 are considered. The equivalence ratios are slightly different for the two fuels to maintain similar flame shape and size. Five different levels of forcing amplitude, from 5% to 25% of relative velocity fluctuation, are examined. The gain of the two FDFs shows the characteristic behavior for this type of flame: a local minimum followed by a local maximum. Moreover, increasing the forcing amplitude, the value of the local maximum of gain decreases. This is verified for the two considered fuels. Analyzing the phase-locked images of OH* chemiluminescence, it is possible to conclude that the size of the vortex roll-up at the tip of the flame plays an important role in the reduction of the value of the local maximum of gain when methane replaces propane.