EXPANDING THE STABILITY RANGE OF A LIFTED PROPANE FLAME BY RESONANT ACOUSTIC EXCITATION

We found that the stability of an unconfined fuel-rich lifted propane-air flame is substantially enhanced and the stability range (in the Reynolds number-equivalence ratio domain) is extended when forced by periodic perturbations at 280 +/- 20Hz and an intensity of 10%. This preferred frequency appeared to be independent of the Reynolds number over the range considered (2500 to 12000), resulting in the corresponding Strouhal number (St) variation from about 1.8 to 0.4. For the stabilization heights below 5 nozzle diameters, the enhanced stability is especially effective in shifting the blow-off limit toward leaner mixtures. From high-repetition particle image velocimetry (PIV) measurements and dynamic mode decomposition (DMD), as well as flame visualization by CH* chemiluminescence, it was found that the flame affected the large-scale ring-like vortices by increasing their convection speed and suppressing their pairing and the consequent subharmonic modes of the flame instability. The resonance frequency of 308Hz (St=0.82) was also found to be the natural frequency of the unforced propane flame, compared to about 127Hz (St=0.38) in the cold jet of the same configuration. Both the forced and unforced flames stabilized on secondary (azimuthal) instabilities associated with the streamwise vortex filaments in the braid between the roll-up vortices, possibly excited by the feedback of heat-release pulsations. This leads to amplification of the first harmonic of the fundamental frequency, which, together with thermal expansion, is believed to cause a more than two-fold increase in the preferred frequency compared with that of a nonreacting jet.