Expanding the Stability Range of a Lifted Propane Flame by Resonant Acoustic Excitation

Abstract

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 ± 20 Hz 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 308 Hz (St = 0.82) was also found to be the natural frequency of the unforced propane flame, compared to about 127 Hz (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.

Keywords:

• Dynamic mode decomposition
• Flame stability
• Large-scale vortices
• Lifted flame
• Particle image velocimetry

ACKNOWLEDGMENTS

This work was performed in the framework of the Lead Scientists Grant from the Government of Russian Federation (No. 11.G34.31.0046, K. Hanjalic) and was also partially funded by the European Community 7th Framework Programme (FP7/2007-2013) under Grant Agreement No. 265695.

Notes

¹Actually, the measurements of frequency with a sensitive microphone showed a slight decrease of St (corresponding to the fundamental jet instability mode) with an increase in Re, from about 0.5 to 0.35 over the range of Re = 2500–10,000. In all cases, the subharmonic frequency was also detected with St = 0.18–0.24. Slight variations were observed depending on the distance of the microphone tip (placed at the jet edge) from the nozzle exit over the range z/d = 1–3. Interestingly, for Re ≤ 5000, the fundamental frequency (corresponding to the average St ≈ 0.4) prevails at z/d = 1–2, whereas for higher Re number, subharmonics at an average St = 0.20 were found to be more dominant, hinting at a stronger pairing of the K-H vortices at higher Re numbers.

²It should be noted, however, that the St number used may not be the appropriate similarity parameter since it is defined with the nozzle diameter d and the jet exit velocity U _b . While valid to characterize the hydrodynamic jet instability of cold jets, strong temperature gradients and fluid expansion in flames may instead require to use the local parameters (velocity, length scale, and frequency) in order to achieve an St similarity in different flames. Moreover, the flame base height H in lifted flames is another length scale that may be employed. This issue is currently under investigation.