Effect of High-Amplitude Forcing on Turbulent Combustion Intensity and Vortex Core Precession in a Strongly Swirling Lifted Propane/Air Flame

Abstract

The present work reports stereoscopic particle image velocimetry (PIV) measurements in a strongly swirling nonreacting jet and partially premixed lifted flame. The spatial distributions of the average velocity and components of turbulent kinetic energy were calculated from the measured ensembles of the instantaneous velocity fields. A pronounced bubble-type vortex breakdown was observed for the studied flows. Based on proper orthogonal decomposition (POD) of the PIV data and on estimates of velocity fluctuation spectra by a laser Doppler velocimetry (LDV) probe, it was concluded that the combustion did not fundamentally affect the type of coherent structures in the strongly swirling flow: a pair of secondary helical vortices was induced by a precessing vortex core in both cases. Because a strongly swirling jet flow is usually insensitive to weak forcing, strong perturbations were superimposed on the flow bulk velocity to force the formation of ring-like vortices in the flow and to investigate the possible outcomes on the turbulent combustion process. The forcing frequency was below that of the precession. Based on the CH* chemiluminescence signal, it was observed that the forcing provided an increase in the turbulent combustion rate near the flame onset, as the entrainment of ambient air to the rich mixture must have increased. Moreover, for a forcing amplitude typically above the magnitude of reverse flow inside the bubble-type recirculation zone, a dramatic suppression of the vortex core precession took place in the reacting case. This effect was accompanied by a quasi-periodical vanishing of the recirculation zone due to interaction of the forced ring-like vortices with the lifted flame.

Keywords:

• Large-scale vortices
• Lifted flame
• Particle image velocimetry
• Precessing vortex core
• Swirling jet
• Swirling partially premixed flame

ACKNOWLEDGMENTS

This work was performed with funding from the Government of Russian Federation (Grant No. 11.G34.31.0046, Federal Oriented Programs “Scientific, research and educational cadres for innovative Russia” for 2009–2013 and “Research and engineering in priority directions for development of Russia's scientific-technological complex” for 2007–2013). The research leading to these results received funding from the European Community's Seventh Framework program (FP7/2007-2013) under grant agreement No. 265695. The authors are also grateful for support from the Russian Foundation for Basic Research (grant N 11-08-00985) and Global Energy Foundation.

Notes

Published as part of the 23rd International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS) Special Issue with Guest Editor Derek Dunn-Rankin.