Experimental Investigations and Scaling Analyses of Whirling Flames

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Hariharan, Sriram Bharath
Gollner, Michael J
Oran, Elaine S
Swirling flows are ubiquitous in nature, occurring over a large range of length scales -- on the order of many tens-of-thousands of kilometers in the case of Saturn's hexagonal polar vortex, to just a few centimeters in dandelion flight. Most instances of swirling flow involve momenta competing in two different directions, axial and azimuthal. Whirling flames (also known as fire whirls) occur at the intersection of vortical flow fields and buoyant, reactive plumes, and they represent a general class of flows that may be considered slender vortices involving axial momentum from heat-release and tangential momentum from air entrainment. In this work, two previously unexplored characteristics of whirling flames are considered over a wide range of scales, spanning three orders of magnitude in length and four orders in heat-release rate. First, emissions of particulate matter (PM) from fire whirls (FW) were measured and compared to those from free-buoyant pool fires (PF). For different pool diameters and fuels, FWs showed higher burning rate and fuel-consumption efficiency, but lower PM-emission rate, leading to lower PM-emission factors. The lower PM emissions from FWs is attributed to a feedback cycle between higher oxygen consumption from improved entrainment, higher average temperatures, increased heat feedback to the fuel pool, which in turn increases burning rate and entrainment. A scaling analysis showed that the PM emission factor decreased linearly with the ratio of inverse Rossby number to nondimensional heat-release rate. Second, the structure of the blue whirl (BW), a soot-free regime, was investigated using dimensional analysis and non-intrusive optical diagnostics. Experimental data of heat-release rates and circulation for BWs and FWs from the literature were used to define the nondimensional equivalents of buoyant and azimuthal momenta. The combinations of these parameters showed that FWs primarily formed in a buoyancy-dominated regime, and that a circulation-dominated regime was required for BW formation, corroborating hypotheses that the transition was caused by the bubble mode of vortex breakdown, resulting in the formation of a recirculation zone. Finally, OH- and PAH-PLIF, OH* and CH* chemiluminescence suggest a triple-flame structure anchored at the blue ring region of the BW, with the rich branch formed by the lower blue cone, and the lean branch by the upper purple haze. These results show that the mixing process occurs upstream of the conical region and that the recirculation zone is comprised of combustion products.