Abstract
ABSTRACT V-shaped premixed flames of propane, methane, and ethylene-air mixtures were stabilized on a triangular-shaped flame holder under three different free stream turbulence levels; 4%, 18%, and 34% rms at a nominal burner exit velocity of 10 m/s. Flames were studied at strongly burning and near blowoff conditions to observe the changes in the overall flame dynamics. Flame behavior at different upstream flow turbulence conditions was visualized by simultaneously capturing the end and side views of flame chemiluminescence using high speed (4 kHz) imaging. A gradual shift from symmetric to asymmetric flame structures was observed with reduction of equivalence ratio approaching blowoff. Particle image velocity measurements confirmed the gradual switch of vortex shedding from symmetric varicose mode to the asymmetric sinuous mode as flame blowoff is approached by reduction of the mixture equivalence ratio. It is found that average flame front position crosses the shear layer from its outer location at strongly burning conditions to within the shear layers bounding the recirculation zone as flame blowoff is approached. Transition of flame behavior from “varicose” to “sinuous” oscillations is documented by way of a symmetry index as a function of equivalence ratio and turbulence intensity for the three fuels studied.
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