Flame structure of steady and pulsed sooting inverse jet diffusion flames

In turbulent buoyant fire plumes, local inverse diffusion flames of air injected into gaseous fuel or fuel vapors occur, but little is known about the tendency to form soot and produce thermal radiation in these flame structures. To investigate these phenomena, steady and pulsed normal and inverse jet diffusion flames of methane/air and ethylene/air have been stabilized on a slot burner, which has advantages over a coannular flame geometry for performing flame imaging measurements in sooty flames. OH and PAH laser-induced fluorescence (LIF), soot laser-induced incandescence (LII), and soot thermal emission at 850 nm have been measured in the lower flame region of steady and pulsed flames. These measurements reveal that the relative positions of these different structural features are very similar in the normal and inverse steady flames of each fuel. Also, the OH signals are nearly identical in the normal and inverse flames. The inverse flame PAH signals and soot concentrations are somewhat smaller than for the normal flames, and the near-infrared radiation is approximately 250/0 lower for the inverse flame. When the central slot is pulsed, the primary buoyant vortex roll-up occurs on the fuel-rich side of inverse flames, resulting in strongly enhanced PAH signals and soot concentrations. The near-infrared radiation also increases in the pulsed flames, but not from the soot within the vortex roll-up region. In general, enhancements in peak signals from soot and near-infrared radiation similar to those in pulsed normal diffusion flames are apparent in pulsed inverse diffusion flames. PAH signals are clearly greatest in the pulsed inverse flames. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.