Investigations of heat release, extinction, and time evolution of the flame surface, for a nonpremixed flame interacting with a vortex

Flame/vortex interactions to a great extent govern turbulent combustion. Fl ame roll-up due to vortices is also one of the most important phenomena dri ving combustion instabilities. An experimental investigation analyzes some fundamental features of a diffusion flame interacting with a vortex ring. A steady nonpremixed counterflow flame of air and hydrogen diluted with nitr ogen is first established. A vortex ring is generated from a tube installed in the lower combustor nozzle and impinges on the flame. In the experiment described herein, the visualization of the flame front is achieved by OH p lanar laser-induced fluorescence (PLIF). The relevance of OH radicals as a marker of the reaction zone is discussed on the basis of direct numerical s imulation (DNS) results. Scatter plots of correlations between OH concentra tion and heat release rate are also presented to derive a criterion of exti nction. A detailed description of the interaction is given, showing a globa l enhancement of combustion due to the interaction with the vortex. Extinct ion processes occurring later are also described. The evolution of the flam e surface during the interaction is extracted from the experimental visuali zations. It is shown that extinctions are characterized by a reduction in f lame surface area and that this decrease may be hidden by flame stretching and/or roll-up. (C) 1999 by The Combustion Institute.