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. Flame roll-up due to vortices is also one of the most important phenomena driving 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 nitrogen 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 planar laser-induced fluorescence (PLIF). The relevance of OH radicals as a marker of the reaction zone is discussed on the basis of direct numerical simulation (DNS) results. Scatter plots of correlations between OH concentration and heat release rate are also presented to derive a criterion of extinction. A detailed description of the interaction is given, showing a global enhancement of combustion due to the interaction with the vortex. Extinction processes occurring later are also described. The evolution of the flame surface during the interaction is extracted from the experimental visualizations. It is shown that extinctions are characterized by a reduction in flame surface area and that this decrease may be hidden by flame stretching and/or roll-up. (C) 1999 by The Combustion Institute.