Effect of soot addition on extinction limits of luminous laminar counterflow diffusion flames

The objective of the present study is to use large activation energy asymptotic (AEA) theory to bring basic information on the extinction limits of non-premixed flames under sooting and radiating conditions. The AEA analysis assumes single-step global combustion chemistry, constant heat capacity and unity Lewis numbers; it also includes a two-equation phenomenological model to describe soot formation, growth and oxidation processes, as well as a generalized treatment of thermal radiation that assumes spectrally- averaged gray-medium properties and applies to flames with an arbitrary optical thickness. The focus of the present study is on the effect of external soot loading on flame extinction, and in particular on the slow-mixing/radiative-extinction limit that is believed to be the dominant mechanism that determines flame extinction in fires. External soot loading simulates non-local effects observed in multi-dimensional sooting flames in which soot mass may be produced at some flame locations and transported to others where it will increase the flame luminosity and drive combustion conditions towards extinction. The AEA analysis shows that external soot loading results in a significant decrease of the size of the flammable domain and that the minimum value of flame stretch at the radiative-extinction limit is increased by more than one order of magnitude compared to a non-soot-loaded-flame case. Multi-dimensional sooting flames are therefore expected to be significantly more susceptible to radiative extinction than the one-dimensional configurations that have been previously studied in microgravity combustion research. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.