Lewis number effects in laminar diffusion flames near and away from extinction

An experimental study of the extinction limits of methane-air and propane-air counterflow diffusion flames is presented. Diffusive-thermal properties (i.e., Lewis numbers) are systematically varied by diluting the fuel stream with helium or argon. For the same level of dilution, the extinction strain rate is considerably lower for He-diluted flames than for Ar-diluted flames over the entire range of dilutions tested. The observed experimental results are successfully explained by considering a weighted effective Lewis number of the counterflow flames as they approach the extinction limit. The flames investigated are also modeled using the OPPDIF code from the CHEMKIN package considering multicomponent transport properties. Experimental extinction results are in very good agreement with calculated extinction strain rates based on the detailed chemical kinetic models of the flames studied. It is also found that in counterflow diffusion flames away from extinction limits (i.e., larger Damkohler numbers), calculated flame temperatures follow the trend of the weighted Lewis number due to the finite strain rate ever present in the counterflow configuration. A parallel investigation of lifted laminar CH4 jet diffusion flames reveals that at the mid-sections of flames, the flame temperature follows the change in the unweighted fuel Lewis number, even for flames near blowout limits. Similar observations are also made in laminar C3H8 jet diffusion flames and the observed differences are discussed. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved