Local extinction Karlovitz number for premixed flames

The extinction of laminar premixed flames under the influence of stretch is studied experimentally and numerically. Experiment involves measuring the velocity field of methane/air and propane/air flames in an opposed jet with a laser Doppler velocimeter and computations are made using several chemical kinetic mechanisms. The major result is that, in contrast to the various previous claims of extinction Karlovitz number being anywhere between 0.03 and 20, it is found to be constant around unity for the mixtures tested. That is, premixed flames are extinguished when the physical flow time approximately equals the chemical reaction time. Here the relevant chemical reaction time is not the one computed using the one-dimensional flame properties as originally suggested in the formulation of Karlovitz number, but rather it is the one obtained using the stretched flame properties which fully reflect the effect of straining on the flame structure.