Heat release and induced strain in premixed flames

Numerical and experimental results are presented to demonstrate some effects of heat release in open premixed flames, especially the strain induced in the reactant flow field. A simple model represents the heat release by treating the flame front as a two-dimensional line source of volume. The velocity and strain rate induced in the flow field are determined and the numerical solution for the case of a laminar double kernel ignition is obtained. Of primary interest is the strain rate induced in the reactants between the expanding flame kernels and, for heat release rates typical of hydrocarbon flames, the strain rate at the plane of symmetry midway between the kernels up to 150 s−1. The kernel size and shape, as well as density ratio across the flame front and the laminar burning velocity, were found to have a significant effect on the induced strain rates. For the case of turbulent combustion the velocity induced in the reactant stream is measured experimentally tangential to the mean flame zone and also in a plane parallel to the flame holder of methane/air open premixed turbulent v-shaped flames over a range of equivalence ratios. Divergent flow fields, with mean strain rates up to 50 s−1, are induced by the heat release within the flame zone and the consequences of this for determining the turbulent burning velocity in this and similar systems, with special emphasis on the use of the double kernel technique in turbulent media, are discussed.