The use of counterflow flames for the evaluation of burning velocities and stretch effects in hydrogen/air mixtures

We briefly review the concept of Markstein lengths with respect to unburned and burned gases and also the problem of defining the correct reference plane to measure flame speed and stretch. We then present numerical simulations of hydrogen/air flames in the counterflow configuration, using full chemistry and multicomponent transport. The values of flame speed derived by extrap-olating the stretched burning velocities to zero stretch and those calculated using planar flame modeling are within 2% of each other over most of the equivalence ratios studied. The Markstein lengths of these flames were computed, as a function of equivalence ratio, with respect to both the unburned and burned gases and can be used to model the response of stretched hydrogen/air flames. We show that the values measured with respect to the burned and unburned gases are different yet in accordance with asymptotic theory. The values from our numerical simulations are then compared to the values published in the literature for the same mixtures. The numerical and experimental values in the literature, obtained from the expansion rates of spherical flames, are in very good agreement with our evaluations of the values of the Markstein lengths with respect to the burned gases in the counterflow configuration. These results also confirm the idea that, for small stretch rates, flames have the same response to stretch arising from either strain or curvature.