Sensitivity of calculated extinction strain rate to molecular transport formulation in nonpremixed counterflow flames

Abstract

The counterflow flame is a commonly used geometry for experimental and chemical kinetic modeling studies of nonpremixed combustion. Because the flame structure is quasi-one-dimensional and more computationally tractable than inherently multidimensional flame geometries [1], detailed chemical kinetic modeling codes for counterflow flames have come into widespread use. An important characteristic of counterflow flames is the extinction strain rate, the maximum velocity gradient which a flame can support and still burn. At strain rates below the extinction value, there exist three solutions to the governing equations which determine the flame structure. Two of the solutions correspond to a stable flame and an essentially non-reacting cold flow, while the third branch is an unstable solution which cannot be physically realized [2]. Various computational approaches have been used to extend stable solutions onto the unstable branch, and thus determine the limits of bistability corresponding to flame extinction and autoignition [2, 3]. The extinction strain rate is important for modeling turbulent combustion using the laminar flamelet approach [4], and is a figure of merit for the effectiveness of fire suppressants in nonpremixed flames [5]. It is desirable for calculations to accurately predict extinction strain rates. Substantial variations in predicted extinction strain rates of nonpremixed methane/ air counterflow flames have been noted between different chemical kinetic mechanisms [6]. Here, the predicted extinction strain rate is found to also be sensitive to the treatment of molecular transport in the computational model. The effect of transport on premixed flame structure and extinction has been recently investigated by Paul and Warnatz [7], and by Ern and Giovangigli [8, 9]. To our knowledge, the effect of transport formalism on extinction of nonpremixed counterflow flames has not been previously reported in the literature. Furthermore, earlier studies reporting computational predictions of extinction strain rates have not always specified the manner in which the calculation handled species transport. In the present study, we employ two computer programs developed for counterflow flames incorporating the CHEMKIN program packages developed at Sandia National Laboratories. Initially, the chemical kinetics [10] and molecular transport [11] routines were applied