Modification of methane-air nonpremixed flamelets by vortical interactions

Abstract

The structure of a nonpremixed methane-air flame interacting with avortex is described by a two-step reduced mechanism. The mixture-fraction field Z for the flamelet is approximated by a stretched parabola. This synthetic mixture-fraction field is used to relate the vortex swirl velocity to the tangential velocity along the flamelet and to identify a rate of scalar dissipation χ appropriate for parabolically shaped flame tongue. The flame temperature and χ along the tongue are calculated in the context of rate-ratio asymptotics. It is found that the peak temperatures, and hence the smallest χ, occur at the flame tip. In addition, analytical estimates are made to evaluate the importance of the transverse convective effects in the reaction zone relative to the usually assumed diffusive-reactive balance. Away from the tip of the flame tongue but near it, the influences of transverse convection are important, but the relative magnitude of these effects is dependent on the curvature κ of the flamelet as well as on the spacing l between the mixture-fraction contour of the oxidizer stream ( Z = 0) and that of the fuel stream ( Z = 1); the convective influence becomes more pronounced as κ and/or l is increased. However, the estimates show that the importance of transverse convection is not as large as would be indicated using activation-energy asymptotics.