MODIFICATION OF METHANE AIR NONPREMIXED FLAMELETS BY VORTICAL INTERACTIONS

The structure of a nonpremixed methane-air flame interacting with a vo rtex is described by a two-step reduced mechanism. The mixture-fractio n field Z for the flamelet is approximated by a stretched parabola. Th is synthetic mixture-fraction field is used to relate the vortex swirl velocity to the tangential velocity along the flamelet and to identif y a rate of scalar dissipation chi appropriate for the parabolically s haped flame tongue. The flame temperature and chi along the tongue are calculated in the context of rate-ratio asymptotics. It is found that the peak temperatures, and hence the smallest chi, occur at the flame tip. In addition, analytical estimates are made to evaluate the impor tance of the transverse convective effects in the reaction zone relati ve to the usually assumed diffusive-reactive balance. Away from the ti p of the flame tongue but near it, the influences of transverse convec tion are important, but the relative magnitude of these effects is dep endent on the curvature kappa of the flamelet as well as on the spacin g l between the mixture-fraction contour of the oxidizer stream (Z = 0 ) and that of the fuel stream (Z = 1); the convective influence become s more pronounced as kappa and/or l is increased. However, the estimat es show that the importance of transverse convection is not as large a s would be indicated using activation-energy asymptotics.