On transient flamelets and their relationship to turbulent methane-air jet flames

Abstract

A series of parametric laminar flame calculations are performed to investigate physical mechanisms that may contribute to the apparent ‘non-flamelet’ behavior that has been measured in turbulent flames. A simplified, one-dimensional formulation of the transient diffusion flame problem is used, and chemical reactions are treated by a ‘skeletal’ mechanism. Three factors affecting the structure of methane-air diffusion flames are considered: i) changes in the profile of scalar dissipation through the reaction zone, ii) temporal variation of the scalar dissipation rate, and iii) elimination of differential diffusion by setting all Lewis numbers to unity. Results indicate that changes in the shape of the scalar dissipation profile and periodic variation of the scalar dissipation rate have relatively minor effects on species mass fractions. However, the mass fractions of OH and CO overshoot their steady state limits when a flamelet is subjected to a sudden decrease in scalar dissipation. The predicted overshoot occurs without the laminar flame being first pushed to the brink of extinction, and recovery of the flame to steady-state conditions is relatively slow. This shows that details of the strain history can be important and that certain types of unsteady strain can contribute to the high OH and CO mass fractions measured in turbulent flames. Significant changes inspecies mass fractions also occur when all Lewis numbers are set to zero. In particular, the peak mass fractions of OH, CO, and H 2 all increase when differential diffusion is eliminated.