Flame propagation in a nonuniform mixture: Analysis of a slowly varying triple flame

For flames propagating through a nonuniform medium a three-flame structure is described. This consists of a fuel-rich premixed flame, a fuel-lean premixed flame, and, starting where these flames meet, a diffusion flame. Such formations have been observed experimentally and probably occur as laminar flamelets in turbulent diffusion flames. A low-heat-release model for such flame structures is developed and solutions are obtained in the limit of slowly varying premixed flames. Under these conditions, it is shown that the Triple-Flame propagation speed depends on the transverse mixture fraction gradient and is bounded above by the maximum adiabatic laminar flame speed of the system.