Numerical Investigation of the Flame Base Structure of Lifted Diffusion Flames.

The flame base structure of lifted diffusion flames identified as the triple flame has been investigated numerically. As the structure related to the flamelet model or the flame sheet model of the turbulent diffusion flame, the detail is required, because the structure may control the downstream flame. Thus, the reacting flows with four types of inlet fuel concentration gradient in the expanded duct examined by Kioni et al. or Hirota et al. are calculated by using the HSMAC method. Results show that the decrease in the inlet fuel concentration gradient leads to the growth of both premixed flames, especially the rich premixed flame, and weakens the diffusion flame. That results in the convex geometry of the lean premixed flame. Moreover, it has been found that the flame base structure of lifted diffusion flames is modeled in the mixture fraction space; the unburnt region upstream of the flame tip, the triple flame structure region surrounded by both premixed flames, the unburnt transient region outside of the triple flame region, and the diffusion flame structure region downstream of both of the triple flame region and the unburnt transient region.