Modeling and calculation of turbulent lifted diffusion flames

Liftoff heights of turbulent diffusion flames have been modeled using the laminar diffusion flamelet concept of Peters and Williams [AIAA J., 21:423–429 (1983)]. The strain rate of the smallest eddies is used as the stretch describing parameter, instead of the more common scalar dissipation rate. The h(U) curve, which is the mean liftoff height as a function of fuel exit velocity can be accurately predicted, while this was impossible with the scalar dissipation rate. Liftoff calculations performed in the flames as well as in the equivalent isothermal jets, using a standard k- turbulence model yield approximately the same correct slope for the h(U) curve while the offset has to be reproduced by chosing an appropriate coefficient in the strain rate model. For the flame calculations a model for the pdf of the fluctuating flame base is proposed. The results are insensitive to its width. The temperature field is qualitatively different from the field calculated by Bradley et al. (Twenty-Third Symposium on Combustion, 1990, pp. 685–692) who used a premixed flamelet model for diffusion flames