A lagrangian simulation of flamelet extinction and re-ignition in turbulent jet diffusion flames

Abstract

In order to understand the complex phenomena observed by Dibble et al., 1 Masri et al. 2,3 and Chen et al. 4 in piloted turbulent jet diffusion flames of methane near extinction, a stochastic simulation of flamelet extinction and re-ignition is performed. The simulation is based on a library of nine flamelet histories, each of them having a different transient behaviour of the scalar dissipation rate. Each of these flamelets is believed to represent a typical, scenario of the conditions to which, a flamelet in a turbulent diffusion flame may be subjected. It is found that the time scales of extinction and re-ignition are typically much longer than the local eddy turn-over times to that a Lagrangian formulation appears to be appropriate Local extinction may lead to a transient overshoot of the maximum concentration of CO well above those calculated for steady flamelets. This effect is kinetically explained by a comparison of the rates by which CO is formed and consumed during the transient. A stochastic presentation of the flamelet data leads to scatter plots which are qualitatively similar to those reported by Raman measurements in turbulent diffusion flames near extinction.