Modeling autoignition in non-premixed turbulent combustion using a stochastic flamelet approach

In this paper, a stochastic flamelet approach is used to model autoignition in an initially non-premixed medium in isotropic and decaying turbulence, using a one-step irreversible reaction. This configuration corresponds to the DNS data from Sreedhara and Lakshmisha [Proc. Combust. Inst. 29 (2002) 2069]. The system can be described by the flamelet equations for the temperature and fuel mass fraction, where the scalar dissipation rate appears as a stochastic parameter. In a turbulent flow, fluctuations of this scalar have a strong impact on autoignition. Assuming a log normal distribution, a stochastic differential equation (SDE) can be derived for the scalar dissipation rate. The decay rate of the mean dissipation rate is taken from the DNS. The DNS data suggest that the normalized variance is close to unity but depends upon the Reynolds number. The flamelet equations for the temperature and fuel mass fraction, and the stochastic differential equation are coupled and solved numerically. The effects of the turbulence are discussed, and the results are compared with the DNS database. The model reproduces the conditional mean temperature profiles and the ignition delay times with good accuracy. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.