Autoignition of turbulent non-premixed flames investigated using direct numerical simulations

The autoignition of a laminar non-premixed flame placed in a field of homogeneous isotropic turbulence has been studied previously using single-step chemistry and/or simplified models for diffusion processes. The existence of a specific value of the mixture fraction, called "most-reactive," and the importance of the scalar dissipation rate to predict the ignition location were demonstrated. The effect of the turbulence intensity on the ignition time was found to be non-monotonic. In this work, we wish to assess the influence of more realistic chemistry and transport models on ignition location and time. To do so, direct simulations are carried out using a detailed reaction scheme, multicomponent diffusion velocities and accurate thermodynamic properties. We observe that the turbulent non-premixed flame ignites always faster than the laminar one, even for the highest Reynolds numbers investigated. The scalar dissipation rate can still be used to predict the ignition site, as was observed in simple chemistry simulations. But the most-reactive conditions must of course be determined using the detailed modeling, and cannot any more be analytically predicted. The interest of repeating the direct simulations to get rid of the influence of random initial conditions is also demonstrated. (C) 2002 by The Combustion Institute.