A Model for the Effects of Mixing on the Autoignition of Turbulent Flows

Modelled transport equations for the conditional mean and r.m.s. temperature increments θ before autoignition in a turbulent non-premixed flow have been developed. The model is based on the recent finding that a well-defined mixture fraction, fMR, first ignites and the concept that the conditional scalar dissipation rate, χ|fMR, controls the heat losses and hence the ignition time. By introducing the fluctuations of χ|fMR, and by modelling the conditional correlation coefficients between χ and θ from results from Direct Numerical Simulations, the model is closed and calculates autoignition times in good agreement with DNS results. Extensions to complex chemistry and the relationship with the more rigorous Conditional Moment Closure are discussed. Finally, the model is incorporated in a Favre-averaged k - ϵcode(KIVA-II)to predict autoignition of a transient fuel jet in hot air with promising results.