Numerical Simulation of Premixed Turbulent Combustion Based on a Level Set Flamelet Model

Turbulent prcmixcd combustion occurs in a wide variety of technical applications. To achieve a profound understanding of the relevant physical and chemical processes involved and to enhance the predictability of these processes, a level set flamclct model for prcmixcd turbulent combustion is presented in this work. As a turbulent combustion model is ultimately aimed at the design process, numerical simulations should give results in relatively fast turnover times without sacrificing physical accuracy.An initial analysis shows that the technically relevant turbulent prcmixcd combustion processes occur almost exclusively within the so-called corrugated flamclct and thin reaction zone regime. In these regimes, the relevant chemical time and length scales arc smaller than the respective turbulent time and length scales. This implies that the important chemical reactions take place in thin, locally one-dimensional laminar layers, the so-called flamclcts, embedded in an otherwise inert turbulent flow field. Hence, assuming scale separation of chemical and turbulent scales, the calculation of the chemistry can be decoupled from the calculation of the turbulent flow field. In practice, the chemical structure of the instantaneous prcmixcd flames is solved in a pre-processing step and then stored in so-called flamclct libraries. Since the chemical time and length scales need no longer be resolved in the subsequent turbulent combustion simulation, the numerical effort is greatly reduced, thus allowing for the calculation of complex reacting flows, even within the scope of an engineering framework. The effect of combustion on the turbulent flow field is then accounted for by reattaching an ensemble average of the previously calculated flamclcts to the mean flame front location with the help of a presumed shape probability density function approach. The position of the propagating mean flame front in the turbulent flow field is defined by a level set iso-scalar surface whose motion is described by the mean level set transport equation. This implies that the turbulent burning velocity is a well defined quantity. It can be calculated from cither an algebraic equation directly or via the solution of the differential equation for the flame surface area ratio.