Turbulent combustion modelling

This paper attempts to review and discuss the current status of the art in turbulent combustion modelling. A description of our present physical and experimental knowledge of the structure of turbulent flames is presented first in order to help the further discussions of models on a physical basis. The basic equations and the modelling problem are described as simply as possible. The problem of modelling turbulent combustion is then broken down. The modelling of the mean reaction rate is considered first; the method using a balance equation for the probability density function (pdf) of reactive species concentrations is described and the modelling assumptions are discussed in connection with their physical significance; a derived method, called the `presumed pdf method', which can be more easily used for complex flow fields, is also presented, and examples of application are given. We consider particularly the case when a complex chemistry has to be taken into account, and after a short review of proposed methods, a detailed description of a so-called `Lagrangian model' is presented. The second part of the modelling concerns turbulence and turbulent diffusion; how it is possible to model these two phenomena for flows with combustion is reviewed and discussed. The modelling of time and length scales of the turbulent field of scalars is the third part of the problem; this aspect is studied in less detail, but the present status is discussed and some provisional conclusions are attempted.