Turbulent combustion models for CFD

摘要

The state-of-the-art of combustion models for implementation in CFD codes is reviewed. Existing closure assumptions suffer in general from an insufficient understanding of the principles of closure hypotheses in turbulent flows. It is shown that for fast chemistry not only the chemical source term but also the scalar transport is effected by chemistry. Therefore the gradient transport hypothesis is also not valid for reacting scalars. Models based on pdf transport require in addition a closure for the molecular diffusion term. These may therefore be justified in the limit of slow chemistry only. A way to circumvent these difficulties is the flamelet concept. It is based on the assumption that all reacting scalars are uniquely related to either one or both of two non-reacting scalars, the distance function G and the mixture fraction Z. The distance function is used in premixed combustion and describes the distance of a point in physical space from the location of the instantaneous flame front. The mixture fraction variable describes the local fuel-to-air ratio for non-premixed combustion. Examples of CFD calculations based on the flamelet approach are shown for a Diesel engine and for a gas turbine combustor.