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Modelling Differential Diffusion in Nonpremixed Reacting Turbulent Flow: Model Development

The modelling of differential diffusion in nonpremixed reacting turbulent flows is considered. The present approach is based on the conditional moment closure (CMC) method. As in the nonreacting case the terms involving the deviations from the conditional averages, the eyterms, cannot be neglected and need modelling. The development of new closures for the eyterms and their validation is aided by DNS studies. Non-unity Schmidt number effects of reacting scalars in homogeneous iso-tropic decaying turbulence are investigated. Chemical kinetics are approximated by irreversible one-step temperature independent and temperature dependent reactions. It is found that the ey-terms and their modelling in reacting flows strongly resemble ey-terms and -models in nonreacting flows (Kronenburg and Bilger, 1997). However, a new definition of a suitable differential diffusion variable is necessary for accurate enclosure in reacting flows. The dependence of the ey-modelling on reaction rate and temperature effects is analysed. CMC generally leads to good predictions of the appropriate conditional average differential diffusion variable which can characterize differential diffusion effects on either species concentrations or enthalpy.