Modelling differential diffusion in nonpremixed reacting turbulent flow: Application to turbulent jet flames

The modelling of differential diffusion effects in a well documented turbulent hydrogen-air jet diffusion flame is considered. The species and enthalpy transport equations are modelled by the conditional moment closure (CMC) method. In the presence of differential diffusion the terms involving the fluctuations around the conditional means, the e(y)-terms, must not be. neglected and need modelling. They have been modelled by an expression which is described in detail in a companion paper. Using these non-unity Lewis number modelling strategies, super-equilibrium temperatures on the lean side of stoichiometric and increased nitric oxide concentrations are predicted giving improved agreement with experiment. The results show that the ey-model which has been derived with the aid of DNS is applicable to real flame geometries. However, it is also demonstrated that differential diffusion is not the only cause of substantial underpredictions of NOclose to the jet exit. This is in contrast to all previous studies on these hydrogen flames.