Measurement and prediction of the conditional variance in a turbulent reactive-scalar mixing layer

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Abstract
Equations for the second‐order conditional moments of reactive scalars are derived. For one‐step reaction with equal diffusivity these can be reduced to a single equation for the conditional variance, which is the same for all reactive species. The various terms in the equation have been modeled in light of the experimental data for a turbulent reactive‐scalar mixing layer. In comparison with conventional second‐order moment closure the conditional means do not vary with the cross‐stream position, the turbulent flux in the cross‐stream direction is negligible while the source term needs special attention. Experimental results of the conditional variance at different points across the mixing layer and at various streamwise locations are presented. At each streamwise location the data in general collapse onto each other although the scatter is large. The modeled equations have been solved numerically and are compared with the experimental data. The prediction generally agrees with the data. Two models have been used for the conditional conserved scalar dissipation. One model assumes that the conditional scalar dissipation equals the unconditional one and the other is derived from a mapping closure. Both models give almost identical results for the conditional means and only a small difference for the conditional variance.