Abstract
The present paper reports an experimental analysis carried on turbulent jet flames with the aim of contributing to improving the knowledge on countergradient diffusion of a scalar in nonpremixed flames. The experimental program includes the characterization of mean and fluctuating velocity and temperature fields, as well as the correlation between velocity and temperature fluctuations. The results quantify that velocity–temperature correlation tends to exhibit zones of nongradient turbulent heat transfer. Conditional sampling and averaging is employed to investigate the statistical characteristics of turbulent transport processes of the scalar. In addition, the conditional analysis reported herein demonstrates that countergradient contribution remains strong even in those regions of the flow where velocity–temperature correlations are in the gradient sense. A further analysis shows that this result can be extended to characterize mixture fraction flux within the region analyzed, and highlights the necessity that prediction of such quantities must be based on a second moment model closure rather than on an effective viscosity.
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