Characteristics of turbulent heat transport in nonpremixed jet flames

The present paper reports an experimental analysis carried on turbulent jet flames with the aim of contributing to improving the knowledge on counterg radient diffusion of a scalar in nonpremixed flames. The experimental progr am includes the characterization of mean and fluctuating velocity and tempe rature fields, as well as the correlation between velocity and temperature fluctuations. The results quantify that velocity-temperature correlation te nds to exhibit zones of nongradient turbulent heat transfer. Conditional sa mpling and averaging is employed to investigate the statistical characteris tics of turbulent transport processes of the scalar. In addition, the condi tional analysis reported herein demonstrates that countergradient contribut ion remains strong even in those regions of the flow where velocity-tempera ture correlations are in the gradient sense. A further analysis shows that this result can be extended to characterize mixture fraction flux within th e region analyzed, and highlights the necessity that prediction of such qua ntities must be based on a second moment model closure rather than on an ef fective viscosity. (C) 2001 by The Combustion Institute.