The turbulent Prandtl number in the near-wall region for low-Prandtl-number gas mixtures

Results of experiments for binary gas mixtures, with molecular Prandtl numbers in the range 0.18-0.7 in flow in a smooth circular tube with constant transport properties were employed to examine models hypothesized for the distribution of the turbulent Prandtl number, Pr-t{y(+), Pr, Re}. These models included approximations of predictions from recent direct numerical simulations of turbulent duct flows. Data for a range of molecular Prandtl numbers for 'high' Reynolds numbers, similar to 3 x 10(4) < Re < similar to 1 x 10(5), were obtained earlier by the authors by utilizing mixtures formed by helium or hydrogen with xenon, argon or carbon dioxide. For the range 0.21 < Pr < 0.72, Reynolds analogy and the variable Pr-t{y(+), Pr, Re} model by Kays agreed with downstream measurements to within about 5%.