EDDY DIFFUSIVITY BASED COMPARISONS OF TURBULENT PRANDTL NUMBER FOR BOUNDARY-LAYER AND FREE JET FLOWS WITH REFERENCE TO FLUIDS OF VERY-LOW PRANDTL NUMBER

It is shown that turbulent Prandtl numbers for turbulent boundary laye r and jet flows correlate well if compared on the basis of eddy diffus ivity. Experimental data for liquid sodium (Pr = 0. 0058) lead to a un iversal expression relating turbulent Prandtl number to eddy diffusivi ty of heat with the same expression applying to boundary layer and jet flows. This suggests that the length and velocity scale dependence of the turbulent Prandtl number is associated predominantly with the edd y diffusivity of momentum. Since turbulent flow codes generally includ e eddy diffusivity of momentum calculations, simple and accurate estim ates of eddy diffusivity of heat are possible without reliance on a va riety of turbulent Prandtl number functional relationships for predict ion of the temperature field. Available results also indicate that tur bulent Prandtl number can be set at 0.9-1.0 irrespective of molecular Prandtl number, provided that epsilon(M)/nu > 3Pr-1 and the structures of the mean temperature gradient and mean shear are similar.