Scale separation in thermal turbulent flows – a specific Péclet number scaling analysis

The breakdown of the coupling between the momentum and heat transfer in a thermal turbulent field is crucial for turbulent transport processes and thermal turbulent mixing. It is the consequence of an irreversible decay of turbulence-induced fluctuations by thermal diffusion due to molecular diffusivity and by dissipation due to molecular viscosity with flow structures of different length scales, leading to a deviation of the smallest scales for turbulent heat transfer from the Kolmogorov scale with varying Prandtl numbers. On the basis of a phenomenological view, a specific Péclet number approach for a scale-dependend derivation of the smallest length scale of turbulence-induced heat transport in turbulent flow with non-unity Prandtl number from the specific advection-diffusion equation is reported.