Relationship between the heat transfer law and the scalar dissipation function in a turbulent channel flow

Integration across a fully developed turbulent channel flow of the transport equations for the mean and turbulent parts of the scalar dissipation rate yields relatively simple relations for the bulk mean scalar and wall heat transfer coefficient. These relations are tested using direct numerical simulation datasets obtained with two isothermal boundary conditions (constant heat flux and constant heating source) and a molecular Prandtl number <i>Pr</i> of 0.71. A logarithmic dependence on the Kármán number <i>h</i>⁺ is established for the integrated mean scalar in the range <i>h</i>⁺ ≽400 where the mean part of the total scalar dissipation exhibits near constancy, whilst the integral of the turbulent scalar dissipation rate ̅ϵ_θ increases logarithmically with <i>h</i>⁺. This logarithmic dependence is similar to that established in a previous paper (Abe & Antonia, J. <i>Fluid Mech.</i>, vol. 798, 2016, pp. 140–164) for the bulk mean velocity. However, the slope (2.18) for the integrated mean scalar is smaller than that (2.54) for the bulk mean velocity. The ratio of these two slopes is 0.85, which can be identified with the value of the turbulent Prandtl number in the overlap region. It is shown that the logarithmic <i>h</i>⁺ increase of the integrated mean scalar is intrinsically associated with the overlap region of ̅ϵ_θ, established for <i>h</i>⁺ (≽400). The resulting heat transfer law also holds at a smaller <i>h</i>⁺ (≽200) than that derived by assuming a log law for the mean temperature