Christoph Irrenfried

The accurate computational modelling of momentum and heat transfer in turbulent wall-bounded flow is still a challenging task, especially for fluids with high molecular Prandtl numbers. Due to the steep temperature gradients, the strong variation of fluid properties becomes an additional important issue. The present Direct Numerical Simulation study specially addresses this problem considering a representative working fluid in terms of a real heat transfer oil with temperature-dependent material properties. Near the heated wall, the increase in molecular viscosity significantly attenuates the turbulent convective fluxes of both momentum and heat, despite the markedly increased enthalpy fluctuations. As the thermal conductivity decreases markedly less with temperature than the viscosity, the dampened turbulent mixing significantly reduces the Nusselt number, whereas the skin friction coefficient only marginally differs from the constant fluid property case. The attenuation of the radial velocity fluctuation consistently reduces the production terms in the budgets of the turbulent momentum and heat fluxes, while the production of the enthalpy fluctuations is only shifted away from the wall without significant change in peak level. The also attenuated redistribution in the turbulent axial stress budget reduces the pressure-strain based steering of turbulence towards isotropy. Facing a consequently more anisotropic turbulent motion poses a further difficulty to standard turbulence modelling mostly developed for isotropic turbulence.