Quasi-direct numerical simulation of forced convection over a backward-facing step: Effect of Prandtl number

Abstract

Quasi direct numerical simulations (quasi-DNS) of forced heat convection over a backward-facing step are performed to study the turbulent heat transfer of low Prandtl number fluids in complex flow regimes. Reynolds number based on step height and inlet bulk velocity equals 4805. The expansion wall is heating by uniform dimensionless heat flux density. Effects of Prandtl number (Pr = 0.01, 0.025, 0.1, 1.0) on mean temperatures, heat transfer coefficients, heat fluxes, turbulent Prandtl numbers and instantaneous velocity and temperature fluctuations, are compared with each other. For low Prandtl numbers, diffusion effect is prominent, thermal boundary layer is much thicker, Nusselt number is smaller and Stanton number is larger. The maximum heat transfer location moves upstream of the reattachment point for larger Prandtl numbers while it is at the reattachment point for low Prandtl numbers. Wall-normal heat flux decomposition shows the turbulent part is comparable to the molecular one in the recirculation zone even for low Prandtl numbers. Turbulent Prandtl number profiles are irregular in the recirculation zone but in the recovering zone they increase first and then decrease. Their peak values decrease as Prandtl number increases. Near-wall instantaneous fields for streamwise velocity and temperature fluctuations show that the streak structures disappear in the recirculation zone but exist in the recovering zone. These highly-resolved data with heat transfer could serve for the validation and improvement of turbulence heat transfer models for low Prandtl media in future.