NUMERICAL INVESTIGATION OF TURBULENT HEAT TRANSFER OVER ROUGH SURFACES AT MEDIUM–HIGH PRANDTL NUMBERS

Abstract

Wall-resolved Large Eddy Simulations are performed to investigate turbulent heat transfer in a rough pipe over a range of Pr. The roughness is generated using a controlled numerical procedure that allows independent specification of roughness correlation length, amplitude, and skewness; a statistically Gaussian height distribution is considered as a baseline case. Simulations are conducted at a bulk Re of 11,700 for Pr = 0.5, 1, 2, and 5, and are extended to additional Re=2000-15000 to assess the robustness of global trends. Time-averaged velocity and temperature profiles, turbulent heat fluxes, and Re stresses are analyzed to examine the interaction between roughness-induced mixing and Pr dependent thermal diffusion. Relative to a smooth pipe, the rough surface produces a systematic downward shift of the mean temperature profile, indicating enhanced heat transfer across all Pr. A sheltering analysis based solely on surface visibility to the incoming flow distinguishes weakly ventilated regions with suppressed heat transfer from exposed windward faces that host intense thermal events. A complementary zonal analysis based on roughness height shows that local heat-transfer enhancement becomes increasingly sensitive to surface elevation as Pr increases. For the Gaussian-type roughness considered here, however, the overall influence of Pr on global and local heat-transfer statistics remains comparatively modest, suggesting a dominant role of geometric sheltering effects. Re sweeps of global metrics further demonstrate that roughness-induced heat-transfer enhancement increase monotonically with Re, confirming that the identified mechanisms persist across a broader range of operating conditions.