Experimental study of convective heat transfer with a multi-scale roughness.

This article experimentally investigates turbulent Rayleigh-Bénard convection (RBC) with multi-scale roughness on the bottom surface, focusing on its impact on heat transfer. Two distinct heat transfer regimes are identified: a first regime with reduced Nusselt number (Nu) scaling exponent γ₁ = 0.4, attributed to fluid entrapment in cavities between obstacles, and a second regime with enhanced Nu scaling exponent γ₂ = 0.32, exceeding that of smooth surfaces. Variation of the Prandtl number (Pr) from 6.4 to 62 does not alter the second regime’s scaling exponent but increases Nu values. Flow measurements reveal that roughness perturbs large-scale circulation and increases turbulent kinetic energy near the rough surface, while the Reynolds number scaling remains near 1/2 regardless of regime. The findings suggest that multi-scale roughness, resembling urban topography, significantly influences convective heat transfer, with implications for natural and engineered systems.