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

The heat transfer in a turbulent Rayleigh-Bénard convection with a multi-scale roughness at the bottom is studied experimentally. Two different regimes for the heat transfer are found. The first regime has scaling exponent γI=0.4 and corresponds to the reduced values of the Nusselt number. The second regime with enhanced values of the Nusselt number has a scaling exponent γII=0.32, which is noticeably larger than in the case of smooth boundaries. Significant variation in the Prandtl number (from 6.4 to 62) does not change the scaling exponent value of the second regime but increases the values of Nusselt number. The scaling exponent for the relation Re∼Raα is insensitive to the change of the heat transfer regime and is close to 1/2 for all values of Ra. The characteristic ratio of the velocity pulsations to the mean velocity does not depend on the Rayleigh number and is characterized by close values (about 0.8). The local temperature measurements support the mechanism of the transition from the reduced Nusselt number regime to the enhanced one, which is based on the formation of flows between obstacles.