Effect of Boundary Layers Asymmetry on Heat Transfer Efficiency in Turbulent Rayleigh-Bernard Convection at Very High Rayleigh Numbers

The heat transfer efficiency in turbulent Rayleigh-Benard convection is investigated experimentally, in a cylindrical cell of height 0.3 m, diameter 0.3 m. We show that for Rayleigh numbers 10(12) less than or similar to Ra less than or similar to 10(15) the Nusselt number closely follows Nu proportional to Ra-1/3 if the mean temperature of the working fluid-cryogenic helium gas-is measured by small sensors directly inside the cell at about half of its height. In contrast, if the mean temperature is determined in a conventional way, as an arithmetic mean of the bottom and top plate temperatures, the Nu(Ra) proportional to Ra-gamma displays spurious crossover to higher gamma that might be misinterpreted as a transition to the ultimate Kraichnan regime.