Strongly turbulent Rayleigh-Benard convection in mercury: Comparison with results at moderate Prandtl number

An experimental study of Rayleigh-Benard convection in the strongly turbulent regime is presented. We report results obtained at low Prandtl number (in mercury, Pr = 0.025), covering a range of Rayleigh numbers 5 x 10(6) < Ra < 5 x 10(9), and compare them with results at Pr similar to 1. The convective chamber consists of a cylindrical cell of aspect ratio 1. Heat flux measurements indicate a regime with Nusselt number increasing as Ra-0.26, close to the 2/7 power observed at Pr similar to 1, but with a smaller prefactor, which contradicts recent theoretical predictions. A transition to a new turbulent regime is suggested for Ra similar or equal to 2 x 10(9), with significant increase of the Nusselt number. The formation of a large convective cell in the bulk is revealed by its thermal signature on the bottom and top plates. One frequency of the temperature oscillation is related to the velocity of this convective cell. We then obtain the typical temperature and velocity in the bulk versus the Rayleigh number, and compare them with similar results known for Pr similar to 1. We review two recent theoretical models, namely the mixing zone model of Castaing et al. (1989), and a model of the turbulent boundary layer by Shraiman gr Siggia (1990). We discuss how these models fail at low Prandtl number, and propose modifications for this case. Specific scaling laws for fluids at low Prandtl number are then obtained, providing an interpretation of our experimental results in mercury, as well as extrapolations for other liquid metals.