STRONGLY TURBULENT RAYLEIGH-BENARD CONVECTION IN MERCURY - COMPARISONWITH RESULTS AT MODERATE PRANDTL NUMBER

An experimental study of Rayleigh-Benard convection in the strongly tu rbulent 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 simil ar to 1. The convective chamber consists of a cylindrical cell of aspe ct ratio 1. Heat flux measurements indicate a regime with Nusselt numb er increasing as Ra-0.26, close to the 2/7 power observed at Pr simila r to 1, but with a smaller prefactor, which contradicts recent theoret ical 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 o f this convective cell. We then obtain the typical temperature and vel ocity in the bulk versus the Rayleigh number, and compare them with si milar results known for Pr similar to 1. We review two recent theoreti cal models, namely the mixing zone model of Castaing et al. (1989), an d 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 P randtl number are then obtained, providing an interpretation of our ex perimental results in mercury, as well as extrapolations for other liq uid metals.