Prandtl and wall effects in hard turbulence convection
Résumé
Using low-temperature gaseous helium close to the critical point, we investigate the {P}randtl-number dependence of the effective heat conductivity ({N}usselt number) for a 1/2 aspect ratio {R}ayleigh-{B}énard cell. Very weak dependence is observed in the range 0.7 < Pr < 21; 2 * 10/sup 8/ < Ra < 2 * 10/sup 10/: the absolute value of the average logarithmic slope delta = ( delta ln Nu/ delta ln Pr)/sub Ra/ is smaller than 0.03. A bimodality of Nu, with 7{\%} difference between the two sets of data, is observed, which could explain some discrepancies between precise previous experiments in this range.