Nusselt number measurements for turbulent Rayleigh-Benard convection

We present high-precision measurements of the Nusselt number N as a function of the Rayleigh number R for a cylindrical sample of water (Prandtl number sigma=4.4) of height Lsimilar or equal to50 cm and aspect ratio Gammaequivalent toD/Lsimilar or equal to1 (D is the diameter) for 3x10(9)less than or equal toRless than or equal to6x10(10). For Rsimilar or equal to3x10(9) the data are consistent with existing results for acetone (sigma=4.0, Rless than or equal to3x10(9)). There the measurements are also consistent with a model by Grossmann and Lohse (GL). As R increases, the measurements fall below the GL prediction. Near R=6x10(10) the prediction is 8% above the data.