Nusselt Number Measurements for Turbulent Rayleigh-Bénard 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 σ=4.4) of height L50 cm and aspect ratio Γ≡D/L1 (D is the diameter) for 3×10⁹≤R≤6×10¹⁰. For R3×10⁹ the data are consistent with existing results for acetone (σ=4.0, R≤3×10⁹). 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=6×10¹⁰ the prediction is 8% above the data.