Heat transport in turbulent Rayleigh-Benard convection

We present measurements of the Nusselt number N as a function of the Raylei gh number R in cylindrical cells with aspect ratios 0.5 less than or equal to Gamma = D/d less than or equal to 12.8 (D is the diameter and d is the h eight). We used acetone with a Prandtl number sigma = 4.0 for 10(5) less th an or similar to R less than or similar to 4 x 10(10) A fit of a power law N = N0Rgamma eff over limited ranges of R yielded values of gamma eff from 0.275 near R = 10(7) to 0.300 near R = 10(10) The data are inconsistent wit h a single power law for N(R). For R > 107 they are consistent with N = a s igma(-1/12)R(1/4) + b sigma(-1/7)R(3/7) as proposed by Grossmann and Lohse for a sigma greater than or similar to 2.