Prandtl-number dependence of heat transport in turbulent Rayleigh-Benard convection

We present measurements of the Nusselt number N as a function of the Rayleigh number R and the Prandtl number a in cylindrical cells with aspect ratios Gamma = 0.5 and 1.0. We used acetone, methanol, ethanol, and 2-propanol with Prandtl numbers sigma = 4.0, 6.5, 14.2, and 34.1, respectively, in the range 3 X 10(7) less than or similar to R less than or similar to 10(11). At constant R, N(R, sigma) varies with sigma by only about 2%. This result disagrees with the extrapolation of the Grossmann and Lohse theory beyond its range of validity, which implies a decrease by 20% over our a range, but agrees with their recent extension of the theory to small Reynolds numbers.