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We present measurements of the Nusselt number N as a function of the Rayleigh number R and the Prandtl number in cylindrical cells with aspect ratios 0ex{0ex}=0ex{0ex}0. 5 and 1. 0. We used acetone, methanol, ethanol, and 2-propanol with Prandtl numbers 0ex{0ex}=0ex{0ex}4. 0, 6. 5, 14. 2, and 34. 1, respectively, in the range 310^710^11. At constant R, N (R, ) varies with 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 range, but agrees with their recent extension of the theory to small Reynolds numbers.
Ahlers et al. (Mon,) studied this question.