  Experimental and numerical data for the heat transfer as a function of the
Rayleigh-, Prandtl-, and Rossby numbers in turbulent rotating Rayleigh-Benard
convection are presented. For relatively small $Ra ~ 10^8$ and large Pr modest
rotation can enhance the heat transfer by up to 30%. At larger Ra there is less
heat-transfer enhancement, and at small Pr = 0.7. there is no heat-transfer
enhancement at all. We suggest that the small-Pr behavior is due to the
breakdown of the heat-transfer-enhancing Ekman pumping because of larger
thermal diffusion.
