Rotating turbulent thermal convection at very large Rayleigh numbers

We report on turbulent thermal convection experiments in a rotating cylinder with a diameter () to height () aspect ratio of. Using nitrogen and pressurised sulphur hexafluoride we cover Rayleigh numbers (Ra) from to at Prandtl numbers. For these Ra we measure the global vertical heat flux (i.e. the Nusselt number - Nu), as well as statistical quantities of local temperature measurements, as a function of the rotation rate, i.e. the inverse Rossby number - 1/Ro. In contrast to measurements in fluids with a higher Pr we do not find a heat transport enhancement, but only a decrease of Nu with increasing 1/Ro. When normalised with Nu(0) for the non-rotating system, data for all different Ra collapse and, for sufficiently large 1/Ro, follow a power law. Furthermore, we find that both the heat transport and the temperature field qualitatively change at rotation rates and. We interpret the first transition at as change from a large-scale circulation roll to the recently discovered boundary zonal flow (BZF). The second transition at rotation rate is not associated with a change of the flow morphology, but is rather the rotation rate for which the BZF is at its maximum. For faster rotation the vertical transport of warm and cold fluid near the sidewall within the BZF decreases and hence so does Nu.