Experiments on highly supercritical thermal convection in a rapidly rotating hemispherical shell

We report experimental results on highly supercritical thermal convection in a rapidly rotating hemispherical shell with parabolic gravity. Using silicone oil as the working fluid and an Ekman number Ek = 4.7 10^-6 we reach Rayleigh numbers up to 1.2 , 10 ^10, over 600 times critical. In-situ temperature measurements show that, at these highly supercritical states where convective heat transfer becomes dominant, the time-averaged temperature in the fluid becomes nearly uniform except in a thin thermal boundary layer near the inner spherical boundary. Heat transfer measurements show that Nusselt number Nu increases with Rayleigh number Ra as Nu Ra^0.4. The measured amplitudes of temperature fluctuations scale well with a model of geostrophic convective turbulence. We also examine convection in a two-layer fluid in the same geometry, using layers of water and silicone oil to produce a stable density stratification. We determine the dependence of heat transfer on the thickness ratio of the layers.