Two regimes of flux scaling in axially homogeneous turbulent convection in vertical tube

Abstract

From experiments of axially homogeneous turbulent convection in a vertical tube using heat (Prandtl number Pr≃6) and brine (Pr≃600) we show that at sufficiently high Rayleigh numbers (Rag), the Nusselt number Nug∼(RagPr)1/2, which corresponds to the so-called ultimate regime scaling. In heat experiments below certain Rag, however, there is transition to a new regime, Nug∼(RagPr)0.3. This transition also seems to exist in earlier reported data for Pr=1 and Pr≃600, at different Rag. However, the transition occurs at a single Grashof number, Grgc≃1.6×105, and unified flux scalings for Pr≥1, Nug/Pr∼Grg0.3, and Nug/Pr∼Grg1/2 can be given for the two regimes.