From experiments of axially homogeneous turbulent convection in a vertical tube using heat (Prandtl number $\text{Pr}\simeq 6$) and brine ($\text{Pr}\simeq 600$) we show that at sufficiently high Rayleigh numbers (${\text{Ra}}_g$), the Nusselt number ${\text{Nu}}_g\sim {\left({\text{Ra}}_g\text{Pr}\right)}^{1/2}$, which corresponds to the so-called ultimate regime scaling. In heat experiments below certain ${\text{Ra}}_g$, however, there is transition to a new regime, ${\text{Nu}}_g\sim {\left({\text{Ra}}_g\text{Pr}\right)}^{0.3}$. This transition also seems to exist in earlier reported data for $\text{Pr}=1$ and $\text{Pr}\simeq 600$, at different ${\text{Ra}}_g$. However, the transition occurs at a single Grashof number, ${\text{Gr}}_{gc}\simeq 1.6\times {10}^5$, and unified flux scalings for $\text{Pr}\ge 1$, ${\text{Nu}}_g/\text{Pr}\sim {\text{Gr}}_g^{0.3}$, and ${\text{Nu}}_g/\text{Pr}\sim {\text{Gr}}_g^{1/2}$ can be given for the two regimes.

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• Convection