Transitions in heat transport by turbulent convection at Rayleigh numbers up to 1015

Abstract

We describe a pressure vessel for conducting experiments in helium (He), air, nitrogen (N₂) or sulfur hexafluoride (SF₆) under pressures of up to 19 bars, and facilities for the study of Rayleigh–Bénard convection inside this pressure vessel. The convection cells, known as the high pressure convection facilities (HPCFs), can have interior heights up to L=2.3 m and diameters up to D=1.2 m. Measurements of the Nusselt number Nu for Rayleigh numbers Ra up to Ra^*=4×10¹³ and a Prandtl number Pr0.8 gave Nu∝Ra^γ_eff with γ_eff0.308. At Ra^* there was a sharp transition to a new regime. The Nusselt number was continuous at Ra^*, but the exponent characterizing its dependence on Ra changed suddenly to γ_eff=0.25. Near Ra=Ra^**3×10¹⁴, there was a further change in the Ra-dependence of Nu. A new state with γ_eff0.17 evolved and there was bistability of the γ_eff=0.25 and the γ_eff=0.17 branches.