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 (SFO) 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 Pr ∼ 0.8 gave Nu α Raγ eff with γ ≃ 0.308. 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 Yen = 0.25. Near Ra = Ra**≃ 3 × 1014, there was a further change in the γeff-dependence of Nu. A new state with γeff ≃ 0.17 evolved and there was bistability of the γeff = 0.25 and the γeff = 0.17 branches. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.