HIGH RAYLEIGH NUMBER CONVECTION WITH GASEOUS HELIUM AT LOW-TEMPERATURE

This article presents an experimental set-up to study the turbulent re gime of free thermal convection in a Rayleigh-Benard cell. Helium gas around 4 K is confirmed in a cell of aspect ratio 0.5. With a thermoco uple technique we can measure temperature differences across the cell as low as 0.2 mK, which allows to test the adiabatic gradient effect. Covering a large range of Rayleigh numbers (10(3) to 5 x 10(12)), Nuss elt numbers from 1 to 10(3) are obtained. The results are compared wit h previous works. They show a departure from the 2/7 power law above R a = 3 x 10(10).