Jom~al of Low Temperature Physics, VoL 104, Nos. 1/2, 1996 High Rayleigh Number Convection with Gaseous Helium at Low Temperature X. Chavanne, F. Chill~,* B. Chabaud, B. Castaing, J. Chaussy, and B. H~bral Centre de Recherches sur les Tr~s Basses Tempdratures, Laboratoire associd ?t l'UniversitO Joseph Fourier, CNRS, BP 166, 38042 Grenoble-Cddex 9, France (Received January 29, 1996; revised April 24, 1996) This article presents an experimental set-up to study the turbulent regime of free thermal convection in a Rayleigh-Bdnard cell. Helium gas around 4 K is confined in a cell of aspect ratio 0.5. With a thermocouple 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 s to 5 x 1012), Nusselt numbers from 1 to 103 are obtained. The results are compared with previous works. They show a depar- ture from the 2/7 power law above Ra = 3 x 101~ 1. INTRODUCTION A Rayleigh-B~nard cell consists of a closed volume filled with a one phase fluid. Various shapes and sizes of the cell can be used: parallelepipedic, cylindrical and in general larger than higher. The lateral wall is thermally insulated in order to have most of the heat transfer through the fluid itself. With sufficiently small heating beneath the cell, the fluid is at rest and heat is only transfered by molecular motion. Increasing the heating, the fluid begins to move, first in regular rolls and then its motion becomes more and more chaotic (in the deterministic sense 1) to finally reach a turbulent regime. The study of this evolution implies four different dimensionless quantities: 9 The Nusselt number (Nu) which measures the efficiency of the heat transfer versus the molecular diffusion Qh Nu=-- 2SAT *Present address: l~cole Normale Sup6rieure de Lyon, 46 All6e d'Italie, 69364 Lyon-C+dex 7, France. 109 0022-2291/96/0700-0109509.50/09 1996PlenumPublishing Corporation