Abstract
We study Rayleigh-Benard convection in a cell which consists of a cylinder of 20 cm height and 10 cm diameter filled with gaseous helium between 4 and 5 K. The new feature compared to previous studies comes from a very accurate thermocouple measurement of the temperature difference across the cell ΔT (resolution of 20 µK). The Rayleigh number Ra ranges from the conductive regime to 5 × 1012 by changing the gas density from 3 g/m3 to 30 kgjm3 and by scanning ΔT from 1 mK to 800 mK for each density. On the figure the Nusselt number N u versus Ra is presented. The high sensitivity measurement of low ΔT allows to evidence and measure the adiabatic gradient influence on the convective threshold (in our geometry Ra = 30000 ± 4000 without this influence); the adiabatic gradient slows down the convection for points at low ΔT as seen on the figure.
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