Non-Boussinesq effects on heat transport in turbulent Rayleigh-B\'enard convection of gases*

In turbulent Rayleigh-B\'enard convection large temperature differences often are used in order to reach very high Rayleigh numbers. This can lead to a breakdown of the Boussinesq approximation which assumes temperature-independent fluid properties. We presents quantitative measurements of non-Boussinesq (NB) effects on the heat transport obtained by using Ethane gas at a mean temperature of 40$^\circ$C and pressures up to 40 bars. At the largest temperature differences of about 40$^\circ$C, where the top of the sample approached the vapor-liquid saturation curve, the Nusselt number was {\bf increased} above the Boussinesq value by several percent. This contrasts with NB effects in liquids\footnote{G. Ahlers, E. Brown, D. Funfschilling, S. Grossmann, and D. Lohse, J. Fluid Mech., submitted.} where the heat transport is reduced below the Boussinesq value.