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As shown in earlier work (Ahlers et al., J. Fluid Mech. 569, p.409 (2006)),non-Oberbeck Boussinesq (NOB) corrections to the center temperature inturbulent Rayleigh-Benard convection in water and also in glycerol are governedby the temperature dependences of the kinematic viscosity and the thermaldiffusion coefficient. If the working fluid is ethane close to the criticalpoint the origin of non-Oberbeck-Boussinesq corrections is very different, aswill be shown in the present paper. Namely, the main origin of NOB correctionsthen lies in the strong temperature dependence of the isobaric thermalexpansion coefficient \beta(T). More precisely, it is the nonlinearT-dependence of the density \rho(T) in the buoyancy force which causes anothertype of NOB effect. We demonstrate that through a combination of experimental,numerical, and theoretical work, the latter in the framework of the extendedPrandtl-Blasius boundary layer theory developed in Ahlers et al., J. FluidMech. 569, p.409 (2006). The latter comes to its limits, if the temperaturedependence of the thermal expension coefficient \beta(T) is significant.

Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point