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Rayleigh-Benard convection in binary-gas mixtures: Thermophysical properties and the onset of convection

Summary: Rayleigh-BeŽnard convection in binary-gas mixtures: Thermophysical properties
and the onset of convection
Jun Liu and Guenter Ahlers
Department of Physics and Center for Nonlinear Science, University of California, Santa Barbara, California 93106
Received 3 February 1997
We present an experimental investigation of Rayleigh-BeŽnard convection in binary-gas mixtures. In order to
interpret the results quantitatively, we determined the necessary thermodynamic and transport properties for six
mixtures He-CO2, He-SF6, He-Xe, Ne-Ar, Ar-CO2, and H2-Xe by a combination of data from the literature,
molecular-theory calculations, and thermal-conductivity measurements. All six mixtures have positive separa-
tion ratios . The Lewis number L the ratio of the mass to the thermal diffusivity is of O(1), in contrast to
liquid mixtures where L O(10 2
). An important feature of the gas mixtures is that their Prandtl number the
ratio of the kinematic viscosity to the thermal diffusivity can be lower than those of the two pure components.
We discuss the physical reason for this and show that the minimum Prandtl number reached by using binary-
gas mixtures is about 0.16. The critical temperature difference Tc for the onset of convection is determined
from measurements of the Nusselt number N the effective thermal conductivity and from the contrast of
shadowgraph images as a function of T. The results agree well with the prediction of linear stability analysis.
In contrast to convection in binary-liquid mixtures with 0, N for the gas mixtures increases significantly
with T/ Tc 1 as soon as the convection starts at the Soret onset and is qualitatively similar to the
Nusselt number of pure fluids. However, the critical Rayleigh number Rc is lower than the value Rc0 1708 of


Source: Ahlers, Guenter - Department of Physics, University of California at Santa Barbara


Collections: Physics