CONVECTIVE INSTABILITIES IN A CLOSED VERTICAL CYLINDER HEATED FROM BELOW .2. BINARY GAS-MIXTURES

Non-reactive binary gas mixtures (Xe + He, SiCl4 + H2) have been investigated for convective instabilities in closed vertical cylinders with conductive walls heated from below. Critical Rayleigh numbers NiRa for the onset of various convective modes (including the onset of marginally stable and periodic flow) have been determined with a high resolution differential temperature sensing method. It is found that the second component can significantly alter the hydrodynamic state of the fluid compared to the monocomponent behaviour. Considerably lower critical thermal Rayleigh numbers for steady and time dependent convective modes are observed. The Xe : He system shows stable oscillatory modes similar to those observed in monocomponent gases (periodic disturbances of the mean flow, T0 ≈ 5 s) from NRa = 713 to 780, where a new mode with T0 = 15 s sets in. The frequency of these slower temperature oscillations can be fitted by an equation of the form f2 = k’(NRa − N0Ra) where k’ and N0Ra are constants, which supports the contention that these oscillations are the result of vertical vorticity. For SiCl4 : H2 the high frequency oscillations occur only as a transient mode eventually evolving into the low frequency mode characteristic of binary gas mixtures. This low frequency state is degenerate with a stable time-independent state over a considerable range of NRα. Finite amplitude perturbations can lead to (1) transient oscillatory phenomena accompanied by reorientations of the roll cells with mean periods of 3–5 min; and (2) stable oscillatory flow at NRa 's considerably below NoscRα. The unique behaviour of these binary fluids is tentatively assigned to thermal diffusion. © 1979, Cambridge University Press. All rights reserved.