Convective instabilities in a closed vertical cylinder heated from below. Part 2. Binary gas mixtures

Abstract

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 numbersNiRafor 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 criticalthermalRayleigh 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) fromNRa= 713 to 780, where a new mode withT0= 15 s sets in. The frequency of these slower temperature oscillations can be fitted by an equation of the formf2=k’(NRa−N0Ra) wherek’ andN0Raare constants, which supports the contention that these oscillations are the result of vertical vorticity. For SiCl4: H2the 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 ofNRα.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 atNRa's considerably belowNoscRα.The unique behaviour of these binary fluids is tentatively assigned to thermal diffusion.