Abstract

Convective instabilities in side-heated infinite vertical slots containing a single fluid are stationary, shear driven when the Prandtl number Pr 12.5 while they are oscillatory, buoyancy dominated with Pr 12.5 due to the diminished influence of the thermal diffusivity with increasing Pr. Here we examine the influence of the concentration field generated by thermodiffusion in a binary mixture of otherwise uniform concentration on this phenomenon in a thermogravitational column. While positive/negative separation corresponds to enhanced/diminished buoyancy and should promote instability/stability, the induced positive/negative vertical concentration gradient of the light component, i.e., stable/unstable stratification, and the nonmonotonic horizontal species separation demanded by the vanishing vertical mass flux, combine to result in the opposite effect. Thus increasing/decreasing the separation ratio ε is found to stabilize/destabilize both instability branches so that the cut-off Pr where there is a switch from preferred stationary to oscillatory states is a monotonic increasing function of ε.