Convective instabilities in thermogravitational columns

Convective instabilities in side heated infinite vertical slots containing a single fluid are stationary, shear driven when the Prandtl number $Pr\tilde {\le }12.5$ while they are oscillatory, buoyancy dominated with $Pr\tilde {\ge }12.5$ 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. 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 nonmonotic horizontal species separation demanded by the vanishing vertical mass flux, combine to result in the opposite effect. Thus increasing/decreasing the separation ratio $\varepsilon $ is found to stabilize/destabilize both instability branches so that the cutoff $Pr$ where there is a switch from preferred stationary to oscillatory states is a monotonic increasing function of $\varepsilon $.