Behaviour of a Ternary Mixture with the Net Separation Ratio Close to Zero

The behaviour of the ternary hydrocarbon mixture of toluene-methanol-cyclohexane in mass proportion 0.62/0.31/0.07 is studied. The direct numerical simulation is carried out for the two-dimensional closed cavity heated from above. The mixture is supposed to be in the gravity field. The behavior of the mixture is predominantly caused by the Soret effect (thermal diffusion). An important parameter, responsible for the Soret effect, is the net separation ratio, Ψ. For the mixture under consideration, the two sets of values of Ψ are known. In both cases, the values of Ψ turn out to be close to zero, but in one case Ψ is slightly positive, and it is slightly negative in the second one. This results in qualitative difference in the evolution of the mixture under consideration. If Ψ > 0, the mixture comes to the mechanical equilibrium. If Ψ < 0, we observe the system to be at the border of stability, under small perturbation there is no instability, but for the perturbation amplitude higher than a threshold value of the convective single-vortex flow arises.