[alireza_ark]

I am trying to simulate natural convection between two plates. The problem is 2D, its geometry and model are shown in figure 1.

1-Do you agree with this flow pattern in figure 1?
figure 1:

2-Although it is steady problem, I am using transient solver. Figure 2 shows mass average versus time. It seems the problem has reached it steady state solution between times 60 and 140. But suddenly it becomes unstable. Do you know what might have been wrong?
Natural Convection Unstable

figure 2: