Mesh Instability

Hello, I am trying to do natural convection via Fluent but I have a really strange problem with meshing. My setup is simple I have 2D geometry and it has 4 walls which left and right wall are adiabatib and top wall is cold wall and bottom wall is hot wall and my temperature difference 10 (303K-293K).My mesh is triangular and it is really good and symmetrical.

My problem is that if I have 12464 element numbers there is no problem it is converged but if I have 12312 element numbers there is oscillation in my residual like in the picture and my Nusselt numbers decrease about 50% from its true value. So you can say 12464 element numbers are good what you want. I have tried many combinations and I found 12464 element numbers with luck so if I set little small element size I had 12925 element size and there will not give great residual and it will has oscillation. Now, I have to do some little change in geometry for aspect ratio but this is a really small changing like just 1 mm increasing the length of opposite walls. So now I did a lot of things for meshing but unfortunately, still there is oscillation in my residual.

I am tried with default Coupled method because in the other methods I can see my Nusselt number decreasing but residuals do not have visible things to understand that some things wrong. So I am solving with Coupled method because I can observe there is a problem or not.

You can see my mesh and you can sure I tried many things like refinement, set about 200.000 element numbers, set about 700 element numbers, quadrilaterals and more. I want to say another strange thing that when mesh has 200.000 element numbers my Nusselt number is about 2.50 and if I have 700 element numbers my Nusselt number is about 2.50 so the deviation is zero. Fluent like obsessed some point and decreases every time that point I can't understand what is problem.

Best Regars,

Alp

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