Hi @azmelfadzil, hope you are well.
So the first thing I noticed when looking at ‘convective heat transfer 1 - run 2’ was the convergence:
This really tells us that convergence hasn’t been met.
I would take these steps to improve this:
make the mesh finner in thin channels:
2 cells in the thin channel aren’t fine enough, you could just inflate boundary layer cells here.
reduce under relaxation values in the numerics, this reduces instability but makes convergence take longer.
Set the initial temperature in initial conditions to be the inlet temperature, if you expect 519.25k there is no need to make the solver do extra work.
add a result control like area averages to the two little plates and monitor their convergence for velocity and temperature, this gives us a good insight as to what is happening within the solution whilst it is still solving
Hope this helps,