CFD Online Discussion Forums

As I know there are no limits, apart that you set in solve->controls->limits.
However, I repeat, that if you set wrong (non physical) boundary conditions, fluent will probably diverge.
In your case, with your boundary conditions, the pressure at inlet should be near 500000 atm and velocity at the restriction near 10000 m/s.
These values are both not physical.

If you just want to do a test, set a velocity at the restriction between 25-30 m/s, so to have an inlet velocity of about 0,0025 m/s.
You should enable cavitation model also, otherwise you will have negative absolute pressure at the restriction, which, again, is not physical.

I don't know how fluent reports quality of mesh: usually I evaluate different parameters directly in preprocessing.
Low quality meshes can cause not accurate results, difficult in converging and divergence.
As far as I know hexa meshes are better than tetra, however in some cases (complex geometries) tetra mesh is the only choice.