Reynolds Number in Micro Diffusers-Nozzle

Hi All,

What is the suitable formula to be used to calculate Reynolds number in a micro diffuser-nozzle structure, as the picture attached?

I am using:
Reynolds number = hydraulic diameter*density*velocity/dynamic viscosity

When I go up to Re = 100 I keep getting an error telling me:

"Failed to find a solution. Maximum number of Newton iterations reached. Returned solution is not converged."

How can I solve that?

Thanks.
Marwan