Flow through a concentric annulus

Hi,

I have some problems for the simulation of a simple flow trough a concentric annulus.

I use an axisymmetric model in which only the annular duct is defined (the model is attached here).
Then, for the study of the laminar flow in the channel, I impose as boundary conditions no slip at walls, a non-zero inlet pressure (P_in = 50 kPa) and a zero pressure for the outlet.

I already performed a similar analysis for a flow between parallel plates, obtaining results in agreement with the analitical model of Poiseuille Flow.
Considering the fact that the annular duct is narrow, I would expect results similar to the case of flow between parallel fixed plates, but when I try to solve the model the following error message appears.

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

Although the pressure seems quite high, I have already checked that even for lower values I always have the same problems, even if I increase the max number of iterations.

Can someone help me to understand where is the error in my model?