solution diverging while using UDF to define porosity
 

Hi
I am trying to simulate a flow through a shell and tube heat exchanger with radial inlet and outlet for the primary(hot fluid) and due to large size of the exchanger i am assuming the total volume to be porous due to large number of tubes.

The problem which i am facing now is when i am simulating without using the UDF (like a simple pipe with radial inlet) the solution is converging, once i introduce the porous domain via UDF there is a drastic divergence in the range of from e-2 to e2.

Please provide your guidance on what could be the parameter affecting convergence

the BC are velocity inlet and pressure outlet and the geometry is like as given in the dagram.