How to determine if equal distribution is reached
 

Hi,
currently i'm simulating an inlet diffusor for a micro heat exchanger.
The goal is to determine how short it can still be without losing a more or less equal distribution across the face area. In this case, the micro channels were modeled using a 'porous body'.
I was thinking of using the standard deviation of the velocity or of the pressure on the area, but i'm not sure if this really gives me what i want.

I would be happy about any ideas and remarks.
Thanks in advance