Form what I understand the volume flow rate should be tha same at inlet and outlet.
In my model there is 1% percent difference between the two.
What could be the problem?
A 1% error is typically more than acceptable for Engineering applications. As systems get more complex we will typically strive to ensure we are within 5% or less.
What error % were you expecting?
Advection Schemes and mesh refinement are typically things to consider when trying to improve this however due to the fact that this is numerical approximation I would not expect to get 100% accuracy.
Are you sure you have converged the solution? How about running 500 iterations more?
Have you monitored the points and assessed their plots? ie, are they flat?
What is your mass imbalance?