Your simplified geometry is not simple enough, there are several faces which are completely un-needed and are only for the finished construction rather than a test case. If you refer to the pictures below you can see where I feel that the features can be simplified.
The box covering the entire center is not needed and can be removed.
Same here as the red highlight is where I would define one of my outlets.
The Inlets and other pipe outlet does not require such a complicated cap, a simple flat closed face with zero thickness to keep the model watertight is sufficient for both ends. Your current geometry has intersections especially at these parts.
Same issue here, you points at the box where the pipe connects has additional features that can be removed. A circular pipe attached the box is fine.
If you keep this simple with no intersections we can start on the simulations.