The idea with the circles is that there would be turbulence at every intersection, This pic might show the layout of the channels slightly better
I thought about another design, although lack of a central inlet might hinder it slightly.
This would be the base, channels 1mm apart, 2mm wide, 2mm deep into a 3mm base.
The inlet and outlet would be mounted on top like this
As well as that, another inlet and outlet could be created perpendicular to that, at 3 and 9 o'clock respectivly.
Thoughts on how good a block this would be?