This is a lobe pump for compressing fluids in hydraulics or pneumatics.
It can also be used as a motor, or a flow metering device.
To put this thing to practical use, you would need a top and a bottom, as well as a pair of same-sized gears to connect the lobes to one another and to your drive mechanism.
As usual, 3D printed parts may or may not hold up under pressure, so take adequate safety precautions.
The official home of this thing is over on my projects blog, here: