Rotary pumps

Only two rotating parts having principle of gear within gear but without crescent.
The inner rotor is located off-center and both rotors rotate.
The geometry of the two rotors partitions the volume between them into N different dynamically-changing volumes. During the assembly's rotation cycle, each of these volumes changes continuously, so any given volume first increases, and then decreases.
An increase creates a vacuum. This vacuum creates suction, and hence, this part of the cycle is where the intake is located. As a volume decreases compression occurs.
The Flow of liquid is axial through the elements.
In built features of speed reduction as pinion is rotated at prime mover speed and outer gear rotates at lower speed.