A lobe pump is a PD pump.

For each turn a specific volume is displaced and "send forward".

The principle of the lobe pump is equal to the principle of the externel gear pump. The lobes however, are considerably larger than the teeth from a gearpump. Because of this difference in size, each rotor consist of 2 or more lobes. One of these rotors is directly driven, while the other rotates with it.

As the rotors rotate, liquid is trapped between two lobes of each rotor and the walls of the pump chamber and carried from the inlet side, to the outlet side. As liquid leaves the suction chamber, the pressure in the suction chamber is lowered, and additional liquid is sucked into the chamber.

The lobes are constructed in this way creating a continuous seal at the points where they meet at the center of the pump. Some lobes are fitted with small vanes at the outer edge to improve the seal of the pump.

So some dependency on fluid viscosity might exist - but i think it will only matter for non newtonian liquids and also depend on the actual pump design.

Best regards

Morten

Viscosity affects slip diferently with viscous (laminar) flow and in a turbulent flow régime, and thus the capacity of the pump.

For an in-depth treatment of the subject, I recommend Chapter 3 (Rotary pumps) on the Pump Handbook by Karassik et al. (McGraw-Hill).
 
To MortenA, I was under the impression that unlike gear pumps, lobe pumps, as described by you, need timing gears because one rotor cannot drive the other.

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It seems like I got them mixed up. I agree with you that in the typical lobe pump both lobes are powered (and not touching - while in the gear pump its common that one of the gears may be "unpowered".

Best regrads

Morten

Agreed.  A measurement of PD pump volumetric flow rate efficiency is actually a measurement on slip rates of the pump, which will vary depending on the size and configuration of the downstream pipe network.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee