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Riddle me this : we are driving an external gear pump (this kind) at a fixed RPM. At start, when the oil temperature is low (and viscosity high), the volumetric flow is much less than when the oil starts to warm up. For exemple, we have a ratio of around 5 in volumetric flow when the oil goes from 10°C to 80°C.

Every pump expert we've talked about has told us that this should be the other way around (and definitely not in that proportion, more like 5-10%) because the slip of a less viscous fluid will degrade the volumetric efficiency of the pump.

Nevertheless, we ran the tests and even made some volume/time measurement (to remove a flowmeter error from the equation) and they are consistent...

Also, the circuit is super simple (tank-pump-restriction-back to tank) and there's really nothing that could explain that...

Thanks for your feedback!

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    $\begingroup$ My instinct is that the "restriction" in 'tank-pump-restriction-back to tank' is the crucial thing here. Tell us more about that restriction. $\endgroup$ Nov 12 '20 at 11:36
  • $\begingroup$ It's only oil sprayers (calibrated holes). Nothing fancy :-) Also, the volumetric flow doesn't only go up, the pressure also goes down! One or the other would already be counter-intuitive but now we have both at the same time... $\endgroup$ Nov 13 '20 at 6:30
  • $\begingroup$ Hmm... What I had in mind was that the decreased viscosity might reduce the head loss in the restriction, which would reduce the head the pump would be required to add to the fluid, which in turn would increase the flow rate. The pressure reduction you observe is evidence in support of this view. On the other hand, your description of the oil sprayer sounds like it might consist of sharp-edged orifices, and they have a head loss that's almost independent of viscosity. Maybe it's the head loss in the pipes? If the flow is laminar, that would be viscosity-dependent. $\endgroup$ Nov 13 '20 at 10:53
  • $\begingroup$ Hi Daniel, Your comment makes sense indeed. That's actually the only physical (part of) explanation I can think of. But at the same time, all of this takes place at a fixed RPM of the volumetric pump so that still leaves the volumetric efficiency swinging from, say 20% to 90% or so... At the pressures involved (2-8bars) for a pump able to go to 400bars, I'm surprised to say the least. $\endgroup$ Nov 14 '20 at 14:10
  • $\begingroup$ Another thing to look at would be if there is a pressure relieving regulator in the circuit and how close you are operating to the upper pressure. If the pressure was increased because of increased viscous line loss, then oil could be diverted from the outlet back to the intake to protect the pump. This is a very common configuration and many pumps have internal reliefs. $\endgroup$
    – ericnutsch
    Nov 19 '20 at 2:38

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