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Why is the curvature of pump rotor blades "backwards"?

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NiHaoMike:
I'm designing a rotor for a small centrifugal oil pump that is to be 3D printed, to circulate mineral oil for cooling electronics. (Before anyone says to just buy one, I haven't found one of the right specs that's anywhere near a reasonable price. It and the rest of the project is also a learning experience.)

When it comes to designing the rotor blades, I noticed that most centrifugal pump rotors seem to have the blades curving "opposite" to what I would expect, in that the convex side is driven into the liquid. I would have expected the concave side to be "pushing" so that it would act as a wedge with a changing angle, initially a shallow angle giving more force to get it moving and then steeper angles to continue accelerating the fluid that is already set in motion by the shallow part. Indeed, most fans are built that way. Or looking at it a different way, most airplane wings are concave on the bottom to accelerate the air downwards.

Thinking it might be some nonintuitive effect of incompressible liquids as opposed to compressible gases, taking a look at boat propellers seems to disprove that since the concave side is pushing, same as for fan blades.

I'm sure there's a good reason why centrifugal pumps use the "backwards" curve, what is it?

bdunham7:
Does the oil accelerate as it reaches the outside of the pump or does it slow down?  :)

Berni:
If you look wide enough you can find both directions, some bend forward some bend backward, or some are just straight. It's an optimization for the job the centrifugal blades are designed for.

The logic behind the backward bent blades is that the pumped medium has to slow back down from the rotation inside the rotor. This is most easily facilitated by the blades bending backwards since it provides a nice gradual transition from the high speed center to the slowly moving outside collection chamber. Eventually the medium has to go into the output flange and that is moving at the output flow rate, no faster, no slower. So such a rotor doesn't waste mechanical energy spinning the medium where it doesn't want to spin anyway.

So then why are not all of them backwards if its so nice and efficient? Well it turns out that if what you want is high flow rate at low pressure (like a lot of air moving fans) the forward bent blades are better. They act as scoops to grab as much air as possible, get it up to speed quickly as possible and fling it out like a shovel. This lets the same size rotor create more flow rate.

As for straight blades, it might be that they are designed to spin in both directions. Can also help if the medium has a lot of solid particles since they have a harder time hitting straight blades. Perhaps they actually needed some compromise in between spot between forward or backward bent designs. Or they didn't bother to optimize it, the flat design was easiest to manufacture and worked well enough for the job.

beanflying:
What type of Oil are you planning to pump? Apart from maybe mineral types of weights seriously look at a vane type of impeller over a centrifugal one in particular as it is likely small. You will likely find the vanes print better than the complex curves.

Commercially look at the small weed or ag type spray diaphragm pumps or the Jabsco and Johnson ranges of flexible impeller pumps.

Or if you have a known flow, pressure and oil type (or viscosity will do) throw it here and I will see what suits it what I did when I had a real job.

armandine2:
if taken to the extreme the opposite curvature would not release the liquid - the centrifugal pump isn't a positive displacement device, it accelerates the liquid. Look at the relative linear speeds of points along the curvature.

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