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

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beanflying:
Centrifugal Pump impellers won't 'pump' air worth a damm they are nothing like a fan (for a gas or air) and are in most cases also not a propeller (axial flow case mentioned earlier is an exception).

In this case of the OP's topic AIR is not some sort of analogue for a non compressible fluid. There is a reason wind tunnels are not used to test Pump impellers.

This is a reasonable description too.


--- Quote ---A centrifugal pump works by the conversion of the rotational kinetic energy, typically from an electric motor or turbine, to an increased static fluid pressure. This action is described by Bernoulli's principle. The rotation of the pump impeller imparts kinetic energy to the fluid as it is drawn in from the impeller eye (center) and is forced outward through the impeller vanes to the periphery. As the fluid exits the impeller, the fluid kinetic energy (velocity) is then converted to (static) pressure due to the change in area the fluid experiences in the volute section.
--- End quote ---

Berni:
Most vacuum cleaners use it to pump air just fine, here is what one of those looks like:
https://www.aliexpress.com/item/4000006766726.html

The thing is that air is much lighter than water, so the centrifugal force experienced by the air being spun around in there is a lot smaller, so the static pressure produced is also a lot lower. The workaround for this is to simply spin it a lot faster (especially since the force goes up with the square of the RPM) to increase this force to a point where it does start producing a significant amount of pressure. This is the main reason why vacuum cleaners are so darn loud, they use series would brushed motors to spin those blades at ridiculous speeds. It is also why a centrifugal water pump will basically stop working as soon as air gets in there, they spin fairly slow since that's what is efficient for pumping water, but is way too slow for air (not that the pump would survive running that fast)

They do work in reverse too. If you force air into it it will start spinning and producing mechanical power. This is exactly what the exhaust side of a turbocharger does.

beanflying:
Just because it looks sort of similar doesn't mean it behaves the same. Put that same impeller at the same speed in water and then air and they will behave NOTHING even close to each other for characteristics as the medium passing over it is NOTHING like the same.

You seem to be fighting to make a case against basic Fluid Mechanics by adding additional requirements (speed) to try and prove air is the same as a non compressible fluid. This is simply not the case.

Berni:
I am not trying to say that they are the same.

What i am saying is that air does act as a low viscosity fluid when it is not exposed to any significant pressure changes that can compress it. So a lot of predomenently air moving designs that just push air along and not compress it will act much like moving a liquid. Obviously air and water do have different properties hence why propeller blades designed for one or the other have a different shape and speed due to being optimized for the particular application.

The thing with centrifugal rotors is that they almost exclusively rely on the density of the medium to impart force on it, so that's the parameter that matters the most. Doubling the density would also rise the produced pressure by roughly double.

However centrifugal rotors are usually used because they can produce more static pressure than a fan blade. This means in a lot of cases the air will get inevetably exposed to large pressure differences that cause it to compress and expand, making it NOT act like a fluid.

Not that testing centrifugal rotors in a wind tunnel is very useful since the housing shape around the rotor is just as important as the rotor shape itself, so where the action happens is hidden. Since the blades are not operating as aerofoils there won't really be much interesting to see anyway. Most of the usefulness of an air tunnel test would be to optimize the shape of the collection chamber around the rotor to direct the flow to the output as smoothly as possible.

beanflying:
You are starting with a narrative you require to be true then manipulating select variables to try and fit that narrative and it simply isn't the case as far as Fluid Mechanics works.

You are also confusing 'density' in relation to Air as it is a compressible fluid and as such varies with pressure making it fundamentally different to Water which does not. All of this is before we get anywhere near laminar or turbulent flow differences between Air and Water due to flow speeds entering in and exiting an impeller.

It is simply a bad comparison.

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