Author Topic: Why is the curvature of pump rotor blades "backwards"?  (Read 2509 times)

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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #25 on: October 15, 2021, 01:46:03 am »
Anyway, no thoughts, about a 'downward directed' air stream, is that real ?

Yes, because you cannot defy the laws of physics.

You can think about this by choosing a frame of reference. You could have a wing moving through the (still) air, or you could have a stationary wing (in a wind tunnel) and blow a stream of air over it. To understand wings, it is easier to use the wind tunnel example.

If the wing in the wind tunnel experiences an upward force, then it of necessity must be "pushing down" against something--Newton's law of action and reaction.

The only thing the wing can "push down" against is the air stream, but air will give way if you push on it. So what must happen is that the air receives a downward acceleration from the wing. To accelerate something downwards you have to push on it, which is exactly what the wing does.

This principle becomes very clear if you stand beneath a helicopter (which is an airplane with rotating wings). You will get blasted by downdraft. This is exactly the same downdraft produced by a fixed wing aircraft moving through the air. Conversely, it would be impossible for a helicopter to lift off the ground if there were no downdraft.

So what about pressure differences, you may ask? What about lower pressure above the wing and higher pressure below the wing? This is true too, but it is simply the other side of the exact same coin. Mathematically, the results are equal, but the formulas are different.
« Last Edit: October 15, 2021, 01:50:35 am by IanB »
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Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #26 on: October 15, 2021, 02:13:19 am »
You also cant really include discussions or a lot of the analogies on compressible fluids and gases in a topic about incompressible ones ;)
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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #27 on: October 15, 2021, 05:17:45 am »
You also cant really include discussions or a lot of the analogies on compressible fluids and gases in a topic about incompressible ones ;)

Even so, the subsonic flow of air over wings, or through simple fans and blowers, behaves in almost the same way as incompressible fluids. As long as the overall change in pressure through the flow path is not that much (say +/- 10%), then the same rules can be applied with minimal error.
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Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #28 on: October 15, 2021, 05:40:30 am »
 :o err NO. Reynolds numbers and if we get into smaller impellers then low Reynolds number issues on top of that are not 'minimal'.
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Offline Berni

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #29 on: October 15, 2021, 05:55:45 am »
Centrifugal pumps don't really use the effect of a areofoil aerodynamic lift, the curve is just there to help shape the flow, not to actually create the pumping action. It's propellers that use this effect, those are basically a wing that is actively forced trough the air. Even then you don't actually need to use wing curvature to make a propeller work, they work just fine with flat straight blades that are turned in the appropriate angle. The wing aerofoil shape just is a more efficient mechanism because it produces less drag, so your propeller can move more air for the same mechanical power input.

And yes air can be thought of as a really low viscosity fluid in a lot of cases. Most fans used to just move air around do not produce enough pressure to compress or stretch air in any meaningful amount. But once you move over to things like vacuum cleaners or turbochargers that are actually designed to produce a good deal of pressure then yes air is no longer just a simple incompressible fluid.
 

Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #30 on: October 15, 2021, 06:24:14 am »
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.
« Last Edit: October 15, 2021, 06:26:24 am by beanflying »
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Offline Berni

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #31 on: October 15, 2021, 06:56:10 am »
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.
 

Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #32 on: October 15, 2021, 07:09:56 am »
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.
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Offline Berni

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #33 on: October 15, 2021, 07:42:39 am »
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.
 

Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #34 on: October 15, 2021, 08:49:25 am »
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.
« Last Edit: October 15, 2021, 08:51:24 am by beanflying »
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Offline Berni

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #35 on: October 15, 2021, 11:51:58 am »
Yes that is my point that in the special case of having fairly constant pressure everywhere air can act very similar to a fluid. Otherwise do have to treat it as a compressible gas rather than just a fluid. If you move supersonic trough air then you once again need to treat it differently, a wing will act very differently past the speed of sound. Then with water if you move it vigorously enough it will cavitate and again no longer act like a regular fluid.

Lots of models have special edge cases like this, but this does not make them completely wrong or useless, they just need to be used within the limits.

As for density for that i mean the 1.2kg/m3 that the air weighs at sea level versus the 1000kg/m3 density of water. This has nothing to do with being compressible. Plain simple something heavier needs more centripetal force to go around at the same speed, that force over an area is pressure and this is what a centrifugal pump is producing. This is why a centrifugal fan can't pump water and a centrifugal pump can't pump air.
 

Offline RJHayward

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #36 on: October 15, 2021, 06:17:37 pm »
Uhh: Ianb, thanks, let's see if I can repeat your thoughts, that an upwards 'hump' is also going to cause air deflection away, which is up. SO, that looks like the case like you said, that BOTH surfaces actually send an air deflection, away from wing volume, so upside down has similar action.
  BUT, and here is the struggle, I still am perceiving a mention of the principal, if you throw an 'anvil', wearing dollar skates, you move, opposite direction, (due to conservation of momentum).
Do that just means, the upper hump shape will get some DOWNWARD FORCE, a little bit. Why can't I say ' You would crash...downward force could exceed the upward force (supposedly caused by reaction to sending some air downward).
Difficult for understanding.
 

Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #37 on: October 15, 2021, 11:24:34 pm »
Yes that is my point that in the special case of having fairly constant pressure everywhere air can act very similar to a fluid. Otherwise do have to treat it as a compressible gas rather than just a fluid. If you move supersonic trough air then you once again need to treat it differently, a wing will act very differently past the speed of sound. Then with water if you move it vigorously enough it will cavitate and again no longer act like a regular fluid.

Lots of models have special edge cases like this, but this does not make them completely wrong or useless, they just need to be used within the limits.

As for density for that i mean the 1.2kg/m3 that the air weighs at sea level versus the 1000kg/m3 density of water. This has nothing to do with being compressible. Plain simple something heavier needs more centripetal force to go around at the same speed, that force over an area is pressure and this is what a centrifugal pump is producing. This is why a centrifugal fan can't pump water and a centrifugal pump can't pump air.

Stop claiming 'special case' and just use Water or whatever fluid is required and don't muddy the WATER further and drag in supersonic flow. Air is a compressible GAS period full stop. It is NOTHING like an incompressible liquid!
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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #38 on: October 15, 2021, 11:37:36 pm »
Uhh: Ianb, thanks, let's see if I can repeat your thoughts, that an upwards 'hump' is also going to cause air deflection away, which is up. SO, that looks like the case like you said, that BOTH surfaces actually send an air deflection, away from wing volume, so upside down has similar action.
  BUT, and here is the struggle, I still am perceiving a mention of the principal, if you throw an 'anvil', wearing dollar skates, you move, opposite direction, (due to conservation of momentum).
Do that just means, the upper hump shape will get some DOWNWARD FORCE, a little bit. Why can't I say ' You would crash...downward force could exceed the upward force (supposedly caused by reaction to sending some air downward).
Difficult for understanding.

Just pick the important points out of what you said here. If you throw an anvil away from you while wearing skates, you move in the opposite direction, due to conservation of momentum.

Now apply this same thought to a wing on an airplane. The wing "throws" the air downwards, which tries to move the wing upwards by conservation of momentum. Same principle. Now the actual air flow over the wing is complicated, it curves this way and that on its journey, but at the very end, it is moving downwards to some extent.

In reality, aeronautical engineers use advanced computer design tools with CFD and such to design wings. Modern aircraft need to operate with very high efficiency and wing design is a major part of that. But still, the same basic principle of conservation of momentum is always present.
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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #39 on: October 15, 2021, 11:39:30 pm »
Stop claiming 'special case' and just use Water or whatever fluid is required and don't muddy the WATER further and drag in supersonic flow. Air is a compressible GAS period full stop. It is NOTHING like an incompressible liquid!

Berni has been correct in the posts above. You might try stopping to think about it instead of arguing so much?
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Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #40 on: October 16, 2021, 12:04:28 am »
Berni and you if you like initially are fundamentally WRONG. Bending or worse moving the rules to suit a narrative is not Science and or Engineering.

And if disagreeing on major FLAWED technical points or claiming 'special case' raised is arguing then you might want to try some basic reading here.

https://www.mvorganizing.org/what-is-compressible-and-incompressible-flow/

https://www.chegg.com/homework-help/definitions/incompressible-and-compressible-flow-5
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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #41 on: October 16, 2021, 02:01:50 am »
:o err NO. Reynolds numbers and if we get into smaller impellers then low Reynolds number issues on top of that are not 'minimal'.

Here, in fact, you answered your own question, and is why the answer is "yes".

The Reynolds number is a dimensionless group, which allows us to apply the principle of similarity between systems that vary in scale and other parameters.

If the Reynolds number, and other relevant dimensionless quantities are equal, then two apparently different systems will behave similarly, and the same performance characteristics will apply.

Under the principle of similarity, air and water can be treated with the same equations when the variation in air pressure is small, such as is the case with a typical cooling fan.
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Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #42 on: October 16, 2021, 02:43:38 am »
Reynolds numbers are dimensionless but when you throw AIR at around 800-1000 less density and viscosities at circa 100 times different at the calculations compared to water it becomes clear that water and air are nothing alike. Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.

That is UNLESS you make up  :bullshit: constraints and 'special case' rubbish such as you are continuing to do.
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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #43 on: October 16, 2021, 03:31:10 am »
Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.

If we do the numbers it is about a 7% change in density. It is always good to do the numbers.
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Offline RJHayward

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #44 on: October 17, 2021, 05:35:30 am »
beanflying: I get your point, about compressible fluids / gases, maybe off OP topic, sorry (I'm at about 1/2 year basic physics expertise).

Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
  Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.
Note the picture features wing totally flat, on bottom, no super curvey.
 

Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #45 on: October 17, 2021, 06:29:21 am »
beanflying: I get your point, about compressible fluids / gases, maybe off OP topic, sorry (I'm at about 1/2 year basic physics expertise).

Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
  Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.
Note the picture features wing totally flat, on bottom, no super curvey.

Discussing wings and Aerodynamics is way off topic so start a topic maybe in the Mech Eng. section? Meanwhile add this to your reading it is sort of the R/C bible for low (non full sized or high speed) Reynolds number data and research :) https://m-selig.ae.illinois.edu/uiuc_lsat/Airfoils-at-Low-Speeds.pdf or the source page here https://m-selig.ae.illinois.edu/

Also this bootleg copy of Martin's book is well worth a read for basic concepts of Aerodynamics. https://archive.org/details/ModelAircraftAerodynamics/mode/2up
« Last Edit: October 17, 2021, 10:07:34 am by beanflying »
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Online beanflying

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #46 on: October 17, 2021, 07:43:46 am »
Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.

If we do the numbers it is about a 7% change in density. It is always good to do the numbers.

You might want to check your maths unless you are working with air at 200+C and even then it is +-7% at around 20C it is +-12 ish or around 25% https://www.omnicalculator.com/physics/air-density

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Offline IanB

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Re: Why is the curvature of pump rotor blades "backwards"?
« Reply #47 on: October 17, 2021, 09:18:39 am »
Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
  Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.

As I explained above, it is two sides of the same coin. When you do engineering, there are sometimes different ways of calculating the answer to a problem. Just because a textbook shows one way, it doesn't mean other ways do not also exist.

If you have doubts, keep thinking about a helicopter. Do you think it would ever be possible for a helicopter to lift off the ground without a downdraft? Have you ever been near to a helicopter that was landing or taking off? It produces an absolute hurricane, which is the only way such a heavy craft could get off the ground.

Quote
Note the picture features wing totally flat, on bottom, no super curvey.

Don't be misled by pictures. Anyone can draw a picture of anything. What counts is what is observed in real world experiments. Be in no doubt, if a wing is generating lift, then air is being deflected downwards.
« Last Edit: October 17, 2021, 09:23:25 am by IanB »
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