stevebubs
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| posted on 31/5/06 at 11:16 PM |
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Cavitation Theory
Is cavitation due to reversed water pump impellor more likely to occur at low or high revs?
[Edited on 31/5/06 by stevebubs]
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MkIndy7
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| posted on 1/6/06 at 12:30 AM |
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I'd guess it would be more severe at high speed but would have thought it would occur at any speed as the blades of the pump are likey to be
designed to run in one direction only.
Sounds a bit of a random question you might be better giving a bit more detail.
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stevebubs
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| posted on 1/6/06 at 12:33 AM |
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quote:
Originally posted by MkIndy7
I'd guess it would be more severe at high speed but would have thought it would occur at any speed as the blades of the pump are likey to be
designed to run in one direction only.
Sounds a bit of a random question you might be better giving a bit more detail.
2 litre zetec running pump the wrong way....
Pretty sure my cooling is fine, trying to gather some theory in case of issues...
Do have water pump with reversed impellor that I can fit if need be...
Stephen
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scottc
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| posted on 1/6/06 at 12:47 AM |
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According to Wikipedia..........
"As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid, low pressure
areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become.
As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically
cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades"
Therefore more likely to occur at hight revs (I think).
Scott.
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speed8
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| posted on 1/6/06 at 04:50 AM |
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Not necessarily at high revs. Large ship propellors can suffer severe cavitation due to bad design. These things only run at a one or two hundred rpm.
It can be so bad as to completely destroy the blade and separate it from the shaft.
When I was at Uni we had a test fixture for studying this (bleedin great circular water pipe to accelerate the water around and force cavitation on
propellors to see the effects). Alas, the details of all that studying elude me now but I did have an entire folder of notes on it at the time.
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britishtrident
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| posted on 1/6/06 at 06:04 AM |
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Running a pump the wrong way will cause a shockwave as the water leaves the impeller because the tips of the impeller will be swept back in the wrong
direction to minimise shock. Some pumps have straight radial blades but a shockwave can still occur because the because the design of the casing is
wrong for the rotation of the pump.
In general cavitation will ocur where the design RPM of the impeller is exceeded.
[Edited on 1/6/06 by britishtrident]
[Edited on 1/6/06 by britishtrident]
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Syd Bridge
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| posted on 1/6/06 at 09:17 AM |
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In car pumps, cavitation is mostly caused by over revving. The pump wants to push(throw) more water out than can get in to it, thus creating a semi
vacuum. Derestricting the intake side of the pump can work wonders. That's why most older cars had bigger bottom hoses than tops. Costs too
much these days.
Ships propellors are a different world. Linear speed at the prop surface causes sheer, or shape can cause non linear pressure difference locally.
Or........ A local ship company owner( who I sail with), had problems with two new ships. Naval architects and science boffins suggested the ships
bottom shapes be changed to enhance flow. When riding alongside the ship in a rib, it was obvious the props were dragging in air, even when the ship
was full down. We welded some big flat sheets (ala racecar type diffuser verticals) to the bottom, and problem solved, at a small cost compared to the
boffins suggestions.( Actual shape of the verticals is triangular in X section, for strength. Just for the engineers reading this.  ) Ship speed
went up and fuel consumption went down.
The point of all this, the reasons for a problem can be many, and explore them all. Have a good look at the situation, before applying science. The
solution may be easier and more obvious than first appears.
The question was about water pumps, so forget the above. In the sprint cars I raced twenty years ago, we just ground off half of the vanes. Worked a
treat. Today people use bigger pulleys.
Cheers,
Syd.
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muzchap
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| posted on 1/6/06 at 11:39 AM |
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Steve - how comes you're running it the wrong way?
I removed my 'serpentine' set-up and put a 1.8 Zetec water pump on - seems to run fine with no problems...?
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If you believe you're not crazy, whilst everybody is telling you, you are - then they are definitely wrong!
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stevebubs
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| posted on 1/6/06 at 09:32 PM |
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quote:
Originally posted by muzchap
Steve - how comes you're running it the wrong way?
I removed my 'serpentine' set-up and put a 1.8 Zetec water pump on - seems to run fine with no problems...?
blacktop....
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