surajit001
Topic Author
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Joined: Wed Dec 19, 2012 1:09 pm

Propeller Blades And Cavitation In Fluid Mechanics

Wed Jan 02, 2013 10:26 pm

As, i working on the propeller blades , i studied this idea of cavitation which is formed in propeller blades of boats in water. The concept given in most textbooks did not really satisfy my mind. So, i tried to use an analogy with the formation of bubble in water and formation of cavitation bubble in water when the propeller blades move rapidly. What i feel is as the propeller blades rotate at a high speed, a high pressure zone is created near the system which increases the temperature of the nearby water. Warm water can hold less air and hence as the air gets warmer the air tries to escape the water by forming bubbles which i believe is cavitation bubbles. I really don't know if my idea is right or wrong. Can someone kindly correct my concept?
 
roseflyer
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RE: Propeller Blades And Cavitation In Fluid Mechanics

Wed Jan 02, 2013 10:45 pm

I don’t think that is the best way to think of cavitation. The system including a propeller blade is for the most part adiabatic (no notable heat transfer).

The bubbles are caused because there are very low pressure areas around the propeller. Cavitation will only occur if the pressure declines to some point below the saturated vapour pressure of the liquid. Basically the pressure of the liquid in certain areas gets so low that it boils at the ambient temperature.

As a 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 vapour pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.

One science class trick that is done to show this is a glass of water is put inside a pressure vessel. If the air pressure is dropped, the water will eventually start boiling even though the temperature is constant.
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nomadd22
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Joined: Fri Feb 22, 2008 7:42 pm

RE: Propeller Blades And Cavitation In Fluid Mechanics

Thu Jan 03, 2013 2:54 am

Cavitation is pretty much just liquid boiling. The lower the pressure, the lower the boiling point. When the pressure on the low side of the vane gets low enough to lower the boiling point of the liquid to whatever temperature it's at, any pump will have cavitation problems. Warmer liquid will cavitate easier, but I don't think the heating by the pumping action is that much of a factor.

[Edited 2013-01-02 18:56:10]
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PITingres
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RE: Propeller Blades And Cavitation In Fluid Mechanics

Thu Jan 03, 2013 3:08 am

Cavitation has little or nothing to do with air dissolved in water, since it happens in other fluids too; cavitation is a significant problem in pumps of all sorts, including rocket engine turbopumps. Take a look at the Wikipedia page on cavitation; I'm not a pump engineer but it looks fairly plausible to me, and in line with what I've read in other sources (e.g. Huzel/Huang on liquid fueled rocket engines).
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vikkyvik
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RE: Propeller Blades And Cavitation In Fluid Mechanics

Thu Jan 03, 2013 5:07 am

Quoting Roseflyer (Reply 1):
I don’t think that is the best way to think of cavitation. The system including a propeller blade is for the most part adiabatic (no notable heat transfer).

That's probably true (I don't remember), but there still can be heat transfer within the system. So you could have localized areas of higher temp / lower temp within the propeller and immediately surrounding water.

That said, I agree that it's not a good way to look at cavitation.
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