Fluid Statics – What is the Maximum Height a Siphon Can Work?

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Here is a photo from an article on siphon in Wikipedia

My first question is what does maximum height of siphon mean? I mean which part of the siphon in below the photo would represent maximum height.

I think CD part represents maximum height , but I am not sure because some websites quotes AB as maximum height. Coming to my next doubt , if siphon is operating at its maximum height is the velocity of water in the siphon zero?

Best Answer

the maximum height of that siphon as drawn is AB. Note that the maximum possible height of a water siphon is determined by the water pressure inside the raised portion of the siphon at its highest point, which will be below atmospheric. When that pressure equals the vapor pressure of water at whatever the ambient temperature happens to be, the water boils into vapor at the high spot and the siphoning action ceases.

Related Solutions

[Physics] Maximum liquid velocity in a siphon

That formula is simplified by

ignoring viscosity
ignoring that $h$ could be greater than what could be supported by atmospheric pressure (about 10 meters in the case of water), at which point the fluid separates.

[Physics] Application of Bernoulli’s equation to siphon

In this case, it is allowed. As a thought experiment, you can replace the top bucket with the siphon hose by an S-shaped tube that has a gradual change in diameter, like this:

           _
         /   \
|www|   |     |
\www|   |     |
 \ww|   |     |
  \w|   |     |
   \|__/      |
              |
              |

With this type of analysis, it is more important that the downstream side of the tube makes a gradual transition to a large cross section. If you don't, then the kinetic energy of the high-velocity stream will be converted to heat rather than back to a pressure rise. This is not an issue at the upstream (tube inlet) side.

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Related Solutions

[Physics] Maximum liquid velocity in a siphon

[Physics] Application of Bernoulli’s equation to siphon

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