Vapour pressure – Piston driven cavitation

This is a demonstration I have been working on this semester to illustrate cavitation. Students sometimes find it hard to believe that you can boil water at room temperature just by reducing the ambient pressure. I have tried this with a syringe but it is hard to get the pressure low enough by hand. It is also only a small volume and therefore hard to visualize. Instead, the technicians in our department built a larger transparent cylinder with a piston and a pneumatic cylinder to drive it.

Equipment

  1. A cup of water
  2. A transparent cylinder with piston (mine has in ID of 3″ and is 11.5″ long)
  3. Pneumatic cylinder connected to the piston to pull it out (mine has a 1″ piston)
  4. An air compressor (I tested it with a 100 psi compressed air line)

Pictures of the test rig with the transparent cylinder, piston, and pneumatic cylinder for decompressing the transparent cylinder.

Photo Mar 03, 11 36 33 AM Photo Mar 03, 11 36 40 AM

Demonstration

Put a small amount of water (maybe a cup) in the bottom of the cylinder with a small gap above the water. There needs to be enough water to in the base of the cylinder so that you can see it boil. Connect the pneumatic cylinder to the compressor and release the air pressure so that the piston is rapidly pulled out of the cylinder. This should significantly reduce the pressure in the cylinder causing the water to boil. If you then decompress the pneumatic cylinder it will drop back and the water will condense.

Picture of the transparent cylinder with water and small air gap. You need to make sure that there is enough room for the piston to move up such that the pressure drops enough to boil the water.

Photo Mar 03, 11 37 57 AM

Analysis

The demonstration is fairly qualitative. The pressure drops, the water boils. There are a couple of things going on though. First, the water boils without increasing its temperature. The students can touch the outside of the cylinder and it will not be warm. If you take the water temperature to be the room temperature you can look up the vapor pressure, and hence the pressure that must have been attained by the cylinder. Second, when the piston drops back down and the pressure above the water returns to approximately atmospheric, there may be some bubbles attached to the cylinder wall. These are air bubbles that came out of solution during the de-compression, just like the air bubbles that form on the side and base of a pan when it is heated but has not yet boiled.

 

Thanks to Danny Metz and Scott Black for building the demonstration rig. An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Advertisements

One thought on “Vapour pressure – Piston driven cavitation

  1. Pingback: Video of “Vapour pressure – Piston driven cavitation” demonstration | Teaching Fluid Mechanics

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s