An article in Scientific American online talks about using sound waves in household freezers for cooling, but doesn’t describe anything about this technology works. How can sound waves cool? It seems like if anything, they could heat as they deliver energy rather than absorb it.
I’m not positive on this but I think this might help.
Consider what it is that does the cooling on a normal refrigerator today…a compressor. When a gas is compressed it heats up, when the pressure is released it cools off. Fans and radiators are used to move heat away from the system so you get a cooling effect.
Accoustic cooling (near as I can tell) is using a similar principle except sound waves are used to compress a gas. Also, it appears the sound waves can ‘move’ heat as well thus the sound pushes the heat away from the system. Acoustic cooling is done in a tube. The sound is generated at one end moves down the tube compressing the gas and pushing the heat. When the gas uncompresses it cools off.
That’s my lay-man’s take on it. Hopefully an acoustic cooling engineer will be along shortly (wouldn’t surprise me in the least if one was on this board).
Hmmm. Guess that engineer never showed up. But the article says that the gas would be replaced by this technology, so there goes the gas.
Acoustic cooling still uses gas in a chamber. The deal here is it need not use gas that is harmful to the ozone layer (or anyhting else for that matter). From the little I’ve read it would seem you can fill the chamber with most any gas you like. Try helium…nice and non-reactive with anything.
As a result a working system would be preferable to a CFC laden conventional cooling system.
Um wait a minute. Didn’t we do this same thread just days ago? I remember explaining it and posting some links and explaining a bit about standing waves. Can somebody do a search? For some reason I can never get any results when I search.
Aha. for once it worked: http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=148271
Ever hear of a Stirling engine? In stirling engines you constantly heat one end and cool the other, and a piston gets driven back and forth.
There’s an acoustic version of the Stirling engine called a “Rijke Tube.” You sometimes see these demonstrated in physics class, where the teacher heats a metal screen inside a stovepipe and the stovepipe puts out an intensely loud howling sound. The air in the tube behaves as the oscillating piston. The air also must flow along constantly, not just wiggle back and forth within the tube. The two motions add a “phase shift” effect (see below.)
Acoustic refrigerators are similar, but instead of the temp-difference causing the air to wiggle, the air is driven back and forth, and its wiggle causes the temp difference.
Rijke Tubes and Acoustic Refrigerators rely on phase shift for their operation: if you just wiggled the air back and forth, it would alternately heat and cool but never change temperature on average. To act as a heat pump, the device must:
- Forcibly compress the air so it heats up.
2 . Move the compressed air to the “hot” end, then move it back. - Expand the air so it cools
- Move the expanded air to the “cool” end and back.
Without the “phase shift” between motion and pressure, the air would move like so:
- Compress and heat up as it moves to one end.
- Expand and cool down as it moves away from that end.
In this second version the air would get alternately hot and cold, but it would average out to zero. As the air wiggled back and forth, it wouldn’t transport heat in one direction like the phase-shifted version does.
Also, note that an acoustic fridge is like a normal fridge in that it’s a heat pump: it heats one end while it cools the other. In theory you could make an acoustic furnace by sticking the “cold” end outside your house.