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Name: Aidan W.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 8/28/2004


Question:
My research topic is 'reducing the noise our kettle makes when it is boiling' my intention was to insert three different sizes of gauze into the base of the kettle and measuring the volume of sound generated by the kettle. I have discovered that small bubbles are generated as the water is heating and this is when the kettle makes the loudest noise. As the temp. of the water rises, the bubbles become larger the noise decreases. I would like to know the physics behind this phenomenon as this is the crux of my investigation.


Replies:
I have heard that the loudest, sharpest noises are not due to bubbles forming, but rather to temporary bubbles collapsing.

When you start heating the water, the bulk is cooler than 100degrees C. The heat is entering through the bottom, so the water there at the bottom surface is hotter than in the bulk above it. When boiling just barely starts, the hottest water at the bottom exceeds 100C and blows a steam bubble. But then, because the bubble is all steam which can directly re-condense into water, it acts as a "heat pipe" from the metal and hot water below into the cooler water above. This cools the metal and all the water down to 100C or below, so all the steam condenses and the bubble shrinks to zero. Because water is incompressible, collapsing vapor bubbles make a sharp, high, pressure spike at the moment they vanish. Since the bubble is up against the bottom of the kettle, there is a net thump of water against that metal membrane, radiating a pretty good amount of sound outside the kettle.

And because these initial temporary bubbles are small, many of them must happen rapidly to be able to carry the fix wattage of incoming heat upwards through the bottom-most layer of water, which is held relatively stationary by being so close to the bottom wall.

You can watch water start boiling in an open pot instead of a closed tea-kettle, and notice that the first bubbles to form (omitting the tiny dissolved-air bubbles) do not ever reach the surface, instead they shrink, vanish, and grow again, quickly repeating. This gives the bottom of the pot spots of fluttery appearance. You will notice the intense rattling noise during the same period.

After some of this, all the water is very near boiling and no longer able to cool the top of the bubbles. So they do not collapse, they keep growing until they float to the surface and emerge as free steam. This growth is rather smooth and makes virtually no noise. The sudden start of each bubble makes a little tap, though, and you can hear each bubble softly burst at the surface.

Gauze on the bottom would help to hold the bottom water still, so I would like to know if your gauze seems to make the slightly noise louder or longer-lasting?

Perhaps you can think of a couple other interventions to help this picture.

good work-

Jim Swenson


Hi Aiden,

This is an interesting subject to investigate. I do not know the reason for putting gauze in the kettle. Did you notice a difference in the sound when you changed the size of the gauze? I would guess the effect would be minor. You might try putting something metallic with many sharp points on it. These would, I believe, encourage small bubbles to form at lower temperature and might increase the noise at lower temperatures.

I believe what is happening is that as the water is heating, small bubbles are formed at the bottom of the kettle where the metal is hot. The bubbles then rise into the somewhat cooler water and collapse, generating the noise. When all the water is at the boiling temperature, the bubbles continue to grow, eliminating the noise.

You could try introducing a cool metal object into the quietly boiling water and see if it gets noisy again when the metal is very close to the bottom of the kettle so it makes the bubbles collapse again. Just a suggestion -- might work...

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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