Why is it that when you run water from the hot tap, it initially makes more noise as it it running through the pipe/tap while the water is cold (having cooled down in the pipe) compared with when the water runs hot?
Is it something to do with the amount of air that can dissolve in cold water versus hot water, and that cold water has more bubbles of dissolved air?
Almost certainly it's due to the drop in viscosity of the water with temperature. Between 15C and 60C the viscosity drops by a factor of three, and although I'm not particularly familiar with turbulence and Reynolds Numbers, I assume that it will be less turbulent, and less noisy in the pipe, as a consequence. See
Yes, it's the first indication that I've forgotten to boild the kettle in the first place!
LOL! You too!
...followed by a lack of colour around the tea bag!
Peter
Glad to know its not just me that usually realises I have just made it with cold water just by the sound alone ;-)
Your kettle must have a well insulated handle!
The handle of my kettle is fairly well insulated and doesn't get warm when there's hot water in the kettle. However there's enough heat radiated off the kettle (or enough conducted through the surrounding air) to feel whether there's hot water in the kettle.
Talking of kettles, ours has a switch which only latches in the on position if there's mains to the kettle. It then switches itself off when the kettle has boiled. About a year ago the latch mechanism failed, so I've been holding it down with a cork heat mat whenever I want to use the kettle, and I've kept thinking "I really *must* buy a new kettle". Then lo and behold the other morning it latched into place, for the first time in a year, and has worked properly (even switching off when the kettle boils) ever since. I wonder what went wrong that has since righted itself. I'd assumed that a plastic lug had broken off inside the switch mechanism, but evidently it was nothing so fatal and final!
It does. Bloody great Bosch thing.
Made in China?
Yes. But it gets a lot of abuse, and soldiers on.
I guess Bosch pay the Chinese a bit more to make a kettle, then the other kettle 'manufacturers' do.
There isn't on my kettle, its insulated. Its about the only way to make them more efficient.
Is it because the the viscosity (mu) of water is bigger when it is hot? That would mean the Reynolds number (v.rho.d/mu) is higher (more turbulent) when the water is cold and smaller (laminar) when it is hot.
Robert
Er...no. See my reply to NY at the top of this thread. Viscosity of water falls with rising temperature.
However, assuming that your Reynolds number description is correct, it means the water is potentially more turbulent when hot. How that relates to the sound in the pipes decreasing, I'm not sure. Cavitation perhaps? But again, cavitation in pipes increases with increasing temperature, along with damage*, as the water vapour pressure increases, so that doesn't look like an explanation.
I give up!
But that doesn't change the explanation as to why hot water poured from a kettle sounds different to cold water. Perhaps not surprisingly, it's a question that's been asked and answered many times and in many places:
Or perhaps that the surface tension of hot water is much lower which will change the behaviour of bubbles?
John
I noticed the difference in sound when making a brew and knowing the water is cold by the sound. Obviously, density.
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