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This noise does not come from the kettle or at least it does not come from it heating mechanism.
It is the water itself that is noisy.
And yet, if you look, there is not much to see.
The water isn't even boiling.
On the other hand, if we move forward in time... There, we see that it is boiling hot,
something is clearly happening and yet, it's quieter! How and why is that?
Where does the sound come from if the water isn't boiling and why does this noise disappear
or decrease thereafter when it boils for good? The question seems trivial and seems like
just a fun fact that would be cool to know but has no practical application.
And yet there is.
The sound of water that isn't boiling is studied to be used as a detector in nuclear reactors.
You didn't think we would tackle this subject, did you? To be honest,
neither did I.
Now is the right time to make yourself some tea I think.
I use a kettle because it's more convenient to see but it works
with a saucepan on the stpve, too.
You will hear a loud noise before it boils in either case.
Have you ever noticed this noise change when water boils?
Or are you finding this out through this video? Note that it should work with
a saucepan or kettle, but I don't think that it works in the microwave or in
an oven, and you will understand why in a few minutes.
I still really don't like tea... When you turn on the kettle, that's
what I will call phase 1, my water is at 19C (66F) and the noise level is low
or basically non-existent.
Within seconds, a background noise begins to arise although there is nothing
to see in the water and the temperature is only at 46C (115F).
We have just entered phase 2.
Little by little, the noise intensifies and we see that there are bubbles wriggling
at the bottom of the kettle.
It's boiling locally while the rest of the water has not yet reached its boiling point.
We are still in phase 2 and the noise is at its maximum.
Then the bottom bubbles grow bigger and come off the bottom of the kettle.
They begin to rise, forming columns.
The noise started to change, it's a a little less loud and more serious.
A transition is starting, this is phase 3.
There is not much time left before we enter phase 4.
That's it, it's boiling with big bubbles.
This time the bubbles are huge and the sound is much weaker than in the phase former.
I hope the next time you boil some water, you will look like this.
In this case, Bruno, whom you've just seen, is familiar with the phenomenon,
I went to see him precisely to find out more on the origin of the noise.
Bruno and I have known each other for several years, he was finishing his doctorate when
I was starting mine.
He was (and still is) in the field fluid mechanics and thermodynamics.
Two rather appropriate fields of expertise for today's topic.
Here, we can already see bubbles wandering around.
Yeah, we see little bubbles wandering around, if you look closely.
The first bubbles we will see, in fact, will usually be caused by
gas that is dissolved in water, they are not caused by the water boiling.
So you can't see it, but the composition of these bubbles which are indeed spherical (if
there are any, it's not systematic) is different from boiling bubbles.
The boiling bubbles that you don't see yet at this stage, those do
have water vapor inside, while the dissolved gas bubbles, like
its name suggests, is gas, and that can be several types of gas such as
CO2 for example.
The heating element brings the water that is closest to it to 100C (212F)
and it changes to the state of water vapor.
This is the wriggling in the background that's so loud: it's a local boil.
It's the bubbles' fault, they are physically noisy.
And so, regarding bubbles, there are two opposing mechanisms: the heating element
produces heat and therefore generates steam.
But that vapor inside the bubble is also cooled by the liquid.
And so, we have a phenomenon of implosion at a certain point.
So the bubble will reach a given size where it will be cooled sufficiently by
the liquid and it will implode on itself.
It's this cycle of bubble making and very fast implosion that creates this noise.
This is why the dissolved gas bubbles from earlier did not make any noise.
Because they did not follow this cycle of creation and implosion.
They just wander around.
Bubbles that make noise work similarly to a loudspeaker.
There is a membrane that moves at a certain frequency, sometimes high, sometimes low.
This very rapid movement induces a movement of air and that's what creates the sound.
The same goes for bubbles, each time they are created, they
create an upwards movement (just like with speakers) and when they implode, it
corresponds to a downwards displacement of air volume (as in the case of the loudspeaker too).
The combination of the two, creation and implosion, maintained for a long time and with
lots of bubbles, creates an audible vibration.
The bubbles here are like thousands of speakers which would be inaudible individually but
there are so many that the intensity of the sound is more than audible! This union makes
the force of the racket.
An uproar which, by the way, may be pleasing to some! This is the kind of noise that
can be used to fall asleep to.
I made a video for you with that sound if it helps you focus or sleep.
The link is in the description.
If you pay attention, you will hear that the sound changes.
At the very beginning, the bubbles barely have the time to build up at surface level
of the heating element before they implode directly because of the cold liquid just
above them.
They implode super fast, probably within milliseconds.
If we had a super fast camera, we would see that this cycle of creation and
implosion occurs before we can even see it.
But as time goes by, more and more of the volume of the liquid above becomes hot,
which leaves enough time for the bubbles which just wriggle at the bottom to increase
in volume before imploding.
We can see that if we look ahead, they get bigger and bigger and
they take up more volume.
Each bubble then lives longer and becomes so big that it scrapes off the
bottom of the kettle and rises to the surface, until you create columns of bubbles.
As the temperature of the heating element continues to increase, the
process where isolated bubbles are generated, turns into those bubbles imploding
and creating bubbles that are stable.
So we're really going to have columns of bubbles.
These columns don't go towards the surface at the beginning, because they too
will implode if the liquid is too cold.
But once all of the liquid is hot enough, they will reach the top.
At this point, you will notice that the sound starts to become less loud
in this column mode.
And for good reason, there are a lot less bubbles that go through this cycle of magnification and implosion
that was causing the noise.
When they manage to go up, they don't make a lot of noise for two reasons.
First, there are not so many bubbles anymore: each bubble produced merges with another bubble.
This results in huge bubbles but few in number.
Second reason, big bubbles no longer implode very quickly,
they explode at the surface every now and then (since there are fewer bubbles).
There you go.
You now know why a kettle is louder before the water boils.
It's because the water in contact with the bottom of the kettle starts to boil
but those tiny bubbles implode, because the rest of the water
is too cold.
It creates a lot of gas volume change which results in noise.
Which no longer happens once all the liquid is at the same temperature
and bubbles can form, grow and rise to the surface, where they erupt
in small numbers.
From this I understand that if you try to boil water evenly
(in a microwave or in an oven), we shouldn't observe this phenomenon because
the noise really comes from the fact that the water heating is not uniform.
It comes from one place.
Okay, but I told you we would talk about nuclear reactors... For that,
we must go a little further, beyond what a kettle can do.
We are missing a phase, the one that comes just after.
This is heat transfer, that is to say the heat that is transferred to the water
from the kettle, depending on the temperature.
This was phase 1, the silent phase, then there was phase 2: pschhhh,
and then there was phase 3: a less strong pscchhh, and phase 4, bloop bloop.
I'll point it out though I think it is quite obvious, but the separation of these phases
and the title I gave each of them... I did that, to simplify
the structure of my explanation.
But if you delve into books on heat transfer, this will be presented differently.
For example, it won't be called bloop bloop.
But anyway, the point of the curve where our kettle stopped doesn't stop there.
It goes further.
You can see it in the boil once we have bubble columns, it's the begining of the end.
This is where the boiling begins, the bubbles get bigger and bigger, etc.
In the curve, what is surprising here is that... These are
bubble columns but what is surprising is that the curve goes down again, because
we were at a heat transfer point here.
It would mean that once we start, when columns appear, we lose heat.
What does it mean? Roughly speaking, it's because the vapor that we produce
starts to occupy so much space, it starts to take up so much surface of
the heating element that it is actually going to cover it, to protect it.
This is going to be result in an inability to trade any more
heat with the liquid medium.
So, it's a bit like adding a kind of insulating sheath around the heating
element which, in some way, is a steam sheath.
So, as soon as we no longer have the jets and columns of bubbles and we start to transition
towards the generation of a film, we come to a point where we can no longer
exchange any more heat with the liquid medium, we are caught in a bottleneck.
You've seen this layer of vapor before.
This is what happens when you pour water on a hot plate.
It beads, wanders and does not evaporate.
Which is quite aberrant since the plaque is super hot.
It’s because there’s a layer of vapor between the plaque and the drop, which isolates the
drop from the heat source.
It’s pretty crazy.
Kind of like a thermos would.
It's an open-air thermos, in a way.
It's called the Leidenfrost effect.
And that's what we fear most about a reactor.
So, in an ultra simplistic vision of what a reactor is, we use the energy from
nuclear fission to heat water, which itself will generate electricity.
But this boiling must be extremely controlled to happen in the right place,
at the right time.
And if it happens in a spot where it wasn't planned...
If the water is boiling, there are a lot of problems that will arise with it.
The first is we could end up having bubble columns
or vapor films and in that case, what will happen?
The nuclear fuel areas which are protected by the vapor film will heat up.
You should understand that what is problematic about nuclear fission
is that it generates heat.
And if the temperature rises, fission will accelerate.
So if there are nuclear fuel parts which are isolated by a layer of
steam, they will be able to heat up more because they would be less cooled
by water and therefore their temperature will increase.
It's gonna produce more energy, so it's gonna have even more steam.
The temperature will rise and so on to lead to what we call a runaway.
This is what we see on the graph.
When the vapor film occurs, the heat transfer from the reactor to the water decreases.
So it goes down because it's not the water that recovers heat, but
the fuel inside of the reactor that's overheating.
This is the runaway Bruno is talking about.
Today there are techniques to keep this from happening of course, but
among the research being done on reactors, I was able to read some papers that discussed
analyzing the sound of boiling very precisely to be able to distinguish
when an unwanted boil could appear: a sound sensor to pinpoint
the different phases of boiling water... or non-boiling water.
I could not tell you if this research will lead to anything tomorrow, I read
some papers on it but I'm not at all an expert in nuclear reactors.
I know, there would still be a lot more to say on the subject, but it is
not the purpose of this video.
My goal is that the next time you boil water,
you look at it, you see each phase and you say to yourself “wow,
the job my ears are doing right now, that could well be a technology
found in nuclear reactors”.
Cool right? Well, I think it's cool.
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