(gentle music)
- This is a backflow incense burner,
instead of burning incense sticks, it burns incense cones,
and through some combination of geometry
and fluid dynamics,
the smoke falls instead of rises.
It's a smoke waterfall or a smoke fall.
A number of things confuse me about this.
Like, smoke rises, it doesn't fall.
Actually, I think I've figured out how smoke can fall,
but there is still something that confuses me here.
You can see that there's a hole
in the the bottom of the incense cone.
And if we cut the thing in half
you can see the hole reaches to about this height.
So as the incense burns down,
eventually the leading edge of the fire reaches the hole.
And when it does, the smoke from the fire
is able to escape into this cavity.
And because the smoke isn't immediately carried
up and away by convection, it has a chance to cool down.
Smoke is made up of tiny solid particles,
which are of course, heavier than air.
So if they are given a chance to cool down,
we should expect them to fall.
In fact, our intuition about smoke rising
is based on confounding circumstance.
Like, when we see smoke in our everyday lives,
those smoke particles are typically suspended in hot gas,
because they've just come from a fire.
Hot gas expands, which makes it less dense,
which causes it to rise.
And when it does,
it will take those suspended smoke particles with it.
Two things surprised me though.
The first is why is so much smoke coming out the bottom
and almost no smoke coming out the top?
The way an incense cone burns is similar to a forest fire.
You've got the burnt parts, the unburnt parts,
and a line of fire in between.
That line of fire is pumping out hot gas and smoke
in every direction.
So we should expect smoke to come out the top
and the bottom of the incense cone.
And if anything, we should expect
more smoke to come out the top,
because the bottom side of the fire is mostly blocked
by solid fuel, with just a little gap in the middle.
Whereas the top side of the fire
is exposed to a very porous the layer of ash.
The answer I think is that actually a lot of smoke
is coming out of the top.
It's just moving much faster because of convection.
So it's harder to see, or more formally,
you could say that for a fixed smoke flux,
if the stream of smoke is moving faster,
then it must be less dense.
So the smoke is coming out of the top.
It's just harder to see.
It's also worth noting
that incense material is quite porous.
Like if I suck on the end of this incense cone,
I can suck air through the material itself.
That means that gases expanding out from the fire line
can be pushed through the unburnt material,
and reach the cavity.
By the way, incense is an example of an excitable medium.
An excitable medium is a medium
that some kind of wave can pass through,
like the fire passing through the incense, like a wave,
but it can only pass through once,
because the medium is spent by the wave.
A forest fire is another example.
A Mexican wave is an example as well.
After a spectator takes part in a Mexican wave,
it's unlikely that they would accept a second wave
in a short time period.
But if enough time passes,
a second wave is possible and that time period
is called the refractory time of the excitable medium.
In the case of people at a sporting event,
the refractory time is usually short enough
that a Mexican wave can just keep going round
and round in a stadium.
My favorite example of an excitable medium
is bubbles on the surface of a drink like hot chocolate.
If one bubble burst, that reduces the stability
of the bubbles surrounding it, so they burst,
and then the neighbors bubbles burst and so on and so on.
A number of people have sent me examples of this, actually,
which I'm very grateful for,
and I'm grateful to be able to include one in a video.
This one from Ryan Cummings shows the effect
with pomegranate juice.
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One thing I'm not totally sure about
is the role geometry plays in smoke falls.
Like, I thought it'd be really cool
to have smoke falling from between my fingers.
So I set up this little copper tube that I could pinch
between my fingers and have the smoke fall from beneath,
but it just didn't work.
The smoke just goes off to the side.
And actually, if I just hold the incense like this,
it floats upwards.
My first thought was that perhaps the copper tube
doesn't provide enough cooling.
Like if you look at this burner,
there's a big chunk of metal there.
That's probably a really good heat sink, but actually,
if you lift the lid off, the same thing happens.
The smoke floats out to the side and then rises up.
So I think the real issue here is not
that the smoke couldn't cool down enough,
but it's an issue of convection.
This thing's pumping out quite a lot of heat.
So, I imagine there must be convection currents
pulling the air upwards from below
destroying the smoke fall.
The glass surround of this back flow burner does a good job
of blocking out convection currents.
And in fact, I can get a nice smoke fall
from between my fingers if I put my hand
in this display case.
What's interesting though, is that you can set
up a smoke fall in another way that's quite straightforward.
Take a post-it note, roll it up into something
about the thickness of a pencil, you can use the sticky edge
of the post-it note to keep it rolled up.
You then create a little stand to hold the post-it note
at an angle and you like the top end,
and look, there's a smoke fall,
but to me it looks like there's nothing in the geometry
to prevent convection currents.
Perhaps it's the angle that helps,
like maybe the fire is far enough away from the smoke fall
that it doesn't disturb it.
Testing an incense cone with a bent copper pipe
seems to confirm that.
I also think it's just a good idea to isolate the stream
from general gusts in the room.
The other thing that I realized I didn't understand
as I was investigating this video is the difference
between a smoldering fire and a flaming fire.
Like, why doesn't an incense stick have a flame?
And well, it turns out that actually,
the flame is the special case,
and the smolder is just an ordinary fire.
It's a solid fuel interacting with oxygen in the air.
And of course that will happen
at surface of the fuel that's smoldering.
But if the fuel contains enough volatile compounds,
then when it heats up and releases those volatile compounds,
assuming they're combustible, then they will burn in the air
and you'll see that as a flame.
So the reason incense burns without a flame
is that when the fuel gets hot,
it doesn't release enough combustible material
to create a sustained reaction with oxygen in the air.
One final, final thing that I think needs to be explained
is like, in the case of the rolled up post-it note,
I would expect the hot air rising
from the lit end to suck air up through the tube.
And so kill the smoke fall at the other end, but it doesn't.
I suspect it's because the creation of hot expanding gases
in the fire is more than enough to counterbalance
convection acting to pull up through the tube,
but either way, these backflow incense burners
look really cool.
And by the way, thanks to Vivek Pathak
for the idea for this video.
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(bright music)