How to grill elegantly: Why does the smoke always chase me?

Author: New Media Center of Institute of Physics, Chinese Academy of Sciences

Reviewer: Luo Huiqian, Associate Researcher, Institute of Physics, Chinese Academy of Sciences

In the dead of winter, one of the happiest scenes is to have a hotpot with friends. If it happens to be snowing outside, you can sit by the window in the house, enjoying the food and the heat, waiting for this feast with joy. The steam from the hotpot rises straight up, and everything looks even more beautiful.

However, when you are enjoying such a relaxing and beautiful time, the mist in the pot keeps coming towards your face for some reason.

There was no way out. You, wearing glasses, stood up. Although you were very reluctant, you had to move to another seat helplessly, hoping to exchange your compromise with the fog for long-term comfort. You sat down again, and a happy smile reappeared on your face. However, at this moment, the fog suddenly followed you as if it was fascinated by you. This time, you finally got annoyed and started complaining, why the smoke went wherever you were. You put down your chopsticks, and planned to (call the editor) start (help you) thinking about this annoying question: Were you a range hood in your previous life?

No, no, no, in fact, similar problems can also occur during outdoor barbecues. For example, the following situation seems very beautiful:

But slowly, the situation got out of control:

Whenever something like this happens, the friends who are having a barbecue together will start to wonder, are the people who are with me cursed? Or are they the reincarnated god of smoke?

No, we don't believe in metaphysics, we believe in science. Curses can be broken, but the laws of physics cannot. Today, I will discuss with you why such helpless things like "I am the same" happen all over the world, and how to have an elegant outdoor BBQ ?

First of all, why does smoke occur? We can think of smoke as a collection of incompletely burned particles (including solid particles, small droplets, and gases) when combustibles (such as wood) burn. Because these particles are too small to be seen directly by the eye, when they gather together, we will see them as smoke .

At the same time, combustion also releases a large amount of heat, and nearby air molecules absorb the heat and begin to move faster.

Since temperature is only a physical quantity that represents the degree of hotness or coldness of an object, it is the intensity of the thermal motion of the object's molecules at the microscopic level. At the macroscopic level, it means that the air around the flame begins to heat up. The kinetic energy of these heated air molecules that are close to the flame is large, and they collide with the neighboring molecules that are still moving slowly at a distance, and send these neighbors to a distant place in the opposite direction of motion.

As a result, a space will be vacated near the flame, allowing the hot air near the heat source to continue to expand. This series of chain reaction events causes the air density near the flame to be lower than the surrounding air density a little further away . Some friends may want to ask, why is this?

We can recall the ideal gas law that we learned in the second grade of junior high school:

pV=nRT

Among them, p, V, T represent pressure, volume and temperature respectively, n is the amount of substance, and R is the ideal gas constant. Applied to this scenario, it can be seen that, under the same other conditions, hotter air has a lower density than colder air.

We often hear that low-density gases tend to rise, and this seems to explain why.

For example, we would normally expect that in winter, the upper floors of a building would generally feel warmer than the lower floors . This is because cold air is denser than warm air, so it stays at the bottom while warm air rises.

For example, we often see that balloons filled with helium will fly away easily , because the density of helium is lower than the gas in our air, which means that a helium balloon of the same size is lighter than an air balloon. The difference in density is the key to the balloon's floating.

But the point is, what we have discussed above only means that the less dense air will rise because it is less dense. For something more in-depth and rigorous, we may need to invite our old guest Newton.

Newton's first law of motion tells us that unless an object is acted upon by an external force, then, " objects at rest will tend to remain at rest, and objects in motion will tend to remain in motion ". This is why simply reducing the density of the air is not enough to cause it to rise, another force must be applied to start the upward movement. This force that needs to be involved is gravity . It is easy to understand that gravity can pull the colder, denser air towards the surface of the earth, but in the process, denser air will also reach the surface of the earth, and it will spread around aggressively, which will also weaken the less dense air. As a result, these "incapable" less dense air will be forced to rise. Of course, this process is just a simple general description here.

Now it makes sense to look at the balloon again: the balloon and the air inside it are being pulled down by gravity, while the air pressure is trying to push the balloon up, but this is not enough to overcome gravity, and the balloon falls. However, when helium (which is much lighter than air) is used instead of air, the balloon's weight is reduced enough that the air pressure can easily push it up.

Back to the barbecue, we mentioned earlier that the hot air around the fire is less dense than the air a little further away. Gravity pulls it down less than the surrounding air pushes it up, so the smoke rises into the sky. This is called convection . Because the thermal conductivity of the fluid is very small, very little heat is transferred by conduction, so this is the main way heat is transferred. Convection occurs in many places. It happens on the sun, on the earth, and in your oven.

But during grilling, the rising air doesn't just rise on its own, it's also pushed by the air around it, because when the hot air moves away from the fire, the air pushing it moves on and fills the void left behind by the hot air, which is where you are.

Are you wondering why I'm sitting still and that's a problem? Yes, you are. Because you are like a wall there, which blocks the normal air flow in the area where you are. If you weren't there, the air from where you are sitting would converge at the fire, just like the air from the other surrounding areas. But your body is blocking the air behind you from moving toward the fire. Therefore, the smoke from the burning fire will start to move in the direction of the low pressure area, which means the air in the opposite direction of you will move directly towards you. It will also bring any smoke or soot from the fire with it. Since you are blocking the air, it doesn't matter if you move once or twice, because you will block the air in that direction no matter where you move, giving the impression that the smoke is following you.

In particular, if you are a group of people staying in one place, it means that more people will block the air flow, which means that a larger vacuum area will be formed . In this way, the mystery of why smoke loves you so much makes sense.

So, what should we do to effectively avoid this problem?

First of all, the first method is called self-sacrifice . You can (pretend to) be spirited, ask yourself to stay on one side, and arrange for others to sit in other directions. But there is a problem with this, that is, gradually they find that the smoke on your side seems to be smaller, so they all come to your side for shelter, and the result is imaginable, which can only lead to a situation where everyone suffers.

The second method can be called the principle of equality of all beings . I suggest that you carry a protractor with you every time you go out for a barbecue. Since we say that one person sitting in one direction will cause a vacuum area, why not just put one person sitting in each direction with the same spacing? In this way, equal amounts of air are blocked in all directions, and there is no vacuum that is more serious on one side.

The third method can be called the smoke is the original sin theory. To be more specific, it is to prevent smoke directly. We have already mentioned the formation mechanism of smoke in the previous article. In this case, if you want to avoid the smoke from entering your eyes and lungs when grilling, you can start from the source and prevent it from being produced in the first place. In this case, at least the result is that the vacuum causes warm air to be transported instead of harmful smoke. Then according to the theory in the previous article, why are there particle aggregates? It 's not because the fire temperature is not enough, but also because there is not enough oxygen . Here we start with the second factor and find a way to let in more oxygen so that the combustibles can burn fully. Specifically, you can build a fire in a way similar to the following:

In fact, if it really doesn't work, just use smokeless fuel , so that everyone can sit wherever they want.

I believe that after reading this article, your mother will no longer have to worry about your outdoor BBQ!

References:

[1]Physical and Chemical Characteristics of Inhalable Particles from Combustion Sources[M]. Science Press, Hao Jiming, 2008

[2] Heat Transfer[M]. Science Press, Zhang Jingzhou, 2014

[3] Thermodynamics[M]. People’s Education Press, edited by Li Chun et al., 1979

[4] Isaac Newton. Mathematical Principles of Natural Philosophy (translated by Wang Kedi). Xi'an: Shaanxi People's Publishing House, 2001.

[5]This is Why … Campfire Smoke Follows You Around

[6]Munson, Bruce R. (1990). Fundamentals of Fluid Mechanics. John Wiley & Sons

This article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.