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VulfSki t1_j5r0tb4 wrote

Reply to comment by Appaulingly in Why does hot air cool? by AspGuy25

This is the answer.

I have used thermal cameras in my line of work work a part in an oven.

And I have literally been able to see the reflection of my own body when I pointed the thermal camera at the metal walls. That's likely the issue here. An emissivity issue.

The coworker's explanation sounds like a misunderstanding of how convection works. Just because air is flowing, doesn't mean it's cooling down the metal. Flow doesn't equal cooler. You need to remove the thermal energy somehow. If it's a closed system the heat isn't going anywhere.

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chillaxinbball t1_j5rd007 wrote

The best way to think about thermal cameras is to think about cameras.

Imagine you are trying to measure how hot something is by how much it's glowing. For example you go to resistive coils and see how bright they get. You can get a decent measurement just by seeing how bright it gets. Now try to measure a mirror reflecting an image of the coils. You'll get a very similar measurement. Is this because the mirror is just as hot? No, you are mostly measuring the reflected image of the coils and not the mirror itself.

It's the same thing with thermal cameras. Metal is very reflective at the wavelengths used to measure thermal radiation. You are essentially looking at a mirror and will not get accurate readings from it in much the same way.

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Glasnerven t1_j5rhfa4 wrote

> Just because air is flowing, doesn't mean it's cooling down the metal. Flow doesn't equal cooler.

Roughly speaking, flow equals better heat transfer. We're used to thinking that flowing air = cooler because we--our bodies--are usually warmer than the air around us. Even if it's hot enough outside that the air is warmer than we are, then we're probably sweating, so we have evaporative cooling going on--and flowing air makes that happen faster.

So, in our personal experiences, we almost always find that it feels cooler when air flows over us. It's really easy to over-generalize that and assume that it's a universal law.

You can easily test it at home, though. Get two little cups of ice, make sure there's the same amount of ice in each one, and then put a fan on one and let the other sit in still air. You should notice that the ice with the fan melts faster--the flowing air is heating it faster than the still air is heating the other ice.

You might also already have a device built around this effect: a convection oven or air fryer.

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VulfSki t1_j5rqk0a wrote

Yes, I understand how all that functions. Humans get cooked down from perspiration, which is because the energy to evaporate sweat is partially comes form the heat on our body, and air flow helps with that.

But airflow itself doesn't equal cooking in the general sense, because it only works if you are removing heat by taking it to something that is at a lower temperature, can't violate newton's law of cooling.

You did provide a great explanation, even though I was already aware of all that.

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