Forever I’ve been told that there are three ways to transfer heat:
But in a recent thread a doper here mentioned a fourth way to transfer heat:
I sort of understand what evaporative cooling is. (And I admit it has never occurred to me that it would be a fourth way to transfer heat.) So is this accurate? Or is evaporative cooling really due to conduction and convection?
I think that this is correct. Based on my limited understanding, evaporative cooling reduces temperature of a substance because it takes energy to transform that subtsance from liquid to gas. If this results in the substance being cooler than its environment then it will draw in heat according to any of the other three methods of heat transfer.
More generically, this is latent heat of transformation. Unlike the first three, which are fluxes of heat, this is a source or sink of heat. I.e., the heat doesn’t move (although conduction, convection, or radiation may cause the heat to move immediately after it’s evolved).
Wouldn’t a chemical reaction also be similar?
That’s the macroscopic view. The microscopic view is that it’s just sorting molecules. In any substance, there will be a distribution of particle velocities. The fastest–i.e., hottest–molecules are the most likely to be ejected from a liquid. Those can then be carried away via air current or otherwise, leaving the liquid colder than it had been.
I’m not sure I’m a huge fan of “there are X ways of doing something” type claims. If I take a hot pizza from the oven to the counter, I’ve transferred heat. It doesn’t fit into any neat category. It’s sorta like convection, except that convection is driven by buoyancy instead of human action.
Phase transitions usually happen along with changes in temperature but I’m not totally convinced they count as a separate heat transfer mechanism.
The energy has to move in or out of the system as the phase changes. If we count convection as just a form of moving mass transferring heat I don’t think we have anything new.
This got me to thinking. Where does the heat energy come from during sublimation (going from solid directly to gas)?
It depends on why it’s sublimating. I take it to assume that it’s because the partial pressure of the gas form of the material is too low for the temperature, and at atmospheric pressure that exists there, a liquid of the material cannot form. Like with carbon dioxide/dry ice.
The carbon dioxide in the dry ice is in crystal form, but there is still a distribution of the energy of the particles, and the intermolecular vibrations are constantly sending the highest energy molecules on outside of the crystal into the air, while carbon dioxide molecules in the air are at the same time colliding with the crystal in a way to latch onto it, and in order for it to not cause just as many molecules elsewhere in the crystal to leave, it had to have had pretty low energy as a gas. When the net result of this is less carbon dioxide in solid form over time, it means that the energy was lost to the particles that escaped the crystal.
That’s my understanding at least. Someone with an actual working knowledge of chemistry that’s not remembering things from 20 years ago might give a better answer.
I think this is the thing, and I agree.
There are a number of distinctions made for various reasons. One of these is that “heat” is not the same as “thermal energy”, it’s “thermal energy moving from one thing to another”. Another is that radiation isn’t a heat transfer mechanism, because what travels isn’t heat or thermal energy, it’s electromagnetic radiation, which is transforming back and forth with thermal energy at the ends of the trip.
Trying to say that certain mechanisms are the official ones always forces imperfect choices. Better is to look at a situation you’re trying to analyze, and being good at thinking of everything that is relevant. People discussing a given case might say “that takes care of conduction, convection, and radiation” and then add “oh, look, there’s mass transfer here too, we have to take that into account.”
To address the OP specifically, yes, there are three canonical heat transfer mechanisms. But it’s not always these three that completely define things.
There is also ablation where the outer part of the surface is simply ‘shed’ to remove heat and does not require a phase change.
Is that heat transfer at all in that case?
There’s heat in one place, then some process happens, then there’s heat in another place. So why not?
Convection is just a special case of mass transfer, driven by buoyancy. Ablation would be another. But there are weirder ones.
A heat pump is exactly as efficient (in principle) as a heat engine, both limited by Carnot. Therefore I can extract energy from a heat source, transferring it electrically or mechanically to a different region, then using a heat pump to make a different area hotter (I’m assuming both areas have access to a common thermal reservoir).
So moving a hot cup of coffee from one place to another is heat transfer?
Yes, at least if you agree that convection counts (also, that’s the same as my pizza example above). Convection is just a chunk of hot matter moving from one place to another.
Fair enough
Evaporative is not a new way of hear transfer. Air is blown across water. It comes in contact with the water. The water absorbs heat out of the air turning into water vapor and cooler air. Heat is remover the normal conduction way.
Refrigeration works similar. Liquid refrigerant inside a tube absorbs heat out of air in contact with the tube. As The refrigerant absorbs heat it will change state.
That was my first thought reading the OP. People love to categorize and simplify things. For example, kids are taught we have five senses but reality is more complicated.
I think it’s more correct to say that convection is the combined effect of advection and conduction. Then, advection is the chunk moving and carrying heat. Incropera makes this point in his classic text. There is a bit of weirdness with Nusselt correlations, which should go to 1 in the lower limit but go to 0 instead; I think people ignore this only because Nusselt numbers close to 0 or 1 aren’t what we use Nusselt correlations for anyhow.
At some point in my science education I remember thinking that almost everything I learned about science in elementary/middle school was wrong in some way: gravity is not really a pull, electron are not little balls, light is not exactly a wave, etc. However I couldn’t come up with any fault in conduction/convection/radiation. I could probably help an 11 year-old with her heat-transfer module without cringing at the textbook explanations.
Perhaps. I’d quibble with that, but really it just illustrates my point. There are multiple interacting phenomena happening at once, and it’s impossible to completely delineate each one. They’re each a macroscopic description of behavior emerging from microscopic rules, and are both incomplete and overlapping.