I’ve done a lot of thermal analysis myself but I don’t think I’ve heard the term “primary methods of heat transfer” - what context / field is it used in?
And as others said, convection and conduction behave differently enough that it’s useful to consider them as separate methods. You can’t just model convective heat flow as conduction to air.
AAIUI it’s just “main methods”, scr4. It’s not a term d’art.
This article http://www.americanscientist.org/issues/feature/kelvin-perry-and-the-age-of-the-earth/1 discusses how Lord Kelvin misestimated the age of the Earth in part due to assuming that conduction was the only method through which heat escaped from the deep interior of the Earth.
Can you please rethink that ? Are you saying if I had a layer of Hydrogen (fluffy gas) over a layer of CO2, there will be no mixing ? Wont diffusion play a part ?
Diffusion will work to mix two different gases over long distances; in this case, each atom/molecule has a persistent identity as “gas X” or “gas Y”, so that despite undergoing numerous collisions, “gas X” particles will eventually diffuse across arbitrarily large distances through a volume of “gas Y” particles (and vice versa). Even so, you won’t see any bulk movement, i.e. there will be no wind or air current.
Diffusion will not appreciably work in the same way to mix two volumes of the same type of gas at two different temperatures. The gas particles do not retain a persistent identity as “hot” or “cold;” the mean free path under atmospheric conditions is on the order of nanometers, and the collisions result in transfer of heat that ultimately manifests as the bulk thermal conductivity of the gas. So “hot” gas particles don’t necessarily tend to diffuse from the hot surface toward the cold surface, but the heat that they initially carry certainly does.
Not sure what the confusion is…
Conduction - the interior of the material is not in motion, therefore heat goes from one part to the next by heating its neighbor. Consider if you subdivide the material you are considering into smaller and smaller cubes, and consider each tiny cube. The one cube resting on a hot plate will eventually (quickly?) heat to the same temperature as the hot plate; of course, as it also transfers heat to its cooler neighbors, it can take a bit of time. It’s neighbor in the inside will not start to heat until the outside cube is warmer; then that warmth will begin to transfer to it. The speed with which this heat transfers is a measureable attribute of the material. This is most typically seen in solids. (We don’t really count expansion due to temperature as “moving” in the same sense as convection)
Convection - the heat transferred to a piece of the material will cause it to move. Typically this occurs with fluids, can be caused by gravity - hot material expands, is less dense, rises, cooler contract, is more dense, sinks. However, a Convection Oven, one of the new nifty kitchen appliances, will hasten and redirect this heat using a fan. Typically, yes, this results in a speedier transfer of heat from one interface to the other. Yes, conduction or radiation will heat one interface of a convection process, but the main distinction is how the heat moves through the material. The boiling water reaching the top surface will lose heat through heating the air and by evaporating some of the water. The heat blown around in the convection oven loses some of it’s heat by conduction as it impinges in the cold turkey and helps heat it up to the same temperature as the oven. The fan just makes sure the hot air moves away faster once it cools in contact with the turkey - a hot wind heats faster that slowly rising hot air. Similarly, fiberglass insulation or Styrofoam creates insulation from heat transfer by using a very thin light material that does not conduct heat well, and uses it to break up the air within to very small cells so that the typical convection does not occur in any significant amount.
Radiation is light- electromagnetic spectrum radiation. it is different from the above two.
Heat is molecules in motion. Conduction happens when the moving molecules bounce off their neighbors, imparting some of that motion energy to that neighbor. If the material is a fluid, the molecules are flowing around randomly; if it is a solid, the molecules are vibrating within their solid matrix as if they were on springs.
IIRC, radiation happens when the energy from a moving molecule causes it to emit electromagnetic radiation.
Everything emits above zero emits thermal radiation, which is the principle, for example, that those night-vision systems can follow people on the ground in pitch dark (and because long IR goes through vegetation more easily, even track them through the forest cover.)
So the main distinction between conduction and convection is how the heat moves through the material, not so much how it is transferred to neighbor material.
Yes but how does refridgeration work ?
What is moving the heat from the cooled area to the heated area ? one of the three states means of transporting heat ??
No. But obviously if we change “Convection” to “movement of substance” then yes thats it.
Does that help the OP? Its old school thought… new school its Movement that is moving the heat.
So perhaps you could have 3 movement 3a. convection 3b. other movement.
or
3 convection
4 other movement.
the important difference is that convection is internally powered and spontaneous and somewhat everywhere in liquids and gases, and hard to stop.
Other movement is probably something like a pump or engine, external , and only in specific cases (eg refrigeration )
Isilder got to the point I’d intended to make this morning.
In the initial discovery of the physics of heat, as with so many other scientific discoveries in the 17th/18th centuries, they implicitly assumed that being at the bottom of an atmosphere in a gravity well was the base case for physics. It isn’t.
As has been well addressed by others, there’s heat transport by photons = radiation, heat transport by acoustic means = conduction, and heat transport by physical transport of heat-bearing molecules. Called advection.
IMO if we wanted to properly label the three “primary” forms of heat transport they’d be radiation, conduction, and advection.
As a special case of advection, if you’re dealing with a temperature gradient in a fluid in a gravity well, it will begin to physically transport (= advect) of its own accord. Which is an interesting enough mode of advective heat transport to have its own magic term: “convection”.
Ref md2000’s comment just above about ovens:
He’s 100% correct in what he says, but it has always pissed me off that a fanless oven is a convection oven, whereas an oven with a fan is properly an *advection *oven.
The extra cooking oomph comes precisely because the fan provides a more vigorous heat transfer mechanism than mere convection will do spontaneously. The marketing folks liked “convection” better because more consumers knew the word and so wouldn’t be intimidated by scary off-putting technical terms like “advection”. And so continues the active deliberate vandalism of our language in the pursuit of increased profits. Grrr!!!
Now it’s time to complain about “cable modems” that neither modulate nor demodulate. Grrr!!! :mad:
There is convection in the refrigerated system: in the air inside the fridge, and in the cooling fluid. And the bit which turns fluid at a certain T into fluid at a lower T is a valve (the one at the bottom); the pump is there to move the cooling fluid around within the pipes, but not to make it transfer heat. And what moves the heat from the objects in the fridge to the pipe to the fluid, and from the fluid to the pipes outside to the air outside is interfacial contact.
Interestingly, I’d never heard the term “advection” until seeing this thread. I’ve always understood any heat transfer involving moving fluid to be “convection,” further subdivided into “buoyant convection” (for flows driven by density differences in the fluid) and “forced convection” (for flows driven by an externally applied force, e.g. a fan).
These usages are different from what I read in Incropera’s classic text, I think. Conduction is the purely diffusive mechanism. Advection is transport due to motion. Convection is the sum of conduction and advection, so in situations where there is no movement, convection equals conduction. The distinction between the temperature difference causing the fluid movement and some other mechanism causing the fluid movement is the difference between buoyant and forced advection.
As a chemical engineer, I have always heard it as either natural convection or forced convection.
Hmmm.
The buoyant / natural convection vs. forced convection distinction take some of the force out of my complaint about marketing speak. IOW, it’s really a “forced convection oven”.
IANA thermal engineer and most of my involvement in the more technical aspects of heat are related to meteorology. Where we’re concerned with wind interacting with terrain as the forcing mechanism of transport. Termed “advection” to distinguish from the buoyancy-driven “convection” mode of transport.
Once again proof that even careful technical terminology is no absolute defense against confusion since different disciplines refuse to cross-standardize their terms.
[boringprofessorvoice] Consider thawing a turkey in a bathtub
Convection is how the heat from the hot water at the tap spreads out to the rest of the water, and to the turkey.
Conduction is heat transfer from the bathtub to the turkey, where the turkey touches the tub.
Radiation is what happens when I drop this piece of sodium into the tub.:eek:
[/boring]
Seems like convection is a bit higher level than the other two, kind of hides the conduction at either boundary, and some thermodynamics, inside the equations. Also, conduction requires at least one solid and happens at the interface. Convection requires a liquid, either liquid between two solids or transfer inside the bulk liquid.
Under normal circumstances household, radiation would mostly be blackbody (heat), but if you had a chemical reaction that emitted photons it would also carry away heat. And this is where lasers start to get involved.
Oh, as far as terminology, weirdest one I’ve found is + (plus). Simple, well-defined meaning in normal life, but in software and in higher mathematics things get… messy,
I dunno, = is at least as complicated as +, though for different reasons.
