I’m very familiar with the three primary methods of heat transfer - Radiation, Conduction, and Convection.
But why is Convection considered a Primary method? Seems to me it’s just (1) conduction from a heat source to a fluid and then (2) conduction again from the fluid to another solid. I have the same problem with Advection, which some people include in the Primary methods of heat transfer.
While we’re at it, if these are called Primary methods, is there a list somewhere of Secondary methods? I’ve never seen it.
Convenction is what happens within the fluid, not what happens at its surfaces. It is completely different from the other two in that its main method of transmission involves matter moving around, carrying the higher energy.
As above - convention requires the movement of matter. Conduction does not.
I would differentiate convention from advection in that convection is a fluid movement that is itself driven by the heat differential, whereas advection may be driven by an external energy source or mechanism.
Convention is responsible for the (wrong) maxim that heat rises. Changes in density due to differences in temperature of the fluid result in differential buoyancy in the fluid and in a situation where the gravity points the buoyancy at the colder end you will get a convention cell generated. Hot lighter fluid rises and colder denser fluid drops. This creates a very effective heat transport mechanism. (Which is probably why it is called a primary mechanism - the primary mechanisms are the dominant mechanisms under normal conditions - for common values of normal.)
Convection is, I think, always involves one of the other two methods at each end of the process - that is, heat is conducted (or absorbed as radiation) into the fluid, then convection occurs, then heat is conducted or radiated out at the other end.
It would be interesting to get a response from someone who can spell “convection.”
Edit: Oh there you are, thank you Mangetout. You can spell. Well, that’s precisely what I’m saying. Convection is just a combination of the other two processes.
English is my third language, in the previous two it happens to be called convención/convenció.
Again, convection is not a combination of the other two processes; they happen within different mediums and different ways. What happens at interfaces is a different question.
It wasn’t meant as a personal insult my friend, it was just funny that the first two responders got it wrong. I also speak three languages, so I understand!
Seems hare splitting … if we irradiate one side of a solid, let the energy conduct through and then have convection move the energy away on the other side … do we say conduction is just a combination of the other two?
I’d say no … one difference between the three is the rate of energy transport … conduction is very slow, measured in feet per hour … convection is faster, measured in miles per hour … and radiation is fastest, measure in miles per second …
We can also observe all three happening at once … the solar energy absorbed by the oceans … some is conducted downward, some is convected sideways by the ocean current and some is radiated back into space … we can’t really say that convection is a combination of the other two here since the other two are move energy different directions and at different rates than convection.
Well, here’s the question (actually, I’m sure I asked this in another thread, but damned if I can remember the answer) - is conduction just radiation on a very small scale? - that is, is the transfer of heat within a solid achieved by radiation of energy from one particle to another, or is it achieved by some other, more direct ‘transfer of vibration’ type of exchange?
that’s where it gets transferred between two different mediums.
But it also gets transferred within a single medium, any time that medium presents an internal heat differential. Let’s say that you are filling a bathtub, and you put the plug in before the water has reached the desired temperature, which is hotter than the temperature when the plug was placed: as the hotter water from the faucet pours into the cooler water below, heat gets transferred within the water until all of the bathwater equalizes, and the main mechanism for that transfer within the water (not of the transfer with the air or the tub) is that the water molecules themselves move around, something which does not happen within the solid tub (no movement involved) or in radiation mechanisms (which involve light* moving in straight lines).
Think of economics as a simile. Trade moves between countries, but also within a country. So does heat.
in the wider definition of the word, not just restricted to its visible spectrum. I was trying to avoid using “radiation” with two different meanings within the same sentence.
If you take the statistical mechanics of the system, it is all about bouncing balls. Molecules, or whatever makes up the basic block of the material bounce off one another and transfer momentum and kinetic energy. No radiation needed.
You might argue that QED mediates the bouncing, but that still isn’t radiation. It includes Paulli exclusion, although I guess it will also include the exchange of photons as mediating electrostatic forces. That isn’t radiation as we usually consider it AFAIK. In particular it I don’t think it works as black body radiation, which is how we usually start thinking about radiation as a heat transfer mechanism.
Does conduction occur at the speed of light? Not usually … conduction is the painstaking process of one atom giving up energy to the next one, then to the next one, then to the next one … rather than the one atom emitting a photon and some other far distance atom absorbing that photon.
ETA: I like the economic simile … then we have a black market for dark energy …
So energy moving from the bottom of a lake to the top isn’t a transfer? Does conduction then not count as heat transfer if it’s from one end of a metal rod to the other?
Only if you are considering the transfer of heat in isolation as an observer.
The actual mechanism makes all the difference once you try to manage or control the transfer of energy.
How do you make an insulator? Depends upon the dominant heat transfer mechanisms you are faced with.
Here on the Earth the three dominants ones are conduction, convection and radiation. If you want to insulate your roof or walls you need to address each of them, and in doing so you need to address each one’s mechanisms. Conduction you address by physical gaps, or use of low thermally conductive solids. Convection you address by preventing convection cells from forming. This usually means some form of lightweight material that stops the air moving (ie fibrous or closed cell insulation.) Radiation you address with low emissivity surfaces - ie foil.
The classic thermos flask addresses the issues a bit differently. A vacuum provides a lack of conduction and convection, and a silvered surface stops radiation transfer.
Double glazing prevents convection by creating an air gap too small to allow convection cells to form. Conduction through the air in the gap still occurs, but this is vastly less than the thermal conductivity of glass. Many windows now employ a low emissivity coating that significantly lowers radiative heat transfer.
In space you don’t have any convection. Thermal management of spacecraft is difficult. Insulation is built out of multi-layered reflective foils. Getting rid of heat can only be achieved through radiation (unless you are prepared to expend mass).
If you want to actively transfer heat the same question arises. How to manage it to best effect. Better thermal conductors (aluminium and copper perhaps), heat pipes (arguably a new kind of mechanism since it involves phase changes of the transport medium.) High emissivity surfaces - ie black bodies to radiate better, dedicated design of airflow systems to utilise convection. Forced air - arguably advection.
Convection would be as you describe (conduction to a fluid at one interface followed by conduction from the fluid at the next interface) if the fluid didn’t move at all. Imagine two large flat horizontal plates with an air gap between them. Upper plate is hot, lower plate is cold. Heat is conducted from the upper plate to the lower one by the air. The air isn’t particularly prone to circulating because there’s nothing driving it; dense air is on the bottom, fluffy air on top, and so you can probably come up with a good estimate of heat transfer rate by considering just the thermal conductivity of the air.
Now consider the same scenario, but with the hot plate on the bottom and the cold plate on top. The air is unstable because the hot fluffy air is on the bottom and wants to rise. Local thermal updrafts will form, circulating warm air up to the cold plate, and cold air down to the warm plate. The rate of heat transfer between the two plates will be considerably more than you would predict based on just the thermal conductivity of the air. In fact, the situation is so different from a fundamental conductivity problem that it requires an entirely different set of rules that takes into account the local movement of the fluid as determined by buoyancy, geometry, viscosity, heat capacity, external drivers, and so on. Convective heat transfer, in all but the simplest of circumstances, cannot be adequately understood/quantified as just two instances of conduction.
Or consider the Sun. The surface of the Sun is hot because of energy transferred from the core of the Sun. How was that energy transferred? Well, part of the way, it was mostly via radiation, and part of the way, it was mostly via convection. And there are also parts of the Sun which are heated mostly via magnetic processes which don’t occur to a significant degree under Earthly conditions.
Depends on how you want to slice it. Ultimately, it’s all electromagnetic force that transfers heat energy, but you could logically make a distinction between short-range repulsion (what we think of as mechanical, or molecules bouncing off of each other) and longer-range wave transmission.
But the main point is that, regardless of how they appear if you look at things from a molecule’s point of view, for humans convection acts dramatically differently than radiation or conduction. And therefore it’s useful to think about it as a separate method of heat transfer.