Cooling things
Physical mechanics of cooling
On the technical side
Moving heat, in general, happens though:
- conduction
- a good conductor spreading heat throughout it more quickly
- something might get cooled if it is in direct contact with something it can move heat into -- but that is rarely a large effect
- conduction helps cooling in the sense that if heat can spread easily, it more quickly gets to areas where radiation and convection can happen
- radiation
- specifically thermal radiation - radiated as EM at the surface
- which in most of our everyday cases means 'if it is hot, it will radiate heat from its surface; more surface make that go faster'
- convection
- fluid passing over something will exchange heat into that fluid
- whether that fluid is air (poor exchange, but it's there anyway)
- or water, or oil (better exchange, but if it's more management to keep it clean)
- as a bonus, that heat exchange may set up more flow (caused by heat changing temperatures and densities - plus gravity) that sustains more exchange for a while
- e.g. warming up air will create a draft
Notes:
- In practice there's more than one of these happening, but often one that counts for most exchange.
- Those three happen easily and all over the place.
- There are a handful more, which usually require some specific design,
- and more when we startnaming specific designs.
- Say, phase change, often focusing on evaporation - as used in the refrigeration cycle.
- The speed at which you can move heat is generally proportional to the amount of temperature difference.
- conduction has some everyday intuitions - that might be distracting
- you know how metal feels colder to the touch than most other materials?
- that's not because it is lower temperature somehow
- but because most materials heat up locally first and we are used to that happening within seconds
- this sense of "metal feels cold" only happens
- because the quick movement of heat means it heats up evenly
- is more pronounced when you touch it when it is below body temperature - you will barely get this sense touching metal when it's 35 C outside.
Passive cooling
Passive cooling tends to mean 'what happens with no moving parts', so whatever amount of conduction, radiation, and/or convection would happen anyway without any help.
In most places, passive cooling doesn't move a lot of heat, in part because the temperature difference is never large.
Passive plus a little help
Heatsink plus fan
A fan on a heatsink is frequently a good idea because it often keeps the temperature difference at the point of transfer higher, so the heat transfer keeps going a faster than if warm air just sits around.
The difference is rarely much -- convection always does this at least a little, when there is temperature difference (if you're in gravity; this is about density differences).
Technically this is active cooling - because you're adding work, so using energy -
but intuitively it feels like it hardly qualifies.
- In particular because heatsink would probably stir the air a little by convection, and we're just ensuring that that happens, a little more, and even if there is a casing in the way.
- Just stirring better is not going to make the world of difference. It also does not feel like not a completely different category.