The JAD V2 Kit Showcase - 1x18650 Heavy-Duty C6

[Wolfman29]

Possibly. That's slightly ambiguous, but my gut says yes. Because, do note that the higher the temperature differential, the faster cooling will take place, so that also means that, while aluminum will drop in temperature faster than the copper initially, at a certain point, I am sure the copper will cool off more quickly because it will have a higher temperature.

 

[Hiemal]

Basically, I'd like to think of it as two buckets.

One, larger bucket is copper.

The other, smaller, bucket is aluminum.

You fill both buckets up with water (heat) The smaller one fills first, then the bigger one. Then slowly pour the water out.

As the water leaves the buckets, the smaller (aluminum) one will empty faster than the bigger (copper) one.

However, the copper one holds more water/heat than the aluminum one.

So, the copper one can be said to be better than the aluminum one.

 

[Wolfman29]

EXCEPT, you have to imagine that the hole at the bottom of the copper bucket is slightly larger than the hole at the bottom of the aluminum bucket - it has a higher thermal conductivity, so it conducts heat to the air better.

 

[gif]

nice clean build ryan, good luck with your JAD KITS bud :beer:

 

[dyeote]

I'd have to check back again, but I am pretty sure the rate of heat transfer is proportional to the product of the two heat conductivities.

EXCEPT, you have to imagine that the hole at the bottom of the copper bucket is slightly larger than the hole at the bottom of the aluminum bucket - it has a higher thermal conductivity, so it conducts heat to the air better.

Have you actually checked this? I'm pretty certain that it's not a product of thermal conductivities, or at least its not just that.
Think of a block of material with infinite k (thermal conductivity): it's going to equalize the temperature across all of the block in zero time, but it's not going to lose all of its heat to the surrounding gas in zero time...

I'm actually very interested to know: What factors determine the heat transfer rate on a metal|gas interface completely? Discount convection and BB radiation and assume that the entire body of metal has the same temperature; I'm interested only in the conduction of heat across the interface.

Please feel free to use complicated terms and link to your citations; I'm trained to handle those

 

[Wolfman29]

I haven't done extensive research in this - I was just going off of what I vaguely remember reading. However, from a brief search, I found what I thought I was citing. Turns out, I was thinking about something else entirely. Rather, I was entirely wrong =p

Turns out, the rate of heat transfer between two solids is inversely proportional to the sum of the inverses of the thermal conductivities of the two solids. That and some other constants, but all else the same, that is the "deciding factor." Clearly, the "thermal contact resistance" plays a huge role too - how good is the interface? Nonetheless, though: the bigger the thermal conductivity of one interface, the faster the rate of heat flow, certainly.

However, from some cursory research (VERY cursory), I can't find anything on gas/solid interfaces.

 

[dyeote]

The 'inverse sum of inverses' coefficient (which is the same as division by the straight sum of thermal resistivities) in solid|solid interface transmission rates is in line with my own findings.
I was struggling with a somewhat dissimilar problem a couple years back for a calculation of convectionless heat transfer on the interface between water (which I ended up regarding as fast ice) and (boiling) liquid Xenon - I had done a less cursory research on solid/gas and solid/liquid heat transfer mechanisms in fast, low-temperature systems back then and could not find anything either :yabbmad:
I ended up writing a makeshift kinetic model for calculating the rate that the last two molecular-level planes of ice can move energy into the phase-changing cryogen :thinking: ... ... :tired: ... hard to believe that no one has published a serious address of the metal/air system though... let me know if you do find something.
My reason for obsessing over this point is a feeling that there is relatively little convection involved in our handheld laser hobby; many hosts are unfinned and most of us don't stand next to a fan at all times :tinfoil:

 

[Wolfman29]

Well, the way I see it (and this is just an intuitive understanding - I haven't deeply studied thermodynamics yet), the key to thermal conduction from a solid to a gas is pretty much this - you have a constant temperature gas and a lowering temperature metal. The gas is constant temperature because it will nearly instantly spread the temperature throughout itself. Free molecules and all that. So then we just have kinetic interactions of molecules with the metal... and I suppose that would be considered micro-leveled convection, no? I mean, no gas can really just "contact" a metal. Really, it's just striking the metal and then bouncing off, taking away some of the energy from the metal. So maybe there is a way to formulate it that way?

I'd like to see the model that you wrote, by the way.

 

[dyeote]

Well, the way I see it (and this is just an intuitive understanding - I haven't deeply studied thermodynamics yet), the key to thermal conduction from a solid to a gas is pretty much this - you have a constant temperature gas and a lowering temperature metal. The gas is constant temperature because it will nearly instantly spread the temperature throughout itself. Free molecules and all that. So then we just have kinetic interactions of molecules with the metal... and I suppose that would be considered micro-leveled convection, no? I mean, no gas can really just "contact" a metal. Really, it's just striking the metal and then bouncing off, taking away some of the energy from the metal. So maybe there is a way to formulate it that way?

I'd like to see the model that you wrote, by the way.

Our intuitions are similar, and it's exactly that "micro-leveled convection" of colliding molecules that I'm pondering :thinking: and hoping we can find a readable article about
Anyway, I'll search for one, and try to find where I left that model (it's a combined analytical/numerical calculation in a Mathematica file), after I wake up; I'm basically sleep-posting now <-- :tsk:

 

[Wolfman29]

Hmm. Interesting. Well, I will do more research tomorrow after work - almost bed time for me.

Also... I noticed you're in Rehovot! Fellow Jew? I was in Rehovot two years back.

 

[dyeote]

Found the model, PMd you about it.
How about you, found anything good on metal/air interfaces?
I'm a fellow human (who happens to also be a Yid ).
What were you doing, two years back, in our "City of Science and Orchards"?

 

[Wolfman29]

Been busy with work so haven't actually looked further. I was on a school trip, actually. I'll go take a look at that PM.