Jetboil Zip Hacks … Part 2

[David Thomas]

Again, I've got Stuart's back on this. His numbers are all correct, although I'd explain it differently.

Almost all resistance to heat flow is in the air film nearest the pot. The metal in the pot is highly conductive – so much so that for the main pot (not the heat fins) aluminum versus stainless steel or titanium makes essentially no difference. Here's an analogy from electrical circuits:

If you put 1000 volts across these three resistors:

1000 ohm – 1 ohm – 20 ohm which represent:
air film – pot – water film

Where will all the voltage drop be? Across the 1000 ohm resistor. The 50 little HX fins are sort of like 50 little 50,000-ohm resistors in parallel with that first 1000-ohm resistor. They carry the heat through that air film more efficiently.

A weaker analogy to hiking:

You are a hiking packet of heat. The air film is a 1000-foot vertical cliff. The metal pot is a 1-foot step. The water film is a 20-foot-high slope. Virtually all the work in getting all the way is in the 1000-foot cliff. With the fins, that becomes a 500-foot-high slope and more hikers (heat) get to the top (the water).

[James Klein]

Ok I looked at the back of my envelope and yeah somehow I got off by 10^3 – FAIL. My BS filter should have caught 700C result, that is a mega heat rate (literaly). I'll hang my head in shame for the rest of the day :)

.5C is tough to imagine (that blue flame is so dang hot) but that is what the numbers show. My speculation is loosing its momemtum. The point about the boundary layer does make sense.

David I had been thinking about the same analagy…just that for estimates for resistance btw the two convection interfaces I put much closer to each other. Actually, I was predicting the water-Al interface was the bottleneck -b/c of natural convection over forced (even considering higher conduction of liquid water over combusion gases). I still bet they are pretty close but my intuition has been letting me down lately…
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Edited to keep from bumping this thread with my nerdy misfires…
Re: where the real resistance is…
deltaT from inside pot to bulk water can't be more than say 20C (based on when I start seeing bubbles)
delatT from inside of pot to outside is less than 1C (thanks Stuart)
deltaT from outside pot to bulk combustion gas must be in the 1000Cs (adiabatic flame temp is 2000C)

Equivalent resistors would be something like 1500ohm, 1ohm, 20ohm…pretty good ballpark David.
Thank you David and Stuart for clearing up my poor assumptions.

[David Thomas]

It's almost all in the air film. I didn't look up the thermal conductivities nor model the natural convections of water in the pot, but those resistance values I gave are in the ball-park.

The water is 820 times denser than the room-temp air. 2,000 times denser than a hot flame

The water has a heat capacity of 1 versus 0.22 of the air.

Liquid molecules are 10 times closer to each other in water than in a gas. Ten times in each of THREE dimensions.

When a slight heat differential develops across the water film (10-15F?) all the heat still stays in the water where you want it.

But with the large heat differential across the air film (330F-500F?, without and with fins) the hot air leaves and is lost forever.

Resistor analogies: Before computers could solve messy differential equations, complicated heat transfer problems would be recreated with massive 3D networks of resistors and the voltages measured. You'd get the steady state solution but that's often easy to calculate. But with some capacitors and inductors, you could also measure the transient solutions which are much trickier to solve. Of course all of this assumes a cheap source of labor – grad students!

[Stuart R]

Another way to think about it it this: the combustion gasses are mostly hot air, plus some CO2, a little steam and a lesser amount of CO. Air we usually think of as an insulator – in down clothing etc. So, once the thin film of combustion gasses in direct contact with the pot (which is never more than 100C) has given up its heat to the pot, that layer of gas then insulates the pot, preventing the heat in the rest of the gas from getting to the pot (unless the pot has fins).

[Curt Peterson]

Updated with new tests….

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?bo=watch&forum_thread_id=59535

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