David,

I don't want to hijack this thread but after carefully reading your latest post I can't help but conclude that you are smarter, a lot smarter, than me. It reminds me of why I didn't major in any of the hard sciences in college.

Daryl

I think the heat transfer difference between smooth and rough cans could be attributed to reflectivity more than by turbulence. In all this discussion, no mention has been made of preparing the surfaces to maximize infrared heat transfer to the can. This includes the windscreen component.

To optimize, I would want a very IR reflective windscreen and absorptive pot. Flat black anodized pot.

I wonder if Ti performs worse than Al because of its poorer reflectivity (which it may not be at IR). And clearly (p.i.) the coleman screens don't reflect as well as Al.

I think these points completely dominate over turbulence/laminar concerns.

Nathan, you are no doubt correct that a flat black pot bottom can help efficiency. There was a prior thread that I know David was involved in that theorized and then tested that very premise. It was about a year ago. I painted the bottom of my Heine pot black with high temperature paint after that thread.

I continue to believe there is a significant amount of convective heat transfer as well. That's why jetboil has the fins. I know I read another thread that David was involved in too that showed some increase heat transfer when thin metal fins were added.

Nathan,

Interesting thoughts.

If I understand you correctly, you think the main factor in the difference in boil times between a regular Foster's can and a brushed Foster's can is because the brushed can is more IR absorbing, rather than the surface roughness. Not sure how to verify empirically. I could buff and polish the can pot so it is shiny and reflective, but that would also make it smooth.

I have spoken to an anodizing company and they can't do a can that has not been emptied. They also cautioned that the anodizing process uses some fairly nasty chemicals that must not be consumed but which would be deposited on the inside of an empty can. I have an appointment with them tomorrow morning to see if they can even do something as thin as a beer can. If they can do it I will get at least one can anodized for experimental purposes.

Painting flat black is easy, and I tried that in early experiments....didn't seem to make any difference. However, I will try it again under more controlled conditions.

Will also look into other processes by which bare aluminum can be blackened.

Will try polishing the inner surface of a windscreen too, to see if increases IR reflectivity. I use stainless steel and titanium for windscreens, so I will try one of each.

Do you know if Ti performs worse than Al as a windscreen, and are wondering about the cause, or are you wondering if Ti actually does perform more poorly than Al for that use?

Check out Caswell for diy anodizing. I'm sure you could make a rig that only anodizes the exterior. Not sure if it is worth the effort. That said, paint doesn't conduct heat so well and it is heavy...

I only just became interested in myostoves (and made 5 different types in an afternoon - love it!) I'm moving on to the windscreen/support, so no knowledge as yet of the difference between Ti and Al wrt IR reflectiveness.

I'm keen on building an accurate test setup that measures heat transfer in degrees/minute, not just boil times. It needs to have a consistent heat source so I'll use a cannister stove and let everything stabilize before I track the temp rise.

I think it goes like this: Set it all up with cold water, then let the temp rise to 100 F and track time to 200 F. Needs to be stirred while rising to prevent convective hot spots, etc..

I would expect that a titanium wind screen would hold the heat better than aluminum, due to its poor heat conductivity.

Will Reitveld, a contributor here on BPL, uses a digital thermometer and digital scale to calculate degrees/minute. His typical test protocol is 500 ml of water and 14 grams of fuel, with water, fuel and air temp all the same and usually cool(40* F - 50* F) to simulate real world conditions. Don't know if he stirs too.

Figured out a way to blacken the outsides of the cans without using any chemicals or paint. Will try some test boils later. I use a meat thermometer and measuring cup, so it's not as accurate as digital, but close enough I think for comparisons and reasonably accurate calculations.

"I would expect that a titanium wind screen would hold the heat better than aluminum, due to its poor heat conductivity."

Gary, why can't you make a windscreen out of titanium wire with a cover of aluminum foil?

--B.G.--

Edited by --B.G.-- on 01/21/2013 15:03:51 MST.

Here is an older post I referenced above that dealt with some of the same issues:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=58270&disable_pagination=1

I thought it had some pretty good info.

Ok, we're back to the titanium windscreen guys.

I added some 1/2" holes and burned some wood in it. I put about 2 hous of actual burn time in to test for strength and it's ability to retain it's shape. All went well.

After the wood test I went ahead and turned it upside down and tested a few esbit cubes in my esbit burner (esbitmizer). I'm able to place the pot onto the support and then put the windscreen in place. Once they are ready I put the cube into the esbitmizer, light it and slide it under the windscreen into place. The lid of the burner stays attached and I'm able to slide it out if I want to reduce the flame height to simmer or snuff it out. If I snuff it out I can let the pot sit on the support with windscreen around it till I'm ready to use the water.

http://www.youtube.com/watch?v=OsUx4P0tWNs

So I read the prior threads regarding painting the can pot bottoms black and putting fins on. I blackened a couple of pots by boiling them - whatever is in the tap water here reacts with the bare aluminum and blackens the cans very effectively.

Boil tests did not reveal any improvement from blackening the cans. Did three test boils using 500 ml of water and 17 ml of fuel with air, fuel and water all at 60* F. Average time to boil plain brushed can = 4:26 min. Average time for blackened cans = 4:35 min.

Tried adding fins at the can base, angled at about 30 degrees to the vertical axis.

Surprisingly, the boil times were measurably longer with the fins. Average of 3 boils under the same conditions was 4:53 min. Messy as heck too. The adhesive on all of the fins burned off.

I'm going to stick with plain, brushed pots. Seems to work the best and certainly is a lot less work than blackening and finning.

I added some 1/2" holes and burned some wood in it. I put about 2 hous of actual burn time in to test for strength and it's ability to retain it's shape. All went well.

After the wood test I went ahead and turned it upside down and tested a few esbit cubes in my esbit burner (esbitmizer). I'm able to place the pot onto the support and then put the windscreen in place. Once they are ready I put the cube into the esbitmizer, light it and slide it under the windscreen into place. The lid of the burner stays attached and I'm able to slide it out if I want to reduce the flame height to simmer or snuff it out. If I snuff it out I can let the pot sit on the support with windscreen around it till I'm ready to use the water.

http://www.youtube.com/watch?v=OsUx4P0tWNs

DavidG:

If you need something for your late Christmas list, add an IR thermometer. My first one was $150, but they are $30-ish now. Look at ebay, J C Whitney, etc.

Here's the take-home message (I learned this from the first human to (1) quantum-entangle more than 2 photons and (2) find an exception to the Heisenberg Uncertainty Principle - my wife and I each have brothers smarter than ourselves): clean metal surfaces are MIRRORS to infrared radiation. That is bad, because heat your pot could absorb will be be reflected away.

With your spiffy, new, IR thermometer in hand: fill your pot with hot water. Check its "temperature" with the IR thermometer. If it is a "blackbody" (good), then it will read the water temp. If it is reflective of IR radiation (bad), it will read room temp. Maybe your treatment made it "black" in the infrared. But maybe not. Test it compared to actual black paint (BBQ grill paint or auto-parts-store paint for an engine block are both high-temp paints). Look at my tests from 13 months ago and I found that white, black and red paint were all "black" in the infrared.

The tabs seemed too big and too low. Big because you're trying to energize only the air film very close to the pot. Low because they were in the very hot gases. You want to "stir up" the somewhat cooler air. I'm guessing there was still a visible flame at that point (most HX was visible+IR radiant transfer) and therefore, the IR radiant HX continued further up the pot. Also, if those tabs fit within your windscreen, your windscreen could be a fair bit smaller. I'd guess you are diluting the flue gas temps with excess ambient air and that will definitely hurt HX.

One approach is "time to boil". That's good but subject to a lot of variables (room temp, stove temp, water temp, etc). Exhaust gas heat is another thing to monitor. If exhaust gases are small in flow rate and cool in temperature, you have captured more heat from them.

But a few careful experiments can often blow the best theories right out of the water.

-David

Who cares about boil times in a beer can: tell us more about the exception to the Heisenberg Uncertainty Principle!

Edited by book on 01/21/2013 22:10:14 MST.

David Thomas,

Groan. I'm not even done paying for the holidays yet, but I can see I've still got shopping to do.

Thanks for the thought-provoking comments.

I'm empirical and Edisonian in my approach. I studied hard sciences in college and have an engineering background so am generally familiar with the theoretical side of things. But I test and verify because practice/reality has an uncomfortable way of throwing unknown variables into the mix.

I've tried the BBQ and black engine block paint, but dozens of test boils did not reveal any advantage to it in terms of faster HX or greater fuel efficiency, at least not with the paint I used. Perhaps the paint acts as an insulating layer, perhaps the top part of the can radiates heat more rapidly than when it is shiny, perhaps it's something else. The IR thermometer will help sort that out. What I can say for sure is that the brushed cans I tested heated more rapidly from the same heat source than unbrushed cans, painted cans, or water-blackened cans I tested.

I use frustum windscreens, so the base is wider than the top. At the top the windscreen is about 1/4" from the can. I will try smaller and higher fins.

I will also try polishing the inside surface of a windscreen, to see if it reflects a measurable amount more heat back into the can.

I'd really like to try putting a ridge like Zelph's cans about 1/2" above the top of the windscreen to see if the ridge jutting into the stream of exhaust gases captures more heat, but I don't have the tools yet. More items for the shopping list.

At the end of the day and we put all our boy toys away we can look back and say we've tried out best for today. Tomorrow will be better. :-)

I'm completely happy when I can get an alcohol stove to boil 2 cups of 65-70 degree water using 1/2 ounce of denatured alcohol.

David Gardners' brushed cans have been brutally stripped of their protective coating allowing them to be exposed to the harshness of exhaust gases, the elements and greasy grimey hands. The brushing also removes aluminum from the very thin wall. Life of the can is diminished.

Way back when, in the days of "radiators" to heat our homes with water it was said not to paint the surface of them. Painting reduced their emisivity or ability to radiate heat. So if it prevented heat from going out I would also think it prevents heat from coming in. Let your cans be as is.

We should keep in mind how small of an item we are working with in terms of heating. Gee whiz, you guys are extreme. Be happy with your stoves that boil 2 cups with 1/2 ounce of fuel. There are so many DIYselfers out there that get freaked out when they read all this stuff about the quantum theory of alcohol stoves and the miricals of Titanium.
The Modified StarLyte burner has a lot of merit when it comes to heating in a confined space, controling the release of vapors. David Gardners' stove on the hand has a lot of open space and the fuel burns in a radical fashion just like a Supercat stove made with a Fancey Feast cat food can. As we know, the Supercat stove can heat 2 cups of water with 1/2 ounce of fuel in 4-5 min.

>Who cares about boil times in a beer can: tell us more about the exception to the Heisenberg Uncertainty Principle!

Jeff: While at UC Berkeley I caved with a physics grad student. He later brought his sister along on our annual gourmet BPing trip. I brought ice cream sundaes and a hot tub 6 miles in on that trip (had some Sherpas to help). He went on to a professorship at Spain National Physics Institute. His sister graduated med school, we married, and moved to Alaska.

An overview: http://physicsworld.com/cws/article/news/2011/mar/24/quantum-probe-beats-heisenberg-limit

An excerpt: . . predicted that the Heisenberg limit could be beaten by introducing nonlinear interactions between the measuring particles. That prediction has now been shown to be true, thanks to an experiment carried out by Morgan Mitchell and colleagues at the Institute of Photonic Sciences at Barcelona. Mitchell's group fired laser pulses into a sample of ultracold rubidium atoms held in an optical trap and measured how the atoms' spin angular momentum caused the polarization axis of the photons to rotate.

Well thank God that's cleared up! I was betting on death or taxes, or both. I'm glad that no imaginary cats were harmed in these experiments. I work near Cal, so here's a shout out from your old digs.

And we thought that The Quest for Fire was complicated.

Whew!



Does anyone remember the KISS principle? Keep It Sublimly Simple.

Fuel goes into stove. Stove is lit. Water goes in pot. Put lid on pot. Pot goes above flame. Windscreen goes around stove and pot. Water boils a little later.

Hint: Do not watch the pot. ;-)

What have we learned from all of this discussion? Raise the windscreen 1/2" to 3/4" or use either large draft holes or rectangular draft vents on the bottom of the windscreen to keep the stove's flame stable and under the cook pot.

30 seconds here or there won't really matter. If it does carry a canister stove and be done in half the time or less.

@Dan,

You have burned more alcohol, wood and Esbit tabs than any 100 people I know. You're always looking for new, lighter and more efficient ways for us to boil water. I am impressed by the way you share your knowledge with do it yourselfers. Thanks for the continued experimentation. Please don't stop.

Party On,

Newton

>"no imaginary cats were harmed"

LOL.

His research group has adopted a logo of a kitten and a rubidium atom. Playing on the same joke.