How important is ambient air temperature in the testing of little DIY alcohol stoves?

You've seen it many times where someone gives the statistics during the course of testing their newly designed DIY alcohol stove.

Beginning water temperature, ambient air temperature, amount of fuel, amount of water etc.

What is your opinion and why?

Ambient air temperature sets the temperature of the alcohol (before it cools by evaporation). So going from 30 C to 5 C ambient means you have to warm the alcohol up more to get it to boil, and burn. That takes more priming time and fuel.

Beginning water temperature ... ah yes. The 1st Jetboil stove was initially tested (by Jetboil) with water starting around 30 C and going to just bubbles, which is why they claimed such a fast boil time and such excellent fuel consumption. Once this had been exposed (by BPL, among others) they modified their advertising somewhat. There was a hilarious article about this in one of our OR Reports some years ago.

Cheers

A BTU (British thermal unit) is a unit of measure that relates to the quantity of heat required to raise the temperature of on e pound of water one degree Fahrenheit.

My opinion / hypothesis and why is...

The lower the beginning water temperature the more BTUs will be required to bring the quantity of water to a boil.

The ambient air temperature can steal away some of the heat from the water container (cook pot) if it is cool to cold. If the ambient air is relatively warm to hot it may have little or no effect.

IMHO the amount of fuel is directly proportional to the amount of BTUs available from a given stove and the fuel used in that stove.

As the amount of water to be boiled is increased the amount of BTUs needed to bring it to a boil increases.

More water requires more heat (BTUs). More heat can come from a larger stove and / or a hotter burning fuel. Less water equals a smaller amount of BTUs needed for a boil which could relate to a smaller stove and / or a cooler burning fuel.

IME on my last 24 hour overnight I needed more fuel to achieve a boil with approximately 50 degree water in ambient temperatures that were about the same than I did using my same stove in my tests at home using 75 degree average temperature water in 80+ ambient temperatures. I've discounted the effects of wind for the simple reason that my tests were done in calm conditions and on my recent hike supper was prepared in dead calm conditions.

Dan you took me all the way back to school and my text books! ;-)

Party On,

Newton

"The 1st Jetboil stove was initially tested (by Jetboil) with water starting around 30 C and going to just bubbles, which is why they claimed such a fast boil time and such excellent fuel consumption."


Interesting, yeah I've been able to achieve 1.5 minute rolling boil with my Jetboil Sol. Used ambient temperature water (75F maybe?) outside in a Shelter. Surprised me because my normal boil time was 1:45 to 2:00 minutes. Still going strong after 6L boiled from a large canister.

"Ambient air temperature sets the temperature of the alcohol (before it cools by evaporation). So going from 30 C to 5 C ambient means you have to warm the alcohol up more to get it to boil, and burn. That takes more priming time and fuel."

There are differing types of alcohol used in DIY / MYOG stoves. Everclear in both 190 and 151 varieties, Heet in the yellow bottle, DNA with varying mixtures of Methanol, Ethanol and "denaturing agents" and Isopropyl in the 91% and 70% varieties. All have different BTU outputs and boiling points.

Alcohol stoves do have the "problem" of difficulty in lighting when the temperature dips really low.

Party On,

Newton

A couple of years ago "Tony" designed a "shadow graph" of sorts that enabled us to view the heat waves along the sides of a pot that was sitting on top of a flaming stove. Ever since that time I had doubts if ambient air temperature needed to be included in our calculations.

The entire pot, including the top was engulfed in a hot column of air. The pot seemed to be protected/insulated from ambient air temperature. Even in really cold weather the pot would be protected from ambient air temps by the column of hot air rising/engulfing the entire pot.

I'll go back to my website and see if I can find the thread created by Tony and see if he was able to determine the temperature of the air around the pot at the time of his graph photos.

Take a look at these photos to see how heat surrounds a pot to isolate it from ambient air temperatures:

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Laminar Flow versus Turbulant Flow
http://www.bplite.com/viewtopic.php?f=25&t=1086

Edited by zelph on 11/07/2012 19:07:03 MST.

Does the EPA test for miles per gallon for cars during hot and cold weather? They must have addressed this ambient air question at some point,

Question(s);

If it insulates the pot in cold weather aren't you using the BTUs produced by the stove to insulate the pot from the cold ambient air and thus using more fuel than you would in a warmer ambient temperature?

Given a warmer ambient temp, wouldn't this same condition have the effect of heating up the pot quicker?

Second law of thermodynamics simply stated this just says that heat always flows from hot objects to cold objects never from cold objects to hot objects.

If the ambient air is cold wouldn't it "scavenge" some of the heat from the stove and require a longer burn time?

Party On,

Newton

Schlieren photographry is the name.
This is well known to astronomy.
If you interpose your hand into an open tube telescope between the primary and aceon..
anyways.. you can see the heat waves coming off your hand.

The laws of thermodymanics always follows entropy.. that is energy of a higher state falls to the lower state.

So the energy you see in the Schlieren pic is being lost beyond what you can see.
The insulating effect is mitigated to some degree by losses radiated away in all directions.
The reason you see anything at all is becasue the pot intercepts and deflects some of the hotter air and refracts the light.
A lot of heat is still lost to (ivisible) radiation besides what you see.

In other words, a slight breeze would negate the effect unless a windscreen were used.. yea that is obvious.

Still a cool effect.

To answer: Either way, heat is being sucked out ny cold OR radiated away.. NOT both.
There is no additive effect.. it is a semantic argument.

>Schlieren photographry is the name

The technique I used to take the photos is called shadowgraph, while it is related to schlieren, it is a simpler method than schlieren and schlieren photography methods.

http://en.wikipedia.org/wiki/Shadowgraph

Tony

In the second photo from the bottom Tony shows the "boundry" layer and the "Turbulent" flow. We can invision that hot air going up the sides of our 4" tall pot at considerable speed. I'll agree some heat is lost from the boundry layer and will not affect the heat right next to the pot.

One of the photos shows the Starlyte stove, it lights easily in zero degree weather. The heat generated around the lip of the stove sustains evaporization. Does the stove body lose heat due to ambient air temps...YES.....does it affect perfomance....YES...to what degree????

I would venture to say that 99% of initial stove testing is done inside our homes and garages to determine if we have a stove worthy to be announced on a DIY forum.

If the stove can boil 2 cups of water with 1/2 ounce of denatured alcohol under ideal conditions we have a winner. Ideal conditions are 70 degree starting water temperature and air temperature. Those are easily controlled in an indoor environment. Some of us do 10 - 15 tests during the course of a day so we need constants to get good results.