Morgan
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Expansion of fuel/air mixtures
Does anyone know the maximum pressure rise on averayge you get with a common fuel/air mixtures in a confined vessel? In my SUV for example there is a
10 to 1 compression ratio before the fuel is even ignited. But I am curious about how many atmospheres you would get if you lit a methanol/air mixture
at ~14.7 atmospheric/STP.
For example 1 volume of fuel/air when lit creates X volumes of gas pressure. If anyone knows a rough estimate taking heat and everything else into
account. I realise flame front speed and the shape and size of the vessel will affect results.
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Picric-A
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Completly depends on what fuel is being used.
For example a 'heavier' fuel will give a larger pressure increase due to more moles of CO2 + H2O(g) being released per mole of fuel.
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Morgan
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If you were flying across the Atlantic, you'd rather get your oxygen from the air than carry it. Jet fuel would take you farther than methanol for
example. And there is that thought that a stick of wood burned would produce more energy than a stick of dynamite. Of course again, you are getting
some of your weight from the air with the wood. And dynamite releases it's energy much faster.
My question stems from wanting to calculate the integrity of a 30 liter vessel before it ruptures. Added to this conundrum is that I have a snorkel of
a certain diameter that is the exhaust/intake of this self-sustaining jet engine of sorts. So I would only be concerned with the initial
"deflagration" in a confined space. If you used kerosene, first it's not very perky and very fussy about fuel/air ratios whereas methanol is very
forgiving. If the vessel is not preheated then kerosene is going to be a dud. If you had a stoichiometric ratio though, you would use a smaller amount
of kerosene than methanol.
The vessel in question will rupture above 60 psi. It is eleven inches in diameter. That means that the top if sealed, which it isn't going to be,
would develop a force of around 5,700 pounds, 1500 pounds more than my SUV weighs. Perhaps there is no danger and yet I have had methanol burn so fast
that it backfires with a great deal of noise when partially confined.
Sometimes too I wonder if a flame front will advance faster in a partially confined space than in a completely sealed vessel. So many things to
consider. Here's a small 2.5 gallon vessel with a 1 inch diameter snorkel. With a larger diameter snorkel it gets pretty energetic, but can be more
fussy to start and very loud when running.
http://www.youtube.com/watch?v=XRaGP6FlZw4
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Morgan
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Some tidbits I came across. My keg is a quarter keg which is the same volume as in this story below.
Exploding Beer Keg Kills Man
"The explosion early Sunday killed Sean M. Caselli, 22, of New Milford. Seven other people were taken to hospitals with burns and shrapnel wounds,
police said. Caselli was struck in the neck by a piece of flying metal.
Sgt. Lee Grabner said investigators interviewed witnesses Sunday to try to identify the person witnesses say threw a quarter-keg of beer into the
flames, and to determine whether criminal charges should be filed."
http://abclocal.go.com/wpvi/story?section=news/bizarre&i...
I saw on the net they are now making some kegs with a rupture disk off to the side of the center hole. So I got to wondering if YouTube would
enlighten me on keg rupturing and I found this lively Mythbbusters episode. My quarter keg is only a 7.75 gallon tank or about three times bigger than
my piglet snorkeler. Another way of looking at it is by comparing it to my tall/narrow 12 ounce quilted crystal jam jars. It's 82 times bigger than a
jam jar. Or you could say it's 82 Coke cans worth. I just wonder if it would be possible to get 60 psi to develop with a short snorkel on the keg?
Naturally the thought has crossed my mind what might happen if I got a "non-uniform" flame front to occur.
Mythbusters Beer Keg Explosion
http://www.youtube.com/watch?v=iFZ15sAQnpw
Funny there is a keg that serves 99 12 ounce drinks. It's called a Wall Keg, (99 bottles of beer on the wall). {See chart for various keg sizes]
http://en.wikipedia.org/wiki/Keg
[Edited on 16-2-2011 by Morgan]
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Morgan
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I guess I could try putting a pressure gauge on some scuba tank or something indestructible. I think they hold enough pressure but I don't know if a
gauge would respond fast enough. I have a tank I unscrewed the valve off of.
The experiment would be to see if an uncompressed methanol fuel/air mixture would rise to ~4 atmospheres/60 psi if ignited in a closed vessel. Maybe
if I had multiple ignition points the test would be more telling.
Seems somewhere there are some studies of the expansion of fuel/air mixtures. I saw some videos of barrels that were exploded for some government
research. But 55 gallon drums are huge and not made for pressure. And they really go off with a bang. They had nets around some to catch debris and I
recall sudden flame flashes as they strained the netting. The larger the vessel the more surface area each psi is going to have to weaken it.
[Edited on 17-2-2011 by Morgan]
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