gregxy
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Potato cannon and explosion physics
A while back I made a potato cannon that burned MAPP gas.
The combustion chamber was 6" dia and about 18" long with
2 spark gap igniters positioned on the axis of the cylinder
1/4 of the way from each end.
If I blocked the barrel with just a crumpled up piece of paper
and fired it the, the cannon would still make a large "boom".
Is it possible that the combustion made DDT? (I don't know
enough about acoustics to know if a burning gas can produce
a sound wave as opposed to a detonating gas).
A moving object like a bullet does not produce a boom until
it goes supersonic, but a vibrating object does produce a
souund and still move slowly. Perhaps the burning gas
just causes the air in the pipe to resonate.
I computed the maximum static pressure as follows:
Assume the fuel, initially at 300K and atmospheric pressure
(15psi) burns instantaneously at 2000K in a sealed chamber.
The pressure (using the ideal gas law) will be :
15*2000/300 = 100psi.
If the gas does detonate, can it generate pressure greater
than this?
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hissingnoise
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A gas explosion such as you describe is not a detonation per se, it is a fast burn or deflagration.
For a gas to explode it must have air or oxygen mixed with it.
Black powder deflagrates but a black powder cracker explodes.
BP has never been known to detonate despite how it may look and sound.
A detonation is driven by a shock-wave moving through the body of explosive, not by the heat of reaction/explosion.
Explosions are not instantaneous, though a nuclear explosion comes close. . .
I would say the pressure generated would be considerably higher than the figure you give.
A stoichiometric mixture of acetylene and oxygen would when ignited appear to detonate but, strictly speaking, it is still a deflagration, albeit at
very high-speed.
[Edited on 17-4-2009 by hissingnoise]
[Edited on 17-4-2009 by hissingnoise]
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gregxy
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Burning air/gas can transition to detonation and
as with primaries it is more likely in a closed space,
probably because incipent shock waves reflect off the
walls of the container.
Here is a paper on the topic:
http://etd.gatech.edu/theses/available/etd-10282005-220716/u...
Based on the paper, if my cannon were burning H2
detonation would be pretty likely, but since it is burning
MAPP it is probably not. Still I think it is interesting that
the cannon produces a "boom" even though there is no
container to rupture an suddenly release the gas.
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Mr. Wizard
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I think most potato guns are about 30 psi at max running propane or alcohol. Discussions about using a similar device and compressed air at 100 psi
say it's a lot louder and goes farther than the normal fuel combustion ones. This makes me think the 30 psi range is about right. I don't think the
air fuel mix does anything but burn. I don't know about MAPP or Acetylene, but I wouldn't try it.
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hissingnoise
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gregxy, I couldn't download that paper for some reason. . .
Bubbles (minimal confinement?) of H2/O2 from electrolysis will ignite with a sharp report but the reaction is propagated by heat transference, not by
shock and is not a detonation in the strict sense.
A paper bag of C2H2/O2 will give a loud bang on ignition, but again, it's just a very rapid deflagration.
People often assume that explosion and detonation are interchangeable but they are not the same thing.
Sorry to sound pedantic. . .
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497
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I know people who have run potato guns with acetylene. They say it works good..
It takes at least 1-2g of tetryl to directly initiate a detonation in air+acetylene mixtures. I'm not sure how easy it would be to get it to DDT..
Probably a long metal pipe would be required..
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grndpndr
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Ive seen the acetylene/oxygen mixed gas explosion during construction slack time just using guestimation and a red hot wire to det., pretty
impressive.All this talk about deflagration versus detonation I decided to check some references about det velocitys of explosives.Whether an
initiator is used to get these figures I dont know.But these are the results from a source found from this site and backed up by another source of
detonation velocitys. acet/02 -2/1, 2660MPS, 2/3 ratio- 2720 mps,3/1-2520MPS.(Detonation velocity and lead block test)http://x40.deja.com Taken from a
sampling are the following sources for the table. K+M-Joseph kohler,Rudolf Meyer,Explosives 4th ed.,Tenny L Davis Chemistry of powder and
explosives,Melvin A Cook The Science of High explosives.
Beyond reproach id think?
The shockwave felt pressure and deafening detonating crack from a small balloon with a guestimated amount of the acetylene/oxygen the crack of the
explosion sounds more akin to detonation than say a deflagrating pipe bomb.So it
seems clear to me at least 02/acetylene MIXED GAS is a detonating explosive quite likely from simple ignition.I think given the Table and its source
the burden lies whith those who would maintain this is a deflagration and not the result of a detonation.All due respect.
Even if you could get a detonating mixture in an unconfined
area that seems very difficult on the other hand an enclosed
structure or cave shouldnt be difficult the result wouldnt be a FAE or nescessarily strictly thermobaric but it seems these devices that increasingly
exhibit combinations of both are becoming more well known though this would seem to lean to the thermobaric?It also wouldnt seem practical except for
larger devices,it seems the smaller warheads are leaning more towards the thermobaric, or enhanced blast warheads utilizing sintered powdered metals
and existing oxygen in the atmosphere or an oxidizer in the surround matrix.?MHO
I still maintain that overpressure/blastvelocity are more than adequate to kill and destroy reinforced concrete is generated by simple coal /organic
dust FAE which i dont think can be argued given the extensive documentation of the same.So whether a true detonation is achieved sometimes is of less
import than damage caused to the military PTB.[Edited on 27-4-2009 by grndpndr] Hissingnoise?
constructive criticism?!497?... comments/mistakes?Eager for replys,where i am mistaken?
[Edited on 27-4-2009 by grndpndr]
[Edited on 27-4-2009 by grndpndr]
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hissingnoise
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I'd class a detonation as a decomposition supported and propagated by a shock-wave travelling through an explosive; a fast deflagration relies on the
(rapid) conduction of heat for its propagation. . .
A purist view, perhaps!
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gregxy
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The reference below documents DDT in hydrogen/air
mixtures in open pipes. The transition (under best conditions)
takes place in a length about equal to the diameter of the
pipe. Ethylene/air can also DDT but longer lengths are needed. Ignition was by an electric spark. No primary is
needed.
I'd still like to know if we can tell anything just by the
sound? If it makes a "bang" and there is no container
to rupture is it a detonation? Does a "bang" imply
a shockwave?
http://etd.gatech.edu/theses/available/etd-10282005-220716/u...
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hissingnoise
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Actually, a bang is a shockwave. . .
You can't have one without the other!
High explosives generally are capable of detonation without any confinement.
A deflagrating explosive can explode without being in a container when the quantity is great enough---in these instances the large mass of explosive,
itself, provides self-confinement.
[Edited on 29-4-2009 by hissingnoise]
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grndpndr
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In the case of the gas mixture of acetylene/02 what passes
as containment is just that, not confinement so no container/confinement to burst releasing an 'explosion'.And while not a HV explosive a high
explosive none the less as the det velocity exceeds by a comfortable margin whats generally accepted det vel of a high explosive.I expect it comes
close to the actual det velocity of many anfo explosions and we all agree thats a high explosive?!
Dust explosions on the other hand are exactly as you describe a rapid 'confined' (to a degree) deflagration i wont argue that point.But as all thats
required to initiate a mixed gas detonation is heat the shockwave theory doesnt follow.
It may be applicable to a solid explosive where a shockwave is required to detonate said explosive that doesnt seem to be the case with a mixed gas to
achieve a true detonation.
What a fine detonation a large quantity of mixed gas would create if there were a way to deliver the mixture effectively
in a reasonably sized warhead.
[Edited on 30-4-2009 by grndpndr]
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WaveFront
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If you use acetylene or hydrogen mixed with oxygen, I would think it is any kind of detonation. I have been able to explode the H2/O2 mix from
electrolysis in small open containers using a lighter.
To see the diference, you can use an antidetonant fuel as a gasoline. The gasoline is made for not to be detonated by the adiabatic heating in the
motor.
If the adiabatic heating produced in the shockwave is enough to produce combustion (or decomposition of the acetylene, which is explosive by itself),
a DDT is feasible.
In my humble opinion, acetylene/oxygen is easy to DDT
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hissingnoise
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Anyway, we know that detonation and deflagration are not the same thing grndpndr, but, how do you think they differ?
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grndpndr
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I would just be repeating the shockwave HE detonation and the confined deflagration until container failure.
Only thing Id add is it doesnt have to be a deflagration for a confinement explosion.Steam engines regularly failed the boilers exploding,ive seen
experiments with iron containers filled with water and plugged explode with great violence.
What Made me curious was by what mechanism does a mixed gas detonate.I didnt believe detonation was possible simply by deflagration?
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hissingnoise
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Many thanks for clearing that up, grndpndr. . .
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grndpndr
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More than welcome Hissingnoise.If that doesnt satisfy your textbook definition a more definitive answer or outright correction is welcome. I AM here
to learn as your aware never holding myself up as expert in all things so any slight or fascetiousness
will be wasted on me, And summarily dismissed as I thought until now, the truce you proposed was still in effect
The question still stands by what mechanism besides the shockwave or deflagration does the mixed gas detonate w/o a initiating shockwave but only a
hot wire etc,rapid DDT?
Supossedly nonexplosive gasoline detonates under abnormal engine conditions of pressure/heat in an internal combustion engine
as well as diesel fuel detonates under extreme pressure in a common diesel engine and Im sure many other examples.So the mixed gas must act as a
primary explosive?
[Edited on 2-5-2009 by grndpndr]
[Edited on 2-5-2009 by grndpndr]
[Edited on 2-5-2009 by grndpndr]
[Edited on 2-5-2009 by grndpndr]
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497
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| Quote: | | Supossedly nonexplosive gasoline detonates under abnormal engine conditions of pressure/heat in an internal combustion engine as well as diesel fuel
detonates under extreme pressure in a common diesel engine |
In both those cases, detonations are specifically avoided... When they do occur because of unsuitable fuel or timing, it trashes the engine pretty
quick..
As far as I know, most fuels will not DDT under normal circumstances.. Of course, factors such as containment, turbulence, cloud size, etc
can greatly effect the possibility of a DDT.
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grndpndr
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I did say abnormal conditions.Otherwise Your right of course particularly in the case of the gasoline engine.The timing of the fuel injection into the
diesel takes advantage of the extreme pressure which generates heat to
explode the diesel combustion cycle which in both cases can result in DDT and destruction of the engine.Gasoline engines detonate due to sometimes
incorrect timing (spark) but more often inadequate fuel octane.The point being even the theoretically nonexplosives like FO and gasoline can
detonate. Yes?
The lightly built gasoline engines obviously dont rely upon the extreme pressures generated in a diesel engine by the sudden injection of fuel into
the extreme(heat/pressure) enviroment of a diesel combustion chamber but relatively gradual deflagration of the gas engine otherwise your diesel could
be built as lightly as the gasoline engine.
[Edited on 2-5-2009 by grndpndr]
[Edited on 3-5-2009 by grndpndr]
[Edited on 3-5-2009 by grndpndr]
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grndpndr
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What Made me curious was by what mechanism does a mixed gas detonate.I didnt believe detonation was possible simply by deflagration?[/rquote]
Obviously my last sentence is incorrect,perhaps i didnt have my coffee but the question stands by what mechanism does a mixed gas detonate?DDT?Akin to
a primary HE?
[Edited on 3-5-2009 by grndpndr]
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497
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Heh, I missed that you said "abnormal".. I was tired..
The following link describes engine "detonation" well: http://www.streetrodstuff.com/Articles/Engine/Detonation/
So going by their definition, the "detonation" in ICE circumstances is not really a deflagration to detonation transition as far as I can tell. It is
more a spontaneous ignition of part of the fuel/air mixture due to extreme heat and pressure. AFAIK there is no supersonic shock wave involved (which
is the definition of a true detonation). In engines "detonation" is merely the spontaneous deflagration of a volume of fuel/air mixture, rather than
the usual controlled progression of a deflagration wave initiated by a spark.
In a diesel a similar principle applies. Under normal conditions the high pressure spray of diesel burns immediately on being injected, causing a
steady increase in pressure. When "detonation" occurs, the fuel spray ignites after a delay, cause is more rapid (destructive) pressure rise. Still,
there is no supersonic detonation wave, thus no true detonation.
Yes most any fuel *can* detonate under sufficiently high heat/pressure, and a large enough cloud. I've found some information on the "minimum cloud
radius" for a fuel/air cloud to DDT from a weak ignition source. For many fuels, the radius seems to be up to 50 meters, depending on conditions.
Thus, I submit that it is not practical to attempt to DDT a fuel in an uncontained environment. In a semi-confined environment like a potato cannon,
it requires less "run up" distance to DDT, but it is still usually many meters for common fuels. Even acetylene may not have enough distance in a
potato cannon.
I just found a file that explains fuel/air detonation mechanisms very well. From what I read, it is somewhat comparable to DDT in a solid explosive.
Now that I think of it, there seems to be considerable analogy between solid and fuel/air explosive mechanics. That file has some really good data,
I'll attach that file as soon as I find it.
EDIT: Here's the file. It has a huge amount of useful information. It is probably the best single collection of information I've found so far. It will
clear up *many* misconceptions I've seen thrown around on this forum.
Attachment: Fire and explosion.pdf (207kB)
This file has been downloaded 791 times
[Edited on 5-5-2009 by 497]
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grndpndr
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Thanks 497 ive downloaded the file , a better understanding
of DDT, etc cant hurt, a days wasted w/o learning something.
Tearing down an engine in which 'detonation' has occured for a time will make you wonder about the 'explosion' velocity.Cracked pistons, bent/broken
rods.Of course if the 'deflagration'/detonation were timed say on the upward compression stroke that would create a tremendous mechanical shock to
the entire engine.
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