Fantasma4500
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The definition of a "detonation"?
so i was talking to an engine specialist, and eventually detonation finds its way into the conversation, about nitromethane, and i explain that theres
no chance nitromethane manages to detonate inside of an engine because the pressures of a detonation really just wouldnt do anyone good, unless they
have some serious masochism.
i remember talking to one youtuber long back who was deep in physics, he explained a detonation as when the shock front propagates faster than the
ignition front and google gives me this "Detonation is a type of combustion involving a supersonic exothermic front accelerating through a medium that
eventually drives a shock front propagating ..."
now i remember that "detonation" velocity doesnt define whether its a detonation, but doesnt it in some way? if you see it through shock front moving
faster than ignition front, how fast can ignition take place? it would by that have to surpass the maximum ignition front velocity right.
so what do you guys got on this? what is the definition of a detonation
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Dr.Bob
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Some people consider a deflagration as a fire front moving slower than the speed of sound. A detonation wave moves faster than the speed of sound,
but that is just one way to look at it. Chemically, if molecules are just burning, then it is a deflagration, but if the molecules come apart
completely, and then recombine, then it is a detonation. eg, TNT can burn like a candle if lit with a wick, but if there is an initiation event
(primer or blasting cap goes off creating a shock wave) that will cause the nitro groups in the TNT to come apart very quickly, releasing the nitrogen
and oxygen atoms, which can react with the other atoms to form N2, CO2, CO, H2O (steam), and other gases.
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unionised
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Detonation- the passage of a reaction front through a material faster than the speed of sound- can happen in an engine.
It shouldn't happen.
When it does it is called "knocking" or some such term.
https://en.wikipedia.org/wiki/Engine_knocking
The speed of sound in the gas mixture in an engine is rather lower than in most condensed phase explosives.
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greenlight
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This is a hard question to answer. I think to understand the difference between the motor detonating and an actual detonation, the speeds of reaction
could be considered.
For instance, when you burn wood the wood is the fuel and the atmospheric oxygen is the oxidiser. This is quite a slow process as the surface area
of the fuel is very limited and the oxygen can only act on the outside layer of wood, resulting in product gases forming at a slow rate without much
pressure, agreed?
Then you have propellants like black powder, flash powder, rocket compositions, etc,, were the oxidiser and fuel can be mixed a little more intimately
as they are in powder form. This results in a steady, faster but still subsonic burning where gases are formed readily and don't have much effect on
the surrounding air unless they are confined in a casing.
Now, with high explosives, the oxidiser and fuel are not just mixed together but within the same molecule, which is the most intimate mix you can get
without going nuclear.
The combustion/decomposition moves through the material at such supersonic speeds, it is now called a detonation wave, forming an extremely high
pressure front of instantaneous product gases that can even act on only the surrounding air and form a shockwave from compressing it.
So at the end of all that, nitromethane would be the fuel and air inside the motor would be the oxidiser. They could mix very well because they can
both be in gas/vapour form, but not within the same molecule well.
I don't believe it could reach the same speed of reaction as high explosives and therefore not a true detonation.
[Edited on 25-5-2021 by greenlight]
Be good, otherwise be good at it 
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Fulmen
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It kinda requires it to be, doesn't it? I know that some lists detonation velocities for low explosives as well, but it's not correct.
The shock wave definition it correct. It's not just about speed, it's about how the initiation is transmitted. In a deflagration the combustion
process is conveyed through heat transfer and thermal radiation. In a detonation (proper) the process is conveyed through a self-sustaining pressure
wave traveling through the material.
We're not banging rocks together here. We know how to put a man back together.
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unionised
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Quote: Originally posted by greenlight  |
I don't believe it could reach the same speed of reaction as high explosives and therefore not a true detonation.
[Edited on 25-5-2021 by greenlight] |
The rate of reaction only needs to exceed the speed of sound in the gas mixture. It does not need to reach the speeds obtained in "high explosives".
But... if you don't believe that detonation can happen in an engine, what causes knocking?
And why is this so loud?
https://www.youtube.com/watch?v=6gkblppESHA
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zed
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Ummm. Detonation is like Pornography. Hard to define, but I know it... when I see it.
Internal combustion engine? Detonation on every stroke.
Knocking, is detonation mis-timed. As is Back-firing.
[Edited on 26-5-2021 by zed]
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greenlight
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| Quote: Originally posted by unionised | | Quote: Originally posted by greenlight |
I don't believe it could reach the same speed of reaction as high explosives and therefore not a true detonation.
[Edited on 25-5-2021 by greenlight] |
The rate of reaction only needs to exceed the speed of sound in the gas mixture. It does not need to reach the speeds obtained in "high explosives".
But... if you don't believe that detonation can happen in an engine, what causes knocking?
And why is this so loud?
https://www.youtube.com/watch?v=6gkblppESHA |
It seems you are are correct indeed, the definition is a combustion wave that has reached a supersonic speed and this can be achieved with fuel air
mixture.
Found this:
" "Detonation This is a supersonic combustion wave. Detonations in gases propagate with velocities that range from 5 to 7 times the speed of
sound in the reactants. For hydrocarbon fuels in air, the detonation velocity can be up to 1800 m/s. The ideal detonation speed, known as
the Chapman-Jouguet velocity, is a function of the reactant composition, initial temperature and pressure."
And this:
"Chapman-Jouguet Velocity This is the velocity that an ideal detonation travels at as determined by the Chapman-Jouguet (CJ) condition: the
burned gas at the end of the reaction zone travel at sound speed relative to the detonation wave front. CJ velocities can be computed numerically by
solving for thermodynamic equilibrium and satisfying mass, momentum, and energy conservation for a steadily-propagating wave terminating in a sonic
point. CJ velocities in typical fuel-air mixtures are between 1400 and 1800 m/s."
It seems the differing factor is really the speed at which the detonation
travels.
Which partly explains why detonation in a combustion engine (~1800m/s) does damage the engine but does not shatter the chamber instantly whereas
detonation of a small amount of nitroglycerine (~7500 m/s) in the same confined chamber will have catastrophic effect.
[Edited on 26-5-2021 by greenlight]
[Edited on 26-5-2021 by greenlight]
Be good, otherwise be good at it
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Texium
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Thread Moved
26-5-2021 at 10:56 |
Fulmen
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The biggest difference between a gas detonation and a solid detonation is energy density. Even with a compression ratio of 10:1 the density will be in
the order of 1% of the solid. And when you factor in that it's 80% nitrogen, the energy per bang isn't that impressive. The real problem with engine
knocking is probably that it will reach peak pressure before the piston reaches TDC.
We're not banging rocks together here. We know how to put a man back together.
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MineMan
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It’s the definition of happiness.
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greenlight
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I think we can all agree on that
Be good, otherwise be good at it
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Pyro_cat
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It is a quite interesting phenomenon for sure.
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Laboratory of Liptakov
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It is clear and simpl.... Detonation is, when women scream...
https://www.youtube.com/watch?v=6gkblppESHA
....in 2:50....
Development of primarily - secondary substances CHP (2015) Lithex (2022) Brightelite (2023) Nitrocelite and KC primer (2024)
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Fantasma4500
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well nitroglycerine can fully and partly detonate, they used to "detonate" it with blackpowder, heatshocking it
tannerite is also half-arsed detonated by bullet impact, but not truly detonated is it? because you oftenly see plumes of NOx on tannerite videos, the
NOx is unfulfilled potential
maybe the completion of reaction, or completion of decomposition is what would define it, because that would tell if it yields full power, but thats
again relative to the material itself and not an objective general measure. Hm.
one russian youtuber set off TACN and tested it to have lowest VoD at 2400m/s, increasing charge size increased the VoD, but i recall hydrogen oxygen
mixture specifically 75-25 (yes, forming "H3O") could reach 4000m/s where the regular 2:1 would reach 3000m/s or slightly above that
so what are we saying, it might be able to detonate, but we can assume it doesnt really because of low density explosion, and because it doesnt fully
react in a detonating sense, and because its not a detonating material? if nitromethane goes off properly its around 6500m/s iirc, obviously engine
knocking does exist without nitromethane
now the pressure of explosives is measured in kBar, thousands of bars. you cant really use that for propulsion as it will just rip right through
metal, higher brisance explosives will have higher pressure and thus higher force, but how many bars does an engine run at, and how many bars does
"engine knocking" create? VoD and kBar is related, i believe kBar and density are the main components to explain VoD
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Fulmen
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That doesn't mean it's caused by "incomplete" detonation. A low OB would cause the same effect.
We're not banging rocks together here. We know how to put a man back together.
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Fantasma4500
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youre correct, the beirut blast is really good evidence of a detonation that progressed as far as it possibly could but with lacking oxygen balance to
"fully detonate" the NOx
so its probably lacking about a thousand metres per second VoD because of lacking fuel
OB is important to VoD, its why EGDN is so powerful, its got a perfect OB
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C6(NO2)5CH2CH(CH3)N(NO2)2
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I think tannerite is AN, about 5% Aluminum, with a few other additives. The perfect OB ratio is I think 80:18 AN:Aluminum to create N2, H2O, and
Al2O3. I wouldn't be surprised if itemits NOx for that reason. It's also possible for small particles floating in the air (alumina in this case) to
create an orange haze, without any nitrogen compounds involved.
As far as what counts as a detonation. I think it's when a blast wave moves through a block of energetic material and compresses it so forcefully it
reaches its ignition temperature. It has to be able to react and ignite fast enough and violently enough to keep the blast wave at that strength as it
spreads through the material. Look at the pressures most of these compounds can create, up to 80kBar for acetone peroxide, I think 220 or 250 for
Nitroglycerin, even more for stuff like ETN or RDX. if you suddenly compressed a container full of water or sand to that pressure it would definitely
increase in density and get hot. Now, if you compress a heat sensitive organic energetic, it's going to instantly ignite and produce gas at even
higher temperature and pressure, and ignite the material around it via compression alone. It's basically a heat engine cycle that doesn't need any
external moving parts besides the EM and combustion products.
Put that in your pipe and smoke it!
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Nitrosio
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Attachment: Tannerite.pdf (305kB)
This file has been downloaded 374 times
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Linus1208
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Personally I like the definition of a detonation having a reaction front moving supersonic more.
The defintion of molecules coming apart and recombining would imply that e.g. AM or Cheddite / Chloratite composition do not detonate, which surely is
not the case.
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B(a)P
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| Quote: Originally posted by Linus1208 |
The defintion of molecules coming apart and recombining would imply that e.g. AM or Cheddite / Chloratite composition do not detonate, which surely is
not the case. |
I am confused, how do these reactions not involve the coming apart and recombining of molecules?
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macckone
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Ammonium Nitrate has an excess OB.
Nitromethane is a negative OB.
Both will incompletely combust without a correction of the OB.
ANFO or tannerite has an added fuel.
Nitromethane gets add oxygen from air in the engine.
Given the proper conditions even wood or grain can detonate (saw dust and grain dust explosions).
As others have pointed out knocking is detonation as opposed to deflagration in the engine.
And as the OP surmised, it is bad for the engine.
ps. no matter how good the OB is, any deflagration or detonation is going to leave residue.
[Edited on 22-11-2021 by macckone]
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Laboratory of Liptakov
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God created the detonation. Maybe just to have a long time to discuss. Many physical quantities are interdependent. Without voltage there is no
current, without matter there is no energy, without time there is no space. And conversely. There is no temperature without pressure and no pressure
without temperature. It's time to put the mass on the anvil. And create in matter the highest possible pressure and temperature. Similarly, as God
did 13.7 billion years ago.
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Nitrosio
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Basilius Valentinus described an attempt he made with a bell jar: equal small amounts of fancy gold and black powder. He places a bell jar under each.
The flashing gold detonated and locally destroyed the base.
The bell jar remained intact. During the deflagration of the black powder, the backing was only
slightly busy, the bell jar hung destroyed.
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SWIM
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Note to self: NEVER buy a used car from Zed.
The timing actually causes the detonation because if too much combustion goes on before TDC the peak pressure and heat in the chamber will be much
higher and gets high enough to cause detonation.
Detonation is going to sound pretty similar even if it somehow happens after TDC because piston velocities are pretty low compared to the wave front
velocity of a detonation.
6000 RPM with a 4 inch stroke means a peak piston velocity of just about 100 feet per second.
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specialactivitieSK
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https://youtu.be/oqSJhrRY-Pc
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