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Author: Subject: The definition of a "detonation"?
Antiswat
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[*] posted on 21-5-2021 at 04:57
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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[*] posted on 25-5-2021 at 09:40


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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[*] posted on 25-5-2021 at 10:05


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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[*] posted on 25-5-2021 at 11:14


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]




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[*] posted on 25-5-2021 at 11:32


Quote: Originally posted by Antiswat  
now i remember that "detonation" velocity doesnt define whether its a detonation

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.




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[*] posted on 25-5-2021 at 12:01


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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[*] posted on 25-5-2021 at 21:44


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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[*] posted on 26-5-2021 at 10:37


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]




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26-5-2021 at 10:56
Fulmen
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[*] posted on 27-5-2021 at 04:52


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.



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[*] posted on 28-5-2021 at 23:22


It’s the definition of happiness.
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greenlight
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[*] posted on 29-5-2021 at 12:02


I think we can all agree on that:D



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[*] posted on 2-6-2021 at 22:03


It is a quite interesting phenomenon for sure.



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[*] posted on 4-6-2021 at 02:02


It is clear and simpl.... Detonation is, when women scream...
https://www.youtube.com/watch?v=6gkblppESHA
....in 2:50....:cool:




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Antiswat
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[*] posted on 5-6-2021 at 01:03


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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[*] posted on 5-6-2021 at 01:32


Quote: Originally posted by Antiswat  
you oftenly see plumes of NOx on tannerite videos, the NOx is unfulfilled potential

That doesn't mean it's caused by "incomplete" detonation. A low OB would cause the same effect.




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[*] posted on 13-6-2021 at 00:06


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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[*] posted on 14-6-2021 at 15:58


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.




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[*] posted on 15-6-2021 at 21:58


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