Subsonic vs. Detonation: Comparing Pressure and Temperature in Fuel/Air Mixtures

[sid_galt]

Two identical fuel/air mixtures are kept in boxes A and B at equal pressure, volume and temperature. The mixture in box A is burnt using a subsonic flame. The mixture in box B is detonated. Which will generate more pressure and temperature?

If fuel/air mixture is forced into a chamber containing high pressure gas (20 atm) at high temp, will the mixture deflagrate or detonate?

 

[Danger]

Which will generate more pressure and temperature?

I would expect it to be equal. The same amount of energy is being released in almost the same amount of time from the mixture. The total pressure and temperature in each box would vary depending upon how much is contributed by the initiating flame and the means of detonation.

 

[sid_galt]

Wouldn't detonation generate more temperature and pressure due to shock waves?

 

[Antiphon]

This is not my specialty, but I would imagine you'd have to lookk at the initial and
final chemistries. If they are exactly the same, I suspect the two are equivalent.
If they are different, then the energy difference should show up there.

 

[sid_galt]

There has to be a difference because if detonation shows no advantage over constant volume combustion, then why would NASA and different companies be investing in pulse detonation engines - a variation of pulsejet only in which instead of deflagration combustion, detonation combustion is used.

 

[ludi_srbin]

I would guess that temperature would be equal but there would be more pressure caused by the explosion because it happens in less time. But again the pressure would build up in the first case...I guess it would be equal.

 

[Astronuc]

Deflagration essentially means 'slowly burn', while detonation means 'explosion'. A detonation is more or less adiabatic with a rapid energy release. With a slow burn, it is more likely that the thermal energy is dispersed (by conduction, convection, or radiation) thus for similar mixtures, the deflagration would produce lower pressure and temperature.

 

[sid_galt]

Does this mean that if a mixture were deflagrated in a very efficiently thermally insulated chamber, detonation would produce about the same final temperature and pressure as deflagration?

 

[Astronuc]

If the total energy is the same and the mass involved is the same, and it is purely adiabatic (i.e. no heat loss), then yes the final temperature would be the same.

 

[sid_galt]

Thank you for the help

 

[Clausius2]

Two identical fuel/air mixtures are kept in boxes A and B at equal pressure, volume and temperature. The mixture in box A is burnt using a subsonic flame. The mixture in box B is detonated. Which will generate more pressure and temperature?

If fuel/air mixture is forced into a chamber containing high pressure gas (20 atm) at high temp, will the mixture deflagrate or detonate?

The answer to this question is not a trivial one. First of all, the detonation conditions are dependent on the nature of the fuel. So it would be impossible to forecast if it will be a detonation or a deflagration if you don't say what fuel are we talking about. Also, detonation processes are very influentiated on how are they initiated. In your example, there must be reached detonation conditions during the chemical reaction of explosion, and this achievement is studied taking into account the heat transfer properties of the vessel. You know, there are also depurated techniques for fabricating explosive vessels, taking into account the ratio of heat chemically released and the heat transferred to vessel surroundings.

The main differences between a deflagrative and a detonative wave are:

i) a detonative wave travels at supersonic velocity. In fact it is a reactive shock wave. It is used in pulsed detonation engines because behind the front of a detonative wave, the pressure and temperature of products are higher than the reactants pressure, so it will push air to rearwards, pushing also the engine by pressure.

ii) a deflagrative wave travels at subsonic velocity. For example, a deflagrative wave is the wave produced by a bunsen burner. It is a steady diffusion flame, but viewed from a reactant reference frame, it is a wave which travels far below sonic limit. On the other hand, deflagrative waves causes a decreasing of pressure behind it, they expand the gases. This deflagrative waves can be seen at explosions in buildings and firings. Deflagrative waves are not reliable for scramjet engines, because the flow is supersonic at the inlet, but they are produced in usual turbojet engines and ramjets.

If the total energy is the same and the mass involved is the same, and it is purely adiabatic (i.e. no heat loss), then yes the final temperature would be the same.
Today 06:42 PM

I don't think so. Both processes deflagrations and detonations are non isentropic ones. Moreover, I will leave sid-galt to demonstrate that a detonation process causes more increasing in entropy. It can be seen graphically with Rankine-Hugoniot curves or analitically. Surprisingly, detonation processes are more difficult to achieve in a naturally way. They must be forced, unless the mixture reaches detonation conditions. So that, a detonation will cause greater final temperature.

For more references, you all can take a look at the book of my signature. It is written by FA Williams (UCSD) and A. Liñan (ETSI Aeronauticos Madrid). A great book .

 

[sid_galt]

Thank you for the reply Clausius2.

The answer to this question is not a trivial one. First of all, the detonation conditions are dependent on the nature of the fuel. So it would be impossible to forecast if it will be a detonation or a deflagration if you don't say what fuel are we talking about.

If for example the fuel is gasoline mixed with air to form a highly enriched mixture.

Also, detonation processes are very influentiated on how are they initiated. In your example, there must be reached detonation conditions during the chemical reaction of explosion, and this achievement is studied taking into account the heat transfer properties of the vessel.

You mean there will be a difference if the detonation is triggered for the gasoline fuel through a spark or compression?

I don't think so. Both processes deflagrations and detonations are non isentropic ones. Moreover, I will leave sid-galt to demonstrate that a detonation process causes more increasing in entropy. It can be seen graphically with Rankine-Hugoniot curves or analitically. Surprisingly, detonation processes are more difficult to achieve in a naturally way. They must be forced, unless the mixture reaches detonation conditions. So that, a detonation will cause greater final temperature.

Is the higher temperature reached through the shock waves in the blast waves which cause compression in the ambient air?

In our example fuel, how much difference could there be between the final pressure and temperature ratio if it is detonation vs if it is deflagrated? What equations are used to perform these type of calculations?

 

[Clausius2]

Sidgalt, all the questions you posted are not answereable in an easy way. First, mixtures of gasoline and air have autoignition conditions tabulated and given by some approximate formulation (see Heywood "Internal engine combustion fundamentals").

Secondly, the problem of triggering a detonation is a current unsolved problem. We know that there are some conditions such a minimum detonation energy to be reached in the mixture, but little is known about the formulation besides the yet known Rankine-Hugoniot equations. Don't rush, you will face with combustion equations if you seek them (i.e. if you enroll in a combustion course). For more references, please go to specialized bibliography as the one I posted.

 

[sid_galt]

I tried searching for the book which you recommended on amazon. They said it was unavailable. Could you suggest more books?

 

[Clausius2]

Yeh, in fact this book is a bit old. FAW also have another good book written by himself, and I have got over here the book of Turns "Intro to Combustion".

 

[sid_galt]

One more question.

If a bomb is detonated on level ground, will the detonation shock wave strike against the ground itself where the bomb is kept? In other words, will a created by the detonation shock wave in case a bomb is detonated on ground?

 

[Danger]

That last question is pretty unclear, sid... and I suspect that you left a word out of the 2nd sentence. Will what be created? I assume that you meant a tremor like a baby earthquake.
Unless it's a shaped charge, the shockwave from a bomb expands pretty spherically. The ground therefore absorbs a large amount of it (the intensity varies with the distance from detonation, since anything not directly under it is struck at an angle.
The shock itself propogates at the speed of sound in whichever medium carries it.

 

[Clausius2]

Both of you shouldn't mix together which a detonation wave means in combustion context and what it means popularily. Detonating a bomb doesn't mean to create a detonation wave.

Personally, I doubt pretty much than no detonated bomb is able to generate a shock wave in the surrounding air. Also, what do you mean with bomb? What kind of bomb?. An atomic bomb? a box filled with TNT?. Anyway if the bomb generates a shockwave I would bet it is dissipated and degenerated into a subsonic pressure wave in a little space.

 

[Danger]

You totally lost me there, Clausius. The whole point of a bomb is to create an overpressure that causes substantial damage to the surroundings. (Or to expel high-energy shrapnel for the same purpose.) This involves high-explosives, which are definitely detonated as opposed to conflagrated, unless it's a primitive bomb that utilizes low-explosives in a bursting container.
Please post a more detailed version of your response, with as few technical terms as possible.

 

[Clausius2]

You totally lost me there, Clausius. The whole point of a bomb is to create an overpressure that causes substantial damage to the surroundings. (Or to expel high-energy shrapnel for the same purpose.) This involves high-explosives, which are definitely detonated as opposed to conflagrated, unless it's a primitive bomb that utilizes low-explosives in a bursting container.
Please post a more detailed version of your response, with as few technical terms as possible.

Ok. I mean a enough powerful bomb can create a shock wave, but the shocks are unstable per se. Take a look at this link:
http://en.wikipedia.org/wiki/Shock_wave

There it is said:

Shock waves are not sound waves; a shock wave takes the form of a very thin membrane (sheet of energy) on the order of micro-meters in thickness. The pressure excursion within the shock wave is so extreme that it causes the speed of sound within the wave to change. Shock waves in air are heard as a loud "crack" or "snap" noise. Over time a shock wave can change from a nonlinear wave into a linear wave, There are two basic types of shock waves: blast waves and driven waves. A blast wave is produced by explosive phenomena. Blast waves can travel out from their source at supersonic speeds.

Well, I don't know the amount of explosive needed to produce a shockwave in the air. But if there is not enough power, there will be the so-called linear (acoustic) wave, which is a normal pressure wave. They are not shock waves, they are merely acoustic waves which reorders pressure in the fluid. As you can read above, the shock may be degenerated into an acoustic wave due to dissipation.

It is said Hiroshima bomb caused a shock wave. Maybe there was a shockwave in the surroundings of the bomb, but far away it degenerated into an acoustic wave.

As you may understand, I am not an expert on explosives.

 

[Danger]

Thanks for the clarification, Clausius. I never considered that there was a difference between a shock wave and a pressure wave other than the magnitude and suddenness of production. I defer to you terminology, but still maintain that the 'acoustic wave' from a couple of pounds of C4 will still kill you.

 

[Clausius2]

Thanks for the clarification, Clausius. I never considered that there was a difference between a shock wave and a pressure wave other than the magnitude and suddenness of production. I defer to you terminology, but still maintain that the 'acoustic wave' from a couple of pounds of C4 will still kill you.

Maybe we could meet and see what happens. For instance, you buy the explosives, and before I push the detonator button to see what happens with you (now you are unawarely preparing the bomb and don't know I will use you as a proof) .

 

[sid_galt]

Thank you for the help.

 

[Danger]

Maybe we could meet and see what happens.

This is the first time I've ever been glad that you're on a different continent.