Sciencemadness Discussion Board

Calculating Heat of Detonation

MineMan - 31-3-2020 at 17:22

I am trying to calculate the heat of detonation for mixtures of NM, AN, and Aluminum. This would be a non ideal explosive, which could make this calculation less accurate or more difficult is my understanding?

How do I go about this? I have the separate value for NM and AN, but a ratio cannot be used since the detonation products will be different at different ratios. As for the Al, I am assuming it strips oxygen from the H20 vapor first, then CO2 giving products of H2, CO, and aluminum oxide.

Is this possible to calculate without thermochemical codes?

Hoping for a quick answer.

Thank you.

Dornier 335A - 1-4-2020 at 08:12

You don't have to worry about ANNMAl being a non-ideal explosive. The reaction may be too slow to deliver all its energy to the shock wave, but it is fast enough to complete during the gas expansion. Thermochemical codes are not necessary, some simple rules are enough to estimate the reaction:

For CHNOAl at high density/strong confinement:
- Al and O reacts to form Al2O3
- C and O reacts to form CO2
- H and O reacts to form H2O
- Excess H, N and O form H2, N2 and O2.

At lower densities, CO is preferred over CO2 and C:
- C and O reacts to form CO
- H and O reacts to form H2O
- CO and O reacts to form CO2

So you will have to calculate the composition of your mixture. For example, 64.5% AN, 24.6% NM and 10.9% Al is 2 mol AN, 1 mol NM and 1 mol Al, or CH11N5O8Al.
Following the low density rules above gives 0.5 Al2O3, 1 CO, 5.5 H2O and 2.5 N2. The enthalpy of reaction is 5.78 MJ/kg.

I recommend "Calorimetric determinations of the heat and products of detonation for explosives" by Ornellas if you are interested in some experimental data!

Pyro_cat - 1-4-2020 at 08:51

I know in thermo nuclear the X-rays race past the speed of the chemical explosion and set of the hydrogen secondary

We know Thermonuclear devices rely on a two-stage process, in which X-ray radition from a fission primary is contained and used to trigger a fusion or fusion-fission reaction in a physically separate, secondary portion of the device. ... X-ray fluorescence causes the casing ions to generates secondary X-rays

https://en.wikipedia.org/wiki/Thermonuclear_weapon

Maybe this thought will lead you closer to your answer.

Pyro_cat - 1-4-2020 at 09:10

Still thinking about this and my observation has always been that aluminium does a great job of blocking heat so that could be detrimental to the heat travelling through the material at optimal speed even if its consumption generates heat in the process.

I can only imagine the old days when they were discovering this stuff. Those scientists in the 1800s must have had a blast.

I got to get my ass back into tesla coiling. *maybe* if I can put down this darn keybord and quit reading science and start doing science I could build that kick ass coil I always wanted.



Dornier 335A - 1-4-2020 at 10:44

A detonation is not driven by heat transfer, it is driven by the shock wave (by definition). The thermal conductivity of the material is completely irrelevant. The aluminium is in fact inert in the shock front of a detonation and thus won't contribute much to the detonation velocity unless it is added in very large proportions or is excessively fine.

What you are thinking about is a deflagration and the closest example I can think of is that some rocket fuels are translucent in the IR spectrum, so they require some opaque ingredient in order to prevent ignition ahead of the burning front.

Pyro_cat - 1-4-2020 at 13:26

Not driven by heat... But interesting with the rocket fuel my idea was legit.

Said this before I think something similar is going on in a detonation shockwave as the slo mo guys breaking glass https://youtu.be/GIMVge5TYz4?t=101

I found that so interesting that glass breaks at detonation speeds . And Prince Rupert's Drops.

Its that pent up energy.

What do we really know, seems to me the observation usually comes first then an explanation is come up with it to explain it later.


MineMan - 1-4-2020 at 19:11

Quote: Originally posted by Dornier 335A  
You don't have to worry about ANNMAl being a non-ideal explosive. The reaction may be too slow to deliver all its energy to the shock wave, but it is fast enough to complete during the gas expansion. Thermochemical codes are not necessary, some simple rules are enough to estimate the reaction:

For CHNOAl at high density/strong confinement:
- Al and O reacts to form Al2O3
- C and O reacts to form CO2
- H and O reacts to form H2O
- Excess H, N and O form H2, N2 and O2.

At lower densities, CO is preferred over CO2 and C:
- C and O reacts to form CO
- H and O reacts to form H2O
- CO and O reacts to form CO2

So you will have to calculate the composition of your mixture. For example, 64.5% AN, 24.6% NM and 10.9% Al is 2 mol AN, 1 mol NM and 1 mol Al, or CH11N5O8Al.
Following the low density rules above gives 0.5 Al2O3, 1 CO, 5.5 H2O and 2.5 N2. The enthalpy of reaction is 5.78 MJ/kg.

I recommend "Calorimetric determinations of the heat and products of detonation for explosives" by Ornellas if you are interested in some experimental data!


Thank you Dornier! I have some additional questions.

What you calculated was the heat of combustion, the heat of detonation I believe if different and we must subtract the values of the energies to disassociate the bonds?

For calculating detonation pressure, I believe non ideal explosives use different equations? The compositions will not be oxygen balanced... adding the complexity that the H2O will have to be disassociated to oxidize the Al.

Dornier 335A - 2-4-2020 at 10:02

The energy to disassociate the bonds is included in the enthalpies of formation for the reactants. You just calculate the enthalpy of detonation like any other chemical reaction.

Other detonation properties are much harder to calculate and especially so for non-ideal explosives since CJ-theory doesn't really apply. There are many different reasons why explosives are non-ideal and there are different ways to get around it...
ANNMAl is non-ideal mostly because all ingredients don't react quickly enough. The simplest way to model this is to only let fractions of the explosives react. Reasonable values are probably 100% of the NM, 50% of the AN and 15% of the Al but these numbers are just my guesses. Then you apply CJ-theory to the mixture of hot gases and the remaining super compressed AN and Al. This requires pretty serious equations of state though. The rest of the AN and Al can react during the expansion, but nothing that burns after the CJ-point will contribute to the detonation pressure.

This approximation will not work for something like TATB which is non-ideal because of complicated carbon condensation effects.