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A Thought On Combustions

A place to ask general spud cannon related questions.
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A Thought On Combustions

Unread postAuthor: Hubb » Tue Apr 15, 2008 6:04 pm

This is just something that I thought of. Here goes:

I understand a fuel burns at a certain rate. Example, propane burns at a rate of about 12fps. What I was wondering was, would that burn rate increase if the chamber was heated prior to combustion?

What I had in mind was wrapping a steel chamber with, say, nichrome wire from a toaster and heating it. The fuel is injected and allowed to heat. A chamber fan will assist in spreading the heated fuel.

I am under the impression that heated air travels faster than normal temperature air. If the same laws apply to other gasses, then the fuel would also. Upon combustion, it should burn more thoroughly and burn faster, right?

If that would not apply to a combustion, what about a hybrid. The chamber heating the added air should have an effect, which would then lead to a new generation in hybrid launchers (and probably DDT :shock: ).

How about some thoughts on this.
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Unread postAuthor: DYI » Tue Apr 15, 2008 6:18 pm

Higher temperatures will lead to faster burn rates, and more power, so long as the chamber is airtight, as they will add to the preignition pressurisation.

The particle speeds of all gases in the chamber will increase as you heat the chamber, but so will the pressure, which causes a decrease in burn rates. Spudblaster did the math, and at higher mixes, the increased burn temperature (and therefore speed) is enough to overcome the decreased burn speed caused by the initial pressurisation. What you are doing is essentially increasing the mix, but through heating more than pressurisation.

Where my knowledge ends is the actual effects that this will have on the properties of the deflagration in the mix. I don't know if the gases will reach a higher temperature peak if they are preheated, or if that property is independent of the preignition temperature. My guess is that you could increase the burn temperature by this method, but I don't know for sure. I would also guess that doing this will decrease the DDT runup distance by creating a hotter and faster burn.

I'll bet Jimmy and Dave will have ideas that involve a lot less guesswork.
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Unread postAuthor: Hubb » Tue Apr 15, 2008 6:54 pm

What you are doing is essentially increasing the mix, but through heating more than pressurization.


That's pretty much exactly what I was thinking would happen.

I am a little confused about the increase in pressure decreasing the burn rate, though.
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Unread postAuthor: STHORNE » Tue Apr 15, 2008 7:17 pm

this will most likely work especially if you use mapp gas.

as mapp burns hotter/faster and is cleaner burning than propane.
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Unread postAuthor: Fnord » Tue Apr 15, 2008 7:17 pm

Here's a related question: Has anyone ever tried plating the inside of a chamber with a paper-thin coating of polished copper/aluminum?

Since copper is an extremely efficient reflector of far-infrared light(heat), it might help to reduce heat losses which rob a great deal of power from combustion motors (...linear motors in this case).
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Unread postAuthor: STHORNE » Tue Apr 15, 2008 7:19 pm

_Fnord wrote:Here's a related question: Has anyone ever tried plating the inside of a chamber with a paper-thin coating of polished copper/aluminum?

Since copper is an extremely efficient reflector of far-infrared light(heat), it might help to reduce heat losses which rob a great deal of power from combustion motors (...linear motors in this case).


that would be a thought....
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Unread postAuthor: DYI » Tue Apr 15, 2008 7:23 pm

I am a little confused about the increase in pressure decreasing the burn rate, though.


I am too, but it seems to be the consensus among the resident combustion "experts". Hopefully one of them can elaborate on it if they stumble upon this thread.

I don't believe much research has ever been done into chamber coatings, as one would need either incredibly consistent equipment (or a very big gun) to notice the results.
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Unread postAuthor: dongfang » Thu Apr 17, 2008 5:00 am

Hi,

Cool idea trying to put a reflector inside the combustion chamber!!

I would try with the heavy kind of aluminum foil that is used together with rockwool insulation (to avoid condensation, as far as I know). IF I had a combustion here.

Regards
Soren
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Unread postAuthor: Ragnarok » Thu Apr 17, 2008 9:06 am

_Fnord wrote:Here's a related question: Has anyone ever tried plating the inside of a chamber with a paper-thin coating of polished copper/aluminium?

I am not aware of that exact situation, however, I do believe that reflective paints have been used for a similar purpose - one cannon I think does is BigBang's Crusader, and as you may well know, that was chronoed as supersonic.

I would link to the spudtech archive, but it seems to be down ATM.

Now, combustions are only about 15% efficent at best. It wouldn't be completely unreasonable to consider that the remaining 85% was all heat loss. (Some energy will go into sound, and some into leaks and friction - but if any significant percentage went to sound you'd know about it)

Cutting heat loss to 80% of it's current value (a little optimistic, but not completely inconceivable) would roughly double the amount of energy available to go into the projectile. The C:B ratio would need adjustment to take full advantage of this, but as you can see, relatively small drops in heat loss will massively increase the potential of a combustion.

It therefore has somewhat surprised me that little attention is paid to the subject when talking about how to improve a combustion's performance.
General advice is just "Chamber fan, better ignition, propane metering and the right C:B ratio", but "reduce heat loss" should really be added to the end of that list. If that were done, the real potential of combustion launchers could be unlocked.

Just something as simple as spraying the interior of the chamber in a reflective silver would have some effect, and wouldn't be particularly difficult or costly. If heat reflective paint could be found, that would be even better.
I was planning on trying it out myself next time I build a combustion.

@Hubb017: Given that most combustions are not air-tight, the heating would of course raise temperature, but I would guess that the launcher would then leak a large amount of gas mixture because of the raised pressure of the higher temperature gas - until the heated gas mixture was then back at atmospheric pressure.

The technique would need to be used with a burst disk to achieve any gain, and the chamber would need to be sealed - this would make any maintenance of the chamber fan difficult. Even then, I suspect any improvement in velocity would be more due to the burst disk than the heating.

Now, consulting GasEq, it becomes apparent that a lack of burst disk in the design would actually lower peak pressure.
For Adiabatic combustion at constant volume, the peak pressure is lowered by about an atmosphere when the mix is heated from 20 degrees C to 60 degrees C.

With the burst disk, the peak pressure rises fractionally, but only by a matter of a couple of psi.
Speed of sound in the gas is raised by about 10 fps, but when it's already about 3250 fps, those last few fps are insignificant.

Therefore, I'd say that no (noticeable) advantage would come from any peak pressure or speed of sound changes.
However, the changes in burn rate might propose a small advantage. My advice would differ though - it would just be simpler and much more effective to build a hybrid instead.
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Unread postAuthor: jimmy101 » Thu Apr 17, 2008 11:16 am

Ragnarok wrote:Now, combustions are only about 15% efficent at best. It wouldn't be completely unreasonable to consider that the remaining 85% was all heat loss. (Some energy will go into sound, and some into leaks and friction - but if any significant percentage went to sound you'd know about it)

Cutting heat loss to 80% of it's current value (a little optimistic, but not completely inconceivable) would roughly double the amount of energy available to go into the projectile. The C:B ratio would need adjustment to take full advantage of this, but as you can see, relatively small drops in heat loss will massively increase the potential of a combustion.

It therefore has somewhat surprised me that little attention is paid to the subject when talking about how to improve a combustion's performance.
General advice is just "Chamber fan, better ignition, propane metering and the right C:B ratio", but "reduce heat loss" should really be added to the end of that list. If that were done, the real potential of combustion launchers could be unlocked.

Just something as simple as spraying the interior of the chamber in a reflective silver would have some effect, and wouldn't be particularly difficult or costly. If heat reflective paint could be found, that would be even better. I was planning on trying it out myself next time I build a combustion.

Heat loss in a dynamic system is a very complex process. I kind of doubt that IR reflective paint will make any difference. Heat loss is probably dominated by direct contact between the very hot gases and the cold chamber/barrel walls.

The very nature of a combustion gun puts the hot gases moving at high velocity (probably turbulent at some times and places in the gun) in direct contact with the cold barrel/chamber. I wouldn't think that the heat loss is a radiant (infrared) process in this system.

Basically, a spudgun is a "stirred system". Very bad for minimizing heat loss. Heck, the entire concept behind the insulation in a typical house is basically "insulate with air but don't let the air move". Everything in home insulation is based on that. In a spudgun the air is moving, and it is moving very fast. To reduce heat loss you have to keep the gases still. That isn't very compatiable with the basic concept of a gas driven gun.
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