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combustion coxial

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Sun Apr 15, 2007 12:28 pm

BC Pneumatics wrote:The best place of ignition is as far from the barrel as possible, so the flamefront can accelerate, as far as anyone has ever been able to show. the whole 'put it in the middle some more is burning' has never been proven as effective. (Most people seem to think the opposite, it hurts performance.


Any proof of that? The whole "put it at the back of the chamber so it can accelerate" has not only not been proven but been disproven many times, though not specifically in spud guns.

The flame front acceleration has to do with the form function (shape of the flame front) and the temperature in the chamber. The flame will accelerate more if the initial ignition event is near the center of the chamber. With a central ignition event you get two flame fronts. It doesn't really matter that one of the flame fronts is moving backwards, it is still consuming fuel and producing heat and pressure. The pressure created by combustion moves in all directions equally, it does not move in just the direction the flame is moving. With an ignition at the rear of the chamber one of the flame fronts burns out at the guns breach and the rate of fuel burn gets cut in half.

It is a common misconception that the movement of the flame front has something to do with the pressure created in the chamber or how the pressure is applied to the rear of the spud. It does not. The movement of the gases in the chamber is almost completely independent on how the flame front is moving.

It is very well established that the maximum burn rate in a close chamber occurs with a centrally located ignition event. That ain't theory, that is a fact determined by laboratories that do this stuff for a living.
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Unread postAuthor: BC Pneumatics » Sun Apr 15, 2007 3:49 pm

As much as I thought that was true, I have tried both ignition points in two cannons, and both cannons preformed better with the rear ignition. Of course, barrel length could have considerable effect on this as well. I can't explain my counterintuitive results, but I know I have seen them.
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Unread postAuthor: willarddaniels » Sun Apr 15, 2007 10:41 pm

Jimmy101, you have convincing reasoning, but your logic and sources are flawed. This is according to my research, experience in addition to BC and others' research and experience.

Here is your proof: The highest flame speed was observed when the gas was ignited in the closed end and the other end was open. In that case the gas ahead of the flame was pushed through the pipe and a lot of turbulence was generated.
Boilingleadbath posted this site a few days ago.
It is very well established that the maximum burn rate in a close chamber occurs with a centrally located ignition event. That ain't theory, that is a fact determined by laboratories that do this stuff for a living.
The research done by this company contradicts your "well established ... fact determined by laboratories" statement.

Also, combustion cannons are not considered closed chambers. The chamber is open, it just has the opening "blocked" by a very low-friction projectile. Otherwise, combustions would be failing/exploding far more often. If we used closed chambers, this website would be about bomb making, not creating produce accelerators.

Furthermore, the reason many use multiple points of ignition is due to combustions being an open chamber: As the gasses combust starting at the sealed end, they push the unburned air/fuel mix ahead of the flame front, combusting as it goes along, creating turbulence and higher pressures. Even with .8/.9:1 ratios, you will have excess fuel escape the barrel before combustion, thus wasting potential energy. With multiple points, you still have this effect, but to a much smaller degree.

Clarification on length of spark in multiple spark set-ups: It doesn't matter if one spark is longer than another, they ignite the fuel the same.

It is a common misconception that the movement of the flame front has something to do with the pressure created in the chamber or how the pressure is applied to the rear of the spud. It does not. The movement of the gases in the chamber is almost completely independent on how the flame front is moving.

If this were true, we wouldn't be using chamber fans. Also, The movement of the flame front is directly related to the pressure in the chamber, as the pressure increases, the flame front accelerates - this is a way how DDT occurs. Just read the explanation given in the website boilingleadbath provided and that I quoted.

I may be wrong on some of this, if I am- let me know and offer a solution, don't just gripe. I have never claimed to be an expert on this, just a thinker.
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Unread postAuthor: aturner » Mon Apr 16, 2007 12:28 pm

jimmy, there is no data to support your claims. You might have all the theory in the world to back you up, but in practice, I can tell you there is no measurable difference between the old in line muzzle loader versus a coaxial.
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Unread postAuthor: jimmy101 » Mon Apr 16, 2007 1:24 pm

Willard: I think you have been mislead by a couple of things.

The Gexcon handbook is describing combustion in an open chamber. A spud gun is not an open chamber. Granted it isn't exactly a closed chamber either but it is clearly not the same as an open chamber.

"Also, combustion cannons are not considered closed chambers. The chamber is open, it just has the opening "blocked" by a very low-friction projectile."

The spud is not a "very low friction projectile". Indeed, I believe it is fairly well established that combustion guns perform better with more friction. A double beveled spud cutter gives a tighter fitting spud, more friction and higher muzzle velocities. A single beveled cutter, with the bevel on the outside of the barrel, gives lower friction and lower muzzle velocities. Higher static friction makes the gun perform with some of the characteristics of a burst disk.

Otherwise, combustions would be failing/exploding far more often. If we used closed chambers, this website would be about bomb making, not creating produce accelerators."

This is not true. The peak adiabatic pressure from propane + air is just ~120 PSIG. Nothing is truly adiabatic, so a real world closed chamber won't even reach 120 PSIG. That is well within the pressure limit of pressure rated Sch40 PVC pipe. In addition, the whole point of a burst disk gun is to get the working pressure up near the theoretical pressure. Burst disk guns are not "bombs", even if a burst disk is used that ruptures at ~120 PSIG. Even with a closed chamber, 120 PSIG is not enough pressure to cause pressure rated pipe to fail.

Combustion in an open pipe is a very poor model of what happens in a combustion gun. For a gun to work effectively (or at least as effectively as a combustion spud gun can operate) the pressure in the chamber must rise. In an open pipe, there is very little pressure rise during the combustion process and the gases can move at high velocities because there is nothing to prevent them from doing so. In a spud gun the gases are not free to move at high velocities. The flame still propagates even though the gases are moving only a small amount.

Looking at the Gexcon data for the 1m diameter open pipe, they get flame speeds approaching 250 m/s (after burning along a pipe 40m long). I would suspect that the gas velocity is similar to the flame front velocity in their system.

In a typical combustion spud gun you are unlikely to get air velocities of 250 m/s (820 fps) since the spud would have to be moving that fast.

It is easy enough to estimate the velocity of the gases in a spud gun. When the spud exits the barrel the small volume of gases immediately behind the spud is moving at the speed of the spud. If the spud is moving at 500 fps then so are the gases immediately behind it. However, the gases in other places in the chamber are moving slower. The small volume of gas at the breech is not moving at all. A slug of gas located near the center of the chamber is only moving at about 1/4 the velocity of the spud. (There is a factor of ~2 for being halfway from the spud to the breech and another factor of ~2 for the chamber having roughly twice the cross sectional area as the barrel.)

If the spud exits at 500 fps then during the time it was actually moving through the barrel is was moving slower. On a time basis, the spud spends much more time moving slowly then it does moving fast. Hence, for most of the time the gases in the gun are also moving slowly.

"If this were true, we wouldn't be using chamber fans."

The chamber fan can not get the gases in the chamber moving in an organized fashion prior to ignition. It is a closed chamber with a large length to width ratio. A fan creates turbulence but does not actually get the gases moving in any particular direction. Even if the fan could get the gases moving in an organized fashion the momentum towards the front of the gun is balanced by an exactly identical amount of momentum that is going towards the back of the gun. It must be, otherwise turning the fan on would make the gun move. So, a fan can not get the gases going in a particular direction prior to ignition. The fan does get the gases moving in a chaotic fashion and that may be all that is required to get the flame front to transition from a laminar front to a turbulent front. It doesn't take much movement of the gases in the chamber prior to ignition for that movement to affect the laminar to turbulent flame front transition. (Indeed, in laboratories that study these types of things, they frequently let the chamber sit for several minutes after turning off the fan to allow the movement of the gases to stop. If they don't do that, then the laminar to turbulent transition will happen even though the fan is not actively running when the mixture is ignited.)

Also, The movement of the flame front is directly related to the pressure in the chamber, as the pressure increases, the flame front accelerates - this is a way how DDT occurs. Just read the explanation given in the website boilingleadbath provided and that I quoted.

Actually, it is the speed of the flame front that is directly related to temperature and pressure in the chamber. Rising temperature increases the flame front speed (roughly as the square of the absolute temperature). Rising pressure actually decelerates the flame speed by a small amount. To get to DDT, the pressure wave must approach the speed of sound and the temperature in the unburned gases must approach the autoignition temperature. Those new DDT papers are very interesting but nobody has ever detected DDT in a propane+air combustion spud gun.

I've been thinking about how a combustion gun works for a long time. The physics are not simple. Here are some links to post I did to the SpudTech forums on the modeling of combustion guns;

Intro: Towards a mathematical model of combustion spud guns
http://www.spudfiles.com/spudtech_archi ... hp?t=15477

Part I: Towards a model of combustion spud gun
http://www.spudfiles.com/spudtech_archi ... hp?t=15478

Part II: Combustion / Compressed Air gun performance disconnect
http://www.spudfiles.com/spudtech_archi ... hp?t=15498

Part 3: How efficient can a combustion gun be?
http://www.spudfiles.com/spudtech_archi ... hp?t=15526

Part 4: Modeling closed combustion chamber
http://www.spudfiles.com/spudtech_archi ... hp?t=15557

Part V: Adiabatic Gun Model
http://www.spudfiles.com/spudtech_archi ... hp?t=15614
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Unread postAuthor: jimmy101 » Mon Apr 16, 2007 1:26 pm

aturner wrote:jimmy, there is no data to support your claims. You might have all the theory in the world to back you up, but in practice, I can tell you there is no measurable difference between the old in line muzzle loader versus a coaxial.


Can you describe the guns and provide performance data?
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Unread postAuthor: aturner » Mon Apr 16, 2007 2:48 pm

Jimmy, you're the one who should be providing data, since you're making unequivocal claims. However, I'll share what I know....

The launchers I've compared are my blackhawk and my patriot. Similar chamber volumes and barrel volumes. Both have a 1.5:1 ratio so they have plenty of power. The only major difference between the two is an in-line versus coaxial design.

For 2.5oz spuds, the chrony gives me speeds in the range of about 340fps on a consistent basis for both of these launchers. Specs are: 2.8-3L chambers, 1.5" by 5.5' barrels, piezo ignition, chamber fan in both launchers, metered propane fuel.

Then, in a "real world" comparison, these two launchers perform about the same. Basically, I observe that both hurl potatoes near the same distance landmark.

I have ideas for some additional tests, but I'm not too motivated since the differences are minimal anyway. So in my opinion it is only a matter of preference.
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Unread postAuthor: aturner » Tue Apr 17, 2007 10:10 am

Jimmy,
You got me thinking. Do you think my results are influenced by the fact that I've looked at fairly high c:b ratios? If so, do you think the effects of energy lost to heating the chamber walls is mitigated by having plenty of additional energy from a larger chamber?

If energy lost as heat to the chamber is a significant factor, then then "ideal" c:b ratio for coaxials should be slightly larger than for an inline. I question whether this is significant enough to matter? If folks just stick to a ratio of 1:1 or greater, then they should always have plenty of power.

Sorry for the double post, but I want to make sure this doesn't get lost in the shuffle.
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Unread postAuthor: jimmy101 » Tue Apr 17, 2007 12:51 pm

Great data Atuner, spud gunners in general don't have data or a theory to back up there claims.

I'm not sure about the affect of a 1.5 C:B on the relative performance of a traditional inline versus a coaxial with the barrel extending into the chamber.

Energy lost to heating the chamber is a significant affect in real guns. A quick google search says that 20 to 30% of the energy in a real gun is lost to heating of the barrel and chamber. I would think heat loss is even more significant in a spud gun since things happen so much slower. Furthermore, spud guns are perhaps half as efficient as real guns so the additional energy lost has to go somewhere.

I am really suprised that your coax gives the same performance as a non-coaxial with the same barrel diameter, barrel length and useable chamber volume. The flame front geometry should be really inefficient in the coax compared to the inline.

I wonder if the coaxial design transitions to a turbulent flame front sooner than does an inline design. (Or, an inline design doesn't transition at all and a coax does.) Turbulent flame front speeds can be 10 times greater than laminar fronts. With all the wall surface in the coax design there would certainly be more turbulence in any gases that are moving.

Kind of off topic, but I believe that essentially the entire cause of combustion guns working best near a C:B of 0.8 is due to heat loss. Using GGDT or Gaseq you can calculate the adiabatic performance of these guns. You need some approximations but they are not too severe. Both models say the ideal C:B ratio should be in the 0.2 to 0.3 range. That is, both models predict that the barrels should be three to four times longer than what we know is best. The biggest factors missing from both models is heat loss. Both GGDT and Gaseq do a pretty good job of predicting various performance parameters of compressed air guns, which really do have optimal C:B's around 0.2~0.3. One big difference between a compressed air gun and a combustion gun is that the combustion gun runs hot and loses heat to the pipe. Compressed air guns actually cool off when they are fired and non-adiabatic behavior makes a compressed air gun work better. (To be more precise, how much a compressed air gun cools off depends on how and when it was pressurized.)
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Unread postAuthor: willarddaniels » Thu Apr 19, 2007 1:54 pm

Jimmy, I appreciate your contributions here, as well as on SpudTech. That said, be careful. Some of your statements contradict what you've claimed in other posts, as are your reasonings for trying to argue with me- an admitted problem you have had with other members. You've shown you are well thought out and have done some great research; however, you've also shown an ability to throw your own theories out as proof to discredit others' real experience- not an accurate or an effective means of creating a learning environment. This tendency seems to blind you to some of the arguments that you have made trying disprove what others have posted and cause you to either misread or misinterpret others. We (I) neither appreciate nor deserve this belligerent treatment. However intended, an explanation of this attribute only shows how truly passionate you are and can be a starting point in aligning our thinking and reasoning. Don't get upset... these are observations of your dealings with others (boilingleadbath pointed the same thing out once), based on the links you provided and the current thread. Please note: this is not designed to flame you openly (well, maybe a little), but to implore you to see what is being said here in a more objective manner and the fact that we are all working towards the same goal and not trying to see who can create the longest post with the most proof or the most complex set of equations and graphs.

Not really willing to debate this much further and setting egos aside, I offer this as a compromise: The main differences between our reference points and reasoning is based on the model we are using: closed vs. open combustion chambers. As you have admirably admitted before, a combustion cannon is not a truly closed chamber and thus computations presented by other researchers on closed chambers cannot be used with 100% accuracy. Likewise, a combustion chamber is not completely open, the barrel opening is typically a smaller diameter than the chamber and is being loosely blocked by a projectile.
These different theories are based on different models and will invariably utilize different areas of research. As all agree, combustion is not completely understood and this types of debates could go on forever, thus the compromise.

On a different note: Going off of boilingleadbath, to increase the performance of a coaxial combustion, one could use of a Shchelkin spirals (used to propagate DDT). This would create quite a bit of flow restriction (50%), achieving higher pressures. Also use multiple sparks all at the breech of the gun, in order to maximize the utility of the Shchelkin spiral. Be wary of DDT, as both of these methods increase the likelihood of this pressure spike: build gun accordingly (sch 80 steel)
If you test this theory based on a PDE on Pulse Detonation Rockets, I suggest you use remote ignition. ie. Do not hold the gun while firing, ever. A 10" spiral inside a 30", 1" ID chamber has been shown to propagate a DDT.
If you don't understand what is being suggested here, don't try it. I understand it and I am still wary of trying it, but it is exciting... 1200-2000 m/s flame front velocities were achieved in PDEs
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