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Combustion dynamics/mechanics?

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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Combustion dynamics/mechanics?

Unread postAuthor: halvamir » Sat Apr 12, 2008 8:12 pm

Hello Spudfiles,
I’m new to this site, this is my first post in fact and I've yet to build a spud cannon. I have recently become interested in the hobby however and as a result have been doing a fair amount of reading on the subject. My goal is to eventually build a hybrid cannon optimized to achieve the highest velocities I can obtain. I don’t have any illusions about how difficult this may be and don’t plan to attempt it for my first couple of guns. I plan on starting simple.
I am a mechanical engineering student and want to use these skills to my advantage. They however don’t offer any "spudcannon theory courses" at my university (which to me seems a grievous oversight). What I hope to do is calculate with reasonable accuracy the dynamic forces that will be acting on my projectile from the point of initial combustion to the barrel exit. I already understand that a couple of different principle branches of physics will be involved and most likely they will involve material I am not yet acquainted with. I am willing and capable of learning.
In reading some of the forum posts on this site it has become abundantly clear to me that a few of you really know what you’re talking about. In fact I suspect that some of you may be engineers, physicist, and or chemist etc. My hopes of building the best potato cannon the world has ever seen are dashed 8(.
What I hope to get from some of you is a little direction into places I can look to get the information I'm after. I understand that force dynamics, thermodynamics, compressible fluid dynamics and chemistry are areas that I'll need to look. That however, is a pretty damn large collection of material to just start searching with my only guide being the question "does this apply to my problem?” Also, I've been unable to find any websites that address this approach to building combustion cannon. Any direction or advice in either of these two areas (or any I’m overlooking) would be greatly appreciated.
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Unread postAuthor: jrrdw » Sat Apr 12, 2008 8:18 pm

Go to the Wiki and go in sit down loosen your tie get comfortable and start reading.

Welcome to Spudfiles.
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Unread postAuthor: starman » Sat Apr 12, 2008 8:40 pm

Welcome to SpudFiles!

I sounds like you've started doing your homework, that's great! I would suggest getting started with some hands-on by building a basic spray-and-pray hairspray cannon with PVC pipe and BBQ ignitor. These are cheap and easy to build and will get you acquainted with materials, fuel, ignition basics. These are more fun than you might think...most everyone builds one sometime in their spud career.

Go over to Latke's site at http://www.burntlatke.com and spend a day or 2 there. Read and digest everything he has there....then read it again. It is really the defacto standard "how-to" for advanced combustions.

You'll find some chemistry and thermodynamics mentioned in discussion here...you won't find it all in one neat place however. Several have written calculation tools and performance modeling programs that are of great help to the builders. D_Hall's tools, GGDT is one of the most used and is for pneumatic guns. His newer program, HGDT is for combustions/hybrids and he's working on perfecting it as we speak...actually it seems like a fairly good tool at the moment.

http://thehalls-in-bfe.com/GGDT/
http://thehalls-in-bfe.com/HGDT/

Read, listen and participate in the group, you'll learn a lot. Also find a project that floats your boat and jump in there. Don't feel like you have to start on the ultimate hybrid from the get-go.

Also, you won't need to understand all of the science from top to bottom to build killer cannons. Much of the theory will come to you as you progress in your education and in your spudding experience. My advice, get your hands dirty and build yourself a basic cannon.. :wink:
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Unread postAuthor: LikimysCrotchus5 » Sat Apr 12, 2008 9:12 pm

I agree with starman as well. Just go and build something really basic, like a spray n pray. The reason why they are called spray n pray is because when feuling these basic combustion cannons, you tend to over fuel or under fuel because you are using areosols as fuel. So you spray, and then pray so it works :D (no religion involved).

Most common ammo is potato, which can be quite powerful. Just look around in this forum and you will find plenty of info to help ypu make one. Usually for fueling cannons, stiochiometry is used so that you get the correct fuel amount, thats when you build a fuel meter and do some math.

Pneumatics can be fun as well. There is a pneumatic gun on this site that can pentrate 1/4 inch normailzed steel (i think its normalized). Thats pretty powerful. We have hybrids that can exceed mach 1.

So if you like destruction and power, i suggest you do this hobby and start with a basic combustion, then a pnmeumatic (not using a ball valve) etc. etc.

If you want to talk to someone about combustion, physics, etc. talk to jimmy101, D_Hall, Spudblaster15, and DYI and good reliable sources.

Welcome to Spudfiles.

P.S. Nice avatar, i like family guy as well.
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Unread postAuthor: DYI » Sat Apr 12, 2008 10:14 pm

jimmy101, D_Hall, Spudblaster15, and DYI and good reliable sources.


Well that's great, but I really know nothing but the basics about combustion dynamics. Jimmy, Dave, and Boilingleadbath are the ones to talk to about that.

Dave's still trying to figure out the exact cause of the "knee" in the Latke data, but it seems he's developed a good understanding of combustion dynamics while making HGDT.

I suggest that, if a high-powered hybrid is your final goal, to get yourself aquainted with some moderately complex high voltage circuits now. That's been my downfall, and it's holding back my current project quite a bit. An "advanced combustion" is a good place to start to acquire some useful skills.
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Unread postAuthor: halvamir » Sat Apr 12, 2008 11:18 pm

Thankz DYI, I didnt even think of circuits. Conveniently I'm taking that course this semester. In the fall I will be taking instrumentation which hopefully will provide me with the tools to gather some useful emperical data on velocities, maximum pressures, rates of pressure change and so forth.
Your absolutely right though. Combustion requires a spark and this requires a voltage sufficient to ionize the atmosphere in which it's to be produced. My eventual super gun will be some sort of burst disc gun with an oxydizer (probably not O2) enriched mix. Higher pressures mean higher voltages are required for ionization. More over, if I understand everything I've been reading multiple spark points result in better combustions?
So I guess you need to wire them in parrallel to get the best results? I wonder if it would be feisable and worthwhile to wire some small electronics transformer into the circuit to boost voltage? I guess thats why you guys like to use stun guns though eh?
Well anyway I'm rambling. Thx for the advice, links and ideas, this gives me some new stuff to look at and think about.
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Unread postAuthor: psycix » Sun Apr 13, 2008 6:19 am

Multiple spark gaps allow multiple flamefronts to be created, this results in a faster combustion. The quicker the combustion occurs, the more powerfull your gun will be.
You HAVE to wire them in series, not paralell.
If you wire them paralell only one will spark. This is due to that one gap will spark just like a nanosecond earlier, ionizing the air and thus making a low-resistance path. Then ALL power will jump trough that gap.

The higher the voltage, the bigger the (total) spark distance can be, a 100.000V stungun does the job very well.
People also just use piezo BBQ igniters or camera flash ignitors.

There is a lot to think about when building a gun as powerful as possible.
Have you looked at chamber fans already?

Get reading and understanding and you will build a powerful cannon within no-time!
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Unread postAuthor: DYI » Sun Apr 13, 2008 10:18 am

High voltage ignition systems can get quite complex for the more powerful guns. For example: my current project will require an absolute minimum of 200kV through the ignition circuit, as it will be jumping through air/fuel pressurised to over 20 bar.

And the reason for multiple spark gaps? Combustion has 4 main modes that are of interest in the spudding world, and those are: spherical, laminar, turbulent, and detonation. The spherical mode progresses from the ignition point, until the flame front contacts the chamber wall, and then transitions to laminar. Since spherical mode progresses more quickly than laminar, it is advisable to keep combustion in that mode for as long as possible. By using multiple spark gaps spaced 1 chamber diameter apart throughout the chamber, more of the total combustion happens in spherical mode.

Turbulence speeds up flame fronts very significantly, and thus it is a good idea to try and create turbulence in the chamber, hence chamber fans. Not much testing has been done on chamber fans in hybrids, but what little results there are indicates an increase in performance.

Detonation is what you generally want to avoid, as it exposes the chamber materials to an extremely violent pressure spike, as flame front velocity rises from a few ft/s, to several km/s in a very short time frame. Only very light projectiles in comparison to the size of the detonation can actually be carried along with the detonation wave, meaning that deflagration is not only safer, but generally more useful.
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Unread postAuthor: D_Hall » Sun Apr 13, 2008 12:13 pm

psycix wrote:You HAVE to wire them in series, not paralell.
If you wire them paralell only one will spark. This is due to that one gap will spark just like a nanosecond earlier, ionizing the air and thus making a low-resistance path. Then ALL power will jump trough that gap.

Only true if you're powering all sparks from one point.

Alternatively, you can run parallel systems (not to be confused with mere parallel spark gaps). Doing this is more expensive in the beginning, but it allows one to use a single design over and over. No scale-ups or redesigns required (so it may save money in the long run).
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Unread postAuthor: D_Hall » Sun Apr 13, 2008 12:14 pm

Oh, and couple more thoughts....

1) IMHO, anyone with even the slightest knowledge of the principles behind spud guns should skip the spray and pray varients and move straight to advanced combustion.

2) Since the original poster is trying to gain understanding of HIS gun and is a university student, he may want to talk to one or more of his profs and see if he can't get his hands on Cheetah. Cheetah is a hydra code put out by Lawrence Livermore National Labs and it's pretty much state of the art in modeling of combustion processes. It'd be wonderful for somebody like the original poster.
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Unread postAuthor: halvamir » Sun Apr 13, 2008 1:38 pm

Yes!! Thank you D_Hall, that is exactly the sort of information I was after. I had already planned to speak with some of the professors in my dept as well.
The combustion, gas expansion, wave front stuff also is good. It’s going to be the most difficult to model. Compressible fluid mechanics involving turbulent flow is a mathematical nightmare. I’m currently studying non compressible turbulent flow its easier (still ugly) but also of no use for the combustion problem. This will likely be the most challenging aspect of my goal to model the dynamic forces acting on my projectile. They also are going to be the most important. I hope to start off with a desired set of performance specifications and design and build cannon capable of achieving them. There is no perfect solution to a problem like this but if your careful and thorough you can make accurate approximations.
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Unread postAuthor: starman » Sun Apr 13, 2008 2:20 pm

D_Hall wrote:1) IMHO, anyone with even the slightest knowledge of the principles behind spud guns should skip the spray and pray varients and move straight to advanced combustion.

2) Since the original poster is trying to gain understanding of HIS gun and is a university student, he may want to talk to one or more of his profs and see if he can't get his hands on Cheetah. Cheetah is a hydra code put out by Lawrence Livermore National Labs and it's pretty much state of the art in modeling of combustion processes. It'd be wonderful for somebody like the original poster.


My recommendation that he start basic came from not knowing his level of "shop" experience...he didn't indicate any confidence there...not that there isn't any of course. Assuming he wants to build his own guns and not just design them, practical hands-on skills with the materials will be essential.
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Unread postAuthor: boilingleadbath » Sun Apr 13, 2008 3:56 pm

...this whole concept of "experience" is really getting old.

Especially when it comes to combustion cannon construction.

Let's see here, skills needed to build spray&pray (in rough order of difficulty):
1) Gluing
2) Threading holes
3) Cutting

And an advanced launcher (with fan, pressure-regulated meter)
1) Gluing
2) Threading holes
3) Cutting
4) Using threaded pipe fittings

The consequences of screwing up when building an "advanced" launcher are not more severe, and the only additional skill is very easy (and, of course, can be redone as long as one has more Teflon tape.)

And you won't learn how to use threaded fittings by gluing pipe together.

************

That said, don't turn to me when asking about combustion dynamics, I don't know squat about modeling them.

I do know a bit about large scale empirically determined things, and like to throw together excel spreadsheets for simplistically determining things... but that's not really helpful in your situation.
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Unread postAuthor: starman » Sun Apr 13, 2008 11:11 pm

boilingleadbath wrote:...this whole concept of "experience" is really getting old.


Huhhh? Never underestimate the value of experience.

@Halvamir: Get your hands on a real-live spud gun project of some sort to go along with your theoretical studies. It won't cost you a bunch of money and you'll be glad you did.
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Unread postAuthor: jimmy101 » Mon Apr 14, 2008 1:14 pm

One of the nice characteristics of combustion spud guns is that you can start simple then add more advanced features to the basic gun. That is a great way to get a good practical feeling for how much various modifications help (or hurt) the performance of a combustion spudgun.

I would start with a basic "squirt-and-screw" gun. Perhaps a 12"Lx3"chamber, 48"Lx1.5D barrel, single central spark gap powered by a BBQ piezo and "engine starter spray" as the fuel (leaves no residue in the chamber, much nicer than hairspray and the like). Sharpen the outside of the muzzle to cut your ammo.

Fire it a couple times to experience, first hand, how hard it is to get the fueling correct.

Upgrade to a simple fuel meter using a syringe (cost ~$2) and propane (or a disposable butane lighter) as fuel source.

Fire it a couple times to experience, first hand, how much more consistent the gun is.

Next, add a chamber fan. Does it help performance any?

Next, double bevel the muzzle. Does it help?

Next, add multiple sparks. Do they help?

Next, add an onboard meter system (won't help the performance any but the "bling-factor" is off scale).

The most advanced combustion spudgun you'll ever see can still be operated in a "beginners mode". Turn of the fan, remove multiple sparks, fuel by "spray-and-pray".
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