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Pressure drop across burst disc plain

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Pressure drop across burst disc plain

Unread postAuthor: D_Hall » Sat Mar 22, 2008 5:06 pm

In the world of pnuematic guns, the efficiency of one's valve is very important. Low efficiency valves result in large pressure drops across the valve oriface which in turn greatly reduces the power available to accelerate the projectile.

This is the world I've the most experience with.

But as I've been playing around with the theory that drives hybrids, what I'm seeing is that pressure drop across the burst disc plain is virtually negligable. The large oriface and simple gas path simply allows for a very efficient system from this perspective.

Which brings me to my larger point....

In another thread (which I'm too lazy to track down at the moment), Jimmy and I were discussing what burst pressure would allow for maximum velocities.

I felt that the highest pressure you could reliably get the disc to go at would be best. The rational being that you wanted to hit the projectile with as much pressure as you could as early as you could.

Jimmy felt that it would probably be optimal to burst earlier. His feeling was that it would be good to get some unburned fuel/air in the barrel for combustion where it would do the most good (in the barrel).

What I *THINK* I'm seeing is that it doesn't really matter provided all fuel burns before the projectile clears the muzzle. If the fuel burns in the chamber, it smoothly moves to the barrel to equilize the pressure across the disc plain. If the fuel burns in the barrel, it smoothly moves to the chamber(!) to equilize the pressure across the disc plain. That's a bit simplified, of course. There WILL be losses in moving gases around.

This is all based on simplified theory and simulation, however.

Thoughts?



Note: Jimmy, if I have mis-represented your position please correct me!
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Re: Pressure drop across burst disc plain

Unread postAuthor: jimmy101 » Sat Mar 22, 2008 7:17 pm

D_Hall wrote:What I *THINK* I'm seeing is that it doesn't really matter provided all fuel burns before the projectile clears the muzzle. If the fuel burns in the chamber, it smoothly moves to the barrel to equilize the pressure across the disc plain. If the fuel burns in the barrel, it smoothly moves to the chamber(!) to equilize the pressure across the disc plain. That's a bit simplified, of course. There WILL be losses in moving gases around.

This is all based on simplified theory and simulation, however.

Interesting. I've always wondered the same thing about standard combustions. Does it make any difference (or much difference) if the fuel is fully burned before the spud starts to move or is it about the same thing as long as all the fuel burns before the spud exits the barrel?

My WAG is that the sooner the fuel burns the better but perhaps it doesn't really make all that much difference.

Now just need someway to test it.
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Re: Pressure drop across burst disc plain

Unread postAuthor: D_Hall » Sat Mar 22, 2008 9:36 pm

jimmy101 wrote:Interesting. I've always wondered the same thing about standard combustions. Does it make any difference (or much difference) if the fuel is fully burned before the spud starts to move or is it about the same thing as long as all the fuel burns before the spud exits the barrel?

My WAG is that the sooner the fuel burns the better but perhaps it doesn't really make all that much difference.

Now just need someway to test it.


LOL... Well, I started this thread in hopes that somebody would either confirm or deny my theories with test data. :D

To address your question... I don't think it matters much provided that it all burns before the spud exits the barrel.

Mind you, my model is incomplete. Much of what I'm saying is extrapolation based on the things I *am* seeing with my model. But remember, that's a model; not test data. It's completely possible that my head is firmly planted up my ass at this moment.

I've got one more feature to impliment and I'll throw it over the fence at you to see what you think of it. That doesn't mean that it'll be a finished product; not by a long shot. But it'll be close enough to (hopefully) say meaningful things about it.
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Unread postAuthor: jimmy101 » Sun Mar 23, 2008 2:35 pm

Cool, it'll be interesting to see if there is any agreement between your model for hybrids and my model for 1x guns.

If you like, I can send you my Excel spread sheet.
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Unread postAuthor: D_Hall » Sun Mar 23, 2008 3:09 pm

While I'd definately like to see the spreadsheet, I think it's a bad idea at this time.

Put it this way... If two independent models agree, odds are there's something to it. But if I look at that spreadsheet... Well, it will contaminate my thoughts. The models will no longer be independent. Agreement will mean very little.

That said, the model I'm working on should do a fair representation of 1X guns as well (I just set the burst disc to go at 0 psi) so we ought to be able to make some pretty direct comparisons.
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Unread postAuthor: D_Hall » Sun Mar 23, 2008 5:47 pm

Ugh....

Just ran into a friggin brick wall with the model. Well, not really a brick wall, just an "oops" that I'd not anticipated.

Basically I *am* seeing the barrel pressure exceed chamber pressure when the flame propogates into the barrel and ignites fuel/air that had made it unburned into the barrel.

The problem: While I've been treating the chamber as two distinct masses (hot and cold), I've been treating the barrel as one homogeneous mass. Thus, when the barrel pressure exceeds the chamber pressure and the flow reverses, I dump cold air into the chamber and the chamber temperature plumets. This should NOT happen as the air nearest the oriface will be the recently burned material. So while it's fair to say that the flow WILL reverse itself, it is NOT fair to say that cold air will be dumped into the chamber.

I'm gonna have to think on this one some. The obvious solution is to give the barrel the same treatment I've been giving the chamber - two distinct masses. But this is quite frankly a PITA. I'm hoping I don't have to do this (but suspect I will have to).
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Unread postAuthor: D_Hall » Sun Mar 23, 2008 5:53 pm

[after a very small bit of thought]

Another approach would be to assume that the fuel/air in the barrel combusts instantaneously (or very nearly so). Hmmmm.... Coming through the oriface I should have very turbulent flow. This may not be a bad approach....

I know where the projectile is at this moment.

I can assume a flame velocity.

Seems like I could uniformly burn the remaining fuel over some time interval where Time = ....


D'oh!

That still doesn't solve my cold air dump into the chamber problem. Grrr....
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Unread postAuthor: Intel Xeon » Sun Mar 23, 2008 6:36 pm

Well, as I have no experience with hybrid cannons, I am talking completely out of my arse here.

To get the most acceleration, I do believe that you would have to have the burst disk burst at the point in time at, or slightly before, the pressure and speed of the flame front reach their peak, so that a massive pressure spike hits the projectile and creates a large amount of acceleration. THIS DOES NOT EQUAL THE MOST MUZZLE VELOCITY! In a 1X cannon, however, a problem that does not occur at hyperbaric pressures is that if the chamber pressure is ≤ the current atmospheric pressure before the projectile exits the barrel then friction and possibly negative pressure counteracts the momentum to some degree.

The following nonsense assumes the projectile makes a perfect seal

In the dark and convoluted passageways of my mind, I see the barrel and chamber as separate BEFORE THE FIRING. As soon as the pressure causes the burst disk to actually burst, it effectively creates two dynamic masses that are separated by the projectile. This ....

Erm... I think I need some advil....
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Unread postAuthor: sniperjosh » Mon Mar 24, 2008 4:04 am

Hi all, just wondering how accurate test is required. I could run a test at 2x mix's using plastic packaging burst discs, ill use one layer test a few shots with that, then use 2 layers and test with that. Barrel length will remain constant and I will re-use the same projectile (glass marble) for every shot. This will be fairly rough way of doing it, but it would be quite simple to do. Let me know if you want me to do the test.

I hope a conclusion comes of this thread, it would be a good breakthrough :o
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Unread postAuthor: D_Hall » Mon Mar 24, 2008 8:27 am

ALL test data is good... assuming there's actual data to go with it (translation: do you have a chrony set up?).
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Unread postAuthor: sniperjosh » Tue Mar 25, 2008 12:20 am

yes I have a chrony :)

on a side note, D_hall do you know of any programs out now which calculate the performance of hybrids (like ur ggdt does for penumatics).

and not to venture too far off course, just wondering if this PDF lends any assistance with your research into flame front propogation and hybrid modelling (noticed a thread you made) http://ctr.stanford.edu/Summer98/haworth.pdf
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Unread postAuthor: D_Hall » Tue Mar 25, 2008 8:57 am

sniperjosh wrote:on a side note, D_hall do you know of any programs out now which calculate the performance of hybrids (like ur ggdt does for penumatics).

Well, that's kinda the whole point of this thread... I'm working on one. A screen shot of what I've got so far (notice that is NOT GGDT)....
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...But I'm not currently modeling combustion in the barrel; just in the chamber. I think I've had my Eureka moment on how to do it in the barrel, I just haven't had the time to get it done.



Don't have time to look at your pdf right now though.
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