Hybrid disk burst pressure effect on performance

Harness the power of precision mixtures of pressurized flammable vapor. Safety first! These are advanced potato guns - not for the beginner.
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jackssmirkingrevenge
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Thu Dec 07, 2023 12:48 am

D_Hall wrote:
Wed Dec 06, 2023 8:28 pm
You need something to induce turbulence!
Some experimentation was definitely in order, so I machined a Delrin cup to fit the chamber with a 1/4" orifice through the base:
20231206_213324.jpg
baffling.png
baffling.png (10.23 KiB) Viewed 13501 times
I fired a series of shots and the results were quite interesting.

The first thing I did was repeat the tests done in the first post, at 16x with 0.25g airsoft BBs and with photo paper burst disks.

1 layer - 1903 fps (up from 1525 fps without baffle)

2 layer - 2012 fps (virtually no change from 2010 fps without baffle)

3 layer - no burst (1808 fps without baffle)

I then upped the mix to 20x, with a single aluminum disk it failed to burst, whereas normally even two layers would burst without the baffle.

With 2 layers of photo paper I got 2075 fps with the baffle and 1400 fps without it.

In the latter case the muzzle energy with the baffle is 50 J up from 23 J without it, just over double, with all other conditions being equal. That being said, just looking at these preliminary results, what the baffle seems to do is make up for performance loss from a disk that bursts too early rather than increase the energy up from the optimum disk burst pressure.

That's enough experimentation for this session but I would also like to try cutting down the baffle to make the "second chamber" smaller to see if that makes a difference, I wonder if it will increase or reduce peak pressure at the disk.

Of course it's not just putting BBs through the chrono, here is some effect on target at various velocities, more to follow eventually:

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Thu Dec 07, 2023 9:04 am

Interesting results. According to the scientific article on this topic, the diameter of the hole can vary the results. As I remember, the second division of the camera was much smaller than the first. Good job Jack!!
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Fri Dec 08, 2023 12:28 am

jimmy101 wrote:
Thu Dec 07, 2023 12:35 am
A mesh screen would kick up turbulence but also suck up a lot of heat.

We have discussed before if running the chamber fan during firing might increase the burn speed. Way back when I measured burn speed in a closed chamber at 1x with/without the fan running and with one or three sparks gaps. There was about a 20% faster burn speed with the fan running. The fan was a generic small CPU fan.
http://www.inpharmix.com/jps/Closed_Cha ... udies.html
We ran such tests with VERA too. Granted, our fans were 18 inches in diameter and air powered but.... Yeah, our results agreed.

What we found was that with the fans on, the pressure ramped up smoothly until the disk burst at about t = 0.15 s. With the fans off, the pressure ramped up smoothly until the disk burst at about t=0.25.

As faster disk bursts were seen as more efficient (less time to lose energy due to heat loss) we ran her with the fans on for a few years. But the fans were a maintenance issue and eventually we decided to run some shots with 'em off just because we were trying to get some stuff done on a deadline and didn't have time to perform a maintenance cycle on the fans. We didn't notice the difference in muzzle velocity. I still believe "fans on" is more efficient, but at least for a gun of VERA's scale, it doesn't appear to be a big enough thing to worry about. VERA's behavior is still dominated by the step function increase in flame propagation rate when the disk goes.
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Fri Dec 08, 2023 12:31 am

jackssmirkingrevenge wrote:
Thu Dec 07, 2023 12:48 am
That's enough experimentation for this session but I would also like to try cutting down the baffle to make the "second chamber" smaller to see if that makes a difference, I wonder if it will increase or reduce peak pressure at the disk.
If my gut is right, making the baffle shorter (increasing volume of the chamber that has the spark gap in it), will decrease performance (decrease the step function effect).

Oh, and what camera are you running. That's some nice video.
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Fri Dec 08, 2023 12:50 am

Very interesting results. I feel like people had to know about this before, considering how old this hobby is. But, here we are, seemingly on the cusp of yet another evolution of the hobby: the pursuit of turbulence-inducing chamber designs. I have a large 2x hybrid (the one in my thumbnail pic) and I was always so blown away by the performance at 2x with even a very weak burst disc, I didn't expect I was missing much. I mean, that thing just cracks so damn hard. Maybe I was right in not caring about more burst pressure. It does have a chamber fan that is run while firing, plus a spark strip running through the center of the chamber maybe increases turbulence quote a bit?
I say the above experiments by JSR and commentary by D_Hall imply more experimentation is in order!

Another thing. I get the feeling that this could lead to a process of design evolution where the logical conclusion is a DDT event in the chamber. Isn't flame-front speed associated with that? Just tell me I'm wrong so I can stop caring. Thanks.
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Fri Dec 08, 2023 11:40 am

D_Hall wrote:
Fri Dec 08, 2023 12:31 am
If my gut is right, making the baffle shorter (increasing volume of the chamber that has the spark gap in it), will decrease performance (decrease the step function effect).
According to the study that inspired hectmarr's other thread, peak pressure should increase, but for a smaller effective volume.
Oh, and what camera are you running. That's some nice video.
That was will my Galaxy smartphone, I went into some detail here. Compare that to the specialized digital camera I was using over a decade ago!


Moonbogg wrote:
Fri Dec 08, 2023 12:50 am
It does have a chamber fan that is run while firing, plus a spark strip running through the center of the chamber maybe increases turbulence quote a bit?
I've always thought that the chamber fan would not do much if the mixture is already pressurized significantly, in any case I'm sure you've never approached the full potential of what your cannon is capable of ;) Only 2x, what is this, 2007? I think your concerns about DDT are misplaced.

Back to the subject of this thread, for the sake of completeness here are the results of further experimentation posted in the dual chamber thread, I made a second baffle that was both shorter and with a 1/8" orifice that looks something like this to scale:

Image

I then did some more testing again at 16x and using photo paper disks and 0.25g airsoft BBs as projectiles, this time I tested with and without baffle in the same session for a better comparison:

1) 1 disk no baffle - 1136 fps / 21 J

2) 1 disk baffle - 1851 fps / 40 J

3) 2 disk no baffle - 2071 fps / 50 J

4) 2 disk baffle - 2133 fps / 53 J

The results are similar to the previous tests. If the burst pressure is low for a given mixture, then the energy gets a very significant boost from a divided chamber, virtually double the muzzle energy in this case.

This is still lower than a single chamber with double the disk layer. Using one disk and a baffle generated just 75% of the muzzle energy achieved with two disks and no baffle.

It's worth noting that the baffle is not fixed in place and is just a tight fit, so I don't know if it's being moved around during the combustion cycle, it could be that what we are witnessing is not "pressure piling" but simply the effect of turbulence.

My conclusion based on this limited data would be that this setup might be useful with a simple combustion that wouldn't normally use a burst disk, but for hybrids there does not seem to be an advantage compared to using the optimal disk burst pressure.
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Sun Mar 31, 2024 7:29 am

Use a check valve instead of a baffle to stop the high pressure flow from the secondary combustion chamber back to the primary, and see what happens to the performance
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Tue Apr 02, 2024 11:52 am

burst disks do give better velocity than valve usally
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Tue Apr 09, 2024 10:01 pm

I have verified it, in my weapons, countless times.
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