Hybrid disk burst pressure effect on performance

[jackssmirkingrevenge]

One of the experiments I've done with this project to to see how burst disk strength influences performance. Intuitively one would tend to think that the stronger the disk, the higher the pressure it will burst at and therefore the better the performance will be; this might be true of a pneumatic but not necessarily the case with hybrids.

In this case I was using layers of photo paper for disks. At 5x with a single disk, the latter did not burst upon ignition, however it did burst when tested at 10x.

I then fired four shots with progressively more disk layers added and got the following results (velocity in feet per second, energy in foot pounds, velocity in meters per second, energy in Joules) and in all cases the shots were at 16x using 0.25 gram airsoft BBs as a projectile.

1 layer - 1525 fps 20 ft lbs 465 ms 27 J

2 layer - 2010 fps 35 ft lbs 613 ms 47 J

3 layer - 1808 fps 28 ft lbs 551 ms 38 J

4 layer - no burst

As can be noted , the highest energy by a significant margin happens with 2 layers.

I've tried to model the observed data in HGDT but was unable to get the performance with three layers match by raising the burst pressure.

1 layer and 10x mix:

disktest10x.png (22.43 KiB) Viewed 36279 times

1 layer and 16x mix:

disktest1.png (23.7 KiB) Viewed 36279 times

2 layers and 16x mix:

disktest2.png (22.86 KiB) Viewed 36279 times

3 layers and 16x mix:

disktest3.png (23.89 KiB) Viewed 36279 times

[mark.f]

Interesting find, we definitely need some more test data on hybrids to improve the modelling others have done.

[hectmarr]

Interesting research. Timing is also important, not just accumulating more pressure.

[Pennywise]

This is very interesting. Intuitively, the highest pressure accumulated before burst would result in the highest velocity.

It would stand to reason, though, that you want the disc to burst late enough to raise the chamber pressure sufficiently, but early enough for there to be unburnt fuel mix being pushed down the barrel behind the projectile to continue raising the local pressure and accelerate it further. Just conjecture from someone largely inexperienced with hybrids.

Am I off my mark?
Has this train of experiment brought any more data or theory?

[jackssmirkingrevenge]

Am I off my mark?

The way I understand it, in a pneumatic the pressure is more or less constant while in a hybrid it's a brief spike.

If we have a disk on a chamber that bursts at 1000 psi, then by filling that chamber until the disk bursts there will be 1000 psi of air to push the projectile.

With a hybrid mixture that peaks at 1000 psi in a closed chamber, that is still going to burst the disk, but this means that the full pressure has been developed before the projectile has even begun to accelerate.

This means that it is possible for a disk to burst too early or too late to develop maximum velocity, and it's a complex dynamic to simulate, so the easiest way to optimize performance is probably to use a relatively weak disk material that can be stacked and do it experimentally.

[hectmarr]

This is very interesting. Intuitively, the highest pressure accumulated before burst would result in the highest velocity.

It would stand to reason, though, that you want the disc to burst late enough to raise the chamber pressure sufficiently, but early enough for there to be unburnt fuel mix being pushed down the barrel behind the projectile to continue raising the local pressure and accelerate it further. Just conjecture from someone largely inexperienced with hybrids.

Am I off my mark?
Has this train of experiment brought any more data or theory?

Indeed, I also understand it this way. I am not sure if the maximum pressure reached is the same, but Yes, the push time of the hot gases is for a longer amount of time. The portion of the firing barrel behind the ammunition becomes part of the combustion chamber. Increasing the volume of that portion of the cannon would cause the maximum pressure to drop very little. The result is greater speed.

[Pennywise]

Correct me if this theory is flawed, but could it be more than simply the effective chamber being larger?
The unburnt portion of the fuel has mass to it, and so as it's being propelled by the expansion of the burning fuel behind it, it pushes the projectile faster and faster. As the flame front catches up to this super-fast traveling unburnt mix, it ignites(and expands), and pushes the expanding wave even faster (or even creates another wave). Imagine driving a car 150mph and throwing a ball forward. You didn't impart enough force to send the ball at 200mph, but relative to a stationary observer, it's still traveling that fast.

If this was the case, though, the ideal chamber configuration would be a spark closer to the rear of the chamber vs centered (to allow for the expanding pressure wave to push the unburnt fuel down the barrel. For example, say 30% from the rear, 70% from the breech). I would imagine the ideal spark placement would be relative to the burst disc rating. You would want the flame front to be reaching the rear of the chamber exactly as the disc breaks, leaving ample amount of unburnt gas to propel the projectile, and progressively ignite as it travels down the barrel, increasing the velocity of the gasses in front of it.

From reading, the optimal chamber shape is instead, spherical, so something in my thought must be incorrect.


*Essentially, if the flame front of a propane/air mix typically moves at X fps, what happens if that mix is in motion prior to ignition?*

[hectmarr]

In my view, that is the most feasible and simplest explanation. The longer the projectile is pushed by hot air that does not lose pressure as quickly, is better than hot air with more pressure that will always decrease if the maximum peak was reached before the projectile begins to move. I have verified it, in my weapons, countless times.

[jackssmirkingrevenge]

Essentially, if the flame front of a propane/air mix typically moves at X fps, what happens if that mix is in motion prior to ignition?

The effects you mentioned haven't really been studied within the community to my knowledge, although there are surely more academic papers out there.

One of the things HGDT accounts for is multiple spark gaps for example, having the ignition happen in multiple locations within the chamber simultaneously is meant to increase pressure and therefore power.

There has also been discussion of a dual chamber where the flame front from the first one would ignite the second, but not much in the way of practical testing.

A long time ago I had made a combustion cannon with a clear chamber/barrel for some semi-serious testing, it would be interesting to find some clear tubing that can take higher pressures and make a low mix hybrid for similar purposes.

[D_Hall]

Late to the thread here (and I've only read the first post) but one of the things I've seen with VERA is that the burst pressure doesn't really make much difference (at least, not on her). Why? Simple, really... Once the disk bursts and the gases start to move, everything becomes very turbulent. With the rise in turbulence, flame propagation rates go through the roof.

In short... We did a whole lot of testing with VERA and what we found was that for all intents and purposes, the moment the disk burst, ALL fuel was consumed. Seriously, it looked like a step function on the pressure traces (we were taking data at something like 5 kHz). The pressure would ramp up nice and smooth as the fuel burned, then WHAMMO it would be at peak pressure; typically within 1% of max theoretical.

And HGDT does NOT model this. I keep thinking that some day I should get back into it and revamp the model, but at the current rate I doubt I ever will.

[jackssmirkingrevenge]

Late to the thread here (and I've only read the first post) but one of the things I've seen with VERA is that the burst pressure doesn't really make much difference (at least, not on her). Why? Simple, really... Once the disk bursts and the gases start to move, everything becomes very turbulent. With the rise in turbulence, flame propagation rates go through the roof.

Interesting point and glad you chimed in. Could it be that with VERA the disk burst pressure was always inferior to this seemingly "ideal" pressure for maximum power? It seems clear from my own experience and others that having the disk burst too early results in lower power.

[D_Hall]

Late to the thread here (and I've only read the first post) but one of the things I've seen with VERA is that the burst pressure doesn't really make much difference (at least, not on her). Why? Simple, really... Once the disk bursts and the gases start to move, everything becomes very turbulent. With the rise in turbulence, flame propagation rates go through the roof.

Interesting point and glad you chimed in. Could it be that with VERA the disk burst pressure was always inferior to this seemingly "ideal" pressure for maximum power? It seems clear from my own experience and others that having the disk burst too early results in lower power.

Question: How smooth is the interior of your guns? In the case of VERA we tried a couple different burst pressures and while the pressure ramp to the disk burst was obviously longer for the high pressure burst, the muzzle velocity of the gun was indistinguishable... Which made sense given that in both cases we saw the afore mentioned step function in chamber pressure. The only rational answer was flame propogation rates going to high velocities when things started to actually move. This effect would be affected by the chamber's interior, however. The more bends, rough surfaces, and such, the faster the gases would go turbulet. With that said, VERA has a fair amount of equipment mounted internally and the construction (steel pipe) means a less-than-glass-smooth surface. By contrast, if you're building a gun out of a straight piece of very smooth interior pipe? OK, maybe turbulence doesn't dominate so quickly.

[jackssmirkingrevenge]

This effect would be affected by the chamber's interior, however. The more bends, rough surfaces, and such, the faster the gases would go turbulet. With that said, VERA has a fair amount of equipment mounted internally and the construction (steel pipe) means a less-than-glass-smooth surface. By contrast, if you're building a gun out of a straight piece of very smooth interior pipe? OK, maybe turbulence doesn't dominate so quickly.

In the case of my testbed it's very smooth, this was also a cylinder in which a piston was moving so it had to be.

Perhaps it's worth adding some basic baffles as an experiment.

[D_Hall]

My gut says we're on to something.... You need something to induce turbulence!

[jimmy101]

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