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Mass flow and De Laval nozzles for your spuddy

A place to ask general spud cannon related questions.
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Mass flow and De Laval nozzles for your spuddy

Unread postAuthor: Gippeto » Fri Dec 05, 2008 12:53 am

I am doing some research on high speed flow, and was wondering if there were any of you using either mass flow (by design), or preferably De Laval nozzles to increase the velocity of your spuddy.

I suppose, anyone using a co-ax as we know it could say that they were using mass flow. It's simple and it works.

To a point.

But a De Laval nozzle is another animal entirely. Taking sub-sonic flow, converting it to mach flow, and then accelerating it from there,

A search for "de laval" (on the forum) turns up nothing useful. Does anyone know of a spuddy using a De Laval nozzle?

(I did find one reference to it on the net, but details were sketchy at best.)

Thanks.

Looks like a female doggy to get it right, and have it fit inside a 6mm barrel. :(
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Unread postAuthor: Lentamentalisk » Fri Dec 05, 2008 1:02 am

I kinda doubt that it would work, as a De Laval Nozzle is designed for gases at thousands of degrees, and the gases cool down substantially. The net effect is that although the gases are going incredibly supersonic, they are at 1atm, which is absolutely useless to a spudder. Also, if you were to permit any backpressure to build up, the entire nozzle would stop doing what you want it to do, if I remember correctly from my readings.
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Unread postAuthor: Gippeto » Fri Dec 05, 2008 1:04 am

De Laval nozzles are also used in wind tunnels.
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Unread postAuthor: Lentamentalisk » Fri Dec 05, 2008 1:09 am

Oh really? I suppose nearly freezing the air is desired for wind tunnels anyways, because airplanes usually fly so high up where its cold...

Still, wind tunnels do not have a substantial pressure ever. It seems as though the entire deal would break down if you try to increase the exit pressure. A big part of the reason the gases accelerate is that they are expanding (ie decreasing in pressure)
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where P is pressure, T is temperature, and V is velocity
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Unread postAuthor: jackssmirkingrevenge » Fri Dec 05, 2008 2:45 am

Surely it's not too difficult to fabricate a 6mm insert with an hourglass shaped cavity and try it out?
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Unread postAuthor: Lentamentalisk » Fri Dec 05, 2008 2:54 am

It could help you minutely in the supersonic range, but look at how the pressure drops. And JSR, its not that simple. There is a whole ton of equations involving all of that stuff, and it will do very little if you don't get them about right.

After rereading the Wiki article on it, all that I read is about it failing to work if the exit pressure is too far below ambient pressure, which is the opposite of our problem here...
None the less, I ave a really strong instinctual feeling that it just can't help out.
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Unread postAuthor: jackssmirkingrevenge » Fri Dec 05, 2008 3:08 am

Lentamentalisk wrote:It could help you minutely in the supersonic range, but look at how the pressure drops. And JSR, its not that simple. There is a whole ton of equations involving all of that stuff, and it will do very little if you don't get them about right.


Fair enough, but personally I don't see it working. Granted, you might be accelerating the gasses, but as I see it, the flow restriction is going to be limiting the amount of air available to push the projectile, so you'll be increasing one thing at the expense of another.

I think airguns are an old enough technology that if such a design made a difference, modern pneumatic rifles would have hourglass shaped transfer ports, but in reality they have cylindrical ones - as close to the barrel diameter as possible.
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Unread postAuthor: psycix » Fri Dec 05, 2008 4:32 am

Indeed. The gases may speed up, but this is a result of the restriction of flow while air tries to push the same amount of volume through per second.
Ofcourse the gases speed up. But it will not be beneficial in any way for us.
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Unread postAuthor: Gippeto » Fri Dec 05, 2008 10:26 am

It's a fairly complex subject isn't it. :)

You guys make valid points though and thank you for taking the time.

If there is no one here that's tried it yet, perhaps it's time someone did.

If only for the sake of interest. :)
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Unread postAuthor: jimmy101 » Fri Dec 05, 2008 1:11 pm

I think Jack made the killer comment on this idea. If any type of a nozzle would help a compressed gas (pneumatic or combustion) gun then there would be guns out there that use the technique. Something between a BB gun, a 16" naval gun or an aircraft carrier's steam launcher, someone, somewhere would have made it work if it could work.

I think the key parts is that a gun doesn't have a nozzle. A nozzle is to get maximum force by accelerating a gas to very high velocities. In a gun, the gas velocity is limited by the projectile velocity. And even Mach limits rarely have much to do with the operation of most combustion driven guns.
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 1:56 pm

jackssmirkingrevenge wrote:but in reality they have cylindrical ones - as close to the barrel diameter as possible.

I suspect that even if it were of some use, the rifling the barrel would fast cause the development of shockwaves within the barrel, slowing the gas velocity again. As PCPs are usually rifled, the rifling would fast slow any supersonic gas flow.

It's worth noting that the same transfer port stuff doesn't apply on springers - they deliberately have sub calibre transfer ports to offer a little protection against dry fire damage.
But you do need to remember that they DO of course use considerably heated gases as a result of the rapid compression though, and the gases in a springer tend to have a speed of sound about twice normal, which is why springers can record particularly impressive velocities with light pellets - well into the supersonic - without the threat of the shockwaves which will prove the anathema of supersonic performance.

I think this is something worth looking into - making the SOS in the gas higher by shock heating. This would need a certain amount of dead volume for air to be shock heated in, but it could be a workable idea.
Unfortunately GGDT doesn't account for this effect, but I'm capable of doing the estimations myself.

Also worth note is that supersonic shockwaves form off barrel imperfections or flow restrictions. A clean, smoothed and polished one piece barrel will reduce the likelihood of this occurring.

Although I have little desire for it on a regular basis, I am looking into ways to try and get supersonic with a pneumatic - I want to see a Mach 1 paintball, mostly to prove my brother wrong - and these are the two things which are the primary "weapons" in my arsenal. Shock heating and ultra shiny barrels. I think with both elements working together it may just be possible.
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Unread postAuthor: ALIHISGREAT » Fri Dec 05, 2008 2:35 pm

you do realize you could probably just use a hybrid with the paintball in a sabot to achive that?

and as for the flow restriction thingy, it makes sense for it to not give any performance advantage, but maybe with a long barrel and a light projectile?
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 3:17 pm

ALIHISGREAT wrote:you do realize you could probably just use a hybrid with the paintball in a sabot to achive that?

Of course I do - I'm the #5 poster on the world's biggest spudgun forum, a student engineer, and no slouch. If I didn't know that, I wouldn't be worth my salt.

If I was totally dedicated to doing that and nothing else, sure, that would be the efficent way to do it.
But there's no challenge in doing it that way. Why do something if it's easy?
Easy is boring.

The other thing is that I don't want it to be a single trick pony. I want the cannon to be up to lots of things. High muzzle energy and supersonic capacity are not up the top of my list. My biggest goal in most cannons is it being convenient, practical to shoot and man portable/hand fired, otherwise it's not really much fun. Basically - big rifles/shotguns in design.

So, I don't really like hybrids. I'm a pneumatics guy through and through.
The only time I'm likely to build a hybrid is if my hybrid BBMG design gets built.
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Unread postAuthor: jackssmirkingrevenge » Fri Dec 05, 2008 3:43 pm

Ragnarok wrote:It's worth noting that the same transfer port stuff doesn't apply on springers - they deliberately have sub calibre transfer ports to offer a little protection against dry fire damage.


It's got something to do with power restriction too, as larger bore transfer ports are a feature present on FAC versions of 12 ft/lb springers.
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 4:13 pm

jackssmirkingrevenge wrote:It's got something to do with power restriction too.

It's really the opposite actually. A small transfer port on a springer increases the life time of everything, and gives a smoother shot by giving the piston more air cushion at the end of it's travel.
Small transfer ports are the ideal, so you use the smallest you really can.

It's not that low power rifles get smaller ports to restrict their power - it's that higher power ones get larger ports so as not to restrict their power.
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