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Better Turbulence Induction Methods

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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Better Turbulence Induction Methods

Unread postAuthor: biggsauce » Mon Apr 28, 2008 1:19 pm

In my combustion I'm working on, I am using a chamber fan. Big whoop. Its mounted pretty standard, right at my cleanout cap, great with venting. To fit it into the chamber, I had to grind down the corners, the blade diameter is almost the ID of my chamber (4"). Just some information before I start.

Now for the sake of this discussion, lets say the fan blade diameter is just milimeters smaller than the chamber ID. If the fan is turned on in a sealed chamber (projectile in barrel and vent end is closed) I can't imagine much happening very efficiently. I don't think the fan would circulate or mix the air very well in the closed chamber, because very little air could flow backwards around the blades to be "blown" through the fan. (I hope everyone gets me) Also, it probably won't make much turbulence throughout the chamber.

Multiple Fans?
For optimum performance, wouldn't 2 or more smaller diameter fans, placed throughout the chamber do a better job of mixing and adding turbulence? Say you had 3 2" fans in a 16" long 4" diameter chamber, with each fan being 4" apart, all blowing the same direction, and all placed on "one side" of the chamber. This would create much more turbulence in the chamber. Or take a 4" fan and mount it midway in the chamber with its rotational axis perpendicular to the symmetrical axis of the chamber. Either of these would allow for much more air to be sucked through the fan, in turn allowing more circulation.

I plan to keep my large fan in the rear of my chamber, but now I'm considering putting a smaller fan midway throught the chamber to mix/induce turbulence. One switch for venting, one switch for mixing etc.

What do yall think? Anyone with a more developed knowledge of flage propagation and air flow within a closed vessel have any input? I'm almost positive this will Increase performance and as soon as I get my metering system finished, I'll do some tests.

Approaching DDT?
Also, (as if I hadn't already typed enough) what about a Schelkin Spiral? They don't appear to be hard to make. C'mon its a spring looking thing. This spiral has been proven to cause DDT, and may increase flamefront acceleration enough to make a noticeable difference.

Again, whatchyall think?

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Last edited by biggsauce on Mon Apr 28, 2008 3:01 pm, edited 1 time in total.
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Unread postAuthor: Fnord » Mon Apr 28, 2008 1:41 pm

You could always cut a quarter inch or so off the blades to allow for better circulation.

I always preferred to use a small enough fan to allow air to pass around it.
In my hybrid the fan is mounted parallel to the chamber rather than across it. In my big combustion I have two fans blowing against the spark gap from opposite directions.
Not good for venting, but so far both work extremely well.
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Unread postAuthor: biggsauce » Mon Apr 28, 2008 1:48 pm

Ah didn't think about cutting the blades, that would save some money...
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Unread postAuthor: psycix » Tue Apr 29, 2008 6:03 am

I dont think having smaller fans to allow flow will improve turbulence.
Flow = turbulence?
I think not.
If you would have big fan(s) with blades perpendicular on flow (thus not being able to generate flow, no angle) it would generate big turbulence around the blades.
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Unread postAuthor: BlackH » Tue Apr 29, 2008 6:14 am

I use a 60mm fan in a sch 80 90mm (4") chamber and it works awsome on 18 volts!!!!
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Unread postAuthor: jimmy101 » Tue Apr 29, 2008 12:03 pm

psycix wrote:I dont think having smaller fans to allow flow will improve turbulence.
Flow = turbulence?
I think not.

Based on what? Flow most certainly = turbulance in a closed cylindrical chamber. To get flow you have to get the gases moving in different directions in different places. That means you've got air masses that are moving in different directions interacting with each other. Pretty much the perfect scenario for generating turbulance. Even if the flow along the wall is too slow to generate turbulance (easy enough to estimate based on the Reynold's number), the deltaV between the gases moving in different directions will have at least twice the Re. Two gas flows in opposite directions interact turbulently even at very low deltaVs.

Back to the op, I've always wondered about the affect of a fan similar in diameter to the chamber. I would think the air stagnates and there is releatively little mixing and turbulance. I would expect that a fan about 1/2 the diameter of the chamber, mounted near the chamber end and close to the wall, would give the greatest flow (and turbulance) during firing.
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Unread postAuthor: biggsauce » Tue Apr 29, 2008 1:09 pm

Ok jimmy, I din't know the science behind it, but I just figured it would be better that way. I talked about using two or three small fans, but I'm goin to keep it to my vent fan and one or two mixing fans.

I rarely see fans that arent the diameter of the chamber and was always doubtful that it had very efficient mixing/turbulence capabilties.
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Unread postAuthor: psycix » Thu May 01, 2008 8:58 am

Okay, new idea:
Rotational flow.

Quick sketch/diagram:
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Just an idea.
Discuss :D
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Unread postAuthor: rcman50166 » Thu May 01, 2008 10:33 am

Hey how about audio? I can't think of a better way to cause turbulence. A speaker essentially just disrupts ambient air pressure. I'm sure more energy can be stored in an audio wave than an air flow. Essentially energy into the speaker gets transmitted directly into the air. Little heat is given off. Also you can amplify the audio waves by finding the fundamental frequency of the chamber. This could be a breakthrough in spudding! Has anyone ever tried this?
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Unread postAuthor: FiveseveN » Thu May 01, 2008 11:23 am

If you use the chamber's fundamental, won't you just get big fat standing waves? I would go with Brown noise or some prime frequency in the bass range (would need to experiment a little there). Thankfully an oscillator + amp is easy 'nuff to build. A transducer that can handle the heat is more of an issue. Perhaps still use a fan and modulate its speed, like that uber-woofer?
Another unrelated thought on the issue of turbulence: stick some small (wood, PVC?) obstacles on the chamber wall to mess up laminar flow. Streamers? :)
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Unread postAuthor: biggsauce » Thu May 01, 2008 11:48 am

rcman50166 wrote:Hey how about audio? I can't think of a better way to cause turbulence. A speaker essentially just disrupts ambient air pressure. I'm sure more energy can be stored in an audio wave than an air flow. Essentially energy into the speaker gets transmitted directly into the air. Little heat is given off. Also you can amplify the audio waves by finding the fundamental frequency of the chamber. This could be a breakthrough in spudding! Has anyone ever tried this?


Really? Huh thats a pretty cool idea, and I don't think anyone has tried that one before. Would that work? Does anyone with the equipment care to exeriment?

Another unrelated thought on the issue of turbulence: stick some small (wood, PVC?) obstacles on the chamber wall to mess up laminar flow. Streamers?


Like a Schelkin Spiral? Its supposed to increase flame "brush" speed.

I forgot who posted it, and it may have been on Spudtech, but Iremembe someone made a "chamba chain." It was a plastic chain attatched at two points inside the chamber. To mix, just shake the chamber. Pretty cool idea.
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Unread postAuthor: rcman50166 » Thu May 01, 2008 12:43 pm

FiveseveN wrote:If you use the chamber's fundamental, won't you just get big fat standing waves? I would go with Brown noise or some prime frequency in the bass range (would need to experiment a little there). Thankfully an oscillator + amp is easy 'nuff to build. A transducer that can handle the heat is more of an issue. Perhaps still use a fan and modulate its speed, like that uber-woofer?
Another unrelated thought on the issue of turbulence: stick some small (wood, PVC?) obstacles on the chamber wall to mess up laminar flow. Streamers? :)


It would create a standing wave but that doesn't mean the wave isn't moving. Even in the wave wasn't moving but ocilating in a single place that would still create turbulence as long as pressure is not at equilibrium. And for the bass and what not, I would go with higher frequencies maybe harmonics and not just fundamental. It requires more and more energy to create higher and higher pitches. This means more energy will be put into the chamber as waves. At one point or another you'll be able to ignite the chamber by sound alone. It won't be audible, probably by factors of 100 (ultrasonic), but at the energy level required, it will almost certianly cause DDT. Think of it this way, the spark gap adds energy to the fuel pushing it over the top to create a chain reaction, or combustion. This energy level is only present at the electron bolt between the gaps. But, if you send audio with enough energy through the chamber that energy sweet spot will be spread to the entire chamber! If all of the mix detonates instantaneously, I believe that is a DDT hs just occured. All of this is theoretical though. I haven't seen audio used for this specific purpose. I know you can heat things with ultra sound to the point of melting, so I guess you would have to make sure you don't hit PVC's specific frequency. Testing will have to be done but using ultasound for DDT seems like a plausible idea. However it may not be practical.
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Unread postAuthor: jimmy101 » Thu May 01, 2008 1:23 pm

I don't think sound will mix the chamber at all. Sound in a closed chamber just creates standing waves at the resonant frequency (and integer multiples of the resonant frequence.) Waves on the ocean are standing waves, there is very little lateral movement of the water and essentially zero mixing. This is why an oil spil dissipates so slowly on the open ocean. Standing waves do very little mixing. Waves on the ocean won't move a boat laterally. (The wind that causes the waves will move the boat.)

The same with standing audio waves in cylinder, very little net movement of the gases. All you get is pressure fluctuations without any real flow. At high enough sound densities you can get local compression that can exceed the autoigntion temperature.
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Re: Better Turbulence Induction Methods

Unread postAuthor: FLONE » Wed May 07, 2008 4:13 pm

biggsauce wrote:IApproaching DDT?
Also, (as if I hadn't already typed enough) what about a Schelkin Spiral? They don't appear to be hard to make. C'mon its a spring looking thing. This spiral has been proven to cause DDT, and may increase flamefront acceleration enough to make a noticeable difference.

Again, whatchyall think?

(


Flame me if I misread or misunderstand this. My understanding from a Jimmy post is that the projectile can't move faster than the gasses behind it and the gasses can't move faster than the projectile they are pushing. Can a spud actually move a lot faster than your gun pushes it now? It is a fairly heavy object with variable friction, probably not much faster. Could a golf ball be pushed faster? Lighter weight, it makes sense to me that it could.
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