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Chamber Fan Effects Separation?

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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Chamber Fan Effects Separation?

Unread postAuthor: D_Hall » Thu Mar 06, 2008 8:10 pm

Everybody agrees that chamber fans are a Good Thing.

Mechanisms appear twofold....

1) Better fuel/air mixing.

2) Induction of a turbulent flame front right off the bat.

...But has there ever been an attempt to quantify these effects?

More to the point: Suppose to had a chamber fan and you mixed the air, turned off the fan, and then let the gun sit for 10 minutes so that the load in the chamber would become quiescent.

I mean, if you turn your fan off and immediately fire the gun you're still going to have turbulence so you're still going to see effect 1 (obviously) AND effect 2 (albeit potentially at a lesser magnitude).

Waiting a significant amount of time(*) should remove effect 2.

So has anybody done anything like this?




(*) I use 10 minutes because of a paper I read in which DDT was being studied. The authors found that they had to wait 10 minutes after fuel mixing to ensure repeatability; the lack of which they attributed to residual turbulence.
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Unread postAuthor: Novacastrian » Thu Mar 06, 2008 8:26 pm

I don't think anyone has completed a detailed study of fan usage, members tend to throw around things like "Heaps more power", "30% more power" or "Just use a fan".
I think it would be a relatively easy project for members with advanced combustions and a chrony the data collected would be infinantly useful.

So, who is up to the task?
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Unread postAuthor: psycix » Fri Mar 07, 2008 7:05 am

jimmy101 did some tests with a chamber fan

It can be found here
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Unread postAuthor: benstern » Fri Mar 07, 2008 9:35 am

Remember that propane is much heavier then air and may sink to the bottom of the chamber and mess with the fuel/air mix.
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Unread postAuthor: Ragnarok » Fri Mar 07, 2008 9:57 am

psycix wrote:jimmy101 did some tests with a chamber fan

As did burnt latke: http://www.burntlatke.com/fan.html

They found a 19% increase in velocity with metered propane (41% more muzzle energy), and a 16% increase with starting fluid (35% more muzzle energy) - the velocities with starting fluid also became noticeably more consistent.

That 30-40% increase in energy sounds like a very fair deal to me.
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Unread postAuthor: D_Hall » Fri Mar 07, 2008 11:47 am

D'oh!

I'd read Jimmy's page before. Dunno how I missed that tabulated data. Thanks for pointing out my blindness.

As for Latke, I don't really see any indication of precisely what was done. He references fan or no fan, but what does that mean? Does fan mean that he mixed the fuel and left it burning while he fired or do it simply mean that he mixed the fuel? Does no fan mean that he did NOTHING with the fan or does it mean that he turned it off before firing?
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Unread postAuthor: jimmy101 » Fri Mar 07, 2008 12:08 pm

In my closed chamber, the fuel burns about 20% faster with a fan. (I let the gun sit for 5 minutes after mixing the fuel with the fan to let the gases stagnate.) Well mixed but stagnant gases burn about 25% faster than do poorly mixed and stagnant gases (in this particular gun).

I havn't looked at exactly how that translates into gun performance yet.

Latke's data is pretty good, though if I'm reading it correctly, he is really measuring two affects at the same time; the affect of perfect mixing + fan during firing versus poor mixing (exactly how poor is unknown) and no fan during firing.

Of course, the affect of the fan is probably dependent on things like the fan size (mine was 40mm, Latke's is 50mm), how far it is from the spark gap (mine was about 6") etc.

I wonder what the optimal fan size is? Latkes fan is pretty big. How much air movement do you get in a closed cylinder with a fan that is nearly the same diameter as the pipe? Would a smaller (say 1/2 the chamber diamter) fan, offset to one side, give higher airflows in the closed chamber? Greater turbulance? (Of course, a small fan won't vent the gun as quickly as a big fan.)
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Unread postAuthor: psycix » Fri Mar 07, 2008 2:13 pm

jimmy101 wrote:I wonder what the optimal fan size is? Latkes fan is pretty big. How much air movement do you get in a closed cylinder with a fan that is nearly the same diameter as the pipe? Would a smaller (say 1/2 the chamber diamter) fan, offset to one side, give higher airflows in the closed chamber? Greater turbulance? (Of course, a small fan won't vent the gun as quickly as a big fan.)


Youve got an interesting point there. I would like it very much if you could research that :)

A full-diameter fan can only yield a one-direction flow over the full diameter. A good thing for venting but it wont build up much flow INSIDE the chamber.
A small fan, however can do a circular flow inside the chamber because the air can flow back OVER the fan (assuming the fan is on the bottom).
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Unread postAuthor: Jared Haehnel » Fri Mar 07, 2008 2:27 pm

Is it really all about flow or just creating turbulence? I like to think that the inside of my chamber with that 4" fan going looks like a water in a bucket when you spin it... everything is plastered to the outside of the walls...
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Unread postAuthor: jimmy101 » Fri Mar 07, 2008 3:19 pm

psycix and Jared Haehnel:

Interesting question isn't it? My WAG is that with a fan of the same diameter as the ID of the pipe the air is mostly stagnant when the fan is running. Some mixing near the fan but not as much as if the fan was smaller and there was a clear recyling path from the front to the back of the fan.

Another thing to add to the list of unkowns as to what makes a combustion spudgun work as well as possible.
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Unread postAuthor: clide » Fri Mar 07, 2008 7:39 pm

D_Hall wrote:As for Latke, I don't really see any indication of precisely what was done. He references fan or no fan, but what does that mean? Does fan mean that he mixed the fuel and left it burning while he fired or do it simply mean that he mixed the fuel? Does no fan mean that he did NOTHING with the fan or does it mean that he turned it off before firing?


That question has come up before, but as far as I know it was never determined what exactly he did. My best guess is that no fan means there wasn't a fan in the chamber at all. Fan means he runs it for a few seconds and then stops the fan right before firing. Here is the section that leads me to believe that:

Spray propellants often enter the chamber unevenly which can create concentrated areas in the chamber which are too rich to ignite completely and other areas which are starved for fuel. The chamber fan mixes the fuel and air together which improves reliability and performance. Tower mounts make it easy to use chamber fans with spray propellants like hair spray and Right Guard. Start with the fan in the off position and spray a short burst into the chamber avoiding the fan then replace the endcap. Turn on the fan for a few seconds to mix the fuel with the air then shoot. Remove the endcap and turn the fan on again to ventilate the chamber.


Plus he made the circuit that turns the fan on for a specified amount of time which would also imply to me that he would fire soon after the fan shuts off.
IIRC when those tests were done fans were used primarily for mixing fuel, and I don't believe it was well known in the community that the turbulence created by the fan also increased performance. So he wouldn't really have much reason to keep it on during firing.
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Unread postAuthor: psycix » Sat Mar 08, 2008 7:11 am

jimmy101 wrote:psycix and Jared Haehnel:

Interesting question isn't it? My WAG is that with a fan of the same diameter as the ID of the pipe the air is mostly stagnant when the fan is running. Some mixing near the fan but not as much as if the fan was smaller and there was a clear recyling path from the front to the back of the fan.

Another thing to add to the list of unkowns as to what makes a combustion spudgun work as well as possible.


Now imagine two fans!
Both 1/2 diameter of chamber, above eachother blowing in the opposite directions.
So many possibilities

Or use a circular (donut shaped) chamber and one big fan :D
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Unread postAuthor: boilingleadbath » Sat Mar 08, 2008 3:57 pm

Might a chamber-sized fan act like a centrifugal blower to some extent?
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Unread postAuthor: psycix » Sun Mar 09, 2008 10:49 am

Or make a fan with blades wich are 90 degrees angled.
They may not blow, but they will mix and turbulence liek hell.
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Unread postAuthor: jimmy101 » Sun Mar 09, 2008 2:40 pm

boilingleadbath wrote:Might a chamber-sized fan act like a centrifugal blower to some extent?

How do you mean? Are you thinking the air is projected towards the cylinder wall and the return flow is along the cylinders central axis? That seems like it would be very effective at generating turbulance. Not sure if it actually happens...
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