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Chamber fan question.

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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Unread postAuthor: Pete Zaria » Sun Jul 01, 2007 4:37 pm

jimmy101 wrote:1. It mixes the gases. This all by itself may well double the velocity that the gun shoots at. If nothing else, it will reduce the shot-to-shot variablity from perhaps 50% to ~10%.

2. If the fan is running during firing it should boost the burn rate and increase the muzzle velocity. Exactly how much is hard to say. An increase in muzzle velocity of perhaps 20% would seem reasonable.

3. The fan helps to air out the chamber between shots. That means you are much less likely to have CO2 and water vapor carrying over from shot to shot. Any CO2 or water vapor in the chamber displaces a like amount of oxygen.


More or less, dead-on. 100% velocity gain is probably an exaggeration... but there is definitely more power with a fan. Your comments about burn rate and venting are completely correct.




The fan is, indeed, exposed to combustion.
The peak pressure inside the chamber will be around 80psi (give or take 5psi), and maximum temperature might reach 400F or so but only for a split second... soon after combustion, chamber temperature drops back down quickly.
The fans will start to melt if you fire a combustion repeatedly, over and over, in quick succession, but aside from that, a good fan should last quite awhile. Mine has seen probably 600+ shots on the original fan, still works fine (though one of the blades on the fan is broken, and I have no idea whatsoever how that happened....).

Don't worry about the fan "blocking" the chamber - this is a GOOD thing, not a bad thing. More turbulence increases flame front speed, which translates to bigger boom and faster spud. When trying to intentionally trigger DDT (deflagration to detonation transition, which is usually something to be avoided due to the extreme pressure spike) baffles and other obstructions are frequently used to attempt to induce turbulence and accelerate the flame front. So having a big fan in the chamber will actually help performance.

The fan should be placed at the very back of the chamber (away from the barrel), blowing towards the barrel. Placing it on the barrel side of the chamber will damage the fan and decrease performance.

I use an 80mm fan "wedged" into 4" pipe (I sanded down the corners of the fan so it would fit very tightly in the 4" pipe). It's a very tight fit, and works wonderfully. If you're using smaller pipe, look at a dollar store camping fan, or 60mm computer fan.

By the way, computer fans run on 12 volts... you can power them on 9v but they won't run as fast (and thus won't mix/vent as well), so you can do one of four things:
1) use 9v and deal with the slower fan speed
2) use a 9v plus two AA batteries wired in series for 12v
3) find a 12v lantern/RC car/etc... battery
4) Use two 9v's in series (18v) and risk frying the fan sooner, but boost performance considerably (fan spins a lot faster...)

I use a 9v and two AA's in series to make 12v.

As to wiring the fan, a very simple circuit consisting of batteries, wires, the fan, and a switch will work perfectly (radioshack...) - if you're a geek, you could build a 555 timer to run the fan for a certain amount of time.

Peace,
Pete Zaria.
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Unread postAuthor: mtronic » Sun Jul 01, 2007 6:25 pm

Im planning on mounting a little 5v cpu fan into my new combustion. I hope that it will increase the fire power :twisted:

Good info, thanks
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Unread postAuthor: hi » Sun Jul 01, 2007 6:28 pm

mtronic wrote:Im planning on mounting a little 5v cpu fan into my new combustion. I hope that it will increase the fire power :twisted:

Good info, thanks


you can get about 1/3 more power with a fan if i remember correctly.
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Unread postAuthor: psycix » Mon Jul 02, 2007 9:32 am

1-Reading all this.
2-Thinks logical for a very short time.
3-Fitting fan. 8)
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Unread postAuthor: SnowFlox » Mon Jul 02, 2007 10:55 am

ok thanks for the info !!

well i have a very small spudgun. and found a little fan about 2,5cm high.

here's a picture of it

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Unread postAuthor: mtronic » Tue Jul 03, 2007 6:11 am

sweet compact mate:). When I get around to mounting the fan I shall post a pic too :) Im still trying to figure a cool name for it :idea:
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Unread postAuthor: dongfang » Tue Jul 03, 2007 7:47 am

Hi,

I had a high quality German cast metal fan in my biggest combustion. After a few shots, it went dead :( But then again, those were shots with 40% oxygen.

For air-propane, most fans will survive for quite long.

Regards
Soren
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Unread postAuthor: psycix » Tue Jul 03, 2007 8:24 am

When I fit a fan that covers the whole diameter of my chamber, how can i spray in gas from the back then?
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Unread postAuthor: chaos » Tue Jul 03, 2007 8:28 am

psycix wrote:When I fit a fan that covers the whole diameter of my chamber, how can i spray in gas from the back then?


turn the fan on and spray it into the fan :D
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Unread postAuthor: psycix » Tue Jul 03, 2007 8:29 am

And what if its hairspray?
My fan will get sticky as hell then :P
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Unread postAuthor: chaos » Tue Jul 03, 2007 8:41 am

its just a fan and its either the chamber walls or the fan (or both) that will end up sticky.
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Unread postAuthor: psycix » Tue Jul 03, 2007 8:46 am

I want to get propane but I cant get all the needed parts in the shops around here :(
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Unread postAuthor: SnowFlox » Tue Jul 03, 2007 11:03 am

mtronic wrote:sweet compact mate:). When I get around to mounting the fan I shall post a pic too :) Im still trying to figure a cool name for it :idea:


name for a fan ?? or cannon? ;)


btw..psycix your from holland ?
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Unread postAuthor: psycix » Wed Jul 04, 2007 3:02 am

yup im from holland :)

How you know and why u ask? :P
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Unread postAuthor: mtronic » Wed Jul 04, 2007 3:21 am

name for a fan ?? or cannon? Wink
sorry..cannon:) I should have specified. I'm painting it with a desert camo scheme so I was thinking like the "desert dueler" but I dont know. I dont want to call it something too poxy:(

Any suggestions?

And what if its hairspray?
My fan will get sticky as hell then

Psycix, mate try starter fluid. I use Nulon "start ya bastard" but thats made in Australia so you probably wont have it in Holland. But go to an automotive shop or similar and ask for starter fluid(spray form) It wont leave a sticky residue on your fan or chamber, and it wont goo up your electrodes either:)

hope this could help you out.
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