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.