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quick 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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quick fan question

Unread postAuthor: covey12 » Fri Jul 23, 2010 2:21 pm

i have a lot of experience with mini combustions and large pneumatics, but have decided to make a large combustion, and regarding the chamber fan i see most people mount theirs in the chamber. does the ignition of the propellants not harm the fans? it seems like it would melt it to be but i'm probably wrong, just making sure :?:
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Unread postAuthor: jrrdw » Fri Jul 23, 2010 2:36 pm

They must take a good bit of punishment because there is very little discussion about replacing burnt up chamber fans.
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Unread postAuthor: SpudBlaster15 » Fri Jul 23, 2010 3:00 pm

The temperature inside the chamber is very high during combustion, so intuition suggests that the fan has the potential to become damaged, but this is typically not the case in a standard combustion launcher. Gases have very low heat capacity values relative to the solid materials used in the construction of the launcher, so although the temperature is high, there's very little heat energy to transfer to the chamber walls and fan. The fan will only heat up by a few degrees during firing, and is thus not susceptible to melting.
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Unread postAuthor: jimmy101 » Fri Jul 23, 2010 5:08 pm

The most common problem with a chamber fan isn't the temperature (SP15 hit the answer dead on), it's the loading on the fan blades, at the instant the ammo leaves the barrel (rapid decompression gets the air in the chamber moving much much faster than it ever does when the ammo is still in the barrel) and then again a few milliseconds later when cool dense air rushes back into the chamber and slams into the fan.

Put the fan pretty close to the breach so that you aren't moving massive amounts of combustion gases through the fan during firing, that'll trash the fan pretty quick. It is rarely a problem though since putting the fan anyplace other than near the breach is a real PITA so the practical place is also the best place for the fan.

I had a gun for a couple years that had a fan blade blown off. It still worked just fine, indeed the missing blade provided "tactile feedback" that the fan was indeed operating when the gun was closed up and ready to fire ... the whole gun vibrated slightly due to the out of balance fan.
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Unread postAuthor: jrrdw » Fri Jul 23, 2010 6:30 pm

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Unread postAuthor: motorfixer1 » Fri Jul 30, 2010 9:11 pm

Good answers guys very informative! Thanks
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