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Fan size

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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Fan size

Unread postAuthor: Phenom671 » Tue Aug 05, 2008 2:00 am

I'm thinking of getting a chamber fan for my gun, but I need to know how big would my chamber have to be to support a chamber fan. Or do the fans come in different sizes? Also how would I power the fan? Batteries?
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Unread postAuthor: Stifler69 » Tue Aug 05, 2008 2:02 am

U can get all different sizes for the fans so it doesnt really matter what size your chamber is

Hope it all goes well
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Unread postAuthor: Radiation » Tue Aug 05, 2008 11:43 pm

I personally have done some experimentation with fans. What I have found is that a BIG fan in a chamber is not better than a relatively smaller fan. What I did is use a fan that is almost the same diameter of the chamber (3" chamber, 2.5" fan roughly) vs a fan that was much smaller that the chamber (3" chamber, 1.5" fan) the results seemed to point to the bigger fan failing to mix the fuel and air very efficiently and the little fan moving the air much better.

I believe the reason is that a big fan chokes itself off. In a closed space like your chamber, a big fan can't move air behind itself as readily, it's taking up most of the space and doesn't establish an air flow as effectively. A small fan on the other hand allows air to flow from the front to the back and recycles the air much easier leading to a more consistent mixture... That is my theory any way, in practice the results for me seem to point to utilizing a smaller fan. Obviously that doesn't mean use the smallest fan you can find every time, perhaps one that is, give or take, half the diameter of the chamber.

So for a 3" chamber, maybe a 2"-1.5" computer fan. BTW check Radioshack they have dozens of different sizes as well as some cool LED options for some added flare. For a 4" chamber perhaps a 2.5" fan, or if you want to try something different perhaps a 2.5" fan and a 1.5" fan pointing the opposite direction or pointed at each other.

To answer your other question a 9 volt battery (or 2 depending on setup) should do the trick to power it, Most people use a simple on/off switch to control the power, just don't forget to turn it off like I do :) , again Radioshack sells a variety of these. If you have any questions about the electronics send me a PM, it's pretty basic, if you understand how a light bulb circuit works, this is just about the same. 8)

PS, whatever you use, only use Brushless fans, a brushed fan can cause static discharge and could be very dangerous if your gun is loaded!
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Unread postAuthor: starman » Wed Aug 06, 2008 12:36 am

Just a quick comment about Radio Shack fans...way expensive. They can be had much cheaper. Here for instance.

I do know that an 80mm 12v fan in a 4" chamber fits perfectly (with some corner sanding) and works much better at 14.4v than at 9v. Also, the weaker the battery gets the less consistant the shot.

It's not as much about sheer airflow as it is about providing "interference" during ignition, then air flow after the shot for clearing burnt gases.

Also, feel free to study the power system I included in these cannons.
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Unread postAuthor: TurboSuper » Wed Aug 06, 2008 8:57 am

Keep in mind that a smaller fan will usually spin faster than a bigger fan to maintain the same CFM at the cost of noise. It won't really matter in this case, you can make a bracket for a smaller one out of some metal.
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Unread postAuthor: jimmy101 » Wed Aug 06, 2008 11:12 am

Another source for cheapo brushless fans is All Electronics, their DC fan page is at http://www.allelectronics.com/make-a-st ... ans/1.html

The affect on fan size, relative to the chamber diameter, has been discussed before. It seems to me that you want the fan diameter a fair bit smaller than the chamber diameter. If the fan is nearly the chamber diameter then the gases in the chamber really won't move much with the fan on since there isn't an effective recirculation path from the front to the back of the fan. A large fan won't mix as quickly and it won't provide as much turblance during firing as a smaller fan. I would say a reasonable "best guess" would put the fan at 1/2 to 2/3rds the diameter of the chamber. D_Hall suggested the area of the fan blade should be about half the area of the chamber, that puts the fan diameter at about (chamber diamter/1.414). That would be a ~2" (50mm) fan for a 3" chamber.
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Small fan, offset from chamber's central axis, may give a significant recirculation path with substantial turbulance where the two streams pass each other.
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Unread postAuthor: daberno123 » Wed Aug 06, 2008 11:59 am

what if you added another fan to the top right hand corner of the chamber in the diagram above?

The two fans would blow towards each creating more turbulence. Might be something worth trying.
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Unread postAuthor: Radiation » Wed Aug 06, 2008 6:05 pm

jimmy101 wrote:
The affect on fan size, relative to the chamber diameter, has been discussed before. It seems to me that you want the fan diameter a fair bit smaller than the chamber diameter. If the fan is nearly the chamber diameter then the gases in the chamber really won't move much with the fan on since there isn't an effective recirculation path from the front to the back of the fan. A large fan won't mix as quickly and it won't provide as much turblance during firing as a smaller fan. I would say a reasonable "best guess" would put the fan at 1/2 to 2/3rds the diameter of the chamber. D_Hall suggested the area of the fan blade should be about half the area of the chamber, that puts the fan diameter at about (chamber diamter/1.414). That would be a ~2" (50mm) fan for a 3" chamber.


This seems to jive very well with my own independent/non scientific tinkering. I am in the process of fitting my larger gun with a small fan to accompany the larger fan as the chamber is pretty big. I just might try pointing them at each other.
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Unread postAuthor: jimmy101 » Thu Aug 07, 2008 1:01 pm

daberno123 wrote:what if you added another fan to the top right hand corner of the chamber in the diagram above?

The two fans would blow towards each creating more turbulence. Might be something worth trying.

The only problem with this setup is that the barrel end fan may well get trashed by the movement of the combustion gases. The barrel end fan has to survive pushing the gases towards the center of the chamber then getting forced to reverse direction as the combustion gases race down the barrel at perhaps a couple hundred miles per hour. Then the ammo exits, the pressure drops, the temperature drops, the pressure drops below ambient and you get a high velocity surge of air back into the chamber, which reverses the fan direction again.

Plus, when the fan dies it'll be a pita to replace.

EDIT: Radiation, sounds like a great setup. Can you get muzzle velocities with the big fan vs. the small fan vs. both fans?
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