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SO much for chamber fans

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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SO much for chamber fans

Unread postAuthor: Moonbogg » Mon Aug 31, 2009 6:17 pm

Well....this is after a single shot. Brand new 60mm fan, no fire occured. I am guessing it somehow got destroyed just from the pressure, but how? Good thing the cannon works well without a fan also.

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Unread postAuthor: Pilgor » Mon Aug 31, 2009 6:21 pm

I'm guessing it is the location of the fan in the chamber. Was it near the front by the barrel or was it in the back??
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Unread postAuthor: D_Hall » Mon Aug 31, 2009 6:28 pm

Failed before shot?

Creep?

How was it mounted? How long before failure was it mounted?
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Unread postAuthor: Moonbogg » Mon Aug 31, 2009 6:31 pm

It was mounted near the back and was actually mounted pretty free fitting. It had a little room to move around, not fly around, but it could wiggle a little. It was running before the shot and the shot destroyed it's ass.
I mounted it like 20 minutes before test fire.
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Unread postAuthor: D_Hall » Mon Aug 31, 2009 6:34 pm

Oh, sorry. I thought you said it failed before a shot. Not after *A* shot. I'll work on reading comprehension while you work on your gun. :)
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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 6:38 pm

I would guess this is from a hybrid. The location of the fan wasn't the issue. This can be seen from the first picture with the exposed motor core. A high mix burning near or with detonation (ping) will do this. The compression flame front got into the motor and blew the plastic out. The compression wave removed the blades as well as shattering the frame. 1X doesn't do this often, but higher mixes do.

This is the same thing that blew out the Sched 80 contest cannon.
The shock wave is strong.
http://www.youtube.com/watch?v=j9xCgNdZPKk
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Unread postAuthor: theBOOM » Mon Aug 31, 2009 6:41 pm

Tech will a fan encased in a metal covering also have this problems and maybe some non-brittle blades?

Hm will something like 300 or 400 psi also beat the crap outta the fan? :?:
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Unread postAuthor: Moonbogg » Mon Aug 31, 2009 6:47 pm

I am going with techs description of events. It sounds way too likely to be wrong. I thought of the shock wave, but then I thought that the pressure occurred everywhere in the chamber at the same time basically, but I guess the pressure propagates throughout the chamber and when strong enough it breaks stuff that gets in the way....I would try an aluminum fan mounted firmly, but I suspect I will discover bent blades and probably another damaged core.
Hmm...a fan would be nice, but maybe I can just do without. At least I have 3 sparks gaps.
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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 6:51 pm

theBOOM wrote:Tech will a fan encased in a metal covering also have this problems and maybe some non-brittle blades?

Hm will something like 300 or 400 psi also beat the crap outta the fan? :?:


There are several ways to combat this type of failure. The first is proper selection of the fan. Some are fiber reinforced stuff that can take a beating and others are cheap injection molded brittle plastic that is much like cheap plastic models. Find a fan where the blades can be bent some like an unbreakable pocket comb. Nylon and fiber filled nylon are very impact resistant.

Another step that can be taken is to carefully disassemble the fan and pack the rotor coil area with epoxy putty so there is little confined space inside for an internal detonation to blow it apart.

The last step is to use as small of a fan as possible.

For fan selection, it's unfortunate that most manufactures simply don't list the material and you can't check for flexibility while in the store blister pack. The best place to get fans is from old computers where you can bend the blades to see if they snap and tap on the frame to see if it's like a nylon "tupperware" or something brittle and rigid.

@ Moonbogg, from the looks of the fan, the mix is just about right.. :D A slightly richer mix may reduce detonation and pinging and keep the power near peak. Slightly lean mixes are more prone to pinging.
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Unread postAuthor: Moonbogg » Mon Aug 31, 2009 6:56 pm

Well I do have a very small fan I could try. Before this failure, the last fan I tried was smaller and thinner and it didn't blow apart, but it did catch on fire and become destroyed. The thin sticker was left on it and it was thin enough to ignite and melted the fan. Maybe I can try a small fan with no stickers and see how that holds up.
Will a small fan with a very small CFM rating actually help with the mixing you think?


EDIT: This just got me thinking...I bet it would be GREAT fun to conduct "crush experiments" on stuff.
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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 7:16 pm

Moonbogg wrote:Well I do have a very small fan I could try. Before this failure, the last fan I tried was smaller and thinner and it didn't blow apart, but it did catch on fire and become destroyed. The thin sticker was left on it and it was thin enough to ignite and melted the fan. Maybe I can try a small fan with no stickers and see how that holds up.
Will a small fan with a very small CFM rating actually help with the mixing you think?


EDIT: This just got me thinking...I bet it would be GREAT fun to conduct "crush experiments" on stuff.


The fact the sticker caught fire indicates the possibility of a lean mix. After the shot, there should be too little oxygen in the chamber to support combustion of the label. A melted label from the heat is one thing. Catching fire indicates an abundance of oxygen after the shot.
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Unread postAuthor: Moonbogg » Mon Aug 31, 2009 7:21 pm

Technician1002 wrote:
Moonbogg wrote:Well I do have a very small fan I could try. Before this failure, the last fan I tried was smaller and thinner and it didn't blow apart, but it did catch on fire and become destroyed. The thin sticker was left on it and it was thin enough to ignite and melted the fan. Maybe I can try a small fan with no stickers and see how that holds up.
Will a small fan with a very small CFM rating actually help with the mixing you think?


EDIT: This just got me thinking...I bet it would be GREAT fun to conduct "crush experiments" on stuff.


The fact the sticker caught fire indicates the possibility of a lean mix. After the shot, there should be too little oxygen in the chamber to support combustion of the label. A melted label from the heat is one thing. Catching fire indicates an abundance of oxygen after the shot.


Oh there was an abundance of oxygen alright :lol: It was my fault. I was trying to "cool" the chamber because it felt unusually hot, so I injected copius ammounts of O2..uh..I mean air to try and cool it off LOOOOOL! That won't happen again. Perhaps the mix was a little too lean to begin with, allowing the fire to start. It was the first time ever firing the cannon, so I was sort of experimenting and I still need to fine tune the mix by trial and error.
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Unread postAuthor: theBOOM » Mon Aug 31, 2009 7:28 pm

Hmm well having a small fan works for me as I have to find a fan to fit into my small chamber... something smaller than .30" in diameter will work.. sadly I'm not sure how good the quality of small fans are....

Anyway I've never seennnnn fans that can be taken apart, specially small ones, also how hard is it to get to that area that requires the epoxy filling??
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Unread postAuthor: Moonbogg » Mon Aug 31, 2009 7:29 pm

theBOOM wrote:Hmm well having a small fan works for me as I have to find a fan to fit into my small chamber... something smaller than .30" in diameter will work.. sadly I'm not sure how good the quality of small fans are....

Anyway I've never seennnnn fans that can be taken apart, specially small ones, also how hard is it to get to that area that requires the epoxy filling??


I'll just stick a hamster in there and have him run around to mix the gasses up.
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Unread postAuthor: Mr.Sandman » Mon Aug 31, 2009 7:34 pm

Moonbogg wrote:
theBOOM wrote:Hmm well having a small fan works for me as I have to find a fan to fit into my small chamber... something smaller than .30" in diameter will work.. sadly I'm not sure how good the quality of small fans are....

Anyway I've never seennnnn fans that can be taken apart, specially small ones, also how hard is it to get to that area that requires the epoxy filling??


I'll just stick a hamster in there and have him run around to mix the gasses up.

Except that you need some food and a sleeping compartment that is gas and heat resistant and houses an oxygen supply. Why not make one with a small motor and an AA baterry or two?
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