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fuel air mixtures left for extended periods of time

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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fuel air mixtures left for extended periods of time

Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 7:48 am

One for you theorists, as above - good or bad thing for combustion? Would it result in a homogenous mix over time (24 hours plus) or would the fuel tend to settle in a particular part of the chamber?
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Unread postAuthor: Hotwired » Thu Nov 06, 2008 8:25 am

Two gases of sufficiently different density say... oxygen and hydrogen would settle roughly into two layers but I wouldn't put any bets on the layers being so distinct as to not ignite if the ignition point was at a far point of one layer.

Same could be for propane but propane would settle below the oxygen unlike hydrogen.
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Re: fuel air mixtures left for extended periods of time

Unread postAuthor: SpudFarm » Thu Nov 06, 2008 9:47 am

jackssmirkingrevenge wrote:One for you theorists, as above - good or bad thing for combustion? Would it result in a homogenous mix over time (24 hours plus) or would the fuel tend to settle in a particular part of the chamber?


i wonder how a rotating chamber would effect that theory...
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Unread postAuthor: Hotwired » Thu Nov 06, 2008 10:04 am

Even if the walls were extremely smooth they'd still cause movement and mixing in and of the gas.
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Unread postAuthor: starman » Thu Nov 06, 2008 11:22 am

I'm with Hotwired. I don't see how a non-moving tank of varied density gases could do anything but promote the separation of the gases.

So to answer the initial question...I believe you would have to worry more about minute leaks in your tank/chamber winnowing out the original mix over time. Otherwise just picking up and moving the gun and certainly turning on the mix fan should undo most of the separation that would have taken place.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 11:47 am

starman wrote:Otherwise just picking up and moving the gun and certainly turning on the mix fan should undo most of the separation that would have taken place.


Ah, but what if your chamber was stationary and fanless...

I suppose a wide, shallow chamber would therefore be ideal in terms of preserving the mix, however this would reduce performance when it comes to combustion.

hmmm...
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Unread postAuthor: ALIHISGREAT » Thu Nov 06, 2008 12:35 pm

why would you leave a mix stationary for 24 hours + anyway? sounds weird!?!
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 1:30 pm

ALIHISGREAT wrote:why would you leave a mix stationary for 24 hours + anyway?


Oh I don't know, what if you were investigating the possibility of a ridiculously over engineered and correspondingly overpowered alternative to one of these :idea: :D
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Unread postAuthor: i-will » Thu Nov 06, 2008 1:32 pm

i'm working on a combustion cartridge system for a homemade m203 launcher. the last one i fired held the mix for about 16 hours. the chamber is only 1 1/2 inch in diameter but i think a larger chamber would still work fine even after the gas settles.

starman has the right idea though. a quick flick of a mix fan will do fine if settling is a problem to u.
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Unread postAuthor: harmon12407 » Thu Nov 06, 2008 1:46 pm

logical thought progression:

First thought-

well, if it is going to be ridiculously over-engineered, you can use a passive photo-detector and op-amp bufer circuit to turn on a mixing fan if something, (aka a pesky rodent), disrupts the sensor. Use this in combination with an amplifier to get more than 50 mA current, and have a limit switch stop the mixing after, say, 10 seconds.

Second though-

You would probably just be better to use something thats pneumatically actuated.

Wait for it-

Springs store energy too!

Ah Crap-

That patent is already taken...
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Unread postAuthor: ALIHISGREAT » Thu Nov 06, 2008 2:29 pm

a combustion mouse-trap? how exactly does your brain work JSR? :shock: how did you possibly think of that :lol:
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Unread postAuthor: LikimysCrotchus5 » Thu Nov 06, 2008 2:46 pm

Hey well others have done it for similar applications as well, such as a problem that i have now that my father is angered about.

Everyone loves thier lawn when its nice but hates a special something that causes mounds and burrows in the lawn. I want to help my father fix this problem, its just that i am lazy and dont want to cause havoc with this unoriginal idea i found on youtube of using combustion to bring cessation to a special something.

I know how you feel JSR, i might go with a combustion system to obliterate this problem that i have :wink:
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 3:14 pm

harmon12407 wrote:well, if it is going to be ridiculously over-engineered, you can use a passive photo-detector and op-amp bufer circuit to turn on a mixing fan if something, (aka a pesky rodent), disrupts the sensor. Use this in combination with an amplifier to get more than 50 mA current, and have a limit switch stop the mixing after, say, 10 seconds.


I might have overstated the point to which the overengineering would be capable of being described as "ridiculous", i don't want a circuit more complex than simple hot wire ignition.

You would probably just be better to use something thats pneumatically actuated.


Probably, but I want to keep costs as low as possible

Springs store energy too!


Ah, but they don't go WHOOOMPH! :D

a combustion mouse-trap?


I must emphasise that this is a purely theoretical exercise and in no way am I suggesting or condoning harm to vermin or any other living creature.

a special something that causes mounds and burrows in the lawn


A bit cruel, but there are spring loaded solutions...
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Unread postAuthor: jonnyboy » Thu Nov 06, 2008 3:41 pm

ALIHISGREAT wrote:why would you leave a mix stationary for 24 hours + anyway? sounds weird!?!

Well say you were going on vacation so you make a bunch of cartridges full of the gasses so you just put on the barrel.

I think the gasses might settle over time like oily salad dressings do but I don't think it would affect performance unless you were using an unpressurized system ie; spay and pray combustions.
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Unread postAuthor: jimmy101 » Thu Nov 06, 2008 4:12 pm

Gases, once mixed, will not seperate spontaneously under normal conditions.

So, once mixed they are mixed, for all intents and purposes, forever.

If the original mixture is not homogenous, then letting it sit will eventually result in a perfectly homogenous mixture. Depending on the geometry of the chamber, the temperature, and how badly mixed the starting mixture is, it may take minutes to days to fully mix.
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