Davy Lamps

[rcman50166]

Hey you guys, reading Popular Science I saw something somewhat interesting. If you guys have ever wanted to shoot something flame sensitive from a combustion, you know where this is going.

I figured it would limit the power. (A possible feature you may be able to use.) Does anyone have any experience with such a feature?

HOLD YOUR FIRE
- A wire screen is all it takes to prevent dangerous gases from exploding

If you were a coal minder in the early 1800s, the light you used was an open-flame oil lapm -- even though the mines were sometimes filled with "fire-damp," a volatile mixture of air and methane gas. Explosions were inevitable, and at times threw bodies from mine shafts like grapeshot from a cannon. Humphry Davy became a national hero when, 1815 he found a remedy: Surround the lamp flame with mosquito screen.

Davy, one of the world's first professional scientists, solved the problem by systematically studying what happened when gases burned. He started with the observation that gas flames would not travel down long, thin metal tubes because the metal draws heat from the flame, lowering the temperature of the gas below the ignition point.

He tried making the tubes thinner and shorter, until he discovered that thin tubes needed to be only about as long as their diameter to prevent fire from traveling their length. The logical end point was fine metal mesh, which you could think of like thousands of very short tubes arranged in a grid.

As bizarre as it sounds (and I really didn't believe it until I saw it with my own eyes), you can blow an explosive mixture of gases through a fine wire mesh toward a candle flame and, when the gas explodes the fire stops dead at the mesh - this despite the fact that the gas came right through the holes in the mesh and is just as explosive on the outsde as it is on the inside.

Davy refused to patent his invention, preferring to bask in the glory of his role as the miner's savior. His lamp remained in use, and his name was a household word, right up until the invention of the electric light. Today his lamp is all but forgotten, but his reputation as one of the first and greatest chemists lives on.

- Theodore Gray


[Captioned pic]
RE-CREATING A DAVY LAMP
Screen test: A fine-mesh kitchen sieve with a candle inside simulates a Davy miner's safety lamp. An explosive mixture of propane gas and air is blwn in from the outside. If the mesh is fine enough, the fire will stop at the screen even as the explosive gas flows through it.

ACHTUNG!
Anytime you're playing with explosive gases and open flames, the result will be dangerous blasts of fire. The Davy lamp does not work with all kinds of flammable mixures, nor does it work under all conditions.

[jrrdw]

It's hard to read all right, and I'm not to sure if it's going in the right direction for Spudfiles. Does this artical violate any posting rules?

[twizi]

no it deals with that flame can not pierce small holes or somthin it may work i no what u talkin about

[rcman50166]

When I said something flame sensitive I didn't mean explosive. Rather, I meant things like t-shirts or balls. As far as posting rules, I don't see any that conflict. If it makes you feel better I can remove the image and type it out. The same information is passed.

Edit: I'll type out the article anyways. For readability's sake. It'll be a few hours though, I'm going somewhere in a moment.

[D_Hall]

Gee, my kid subscribes to PS and I can type on the order of 90 wpm so.....

HOLD YOUR FIRE
- A wire screen is all it takes to prevent dangerous gases from exploding

If you were a coal minder in the early 1800s, the light you used was an open-flame oil lapm -- even though the mines were sometimes filled with "fire-damp," a volatile mixture of air and methane gas. Explosions were inevitable, and at times threw bodies from mine shafts like grapeshot from a cannon. Humphry Davy became a national hero when, 1815 he found a remedy: Surround the lamp flame with mosquito screen.

Davy, one of the world's first professional scientists, solved the problem by systematically studying what happened when gases burned. He started with the observation that gas flames would not travel down long, thin metal tubes because the metal draws heat from the flame, lowering the temperature of the gas below the ignition point.

He tried making the tubes thinner and shorter, until he discovered that thin tubes needed to be only about as long as their diameter to prevent fire from traveling their length. The logical end point was fine metal mesh, which you could think of like thousands of very short tubes arranged in a grid.

As bizarre as it sounds (and I really didn't believe it until I saw it with my own eyes), you can blow an explosive mixture of gases through a fine wire mesh toward a candle flame and, when the gas explodes the fire stops dead at the mesh - this despite the fact that the gas came right through the holes in the mesh and is just as explosive on the outsde as it is on the inside.

Davy refused to patent his invention, preferring to bask in the glory of his role as the miner's savior. His lamp remained in use, and his name was a household word, right up until the invention of the electric light. Today his lamp is all but forgotten, but his reputation as one of the first and greatest chemists lives on.

- Theodore Gray


[Captioned pic]
RE-CREATING A DAVY LAMP
Screen test: A fine-mesh kitchen sieve with a candle inside simulates a Davy miner's safety lamp. An explosive mixture of propane gas and air is blwn in from the outside. If the mesh is fine enough, the fire will stop at the screen even as the explosive gas flows through it.

ACHTUNG!
Anytime you're playing with explosive gases and open flames, the result will be dangerous blasts of fire. The Davy lamp does not work with all kinds of flammable mixures, nor does it work under all conditions.

[Moonbogg]

Sounds like a flashback arrestor.

[rcman50166]

Gee, my kid subscribes to PS and I can type on the order of 90 wpm so.....

Thanks, that saved me a bit of work. I'll post that in the first post.

Anyways, has anyone ever experimented with this phenomena? It seems like it could have real potential in a spudgun. Another question I have is how drastically this would effect efficiency? How large of a difference is the ignition temperature for various fuels to their combustion temperature. How does pressure (hybrid) affect this. Would it work in a hybrid?

Just some thinking out loud. Also a penny for your thoughts.

[Insomniac]

To be honest, I don't think this would help protect flame-sensitive projectiles much at all. The lamp works because the cool steel mesh can cool the flame front to the point where it will no longer ignite further combustible gas mixtues. This is all well and good for the way the lamp is used: It's a fairly cool, yellow flame surrounded by quite a large amount of mesh, and the resulting deflagrations of the gasses that collect inside are short-lived and cooled by the entirety of the mesh,

In a spudgun, you have a fairly large deflagration being concentrated down to a very narrow apeture, with a high pressure differential forcing a LOT of very hot gas through the barrel in a fraction of a second. I get the feeling that the small amount of mesh would simply heat up to the ignition temperature of the gas, and simply wouldn't be able to cool such a large amount of gas quickly enough to make any measurable difference (although it would probably reduce performance as it restricts the gas flow somewhat).

[jimmy101]

The affect of a small orifice has been discussed before in relation to jet accelerator type setups. If the orifice is too small then a flame won't propagate through it. Of course, the orifice has to be pretty darn small.

It also points out just how important heat loss is to a flame. The flame won't propagate through the mesh because the mesh cools the gases off too much. Same with a combustion spud gun. Too much surface area, or too much surface area for things like the spark gaps and chamber fan, robs the gun of power.

I'm not sure if insomniac is correct. I doubt a 1X combustion gun would heat a mesh screen up all that much. Perhaps a few tens of degrees F but it won't get anywhere near the autoignition temperature of the fuel mixture. There just isn't enough total energy in the combustion mix to heat up a piece of wire mesh all that much. A the walls of a closed combustion chamber only warms up by a few degrees.

[rcman50166]

So then we aren't sure whether it could work? Could someone try to set up an experiment? Perhaps someone with a clear combustion chamber potato gun could put mesh in the chamber and videotape the results, preferably with a high speed camera. Could anyone perform this experiment? I know I can't since i dont own either

[D_Hall]

I'm totally lost as to where/how you think such a device is useful in a launcher.

[Insomniac]

I think an easier way would be to get a gun with a removeable barrel, put mesh over the exposed area and fire it.

Also, a high speed camera won't be nessecary for this test. HS cams tend to need lots of light and arn't actually (usually) that great at picking up flames and whatnot. A regular video camera will capture the flame just fine... sure you might only get one frame of it, and that frame may have been exposing for the duration of the shot, but it will still tell you if a flame did, or didn't manage to escape.

I suppose I could try this with a mini spudgun, but I'm not quite sure if this is an experiment that will scale correctly. If I end up trying it I'll film it with a couple of different methods and post the results.

I'm totally lost as to where/how you think such a device is useful in a launcher.

I think he wants to use it to assist in firing projectiles which can be damaged by the heat and flame of a combustion cannon, so tennis balls and the like. In my mind though, if you cool down the gasses to an acceptable level, won't you be robbing the combustion of a huge chunk of it's power, as combustions arn't powered by the volume of the products of combustion, but the expansion of said products when heated due to the deflagration?

[rcman50166]

I think he wants to use it to assist in firing projectiles which can be damaged by the heat and flame of a combustion cannon, so tennis balls and the like. In my mind though, if you cool down the gasses to an acceptable level, won't you be robbing the combustion of a huge chunk of it's power, as combustions arn't powered by the volume of the products of combustion, but the expansion of said products when heated due to the deflagration?

You are correct about what I'd like to use the phenomena for. Also there would be power robbed... from the flow restrictions. But how are the flame arrestors set up in a jet engine? I'm assuming it uses the same principle and the engines are high flow.

[Insomniac]

I think he wants to use it to assist in firing projectiles which can be damaged by the heat and flame of a combustion cannon, so tennis balls and the like. In my mind though, if you cool down the gasses to an acceptable level, won't you be robbing the combustion of a huge chunk of it's power, as combustions arn't powered by the volume of the products of combustion, but the expansion of said products when heated due to the deflagration?

You are correct about what I'd like to use the phenomena for. Also there would be power robbed... from the flow restrictions. But how are the flame arrestors set up in a jet engine? I'm assuming it uses the same principle and the engines are high flow.

I think you may be misunderstanding the purpose of the flame catcher in a turbine engine. This is all from memory btw, so I may be well off the mark, or thinking of a different component entirely, so take it with a grain of salt.

Anyway, in a turbine engine there is a device called a flame arrester or flame catcher just behind the fuel injection point. When the air flows around this, it causes turbulence and some slow-moving air directly behind it. This allows a small area in which a flame can self-sustain without being blown out by the immense flow of the jet. The purpose of this is to stop the engine from flaming out... without it, the flame front wouldn't be able to keep up with the flow of fuel and air through the engine, and would be spat out the back. The flame directly behind the flame catcher acts like a pilot light and keeps re-igniting the gas, stopping the flame front from dropping behind. There is a similar device used on the nozzle of most blowtorches... the centre, hot blue flame would not stay lit on it's own due to the high speed flow of gas, so there is usually a small ring of tiny, slow jets to keep the main one ignited.