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clear combustion for high speed footage testing

Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 4:17 pm

As mark pointed out:

Code: Select all
The central ignition point produces a spherical flame front at first, but as it meets the chamber walls it forms two laminar ones.
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Unread postAuthor: saefroch » Sun Dec 05, 2010 4:25 pm

Ah. I misinterpreted "laminar" to mean a hemisphere shape. Any ideas why this occurs, why the flame front against the chamber walls don't just extinguish?
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 4:39 pm

Why should it extinguish as opposed to being redirected down the chamber tube?
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Unread postAuthor: saefroch » Sun Dec 05, 2010 5:24 pm

I'm referring about the flame front that is "stationary," I guess against the chamber walls...
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Unread postAuthor: ramses » Sun Dec 05, 2010 10:18 pm

They would continue as you expect if we ignore air currents, which are significant. Even in a sealed chamber, you have temperature gradients, climate gradients, composition gradients, etc. When the projectile starts to move, you have a serious air current there. It lags behind close to the walls presumably because of a lower temperature and higher viscus ("skin") drag.
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Unread postAuthor: ChriL » Tue Dec 07, 2010 11:00 pm

holy crap....
those videos are sooooo cool. really helps explain just what is exactly happening.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 08, 2010 12:17 am

ChriL wrote:really helps explain just what is exactly happening.


That was the general idea of this launcher ;)
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 23, 2011 4:47 pm

A couple of shots using hot wire ignition as opposed to a spark arcing across a gap, two shots in the video below were ignited with a piece of nichrome wire connected to a 12 volt 1000mA power supply. The second shot uses a shorter length of wire, meaning it gets hotter and ignites the fuel/air mix quicker. Filmed at 420 frames per second in deliberate low light.

[youtube]http://www.youtube.com/watch?v=OmLNm6Myg7s[/youtube]
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Unread postAuthor: Fnord » Sun Jan 23, 2011 5:08 pm

Hey, same day delivery! Nice work!

What kind of delay are you getting with the shorter length?

I found a chart for approximate temperatures, where does your ignitor fit on here?

Or, alternatively, how 'hot' does it look visually when your mix ignites? Cameras don't really do an adequate job of capturing high-contrast incandescent glow.
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Unread postAuthor: Technician1002 » Sun Jan 23, 2011 5:47 pm

http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html

I would guess somewhere near 900 F. Chart above is Minimum.
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 23, 2011 11:52 pm

Fnord wrote:Hey, same day delivery! Nice work!


Well Sir, we aim to please *snarfle snarfle*

What kind of delay are you getting with the shorter length?


The wire starts to glow almost instantaneously. I'm not sure what the playback rate for high speed is, I assume it's 30 frames per second - in which case 14 seconds of video represent 1 second in real time, you can calculate the delay from the footage.

I found a chart for approximate temperatures, where does your ignitor fit on here?

Or, alternatively, how 'hot' does it look visually when your mix ignites? Cameras don't really do an adequate job of capturing high-contrast incandescent glow.


I'll measure the gauge of wire when I get home and let you know.

So, here on this site we destroy food by shooting it with other food using fuel meant for cooking and pipes meant for drinking water and sewer systems??

If that doesn't crap in the face of starving third-world countries I don't know what will


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Simples ;)
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Unread postAuthor: Technician1002 » Mon Jan 24, 2011 1:31 am

If you want to use hot wire ignition and don't want a long lag time for it to heat up, you can pull a trick I first saw on a Poloroid instant camera.

To get very low warm up time (when strobes were expensive, not disposable) they ran extreme current at high voltage into a low voltage bulb. They limited the time the current ran by using a charged capacitor. By the time the bulb came up to temperature, the capacitor was discharged so it didn't overheat and instantly burn out.

Here is more info on them.
http://www.photokb.com/Uwe/Forum.aspx/photo-35mm/4160/The-Polaroid-Wink-Light
http://www.flickr.com/photos/californiagirl27/2800914287/

It may be possible to match a length of Nicrome wire to a disposable camera flash capacitor. When the wire is hot, the cap is dead.
It would take some experimentation to match the wire size and length to the energy of the cap.
A short 160 volt discharge into a short piece of wire would heat it quickly.
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 24, 2011 1:48 am

Technician1002 wrote:It may be possible to match a length of Nicrome wire to a disposable camera flash capacitor. When the wire is hot, the cap is dead.
It would take some experimentation to match the wire size and length to the energy of the cap.
A short 160 volt discharge into a short piece of wire would heat it quickly


Interesting possibility, my fear would be that the discharge would be enough to melt the wire. If I could get this type of ignition to work reliably in a hybrid it would be perfect, especially as spark gaps are quite an issue the smaller you get. Ideally if for example this was used in a cartridge the wire would not need replacement after every shot.

On a different note, after cannibalising a couple of CRT monitors for parts, some shots with flyback ignition:

[youtube]http://youtube.com/watch?v=SNBo8sW2Xb8[/youtube]

edit: 1000 fps shot with fan running, impressive:

[youtube]http://www.youtube.com/watch?v=0D-QZ2ws9jk[/youtube]
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