Slow Motion Combustion Video

[ToasT]

Came across this on youtube.

[youtube][/youtube]

Several things here, that if not of general interest, could be useful when designing combustion cannons.
Like the wall cooling effect.

[wyz2285]

The guy has a freaking good slow-mo camera.
Nice demo of ignition position versus efficiency, too bad his mix isn't consistent.

[jimmy101]

Great example of why putting the sparker in the cleanout is a bad idea. Almost takes his hand off with the recoil.

[starman]

Nice to see this stuff with the high frames per second camera. We've operated for years knowing that multiple spark gaps will allow a more complete and faster chamber burn. Also it was interesting to see the forces and "back waves" of pressure in the chamber even before the potato left the muzzle.

Also, One thing a burst disk does is allow a pressure build up and also more complete burn before the projectile begins down the barrel. I would LOVE to see the high fps with a burst disk installed.

[wyz2285]

[youtube][/youtube]
I did the same with a burst disc combustion, but my camera is not nearly as good as his.

[matti]

I did the same with a burst disc combustion, but my camera is not nearly as good as his.

nice work How many fps? what camera do you have, Casio?

[wyz2285]

Bought the Nikon J1, felt like a better bang for the buck. A wonderful camera to have around really.
The video was recording on 1200 FPS I think, the J1 can record 400 FPS or 1200 FPS so I'm not entirely sure, it has been a few years.

[matti]

Bought the Nikon J1, felt like a better bang for the buck. A wonderful camera to have around really.
The video was recording on 1200 FPS I think, the J1 can record 400 FPS or 1200 FPS so I'm not entirely sure, it has been a few years.

Ah ok, I have Casio (1000fps) : http://www.exilim.eu/euro/exilimzrserie/exzr1000/

[Futterama]

So basically, it's not the best idea to put the spark gap in the middle of the chamber because this can or will starve the combustion from oxygen? Or did he just have a rich mix in the chamber?

[mrfoo]

I think he just had a rich mix, what you're seeing is the same effect you get in the Reynst combustor / jam jar pulse jet, and you'll see the same reignition of residual fuel thing with the ignitor at the end, although that may blow more of the unburned mixture out the end of the barrel.

If anything, I would suspect having the ignitor in the middle would give better perfomance overall, including better combustion, as the pressure rise will be approximately twice as fast with 2 flame / shock fronts.

It's a neat video, but for someone who's pushing "SCIENCE", they did it all in a remarkably slapdash way. No metering, no pressure monitoring, no frame count of ignition to projectile leaving the barrel, not even a goddamned chrony. Cool flame video content - 10. Science content - 0.

[jimmy101]

He probably had either a rich mix or a poorly mixed mix. It is a "spray-n-pray" fueling method.

In my crappy videos of a clear chamber I noted that you get odd little licks of orange flame that appear to be associated with either dust or imperfections (like scratches) on the chamber wall. http://www.inpharmix.com/jps/Propane_co ... ycarb.html

You can extract some rough timings from the video. I get ignition at 5:02, projectile (rag) starts to move at 5:15 and projectile exits barrel at 5:34. You can't actually see the muzzle so I estimated exit time as the point when the gases in the chamber made a really big jump in velocity. If the frame rate is 20,000/sec, and the video frame rate hasn't been further fudged in the video and is standard 30frame/sec, then from ignition to start of projectile movement is (5:15-5:02)/20,000*30=19.5mSec. Transit time is (5:34-5:15)/20,000*30=28.5mSec. Total time from click-to-bang 48mSec, which is consistent with other measurements and the models. It looks like the barrel is a good 5 feet long so a rough estimate of the muzzle velocity is twice the average velocity; 2*5'/28.5mSec=351 FPS, which is reasonable (though seems a touch high to me).

It is cool to see that while the projectile is moving through the barrel the front flame front is moving forward faster than the rear flame front is moving backwards. That is what the combustion models we did years ago predicted.

I agree, coolness factor and video camera=10/10, actual thought put into doing the correct experiment=2/10. Kind of like Mythbusters.

[ToasT]

My take away from this is you ideally want to minimise the surface area to volume, ie as spherical as possible, and ignite the mix from the centre ish. This gives you the greatest flame front area, which i take to mean the greatest rate of combustion and therefore pressure.
I say ish, because maybe it is best to have it very slightly forward biased to reduce the amount of fuel combusting in the barrel. (keeping a large flame front)

This all means the wall thickness needs to be greater to support the greater stresses due to the slightly higher pressure acting on a much larger surface.

[jimmy101]

Or, just use multiple sparks. You can decrease the total burn time by a lot with three of four ignition points. With a multiple ignition points the cylindrical chamber basically acts like a sphere.

An actual spherical chamber probably really wouldn't change the performance all that much. Indeed it might even decrease performance because the gas flow pathway wouldn't be as good as in a cylinder.

As always, the most important thing is to (1) properly fuel and (2) mix the gases.

[ToasT]

Yea i forgot to mention that the effects here are maybe the last few things to optimise to get the best performance. These are still things that you might want to consider even after you have picked the low, and big fruits, of additional spark gaps.

Id still be interested in seeing a simulation of spherical vs cylindrical chamber, 1 spark vs 1 spark.