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I recently changed the dessicant cartridge on our FT-IR spectrophotometer at work, decided to keep the acrylic tube it was housed in. Turns out it's a perfect fit for a tennis ball, so I made a simple launcher (ie closed one side with an epoxy endcap fitted with electrodes) do do some high speed testing.
By altering the position of the tennis ball, I was able to show the effect of having ignition at the rear, centre and front of the chamber. Note that it was deliberated filmed in low light in order to better show the progress of combustion, enjoy
Forward ignition at 420 frames per second:
Forward ignition at 1000 frames per second:
Central ignition at 420 frames per second:
Central ignition at 1000 frames per second:
Rear ignition at 420 frames per second:
Rear ignition at 1000 frames per second:
Finally, here it is with central ignition in real time, if you watch closely the spark is visible:
edit: some closeups of the ignition at 420 and 210 frames per second, you can clearly see the flame front expanding radially from the spark gap
Showcasing the impressive penetrating power of this beast followed by a test shot with no projectile but the mouth of the chamber sealed with duct tape:
Here's the simple setup with piezo ignition, note the somewhat singed surface of the tennis ball
Last edited by jackssmirkingrevenge on Mon Dec 06, 2010 5:20 am, edited 1 time in total.
Very nice! loved those vids! rear ignition FTW?
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It certainly gives the most power (and I use the word "power" quite wrongly ), but then again it's a bigger chamber volume (injected 8.8mL of butane as opposed to 6.6mL in the central ignition test) so you can't really make a direct comparison.
I especially like the one which shows the radially expanding flame front.
I think the differences between rear, mid and central ignition are not proving much in this case, as the chamber volume differs greatly.
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You'd need a proper setup with adjustable ignition and measure shots on a chrony in order to adequately evaluate the effects of gap placement on performance, really I made this just to have some cool footage oh, and playmobil abuse:
Very cool! Looks somewhat similar to the tests I did a while back, though your more accurate fuelling seems to make the burn go a bit quicker...
Have you tried some intentionally rich mixes to see if that slows down the flame propogation a little bit?
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Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before.
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really nice, it be cool if you could try one with 2 ignition points
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That was the inspiration
I was filling at 3.5%, here it is a 5.5%:
... and 6.5%, bigger flame but power clearly suffered and look note how the extra gas ignites as soon as the projectile clears the muzzle:
I'll see if I can give it a go.
Very nice. I read you're using 3.5% for fueling so are you using butane?
Theoretically, the best performance would come from a series of ignition points spaced evenly and centrally in the cross-section of the chamber so as to produce multiple spherical flame fronts which burn the fuel faster than a laminar flame front. Feel like some fiddling?
The central ignition point produces a spherical flame front at first, but as it meets the chamber walls it forms two laminar ones.
So is that rebounding flame front from the unburned fuel completing oxidation after the projectile clears the muzzle and allows more oxygen to flow back in? I see a returning flame front on ALL of the videos, so does that indicate a rich mix on every one? Could you try one with a slightly lean mix?
Also, noting that the colour of the flame changes as the projectile travels down the muzzle, is that caused by a decrease in the temperature of the burning mix? Would that mean that a higher-temperature ignition would cause greater efficiency?
So the rear ignition point produces more energy at muzzle than the forward ignition point? Doesn't that go against theory that the quicker the chamber burns (the more spherical the flame front is), the faster the projectile exits the muzzle?
Well the chamber volume was much greater in the rear ignited test, so it's not fair to compare them.
Ah, so any effects from ignition points would be rendered insignificant by the volume difference?
That's the theory.
Compare the 3.5% mix and the 6.5% mix - in the first one, combustion is practically complete and the flame doesn't go past the muzzle when the projectile exits, unlike the rich mix which also burns for longer.
I think multiple ignition points would be more effective than higher temperature ignition, however I haven't made this adjustable enough to test it and in any case I doubt the piezo has enough juice to effectively jump multiple gaps.
In this case, the central and rear ignition tests had a fuel volume 150 and 200% respectively that of the front ignition (because I altered the projectile position) and C:B ratio also changed so it is meaningless to compare energy in this case.
So I have another question, mostly regarding the closeup video. If the flame front propagates in a sphere, and we assume no other factors, as soon as the diameter of the sphere reaches the diameter of the chamber, there should be no more flame front to the sides, and it should just be a hemisphere propagating down the chamber, correct? So why does the flame front (especially when still blue) take on an elliptical shape? Is it because the flame front decelerates as it approaches a boundary, does it "sit" at the boundary for an extended period of time, or is there something I'm not aware of at work here (most likely )?
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