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Very telling, those different flame front speeds.
I guess the fact that the air ahead of a flame front starting at the rear of the chamber is accelerating improves flame front speed significantly.
That said, I'd be interested to see what effect a barrel and projectile would have on the situation.
If a rear spark gap is that much more effective, it could act as part (only part, mind) of the explanation for why launchers such as Crusader recorded such unusually high muzzle velocities.
I'd like to try and replicate Crusader at some point - but I have too many things I'm not doing already.
Does that thing kinda look like a big cat to you?
I would be very interested in seeing the effect of a fan on the flame front in real speed. Particularly the difference between having the fan run only before the shot, and having it running during the shot. Also, multiple spark gaps could be interesting.
I have videos of just that on my other computer awaiting rendering and upload. Now I just have to kick my sister off the computer...
Unfortunately, it was spray and prey fueling, and some shots were made with partially melted bottles from overfueled shots. I almost ran out of clear bottles.
Last edited by ramses on Thu Dec 31, 2009 11:07 am, edited 1 time in total.
Just a note to thank everybody. At the office we have high dollar high-speed cameras at our disposal (no kidding, 6 figure price tags). They do a magnificent job but..... Well, there are times when we'd like to take some high speed video but the location is such that we're not willing to risk the destruction of our camera.
Consumer level cameras like these? They're disposible! We may very well be buying a bunch of these and using them in high hazard locations. If we get usable video, GREAT! If the camera gets blown up... Meh, no great loss.
In any event, thanks for bringing these cameras to my attention.
Oh how I envy your job...
Fascinating stuff! and lol at the Greenday poster
It's interesting that when there's no burst disk, a rear-mounted spark seems to be best, but once you put a burst disk on a mid-mounted spark seemed to do a perfect job of getting the flame front to reach both sides simultaniously. I wonder how an actual projectile would affect it?
Also, I laughed at the fact you had somehow managed to get a full sized pc fan into a soda bottle.
Not difficult to predict and is discussed in the literature for pipe fires. Basically, if air is physically moving the movment of the air dominates the flame propagation. An open container means that air will be rushing out the opening. That means the flame front follows suit. No opening? The air has nowhere to go and different mechanisms dominate.
nah, only an 80mm... but please note that the bottle was melted and reduced in one dimension by around 1/3. I got it wedged to provide some kind of axial flow and said screw it. I was PISSED when the trip it took across my room dislodged the fan.
I have a 2 liter hybrid noisemaker, but I can't use that in my room (2 windows, plus my ears). Maybe once it warms up...
For some reason, it takes a perceivable amount of time between when the camera flash ( switch for an ignition coil) fires, and when the bottle explodes. Almost as if the bottle held the pressure, and needed the heat to weaken it until it burst. My actual hybrid sometimes displays this too. I wonder if the flame fronts move that slow until the valve opens to allow the air movement to move the flame front.
Anyone have some transparent aluminum I can borrow?
Yeah, it's pretty much what I would have expected to happen, but it's still interesting that the disk burst at just the right pressure, not too early or too late.
I have noticed that usually the pressure stays fairly low for the majority of the burn, and only spikes up right at the end. When I've experimented with 'whoosh bottles' (small amount of alcohol in a soda bottle, with a small hole in the lid so it acts like a rocket when lit) I found that they would burn almost completely without producing any real thrust, and then would suddenly lift off right as the burn ended. I suppose this would explain why the disk burst at just the right moment even though there probably wasn't much effort put in to getting it to burst at a specific pressure.
If you're referring to the burst disk in my video, it was 1 layer of duct tape applied to the mouth of the bottle, mostly to keep the axial fan from screwing up the mix by blowing the fuel out of the bottle.
Some people here are seriously overlooking a few aspects on the front/back ignition.
Don't start assuming an ignition from the back is better.
With the back ignition, unburned fuel gets pushed out of the chamber. When the flamefront has reached the middle of the gas, the gas has expanded out of the bottle, and the whole bottle is lit up. So it LOOKS like a faster burn, but that doesn't mean it is faster.
Front ignition guarantees complete combustion. The difference is that it LOOKS like a slower combustion because all flames exit the chamber and the remaining fuel/air stays in the chamber instead of pushing the unburned fuel/air into the barrel and filling the chamber with flames.
Think about it. I'd rather have hot gases in my barrel then in the back of my chamber, which doesn't need to go anywhere anyway.
Adding a barrel with a projectile will certainly change things. This will limit the flow, the more flow is limited, the more the combustion will approach a "closed chamber combustion". From which we all know ignition in the middle is the most powerful.
Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!
Spudfiles steam group, join!
There seem to be a lot more amateur high speed videos online these days courtesy of the Casio series.
My current Fuji seems to have reached the end of its useful life, perhaps it's time I invested in one
What does the board think of this one? On eBay including postage it's at half the price of the same model available locally
Just to be an annoying arse....
When this thread started I was more than a little bit stunned at what was doable with consumer-level cameras. I knew what I'd seen at the office and such (a popular frame rate for us is indeed 1000 fps) and I was wondering WTF was up with the difference in price. Obviously we had better lenses and longer record times but... Well, we were paying enormous sums of money for our toys.
So I asked one of our photographers what our current cameras were capable of. They can be configured for all sorts of frame rates and resolutions, of course, but some interesting answers....
Maximum frame rate at full resolution (1024x1024 pixels) with 24-bit color images. 7000 fps.
Maximum frame rate, period: 1.4 Mfps (but resolution is like 64x64).
The good news, though... It wasn't THAT long ago when 640x480 @ 1000 fps was kick ass even by professional standards. As such.... I imagine it won't be THAT long before similar rates are available at the consumer level.
edit: Oh, and again thanks to ya'll and this thread, we're probably going to buy some of the Casio cameras and use them as disposibles for use in high hazard locations.
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