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Clear Chamber Combustion Video Fun

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: Jared Haehnel » Fri Feb 29, 2008 1:36 pm

Some of you guys take this hobby to a whole new level....I'm jealous
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Unread postAuthor: STHORNE » Fri Feb 29, 2008 2:37 pm

Yeah, me too. You guys have introduced me to a new door to open. Unfortunately, I'm too lazy to open the damn thing. lol :wink:
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Unread postAuthor: D_Hall » Fri Feb 29, 2008 5:58 pm

jimmy101 wrote:Any thoughts on how you are going to do temperature? I've considered trying a thermocouple but the heat capacity and response times seem to be totally unuseable for combustion gases in a closed chamber or gun.

There ARE fast response thermocouples out there, but they're FRAGILE and as such highly unreliable. In other words, I agree with your conclusion that thermocouples are ill-suited.

So for temperature I'm thinking of using your technique with the accoustic analysis. I think that was shear genius on your part.


On a related note, I wanted to respond to something you said in another thread but I've not been able to find it. Somewhere you mentioned that you have to use roughly double-speed flame propogation as a starting point to get your mathematical model to work. IE, 0.8ish as opposed to 0.43 m/s.

I think I've an idea as to why.

As I understand your webpage, the 0.43 number you cite is for propogation through quiescent air. In real world terms, this would be the flame propogation rate of an open pipe lit at the open end.

But that's not what we do. We light elsewhere. For the purpose of simplicity, let's say that we lit that open pipe at the closed end.

For a brief instant everything is sitting still and the flame rate would indeed be in the range of 0.43. But here's the catch... The hot gases are expanding. They push the cold gases out in front. The flow is laminar, but it is a flow. It is NOT quiescent. So.... If we attempt to view it as quiescent air we must move our reference frame at the same rate as the flow near the flame front. How fast will that be in the immediate aftermath of igniton? It'll be very nearly the same as the flame propogation rate as the hot gases have a density that is a fraction of the cold gases. Result? Flame propogation rate as measured by a stationary observer will be on the order of 0.8 m/s.

Of course, the flow will quickly become turbulent and all hell will break loose, but at least at the moment I'm thinking that the difference between 0.43 and 0.8 is literally nothing more than the difference of which end we light the tube from.
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Unread postAuthor: jimmy101 » Sat Mar 01, 2008 12:35 pm

D_Hall wrote:On a related note, I wanted to respond to something you said in another thread but I've not been able to find it. Somewhere you mentioned that you have to use roughly double-speed flame propogation as a starting point to get your mathematical model to work. IE, 0.8ish as opposed to 0.43 m/s.

I think I've an idea as to why.

As I understand your webpage, the 0.43 number you cite is for propogation through quiescent air. In real world terms, this would be the flame propogation rate of an open pipe lit at the open end.

But that's not what we do. We light elsewhere. For the purpose of simplicity, let's say that we lit that open pipe at the closed end.

For a brief instant everything is sitting still and the flame rate would indeed be in the range of 0.43. But here's the catch... The hot gases are expanding. They push the cold gases out in front. The flow is laminar, but it is a flow. It is NOT quiescent. So.... If we attempt to view it as quiescent air we must move our reference frame at the same rate as the flow near the flame front. How fast will that be in the immediate aftermath of igniton? It'll be very nearly the same as the flame propogation rate as the hot gases have a density that is a fraction of the cold gases. Result? Flame propogation rate as measured by a stationary observer will be on the order of 0.8 m/s.

Of course, the flow will quickly become turbulent and all hell will break loose, but at least at the moment I'm thinking that the difference between 0.43 and 0.8 is literally nothing more than the difference of which end we light the tube from.

Damn, that makes a heck of a lot of sense. I was just figuring that the factor of 2 was to account for the flame front converting to turbulent at some point. The model doesn't take that into account so the "2x factor" in initial flame speed was just a very crude fix. It overestimates the initial flame speed (but that is probably OK since anything that happens before the spud starts to move to relatively less important) but gives a better estimate of what is happening at higher combustion fractions when the spud is moving and the flame front is more likely to be turbulent.

In my model the two flame fronts are treated seperately and they propagate at different speeds (relative to the chamber).

One thing I struggled with in the model is what happens to the gases as the projectile starts to move. Seems to me that the "ball of combustion products" has to start moving once the projectile starts to move. Or, to put it another way, the center of the combusted gases moves away from the spark position once the projectile starts to move. The muzzle end flame front has to chase the fuel towards the barrel and ultimately down the barrel. The breech end flame front is moving into the expanding fuel at the guns breech. So the breech end front is moving slower relative to the chamber but faster relative to the unburned fuel, the muzzle end front is moving faster relative to the chamber (and is more likely to be turbulent) but is moving slower relative to the unburned fuel. Which brings up the long standing discussion/debate about where the best locations is for the spark in the chamber. Some hard data on performance versus spark location sure would be nice.

Interesting that, from the videos I did with the polycarbonate "semi-closed" chamber, the yellow flame plumes suggest the gases are moving in the opposite direction of the flame front late in the combustion process. That observation is probably not directly relevant to a spudgun but it does makes me wonder exactly how the flame front speed should be measured; relative to the chamber or to the gases? Relative to the chamber the flame front speed peaks out at about 30FPS. The plumes indicate the gases are going about that speed in the opposite direction so is the flame front speed actually ~60 FPS?
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