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Any theories for Latke's data collapse?

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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Any theories for Latke's data collapse?

Unread postAuthor: D_Hall » Tue Apr 01, 2008 9:04 pm

Image

Look at the [left] hand side of that plot... What would make the velocity drop off so dramatically?

The rapid climb on the [right] can be explained by ongoing combustion. But the collapse on the left.... That one has me scratching my head.

Ideas?
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Last edited by D_Hall on Tue Apr 01, 2008 9:31 pm, edited 1 time in total.
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Unread postAuthor: ammosmoke » Tue Apr 01, 2008 9:14 pm

There isn't enough air for the fuel? Isn't it like a rich combustion mixture vs a lean one in a car? A lean one burns much hotter, and with slightly more power, while a rich one burns cooler, with slightly less?
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<img>http://www.speedtest.net/result/309559995.png</img>
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Unread postAuthor: DYI » Tue Apr 01, 2008 9:23 pm

The careful observer will notice the x-axis label of "C:B Ratio", which rather unsuprisingly has absolutely nothing to do with fuel ratios :roll:

The farther right you go, the less acceleration time the spud has. As for the sudden drop, I'm a bit too tired to try and figure it out. There's obviously something affecting it that isn't as simple as less acceleration time when it declines that suddenly.
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Unread postAuthor: D_Hall » Tue Apr 01, 2008 9:30 pm

DYI wrote:The farther right you go, the less acceleration time the spud has.

LOL... Oh, crap.

I have that same plot in Excel, but I have the X-axis as barrel length. I figured I'd cheat and use the already-online version but missed the CB vs. barrel length significance.

OK, let me rephrase....

The drop on the right can be explained by ongoing combustion.

But why the drop on the left?
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Unread postAuthor: starman » Tue Apr 01, 2008 9:55 pm

D_Hall wrote:But why the drop on the left?


Is it possible that the relative long barrel length is fully absorbing the expansion energy from the combustion (at least in his 1x standard combustion). In one of his extreme tests at (.2/1 I believe) it sent the potato partially down the barrel then it suddenly sucked back down the barrel and into the chamber from the vacuum....blew out the fan from the suck-back tater blast.

Maybe stray air is being sucked in during combustion on the long barrel shots and leaning out the fuel blend too much, essentially putting out the fire so to speak....

Just thinking out loud here...just my common sensical feel... There's obviously some limit to barrel length...infinite barrel length can't happen no matter how much combustion power you apply.
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Unread postAuthor: Lentamentalisk » Tue Apr 01, 2008 10:07 pm

by drop i assume you mean very steep increase, because the slope is positive... That is because the combustion has not entirely happened, and the spud is still being pushed, so the longer you push it the faster it will go.

The drop on the right (this time it is a drop, because the slope is negative) is due to the fact that the combustion is done and because the gases have so little heat capacity, they cool down rapidly, shrinking, and creating a vacuum, pulling the spud back down the barrel.

hope that clears it up.

edit: I think I know what you mean.

the reason that the speed "drops off" is because it is not talking about taking a standard length barrel and changing the size of the chamber, in which case the bigger the chamber the better. They are taking a standard sized chamber, and changing the barrel length. There is a very technically minor difference, but huge effect difference.
To clarify the graph they should say that they are finding the ideal barrel length for a specific chamber, not vice versa.
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Last edited by Lentamentalisk on Tue Apr 01, 2008 10:12 pm, edited 1 time in total.
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Unread postAuthor: D_Hall » Tue Apr 01, 2008 10:09 pm

Oh, I know he's pulling a vacuum. My own data shows that effect. But there seems to be a hard knee in the curve that implies something dramatic is happening. I've no idea what it could be. I thought that it might be water vapor condensing (thus making a disproportionately strong vacuum), but if HGDT is anywhere close to reality, it's not getting anywhere close to cold enough for that to be a problem.
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Unread postAuthor: starman » Tue Apr 01, 2008 10:12 pm

Lentamentalisk wrote:by drop i assume you mean very steep increase, because the slope is positive... That is because the combustion has not entirely happened, and the spud is still being pushed, so the longer you push it the faster it will go.

The drop on the right (this time it is a drop, because the slope is negative) is due to the fact that the combustion is done and because the gases have so little heat capacity, they cool down rapidly, shrinking, and creating a vacuum, pulling the spud back down the barrel.

hope that clears it up.


Got your lefts and rights turned around there....
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Unread postAuthor: Lentamentalisk » Tue Apr 01, 2008 10:15 pm

starman wrote:Got your lefts and rights turned around there....

um no... it increases from left to right, until it hits around the center, then it decreases from left to right...

read my edit ^^
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Unread postAuthor: starman » Tue Apr 01, 2008 10:18 pm

Lentamentalisk wrote:
starman wrote:Got your lefts and rights turned around there....

um no... it increases from left to right, until it hits around the center, then it decreases from left to right...

read my edit ^^


Umm yes, longer barrels represented on the left (.2/1), shorter barrels on the right (1.6/1)
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Unread postAuthor: D_Hall » Tue Apr 01, 2008 10:20 pm

Lentamentalisk....

Trust me, I'm way ahead of everything you've said. You're not grasping the question. Don't just look at the data. Don't just look at the trends. Look at the NATURE of the trends. Something dramatic happens at a CB of about 0.5. It doesn't just start to slow down, it slams on the friggin breaks! Hard vacuums don't just suddenly form like that. At least, not without something weird happening (like a phase transform or something). The question is, what's "something weird" in this case?
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Unread postAuthor: Lentamentalisk » Tue Apr 01, 2008 10:26 pm

oooooh, now I finally get you, you were talking about a drop in the increase...
I think that that 525fps is the expansion rate of combusting propane in their combustion chamber. That would account for it leveling off, but I do find it weird that it is such a sharp stall... This requires further research.
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Unread postAuthor: starman » Tue Apr 01, 2008 10:27 pm

I would be interested if the same knee happens at .5/1 with a hybridized combustion, 2x or greater....or if the ratio changes...drops lower maybe. Have you considered the c:b resonance possibility I raised earlier?
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Unread postAuthor: Lentamentalisk » Tue Apr 01, 2008 10:33 pm

The other thing that would be nice to test would be to see if the effect was different if lighted from the middle versus the end (double the flame front, so faster burn) and how the diameter of the combustion chamber/barrel effects it (also larger flame fronts)
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Unread postAuthor: D_Hall » Tue Apr 01, 2008 10:40 pm

Lentamentalisk wrote:oooooh, now I finally get you, you were talking about a drop in the increase...
I think that that 525fps is the expansion rate of combusting propane in their combustion chamber. That would account for it leveling off, but I do find it weird that it is such a sharp stall... This requires further research.

The leveling off is due to combustion completing and pressure equilizing before vacuum begins to form.


For contrast, here's the data presented in a barrel length format rather than the CB format (which can be confusing).
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