Thermal changes in geometry, and the effect on starting, etc

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Post by Mike Everman » Fri Feb 20, 2004 2:40 pm

Ok, so The Deadline is what drives you; so prepare yourself. I suspect that you will not want to show up to the Pulsejet Historical and Utilization Conference (PHUC) without a new engine to show!
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Post by Bruno Ogorelec » Fri Feb 20, 2004 5:09 pm

Mike Everman wrote:I suspect that you will not want to show up to the Pulsejet Historical and Utilization Conference (PHUC) without a new engine to show!
You're right. It wouldn't be worth a PHUC without one. :o)

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Post by vhautaka » Sun Feb 22, 2004 9:56 am

Hmm... to return to topic somewhat. :)


Has anyone thought that maybe the "boundary layer effect" some have mentioned is actually a good thing?

Kazoo has constant wall area from front to end.

Tharratt suggested corrugated intake opening walls to reduce thermal geometry problems. Now, corrugations only add to the wall area and provide good "nests" for stale air layers, if you think it that way...

The Coanda effect states that fast, hot, turbulent gases will prefer sticking to the walls once on their way out. This may actually be of help.

Then think of the fresh air coming in, especially in the intake it gets to fill a partial vacuum right after an explosion that flashes partially out of the intake. The suction should be fast enough to get the air mixed with fuel and deep into the CC in a fraction of a second, more like implosion than suction, so shouldn't it also be strong enough to suck any "boundary" gases right back in?

Here's where I connect these ideas with various LH engines that seem to start or even auto-start when cold, but won't start hot. And to the fact that Lockies really don't seem to run well in winter temperatures - it probably has something more to it than just propane.


It is obvious that the air, while being sucked in, warms up a bit and expands somewhat.

Now think that air is way much colder than the engine, also more humid like it is in the winter. It expands way more, and more explosively, filling the chamber with thinner air and making it impossible for the engine to get in enough of air/fuel/both or even making it blow some of the mixture away before next bang arrives.

Air does expand/contract very fast under a big enough temperature difference, and just ten degrees C can make a noticeable difference in the expansion.

This first shows up as starting problems, when there's no resonance yet and the next explosion doesn't arrive quite fast enough, then when the temp difference gets bigger, the xtra xpansion may get so severe that it may halt a running engine.

Is this a plausible explanation? Or just plain obvious? And is there some known way around?

I would think someone in the 1940's - 1960's has already thought this out, if the engines were being eyed for military/commercial purpose.


- ville

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Post by Mike Everman » Sun Feb 22, 2004 5:29 pm

Ville-
I'd like to think that there is an advantage to the extra surface area in Kazoo, and I've thought it might even be good for any engine to have dimples on all inside surfaces, like a golf ball. Unknown is whether it has any advantage for hot, reversing, pulsating flows.

We had a discussion a while ago about the heat transfer in the intakes, and I think the correct conclusion was that:
A: the few miliseconds the fresh air charge is in contact with the intake is too short for any significance, and,
B: the intake tract is fairly cool, so little delta T to begin with

The latter conclusion though should have the cards cut on it as the incoming air cools the intake, so there must be thermal transfer, but the outgoing blast is subject to the first conclusion (not there very long), so is not contributing much to the heat of the tube(?) Conduction from the CC is another thing, less a factor on stainless.

If hot starting problems can be solved with a temporary change in geometry, then we can learn this with a Kazoo setup that allows you to squeeze or pull open a couple of dimensions. Any suggestions on which dimensions?

Chris (Resosys) says his start first pop hot or cold, if I remember right. Let's consider the ratio of intake diameter at the CC to exhaust throat diameter as a start. Then compare this ratio to someone's engine that does not start hot. Might be illuminating.
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Post by tufty » Sun Feb 22, 2004 6:46 pm

brunoogorelec wrote:Meanwhile, who was it that applied to have the first crack at the SNECMA stuff obn CD that George has sent me? I am through with it and will send it to teh next user. I lost track of who was the first to claim it. Let me know.
That would have been me, I suspect. Drop me a mail to simon at tufty dot co dot uk (email address deliberately obfuscated in a vain attempt to at least keep constant the enormous quantity of spam I receive every bloody day) and I'll ping you my snail mail address. If the next lucky recipient would do similar, I'll image the disk here on receipt (So I can work on it) and forward it on directly.

Simon

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Post by Mike Everman » Sun Feb 22, 2004 11:48 pm

I believe Chris Brick (resosys) offered to host it on his server. How about if he is next?
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Post by resosys » Mon Feb 23, 2004 5:43 pm

Mike Everman wrote:I believe Chris Brick (resosys) offered to host it on his server. How about if he is next?
Offer still stands but there was some question as to whether it would get anyone in trouble. I can password the site as well, instead of simple posting it and leaving the link on the forum for all to find.

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Post by Bruno Ogorelec » Mon Feb 23, 2004 5:51 pm

Chris, e-mail me your postal address. Frankly, judging by what George was saying, SNECMA doesn't give a horse prick about this stuff anymore.

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Post by Mark » Mon Feb 23, 2004 6:14 pm

Mike Everman wrote:Ok, so The Deadline is what drives you; so prepare yourself. I suspect that you will not want to show up to the Pulsejet Historical and Utilization Conference (PHUC) without a new engine to show!
He's bringing pencil and paper. I think Bruno should be forced to make something so we can chew on his leg for awhile.
Mark

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Post by Mark » Mon Feb 23, 2004 6:23 pm

To be fair, he's a good compiler but not overly zealous in the manufacturing department. The day he tries a jam jam, I might faint.
Mark

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Post by Bruno Ogorelec » Mon Feb 23, 2004 6:28 pm

Mark wrote:[I think Bruno should be forced to make something so we can chew on his leg for awhile.
OK. I guess I could recreate my 1975 design -- a side-ported Logan made out of water pipe, with propane fed at about atmospheric pressure, unlikely as it sounds. It breathed propane from a great big plastic refuse bag. It worked -- noisy as hell and belched big flames, scaring passers-by, but rocked so much that it quickly destroyed my fuel supply apparatus. Got some nasty burns, trying to put everything together and prevent a major conflagration. Great fun but local police didn't like it much. Maybe the Oxfordshire police are more broadminded.

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Post by Bruno Ogorelec » Mon Feb 23, 2004 6:34 pm

Mark wrote:To be fair, he's a good compiler but not overly zealous in the manufacturing department
Well, Hawking made himself no telescopes, just squinted his eyes and thought hard. Yet, he worked out an impressive number of secrets of the Universe. I can squint my eyes pretty well.

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Post by Mike Everman » Mon Feb 23, 2004 11:43 pm

Well, Hawking made himself no telescopes, just squinted his eyes and thought hard. Yet, he worked out an impressive number of secrets of the Universe. I can squint my eyes pretty well.
Oh, my, aren't we in good company! ;-)
I'd think twice about trying to get a plumbing pipe engine to the conference... Best take it all apart for shipping!
C'mon, Bruno, give the muffler shop guys a shot at it.
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Homer the hero

Post by Mark » Tue Feb 24, 2004 2:35 am

Mike Everman wrote:
Well, Hawking made himself no telescopes, just squinted his eyes and thought hard. Yet, he worked out an impressive number of secrets of the Universe. I can squint my eyes pretty well.
Oh, my, aren't we in good company! ;-)
I'd think twice about trying to get a plumbing pipe engine to the conference... Best take it all apart for shipping!
C'mon, Bruno, give the muffler shop guys a shot at it.
I'd bring my pulsejet in a briefcase/attache case, like some snipper rifle disassembled in foam-fitting compartments. One of the spaces would hold a pipe wrench for tightning.
Bruno may be the smartest of them all, I have to wonder where we are all headed on this long wandering voyage of The Odyssey. I guess the journey is what it is all about, to come back and recount what worlds you have seen/faced and survived without losing your sanity to the sirens.
Mark

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