Stainless FWE with small intake runs at -3 F !!!!!!!!!!!!!!!

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Graham C. Williams » Thu Jan 27, 2005 10:53 am

"What average tempature is a propane fueled pulsejet generating in the exhaust? (a basic estimate)"

Very Low, It's mostly Cold air mixed with a little of the hot stuff. Without knowing what part of the cycle I can say the temperature ranges from 300K to about 1000K or above, Belfast suggests the upper limit is 1600K. The average is on the low side. The motor design changes things a lot. I don't have the figures to hand but I think one source suggests that for the Escopette 70% of the mass of air ingested over a cycle is cold air down the exhaust pipe. The motor I'm currently looking at spends a little over half its cycle sucking cold air in the tailpipe and a little over half of the remaining part of the cycle pushing that same cold air back out again.

What harmonic is it generating or is this also controversial?

Personally, I'd only use the idea of Harmonics as a general descriptive term. It's all down to wave action and interacting variables. If you must press me for an answer - Most Pulsejets are strongest as half wave resonators, but not all.

And what is average expected frequency of operation at a given length?

What motor and what loading do you want to use? Put the numbers into UFLOW and subtract 10 to 15% from the answer.


In starting a pulsejet do you ever get a change in the harmonic from say 1st to 2nd?

Do you mean from Half wave to Full wave for example? I hope so but this is an incorrect way of looking at things. It's all about waves again; transition to steady running is the interesting phase when the energy associated with the waves changes. It has been suggested that over the time of ignition the combustion chamber acts as a White noise generator, Pink or Brown may be better. At the end of the day it must be about adding Thermal and Mechanical energy and it has to be done in a nonsteady way. Principally, the location of combustion and the total combustion time as a percentage of cycle time change the amount of reinforcement and damping of the waves in the motor. The work of Putman and Dennis is worth looking at in this respect.

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Mike Everman » Thu Jan 27, 2005 2:43 pm

a one meter long valveless will be a bit over 200 Hz, inversely proportionate to length.
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby mk » Thu Jan 27, 2005 6:10 pm

To you Eric and Adam: Nice to see you calmed down. Exact clarification is a must. Indeed.

All of the following IMHO:

Ogge wrote:What harmonic is it generating or is this also controversial?

Basically the pulse combustion device should produce the whole spectrum of sound, with peaks appearing at the harmonics. The opperating frequency should represent the largest peak.

Changing the operating frequency to another harmonic overtone (applying standing wave at operation) might be a way improving wave-interaction of the small single travelling waves through supporting the applying standing wave, for optimizing the whole engine towards (thrust) efficiency.
That's briefly what I understood under "lock-up".

In general:
Standing waves are just a result of a (n-times) reflected travelling wave. So basically I'd go with Adam, that the single small travelling waves are the things to be optimized -- brought into phase if you will. This is much easier when forming relatively long waves (a lot of cycles), e.g. through x-reflections. The applying standing wave should then be the aim for the resulting sum of the small single waves, always with regard to the conditions.

Ogge wrote:And what is average expected the frequency of operation at a given length?

Regarding to straight engines with opposed openings (Lockwood style)?

Ogge wrote:In starting a pulsejet do you ever get a change in the harmonic from say 1st to 2nd?

I haven't ever noticed something like that. Just a smooth and slight increase in frequency, but no kind of step-up. I can't say "yes" or "no" 100 percent.
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Mike Everman » Thu Jan 27, 2005 6:49 pm

some of the french engines were designed to be what you would call "second mode", where you can imagine the sum of harmonics conspiring to make a node at near the thermo-acoustic mid-length, though the operating cycle is still on the fundamental. it amounts to having two induction cycles at each end for every pop; a simplified way of looking at it.
the Lockwood types have nodes at each end and a standing wave with the largest excursions in the CC at about .25L from the inlet.

in a perfect world of unchanging density and temperature, you can plot the sum of the first few modes like the attached diagram, to see the hump. certain of the modes damped because their pressure anti-nodes would prefer to be somewhere other than where the pop happens. for instance, the fundamental wouldn't be damped at all if the pop was at the mid-length. the second mode ends up with the greatest amplitude, as it's "Pantinode" is at .25L, about where the pop is.
Attachments
modesum 1-4.JPG
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Mike Everman » Thu Jan 27, 2005 6:51 pm

In this one, I arbitrarily damp the fundamental by half because the pop is 90 deg from it's pantinode.
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby mk » Thu Jan 27, 2005 7:12 pm

Cool.
Thanks for the graphs, Mike (E.)!

It's something like that I had in mind, too. Different, but it illustrates everything much better, at least when considering (already standing) waves being in phase already.
But I'd say the chemical (-- through reaction process created ones if I understood Adams statements right --) traveling waves must reach a special number of cycles and reflections, refractions etc. for making a phase gap trifling.

Something like a post-1st or pre-2nd harmonic running, eh? I smell fractals somehow...hehehe...just kidding.[/quote]
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Ogge » Thu Jan 27, 2005 7:41 pm

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Bruno Ogorelec » Thu Jan 27, 2005 8:09 pm

I am finding it difficult to understand what waves some of you are talking about. Until I do, I'll stick to Mike Everman's picture, which explains a lot of things in a pulsejet to a careful observer.

This is standard pulsejet fare, described in fair detail in NACA Document 1131 and tested on myriad examples in the industry and reasearch labs over the next 30 years or so by some very smart people with a lot of money and other resources behind them. For decades, pulsejet researchers have not found reason to stray from the standing wave orthodoxy.

Frankly, I doubt that any one among you has suddenly stumbled on a bright new way to tame the waves inside the tube and make them jump through hoops in a novel way. But, I'll be tickled pink if I'm proven wrong.

Among the best illustrations of pulsejet behavior one can find are the 'method of characteristics' pressure charts one can find in dozens of pulsejet research papers. Enthusiasts often skip those, for they look really incomprehensible, but believe me, they offer wonderful insights into the rumbles inside the pulsejet gut. Once you get used to the dense network of intersecting lines, you can trace the details of events along a single cycle or several repeating cycles and see how multiple reflections really work. It teaches you a lot.

For anyone wishing to delve into that, Franco Marcenaro's pulsejet theory paper is a wonderful introduction. He simplifies things perhaps too much, but initially it may help you get to grips with those charts.

Graham, do you know of a better primer than Marcenaro's?
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Bruno Ogorelec » Thu Jan 27, 2005 8:33 pm

Ogge wrote:IMHO a standing wave is just bleeding energy out of the working cycle. The standing waves are not moving (ie standing), they are pulsating pressure transveresly, inducing vibrations in the walls of the tubes. The walls of the outer tube thus induce traveling waves into the air in the form of sound to the outside observer.


Adam, we are not getting to you, are we? You are having opinions based on what?

You do not seem to understand the nature of resonance at all. It is a complete opposite of what you seem to think it is.

No it is not a waste of energy. It is conservation of energy. The standing wave is a way for the energy of pulsation to be retained at least partly inside the engine, so that it can be used for aspiration.

What the hell is it that gets reflected back into the engine from the end of the tailpipe but energy? What the hell are waves but energy, pure and simple? It is mechanical energy stored in the gas. We use it to drive teh pulsejet cycle.

The fact that PDEs are not really pulsating, but are single-shot machines has clouded your thinking. Pulsejets pulsate. Their cycle is recurring, a self-excited relaxational oscillation.

Pulsejets thus provide their own automatic aspiration. Without a standing wave, this is extremely difficult to achieve. Without it, you have to have ancillary equipment to purge your engine of combustion products and to refill it with fresh charge. You seem to be thinking purely of combustion, but combustion is only half the cycle. What about the other half?

No, the standing wave is not a by-product. It is the part and parcel of pulsating combustion. The only non-acoustic pulsating combustion device I know of is the Reynst combustor -- the original pot burner he patented in the 1920s. Add a resonator to it and its operating frequency goes up by an order of magnitude, while at the same time increasing in amplitude. Figure out how much the energy utilization improves. So, where do you see the loss? I can only see amazing gain.
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Ogge » Thu Jan 27, 2005 9:43 pm

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Graham C. Williams » Thu Jan 27, 2005 11:22 pm

Forget all this business about Standing waves. As an initial look it can be helpful, if you want a full understanding It's a waist of time. You only need consider Compression waves and Expansion waves and the wave that is the massflow. It is the interaction of these Compression and Expansion waves with themselves and the boundary conditions that produces the pressure changes we see in the pipe. Local temperature and gas velocity change the speed of sound for these waves. Sometimes you will get a discontinuity in the wave structure, its called a Shock but most of the time you can forget them. The massflow of gas in any part of the tube is in main the result of the passage of Compression and Expansion waves.
To achieve cyclic operation you need a special interaction between the waves and heat addition, The Rayliegh Criteria.
During steady running I think it correct to think of a wave structure in the pipe but never forget it is the interaction between travelling waves and the reflections.
Numerical solutions to the Euler equations and there extensions provide solutions to the interaction of these waves. Other methods have to be used to include Combustion.

I suggest you read Kentfield 'One dimensional nonsteady compressible flow' I think it's called. It'll give you most of what you need to know.

Any book dealing with nonsteady gas dynamics is worth reading. I’ve given a full listing in previous postings.

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Eric » Fri Jan 28, 2005 12:13 am

Im sorry Bruno, but you are obviously wrong. Standing waves arent really waves at all, they are really just a distortion of the space time continum when the force of gravity between interacting chemicals reaches the schwartschild radius of said chemical reactions. The resulting black holes formed from the chemical reaction cause compression. Now the frequency proceeding said chemical reactions is not limited to, nor confined by the laws of physics or any other laws for that mater, but strictly by the free will of the singularities themselves.

Once resonance is achieved the sub atomic quark gluon mixture present in the combusting gases will set up resonance in the pipe itself, which obvioulsy you must know are also not waves either, but energy being transfered like a superconducting magnetic gyroscope inside a small faraday cage. Once this happens you can very easily derive an equation to govern the laws of the point particle universes inside the combustion chamber resulting from the expansion of the black holes. Well perhaps I should stop asking questions here, because obviously I have such a profound knowledge of things taught in basic highschool physics that I should just sit around and correct people on their inept knowledge of physics and their horrendous blunders like confusing standing waves for an actual wave form. Hahahaha, to think someone would do that, although I myself probably couldnt tell the difference between a transverse and a longitudinal wave if one knocked me on my ass.

And bruno, dont forget about the equations coeficient of bullshit, naturally the higher the coeficient of bullshit the more pure it will be and the better it will integrate into the equation. Sometimes you have to take the derivitive of bullshit^(toomuch) power to realize it is all just too much bullshit before you try to integrate it.



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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Ogge » Fri Jan 28, 2005 1:21 am

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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Mark » Fri Jan 28, 2005 3:08 am

Mankind: Responding to one kind of ignorance with even more of another kind of ignorance ever since there were dumb people[/quote]

Well if you think of the tree of knowledge, I don't see how an apple could impart more intelligence than an orange let's say, but if you play along, we were given knowledge, but as Mark Twain says not much in the way of useful knowledge. A patron came in the library today and said he was of the mind set that a little knowledge is worse than no knowledge. I tend to agree that we exalt ourselves far too much, given a million years from now what will we become? What will be the goal? Would you want to live for enternity, that's a very long time I think. And pulsejets will be completely understood and fall by the wayside. Imagine how far we have come in the last 2000 years, imagine 50 trillion years from now, imagine forever.
For those who found interest in the strange occurance of the tsunami, here's another kind of tsunami to think about in the grand scheme of things. What's 50 million people in the limitlessness of time.
http://www.forbes.com/forbes/2005/0131/048_print.html
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Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Postby Ogge » Fri Jan 28, 2005 7:13 am

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