Straight tailpipe length with new(?) valve arrangement...

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PulseDuuude
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Straight tailpipe length with new(?) valve arrangement...

Post by PulseDuuude » Thu Aug 19, 2004 1:30 am

...technical question.

I am looking to the Tiger-Jet Engine plans for the tailpipe dimensions in my first engine design (I already have tubing of the proper diameter).

I'm working on an 'inverted' valve layout that will (I hope) eliminate the need for a cone/larger diameter section to house the valving (When the solid model is competed it will be posted).

My question is: Do I use the original combustion tube assembly length of 18-7/8" or do I use the 14-7/32" length of the straight tube portion?

Thank you!
Jim

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mike Kirney » Thu Aug 19, 2004 1:43 am

It is the overall length of the duct behind the valves that determines your resonant frequency, so whatever that is, that's what I'd use. If your tailpipe area is the same as the original, then I would guess your effective valve area should be the same as the original too, even though there is no combustion chamber as such. The actual geometry of the duct has little effect on its ability to sustain combustion, as long the length-to-diameter ratio is within a reasonable range.
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Re: Straight tailpipe length with new(?) valve arrangement..

Post by PulseDuuude » Thu Aug 19, 2004 2:05 am

Mike Kirney wrote:It is the overall length of the duct behind the valves that determines your resonant frequency...If your tailpipe area is the same as the original, then I would guess your effective valve area should be the same as the original too...
wOOt! Thank you...that's what I was thinking, I wanted some input from the experts here.

I had intended to, and am going to keep the origional breathing requirements as well.
Jim

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mark » Thu Aug 19, 2004 1:37 pm

If you have a finger pump spray bottle and some methanol, you can do some interesting experiments with straight tubes. Simply spray a shot or puff of methanol in your length of tubing, say Dynajet or Tigerjet exhaust tube diameter, and seal one end with the palm of your hand and wave the open end of your tubing over an open flame. The tube will bark from the very fast combustion, it's so fast your hand won't even feel any heat.
You can really make some loud almost crack/bang sounds. You find that too long of a pipe and the sound becomes weaker and spongy. Too short and it is a sharper sound but weaker as well. When you find the loudest, most pronounced rapping sound, that might be close to the length you would want.
If I take the head off my Dynajet and seal the broad end with my hand after spraying/priming the body with methanol, it really makes a terrifically loud near explosion if I light the tail with an open flame, and again, your hand won't even feel a bit of heat, it's so fast. Some people I have tried to get to try this are such chickens, they think their hand is going to get burned but it won't. But you can just as easily use a rubber stopper for your smaller experimenting.
With copper tubing of 3/4 inch or 1 inch diameter cut to about 16 inches, you can make beautiful green flashes of light as the methanol gets traces of copper in it each time you fire off a shot as mentioned in the above experiments.
Methanol is your best best friend for this kind of experiment. Gasoline or propane and you would be hard pressed to get the mixture just right or produce anything more than a loodle loodle sound.
Mark

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by PulseDuuude » Thu Aug 19, 2004 9:24 pm

Thank you Mark,

That is a darned usefull bit of advice...I'm printing and saving your post.
Jim

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by evildrome » Sat Aug 21, 2004 12:11 pm

Hi Mike,

>The actual geometry of the duct has little effect on its ability to sustain combustion, as long the length-to-diameter ratio is within a reasonable range.

I'm going to build a valved PJ soon & having a waisted design where the exhaust is narrower than the CC would be much harder for me to build than one where the pipe is of uniform section.

I have to wonder though.... why does everyone build waisted design PJ's? Is there an efficiency penalty having it with uniform section and if so how big a penalty are we talking? 10%, 20%, 50% ?

Thanks,

Wilson.

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mark » Sat Aug 21, 2004 7:44 pm

I've made a very small pulsejet run as only a straight tube. It was weak but it ran, I had to start it by priming the duct, corking the tail end and sparking it to get a compression impulse perky enough to initiate and sustain the reaction.
With a long tube about ~2.5 inches in diameter, nicely flared at one end, chromed car exhaust pipe, I got some fierce rev up sounds if I used forced air blown into the reeded intake. Maybe something in flight would work well having forward air speed. It's said, forward air speed is in effect the same as having a larger reed area. The sound the straight duct made was definitely ear muff material, it was so loud. One time it shot out a super fast moving smoke ring when it backfired.
Wouldn't it be funny to find all the struggling we have gone through to make a good pulsejet, when it might just be possible to get away with a simple straight tube/duct? Some helicopter "pulsating ramjets" or valveless pulsejets have been made with very short length to diameter ratios. And it seems like they have made or tested very short L/D reeded pulsejets on the tips of helicopter blades as well.
Mark

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mike Kirney » Sat Aug 21, 2004 8:14 pm

Mark wrote:Wouldn't it be funny to find all the struggling we have gone through to make a good pulsejet, when it might just be possible to get away with a simple straight tube/duct?
I thought it had been done. I thought Schmidt's first successful experiments were with a simple straight tube. I was under the impression that pusejets had an enlarged front end mostly to accomodate the large area of the intake valves, and that with a geometrically efficient valve design, you could just use a straight tube. I remember several sources quoting an effective valve area to exhaust area ratio of about 0.4922 to 1, or in other words, your effective intake area should be about half your exhaust area. I'm sure there are quite a few posts regarding just this topic (straight tubes anyway). PulseDuuuuuuuuuuuuuude, try using the 'search' button at the top of the page !!!
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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mark » Sat Aug 21, 2004 9:43 pm

Schmidt did it, but his pulsejets were more difficult to start or less reliable in starting, not as structurally strong as the three piece pulsejet, and the head was very complex and trickier to make. Other than that it was a winner.
Mark

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mike Everman » Sat Aug 21, 2004 10:18 pm

It's interesting to note that Schmidt's straight tube was driven by a reciprocating detonation tube at the front. The valves were there, but the driver was in the middle and the tube did not rely on the return wave. They discovered the autoignition was giving them twice the frequency that they were pulsing the detonation tube, and also that the test apparatus would run a few pulses after the driver was turned off. This must have been a very exciting time for them!
Mike
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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mike Kirney » Sun Aug 22, 2004 1:03 am

Well, Schmidt's tube actually had a bit of shape to it:

http://www.pulse-jets.com/phpbb2/viewtopic.php?t=589
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=414

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Mark » Sun Aug 22, 2004 2:23 am

Some of Schmidt's work included experiments with straight ducts. Here is a straight duct with an intake of sorts, a valveless straight duct I suppose you could say.
It's not a Schmidt, it's out of my symposium on pulsating combustion book. I don't know what it is really, I must read up about it.
Mark
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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Bruno Ogorelec » Sun Aug 22, 2004 8:47 am

Guys, first of all, do not compare valved pulsejets with valveless ones. Remember that the valved pulsejets have variable geometry. Their port area during intake is much larger than during expansion. So, their behavior as Helmholtz resonators differs greatly from the behavior of valveless pulsejets, which have a constant port area.

Valve closure provides instant increase in mixture containment and a hammer stroke to boot, making for a very satisfactory pressure peak. You don’t get that with a valveless engine. Valveless engines need greater containment and thus have a much more pronounced chamber bulge than most valved engines do.

I think it just might be possible to build a straight-tube valveless engines, but very special tricks would be called for. First, it would have to be BIG. It would have to be very long. It would have to employ laminar flow (no turbulence). Even with everything perfect, it would certainly be a tricky engine that would refuse to self-ignite if you gave it a wrong look.

By the way, the Schmidt engine displayed sectioned in the Deutsche Museum in Munich is not aided by a detonation tube. Only the first experimental layouts built by Schmidt had a detonation driver and they were not aimed at development of a pulsejet. That came along as a surprise. It took Schmidt some time to realize that what he had in his hands was a viable pulsating combustion jet engine, rather than a failed PDE. The tube in Munich is a proper pulsejet, model 510 if I am not mistaken. A huge contraption. A grown man can fit inside that tube. The cylindrical object in the center of the conical valve array is the fuel injection device.

Here's a crude sketch of the 510.
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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Hank » Sun Aug 22, 2004 11:18 pm

[


I think it just might be possible to build a straight-tube valveless engines, but very special tricks would be called for. First, it would have to be BIG. It would have to be very long. It would have to employ laminar flow (no turbulence). Even with everything perfect, it would certainly be a tricky engine that would refuse to self-ignite if you gave it a wrong look.

Hello- The difference in volume between the combustion chamber and exaust duct in other than a straight duct engine is there for a reason. This reason is to use the pressure differential of the intake part of the cycle to readmit heat to the engine to sustain combustion. Please note that the engine breathes in through the exaust end as well as the intake end. This is proved by watching a pulsejet engine in operation. The last third or so of the exaust does not glow from heat as the combustion chamber does.It is cooled by ambient air.
There is a near constant flow in from the exaust end that interfaces with the thrust portion of the cycle. There can be no laminar flow due to this interface. While a straight tube engine can be built its weak operating parameters make it a lab curiosity rather than a prime mover.
Hank

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Re: Straight tailpipe length with new(?) valve arrangement..

Post by Bruno Ogorelec » Sun Aug 22, 2004 11:37 pm

Hank wrote:There is a near constant flow in from the exaust end that interfaces with the thrust portion of the cycle. There can be no laminar flow due to this interface. While a straight tube engine can be built its weak operating parameters make it a lab curiosity rather than a prime mover.
I am not so sure. Of course, there can be no true laminar flow, but you can get quasi-laminar flow from the intake for an appreciable part of the cycle with very careful design (e.g. high intake speed, smooth duct, no diffusion etc.). Schmidt did it with his large bore pipes. He did not even have to use great length to provide a mesure of containment.

Opinions of his engines are mixed. The Argus people hated them, but they had a vested interest, pushing their own designs. Reynst was hugely enthusiastic about them, despite pursuing designs of his own. I have not seen an analysis with copious data. Reynst provides some, but not much.

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