Focused wave engine runs!

Moderator: Mike Everman

Re: Focused wave engine runs!

Postby larry cottrill » Thu Sep 23, 2004 12:12 pm

hinote wrote:My opinion is that you're running this engine well in excess of its max rated thrust capability. Please consider running the engine at lower power levels--especially until you can verify thrust numbers, and thrust related to fuel consumption.

I'm betting that you'll find the max thrust can be obtained at considerably reduced fueling rates. Remember, the max thrust isn't related to how much fuel you can feed the engine. You may be exceeding the max fuel/max thrust value by a considerable amount.

Practical considerations (extended material life) will usually dictate an operating limit at something like 75-80% of max thrust--so you're 'way beyond that value.

Steve -

You need to listen to what Bill is saying here - it is the voice of experience.

I don't know whay I didn't notice it myself - I've seen it enough times! The large, blossoming flame means you're running rich, basically feeding fuel at a rate that the pumping of the motor can't match in terms of air draw. That's OK for starting, but then you need to lean out the mixture to the point where the flame ejection is practically straight, not blossoming out in all directions. Lean running is a lot quieter, and looks much less impressive [the flame seems thin and 'wimpy', not at all bright], but it is the right way to run. The standard Dynajet, for example, is not ported to yield maximum thrust, but rather, a compromise between thrust and reasonable fuel utilization.

I would guess that what you're running here is about 10 or 15 pounds of air for every pound of fuel consumed -- an engineer will try to design a low-pressure combustor for 30 lb/lb, minimum !!! I think some of the problems we amateurs have is in always asking the wrong question: What's the maximum thrust? Instead, we should look for good lean running at a reasonable thrust specific fuel consumption [FSFC]. That's real success - but it usually won't impress your friends. This is why measuring thrust AND fuel consumption becomes important at some point.

Something I think you should do right away is a tail-end flashlight inspection. See if, after the kind of running you've been doing, there is any observable erosion or 'flame polishing' of the inside end of the intake pipe. You don't want to sustain much damage there, because that end point is so critical to the timing of your engine.

In messing with the Elektra II, I observed an interesting phenomenon: When I leaned out the mixture, I reached a point where there was an audible 'shifting gears' and the intake flame suddenly disappeared back into the pipe as the sound of the engine suddenly quieted and smoothed. I would be interested in knowing whether you find a similar point of operation in the focused wave design. Of course, in my case, it wasn't sustaining - I was still pushing air in, though not much.

Keep firing! Don't forget my request for a quote on a couple of cones and domes ...

While you're at it, figure out what you would have to charge to do the rolling and tack welding on cones and domes in lots of, say, 20 units, if I supply the laser-cut flat pieces; in other words, rolling and tacking then shipping to me is all you'd be doing. Also, would the rolling be successful if the oval hole were already cut in the pieces? [You might not have the experience to answer that -- if not, try it on one of the ones you make from scratch for me, and see what happens.] I could get you cut pieces in mild steel, and probably in high-strength steel and stainless as well; of course, those will be harder to roll, but I should be able to get them in thinner stock. If you are not of legal age to sign contracts, your dad or mom would need to get involved. IMPORTANT NOTE: Don't even think about this if there's any chance it would impact your education or other work! The lots would be small, but I would want quick turnaround. I expect a price that will make you some money, not a giveaway!

All the best,

L Cottrill
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Re: Focused wave engine runs!

Postby yipster » Thu Sep 23, 2004 12:38 pm

Congrats on that first time running Focused Wave Engine guy's! and i'm following the rest with great interest
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More focused wave builders needed!

Postby larry cottrill » Thu Sep 23, 2004 1:10 pm

It may seem obvious, but I would like to encourage anyone else who might be interested in building the Focused Wave Engine to do so. It will not be a difficult project for anyone that is tooled up to roll the cone and dome of the front end. All I ask is that you report your findings on this forum when you get your engine running. For propane fueling, the pinch in the intake pipe shown on my original drawing is not needed.

Note that ALL the information you need to build it successfully has been presented in my 'Short Lady' threads. There was absolutely NO private communication between Steve and myself during his building of this design - there are no secrets, everything is in these threads of the forum. Note that Steve did post a correction to the dimensions of the intake, due to my error in sizing the ID of the tubing used. So, make sure you study ALL the posts carefully to get the full picture of how to build. You DO need to follow the final dimensions carefully to be successful.

Independent construction and testing would ensure that we have full validation of the concept, and will give you a low cost, lightweight and compact engine that I guarantee you will find exciting to build and run!

I plan on building one myself just as soon as I can ... ;-)

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Re: More focused wave builders needed!

Postby steve » Thu Sep 23, 2004 1:16 pm

Larry Cottrill wrote:
I plan on building one myself just as soon as I can ... ;-)

L Cottrill


Or rather, as soon as I finish rolling your cones!
(I'm working on it! be patient ;-)
Image
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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Thu Sep 23, 2004 6:35 pm

Before I try to pretend I know something and understand things, let me congratulate Larry on the design and Steve on great work in building the Short Lady.

Exceedingly well done, guys. As Bill has pointed out, quick success in the birth of a new pulsejet design is a rare occasion indeed. This one will go down in the books.

But, I just can't refrain from saying 'I told you so!' To quote myself: "I love it. Larry. I have good reason to believe you are onto a good thing with this. I am not at all surprised that Uflow shows it to be good, too."

Now I have to go knit my brow and think of something profound to say about the functioning of this beautiful little engine. Can't have Bill take all the glory of constructive criticism... :o)

First hint to Larry -- try to work out where the focus of that mirror is located.
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Re: More focused wave builders needed!

Postby hinote » Thu Sep 23, 2004 8:34 pm

Larry Cottrill wrote:It may seem obvious, but I would like to encourage anyone else who might be interested in building the Focused Wave Engine to do so.


It sure would be fun to try a larger version of this--say, with a 6-inch combustion chamber.

Also, I would experiment with adding a reverse meg in the tailpipe assembly.

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Re: Focused wave engine runs!

Postby Hank » Fri Sep 24, 2004 2:56 am

[quote="steve"]I have been thinking about it for a while and I can't figure out why the tailpipe dosn't get hot enough to glow.
As with any tuned pulsed combustion duct there is a complex wave pattern induced by the alternating high/low pressure of operation. The engine is breathing through the exaust end as well as the intake. There is a layer of ambient air preventing the exaust section of the duct from getting enough exposure to heat to glow.
You may want to get a measurement of the length your intake is spitting out flame and increase the length of the intake that amount.
Neat engine, any idea how much thrust you are producing with it?
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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Fri Sep 24, 2004 8:55 am

steve wrote:I have been thinking about it for a while and I can't figure out why the tailpipe dosn't get hot enough to glow. I't makes no sense to me whatsoever! Obviously it is a good thing that the hot zone is so localized, but even that has something very odd about it- you probably couldn't tell from the pictures but the hottest area on the entire engine was directly in the path of the intake air. This also defys conventional pulsejet wisdom because anyone who has ever seen a picture of a running chinese knows that this is one of the coldest areas on the engine. (WHAT is going on here!!!!????!!)
The cold tailpipe really bothers me, especially since I clearly saw flames shooting out of it. Unless fresh air somehow flows all the way back to the combustion chamber through the tailpipe (as in the intake) I don't see how It could stay so cool. It defys logic! (AAAAHHHHHH!!!) That is another strange thing- why are there flames comming out the tailpipe? I have never seen this happen on any other VALVELESS engine EVER! (the lockwood dosn't do it, the chinese dosn't do it, the escopette dosn't do it, the list goes on and on) (Scratch that- I have seen a little bit of flame comming out the elektra1 tailpipe). Bruno (or anyone) could you attempt an explanation? am I going insane?


Steve, the last statement is probably true. No one in his right mind wastes his youth on pulsejets instead of girls. But, this is probably not what you were asking, was it? You are more interested in the technical issues concerning the Short Lady.

The Short answer (pun intended) to your questions is, ‘I don’t know’. Now, let’s see exactly how little I know.

The location of the hottest part of the engine is truly curious. I have two possible explanations. One is that this is where the focus of your concave mirror on the front end of the engine is located. Ideally, that surface should be parabolic, so that it reflects all the energy that hits it in a straight line towards the exhaust. Further shaping of the energy flow should be done by the nozzle shape of the entire chamber. Your mirror is obviously not parabolic but what I call ‘potatoid’, meaning that it probably concentrates the reflected energy in some relatively narrow area. I am familiar with some experiments in this direction and I know that focusing does work to a certain extent.

In the Short Lady, the effect may be exacerbated by the fact that the inside tip of the intake stack is the place of the strongest turbulence in the entire engine. We know that turbulence can promote heat release. It that is what happens, you may have two effects working together towards concentrating the heat in a narrow spot.

The coolness of the tailpipe is perhaps even more vexing. However, the fact that my real understanding is limited liberates my imagination wonderfully and I can conceive a number of reasons, one better than the other.

First, the intake, which Larry presumed would mostly serve an acoustical function during the exhaust phase, is obviously pushing out a strong gas flow, too. This in itself diminishes the amount of gas (and heat) that would otherwise have gone into the tailpipe. Obviously, it takes the tailpipe rather longer to heat up thoroughly than if it received its ‘full portion’.

Next, if the mixture is too rich, the engine is running cooler to start with. Ignition is probably easier. That means that the ignition may start at the high-turbulence part of the engine (chamber bottom and end of intake area) too early -- while suction is still going on at the junction between the chamber and the tailpipe. This would imply that the first part of the process of expansion will be pushing cooler gases through the tailpipe, followed by the really hot gas only later.

Finally, as a reminder that pulsejets are complex beasts, let me point out that at the tailpipe, the flow is not unidirectional. Careful observation of the events has shown that – rather curiously – suction into the pipe begins before the expansion is over. While hot gas is still getting out from the tailpipe in the central, core section of the pipe, cool air is already entering along the walls. Of course, I would hesitate before proposing that the effect can stretch a long way inside the engine – long enough to affect most of the tailpipe – but it must have some effect on the flow pattern and influence the heat distribution.

The Short lady is obviously a very interesting engine that might handsomely repay further tinkering. Again, my congratulations to Larry for the concept and to you for the building and testing of the prototype. Very well done!
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Re: Focused wave engine runs!

Postby larry cottrill » Fri Sep 24, 2004 2:11 pm

Bruno Ogorelec wrote:The Short answer (pun intended) to your questions is, ‘I don’t know’. Now, let’s see exactly how little I know.

The location of the hottest part of the engine is truly curious. I have two possible explanations. One is that this is where the focus of your concave mirror on the front end of the engine is located. Ideally, that surface should be parabolic, so that it reflects all the energy that hits it in a straight line towards the exhaust. Further shaping of the energy flow should be done by the nozzle shape of the entire chamber. Your mirror is obviously not parabolic but what I call ‘potatoid’, meaning that it probably concentrates the reflected energy in some relatively narrow area. I am familiar with some experiments in this direction and I know that focusing does work to a certain extent.

Bruno, Steve et al -

I agree with the logic of everything else you've said, Bruno, but when you talk about parabolic reflectors [actually, paraboloidal, technically speaking], you're talking about old friends of mine, and I really need to speak up. Yes, I'm sure that focusing reflectors can do something in pulse combustors. However, I am 99% sure that they don't do what you seem to be expecting. Bear with me, here -- this will take a while.

A paraboloid, used the way you describe it, is performing the work of a big searchlight mirror. The mirror has a precise 'focal point'; that is, the point at which the image of some 'infinitely' distant object, like a star, would be formed if it were used as a telescope mirror. Here, the situation is exactly reversed; waves are propagated at a nearly infinitesimal point very close in, radiating out in a spherical pattern. Some of these waves are intercepted by the mirror's surface, and reflected back. The "magic" of the paraboloid is that, IF the wave source is at the exact focal point, the reflected waves are perfectly re-shaped to absolutely flat wave fronts that proceed forward in a perfectly straight path whose direction is at a right angle to the fronts, just like in those classic textbook photos of straight waves moving in a wave tank. Think of all those old war movies where searchlights are used: those things were so perfectly adjusted that the 'beam' [actually a slug of moving wave fronts] maintained a five-foot width from the light housing practically to infinity. Only a paraboloid is capable of doing this.

However, a paraboloid is NOT the correct shape for other geometries of wave flow. Suppose, for example, that we want to propagate waves from a point in space back to a point right beside it. We could take our paraboloid and move it gradually farther and farther from the point of light propagation, and what we would see [if the air has a little smoke or dust in it, as it almost always does] is that the projected 'beam' of light starts to narrow in as a long cone. As we move our mirror farther and farther back, the cone of reflected light becomes shorter and shorter. Finally, when we reach a point where the light source is precisely 2x the focal length from the central spot of the mirror's surface, the point of this reflected cone will be at [or right beside, if we just tweak the aim of the mirror a bit] the point of propagation. However, now we have a problem.

The paraboloid turns out to be the wrong curve, in this case. If our point of propagation is as small as we can make it [say, a tiny pinhole in an opaque housing with a bright light inside it], we may be surprised to find that the reflected image is not very good. If we would analyze it further, say, by masking off parts of the mirror, we'd find that the reflected cones of light from the outer edges of the mirror 'focus' to a point a little beyond the pinhole, while reflected cones from the central zone focus a bit short. On a big short-focus reflector like a 60-inch searchlight mirror, the deviation from edge to central reflection would not be small -- probably an inch or two!

While we're pondering this, we might notice something by further experimentation: if we mask off more and more of the surface by working in from the outside edge of our mirror, the remaining area in the center produces better and better images of our pinhole source; in fact, we can narrow it down to where we don't see any distortion of the image at all; that is, the whole central zone that's left seems to bring all the light back to a perfect point image. At this point, we could try to make some physical measurements of the surface of the central zone that's left. It might astonish us to find that, if we take a small enough section of the center of the paraboloid, we can't tell that it isn't simply a segment of a sphere! Of course, we know mathematically that it can't be spherical, but if the piece we're studying is a small enough central region, you can't measure the difference with ordinary tools.

Suddenly, it hits you: if you want to focus a perfect image at the same location as the wave source, the perfect shape would be a sphere! This stands up under rigorous scrutiny: A wave front radiates out from a point source as a spherical front. If a portion of that wave front is reflected by a spherical surface whose center of curvature coincides with the point source, ALL of that front will be reflected by the spherical surface at precisely the same moment in time, and further propagation of the wave will be along paths of convergence toward that point! A paraboloid cannot to that -- its outer zones are too far away, and will reflect the incoming spherical wave front too late for them to converge at the same distance!

It is this phenomenon that was quantified theoretically and validated empirically by the great French physicist Lean Foucault, and has come down to amateur telescope makers everywhere as the 'Foucault Test'. In skilled amateur hands, it can measure differences between a sphere and the nearest matching paraboloid of four millionths of an inch or better!

Now, suppose we have a different problem yet: we'd like to use our 5-foot 150lb paraboloid as a photo enlarger. Let's say we want to enlarge an image 2x its original size. So, we start moving our pinhole around to see what we can do. Intuitively, we realize that to do this, we need to move our point source back in closer to the mirror -- not as close as when it was a searchlight, but closer than when the light was reflected back to a point in space right beside the source. Let's say we now drill out our pinhole to a 'porthole' 1 mm in diameter. We can move it in until the reflected image is right at 2mm diameter -- a 2:1 enlargement. What we will find is that the distance from the source to the center point of the paraboloid is precisely half the distance from that central point out to the reflected image. Except for one little thing: Once again, the image isn't perfect, with the outer zones of the mirror throwing their light cones further than the central zone. If we again start to 'mask down' our mirror from the edge inward, we again finally get to a place where the image seems perfect; we notice, however, that the uncovered central zone is now a lot larger than it was for a perfect image at a 1:1 ratio.

If we now make careful physical measurements of the exposed central mirror surface, we are surprised to find that we cannot fit it closely to any sphere -- the deviation is just too large. We are seeing the effect of the parabolic cross-section; yet, the paraboloid can't be quite right, because we did have to trim off the outer zone ... hmmm ...

Again, intuition finally kicks in, and we measure the distance between the source, image, and central zone of the mirror. The maths of analytic geometry will verify that what we have is a classic ellipsoid: its section is an ellipse with one focus at the point source and the other at the image point. And, there is some portion of the center of a parabola that we cannot sensibly distinguish from this elliptical curve -- it happens to be a much larger section than the part that seems to be a sphere.

It is a truism that the laws of wave mechanics hold for acoustics exactly as they do in optics, as long as appropriate media are available. So, it seems natural to use reflection and other 'optical' properties in dealing with acoustic phenomena. The only difference is in the effect of the HUGE differences in frequency and wavelength. However, in practice for our present case, this turns out to be of monumental importance.

There is a single basic reason that we are able to easily measure the differences in the ideal curves in our searchlight mirror example using light: the diameter of the reflector, no matter how tightly we mask it down, is MILLIONS of wavelengths. When we deal with our pulsejets, no matter what the specific geometry, we are dealing with FRACTIONS of a wavelength. This is not a mere technical detail; it changes everything, radically.

You may remember Mike E, Bill, Bruno and others mentioning the 'Rayleigh Criterion', which has to do with exactly what condition has to exist for explosive combustion to occur. What you probably don't know is that the same Lord Rayleigh provided another Rayleigh Criterion, for us amateur telescope makers to use. It states that, for a perfect image to be formed, a reflective surface must have a curve that falls within one quarter wavelength of the theoretically perfect curve at all points. This is why it's so important to be able to measure millionths of an inch on a telescope mirror's surface. What it means in our case is that defects in a reflector of less than one quarter wavelength magnitude have no significant bearing on the accurate focusing of wave energy!

If this is true, how can Bruno's statement, "I am familiar with some experiments in this direction and I know that focusing does work to a certain extent," be correct? The answer is, you have to understand the "to a certain extent" part. The difficulty is that optical problems can be attacked by 'ray tracing', where you think of the direction of propagation as straight-line 'rays' whose change in direction can be deduced crudely by graphic methods or with unbelievable precision by mathematical methods. When people talk about 'reflectors' in engines, they imagine these kinds of 'ray tracing' solutions. Where the explosion happens is a 'source' -- find where you want the energy to converge, and this is the 'image'; draw a curved reflector that makes the lines radiating from one point all converge on the other, and presto! But, it just can't work exactly that way when the clear apertures [our pipe diameters] are so small relative to wavelength!

When you are meters away from a small explosion, it is perfectly acceptable to think of it happening 'at a point' -- but not when you are only millimeters from it. Does conflagration in a confined space start at a neatly contained 'point' and radiate smoothly outward? Not a chance. At a sub-wavelength level, it is more appropriate to imagine a major region where the pressure and temperature suddenly rise dramatically throughout. It is not any more a wave front expanding spherically than it is a quasi-rigid piston moving in a single direction -- you simply can't describe it properly in any 'macroscopic' terms! The same thing applies to wave motion in the pipe. A mental picture of ocean waves moving toward a beach is totally inadequate; it is much more like the action of peristalsis in the esophagus or colon! But even that is incorrect, since mass motion, rather than wave propagation, is suggested. I'm 57 years old and have been interested in Physics most of my life; and the only physical phenomenon I've ever seen [meaning seen, as with my eyes] that is anything like it was compression waves sent from one point to another through a long Slinky(TM).

I am sure, with Bruno, that reflective focusing works and that, of course, is partly what I'm relying on at the front end of the Focused Wave Engine. But I am not at all sure that we can EVER accurately describe how it happens by simple acoustic theory. I think the only accurate theoretical description of reflection inside pulse combustors would be by an expert with highly detailed knowledge of fluid wave mechanics on a microscopic level, where you're dealing with the subtleties of pressure distribution, molecular interaction, transfer of molecular momentum between molecules and the pipe wall, etc. I really believe it is inherently impossible to accurately describe it in macroscopic terms.

For the rest of us, all we can do is experiment, observe and document what we see.

Of course, I could just be ... wrong.

My theory of how the Focused Wave Engine chamber should work is that the pressure wave should be concentrated on the region just a little behind the front end of the straight pipe, pressurizing that region uniformly to start the traverse of the wave down the pipe. I feel that most of that will be accomplished by the unbroken conical shape and very gradual slope of the long conical chamber. The front reflector is mostly a pressure cell for the buildup of the initial front formed at the time of 'constant volume expansion'. So what we should visualize, I think, is basically a rapid pressure rise in the front of the chamber followed by a simple 'funneling' of the pressure wave into the pipe, over a finite time interval. That was the idea.

I personally doubt that the shape of the front dome has much to do with the action, and I doubt that subtle changes in it will profoundly affect what happens in there. This is why I didn't press Steve on exactly how to make that part. It's entirely possible that Bruno's description of what's happening in there is basically valid, but at this point I can't buy that the exact shape of the reflector is really responsible for it.

But, somebody out there might convincingly show that I could be wrong about that, too. It has been known to happen ...

L Cottrill
Last edited by larry cottrill on Fri Sep 24, 2004 2:54 pm, edited 14 times in total.
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Re: Focused wave engine runs!

Postby Mike Everman » Fri Sep 24, 2004 2:19 pm

I'll pretend to know and add that there is no expansion going on in the exhaust phase, a primary method of removing heat from the gas, and into the duct walls, so it would be comparitively cooler.
On the intake phase of a LH/Snecma type, the tail fills with cool air on the order of 10-25%. Lady has quite less length for the CC volume, and the tail quite less volume itself for not being a megaphone. I suspect it is substantially filling with cool air, perhaps all the way to the CC.
This and the chinese are so fascinating in their ability to be short, mainly. Acoustically they are fascinating intheir ability to have two openings but act as a closed end tube.
I'm going Chineeezy soon!
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Re: Focused wave engine runs!

Postby hinote » Fri Sep 24, 2004 2:59 pm

Mike Everman wrote:. Acoustically they are fascinating intheir ability to have two openings but act as a closed end tube.


I think it's important to get a handle on the operating frequency of this engine--as soon as possible. It will tell us a lot of things about what's actually going on.

Steve, can you get a tentative frequency number for us to ponder? There are several simple ways to do this--I won't go into any details now, but let us know if you have any problems with this request.

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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Fri Sep 24, 2004 3:05 pm

Larry,

Can't argue with what you've said except in practical detail.

I am talking of focusing light, rather than sound. I should have made clear that I did not expect the sound (pressure wave) to be focused -- I was thinking of heat, that is, infrared radiation. Here, the wavelengths are still small enough.

OK, I know that in a pulsejet, the source of radiation is not a point. However, the Short Lady does display a remarkable concentration of heat. I suspected that the dome is closer to spherical than to paraboloid (though it is almost certainly neither, but some kind of potatoid instead) and that it would tend to concentrate the reflected radiation rather than making it into a parallel beam.

The cummulative result of all those partially satisfied conditions might well be the amount of concentration of heat in a fairly narrow region of the chamber -- very far from perfect but greater in its degree than in almost any other pulsejet.

Remember that pulsating combustion is greatly dependent on initial conditions. This new factor in the equation -- a wholly peculiar distribution of heat -- might well be enough to trigger a pattern of pulsation rather different from what you'd normally expect.

An additional factor here is time. Infrared radiation travels instantaneously over such a small distance. That means that the very first emanation of heat from the area near the focus of the mirror -- and I am almost certain that the combustion is most likely to start in the vicinity of the peak of the intake stack -- generates instantaneous feedback. It is a much quicker mechanism than even a shockwave, not to mention the sluggish pressure waves generated by deflagration in the pulsejet chamber.

So, it seems possible that you and I are both right, but for different reasons and with different implications for the Short Lady.
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Re: Focused wave engine runs!

Postby larry cottrill » Fri Sep 24, 2004 3:19 pm

Bruno Ogorelec wrote:I am talking of focusing light, rather than sound. I should have made clear that I did not expect the sound (pressure wave) to be focused -- I was thinking of heat, that is, infrared radiation. Here, the wavelengths are still small enough.

OK, I know that in a pulsejet, the source of radiation is not a point. However, the Short Lady does display a remarkable concentration of heat. I suspected that the dome is closer to spherical than to paraboloid (though it is almost certainly neither, but some kind of potatoid instead) and that it would tend to concentrate the reflected radiation rather than making it into a parallel beam.

Bruno -

That's a lot of speculation.

It is also ... wonderful!

That is so good a hypothesis that it makes me profoundly sorry that I didn't come up with it -- I salute you!

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Re: Focused wave engine runs!

Postby hinote » Fri Sep 24, 2004 3:48 pm

Bruno Ogorelec wrote:



The coolness of the tailpipe is perhaps even more vexing.


Finally, as a reminder that pulsejets are complex beasts, let me point out that at the tailpipe, the flow is not unidirectional. Careful observation of the events has shown that – rather curiously – suction into the pipe begins before the expansion is over. While hot gas is still getting out from the tailpipe in the central, core section of the pipe, cool air is already entering along the walls. Of course, I would hesitate before proposing that the effect can stretch a long way inside the engine – long enough to affect most of the tailpipe – but it must have some effect on the flow pattern and influence the heat distribution.



I just went back and reviewed the pressure history of this engine in Uflow, and compared it with some other (running) engines. The results are pretty revealing.

The typical (?) L-H type engine experiences a minimum pressure of about 73 KPa in the combustion chamber area, assuming the "classic" value of 220 KPa is used as the starting value.

OTOH Larry's engine doesn't experience a minimum value and then rebound--it just continues to decline, to amazingly low values! For example, if we use a frequency of 263 Hz (my assumption) we get a duty cycle time of .0038 sec.; at this time point the CC pressure is down to 27 KPa, and still dropping. The minimum value shown in the Uflow simulation is 15 KPa at .0050 sec.--not realistic, since the cycle time is shorter, but it illustrates how heavily this sucker is breathing.

Of cousre, the sim is inaccurate due to other factors we can't include (such as the influence of the intake), but I think it still shows the power being created by the gas dynamics involved.

Also, it begs the question: Is a reverse-meg tailpipe going to create a performance increase in this engine? Or, is the combustion chamber better off doing its thing without?

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Re: Focused wave engine runs!

Postby larry cottrill » Fri Sep 24, 2004 4:21 pm

hinote wrote:OTOH Larry's engine doesn't experience a minimum value and then rebound--it just continues to decline, to amazingly low values! For example, if we use a frequency of 263 Hz (my assumption) we get a duty cycle time of .0038 sec.; at this time point the CC pressure is down to 27 KPa, and still dropping. The minimum value shown in the Uflow simulation is 15 KPa at .0050 sec.--not realistic, since the cycle time is shorter, but it illustrates how heavily this sucker is breathing.

Bill -

Keep in mind that the radical pressure curve you're referring to there is probably suspect. This happened when a small flare was added to the tail end -- a 'micro megaphone' as it were. The action this has on the curves turns out to be highly specific, in the sense that you don't get anything like that with a little bigger flare or a little smaller one. It really looks like it might be a math anomaly in UFLOW.

Of course, on the other hand, Steve built that little flare in as accurately as he could, and ended up with a critter whose behavior no one can quite explain. So, maybe the anomaly is in the laws of fluid dynamics, and UFLOW just happens to reveal it perfectly ;-)

Seems to me that we probably ought to go back to the UFLOW curves I published earlier on, unless we somehow can determine that the later run points to something real. At this point, I don't want to be guilty of sending everyone down a rosey path to ultimate disillusionment ...

L Cottrill
larry cottrill
 
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