Singing Pig #19

[Bruno Ogorelec]

This is something I have been thinking about for a while.

I have developed this concept for several reasons that melded into one.

The first was the wish to develop an acoustic Reynst pot with a single orifice. With the original Reynst, one can argue that the intake slit is one port and the resonant tailpipe another. As I have pointed out elsewhere, the original Reynst is a kind of Chinese rotated along the central axis. In his works, however, Reynst argued in favor of a combustor with just one orifice, which would alternate between being the intake and the exhaust.

He gives reasons in favor of such a layout, which I will not reiterate here. Let me just say that I find them convincing and attractive. I am convinced that it will be more efficient in aspiration and propulsion than the slit-type Reynst.

I really wanted something along the lines of Mark’s ‘snorkelers’ but with a less restrictive duct.

An additional reason I was thinking about this one was to remove the trace of doubt about the ignition of the charge before it enters the chamber. Some of you may remember that I have maintained that the pre-mixed charge waiting in the mixing chamber of a gas-fired Reynst will not ignite from the passing hot gas, even though it seemingly waits at the combustor entrance. Some of you disagreed strongly, however. This will probably look better to those who disbelieve my theory of separation of charge by the blast wave. In this thingy, gas mixes with air only upon entering the chamber.

Furthermore, this harks back to my first efforts at developing a valveless pulsejet, way back in the 1970s, when I wanted fresh air sucked by the engine to suck propane with it and mix with it in a set proportion. I still like that layout and think it should give better mixing that the usual injection of propane through a jet.

Next, I wanted a layout as simple geometrically as possible, with the fewest amount of stray wave reflections – the simplest wave scenario one can get in practice.

Finally, I wanted the engine to be as short as possible. After all, it breathes through the tailpipe. I didn't want to make the breathing pipe too long.

After going through several iterations, I have settled on the layout shown below. It approximates what I would like to build. Alphanumerically, it is tagged SP-19, for Singing Pig #19. I have not come up with a “realâ€

[Eric]

Bruno,
Great minds must think alike ;). Im actually building a conical combustiong chamber thermojet/fwe with the intake like you have in the drawing, and a long conical tailpipe flare. The intake is a bit different for my final design but I think we are basically thinking of the same engine.

The current engine is like the engine on the bottom. I like the bottom one best for prototyping because I will be able to slide the tailpipe in and out to tune it like you said, where as the other will probably come later when I figure out exactly what lenghts I will need since the top design cant really be tuned by moving the pipe in and out.

Thats a very good point that the preheating of air will help mixing. I was thinking this same thing when I was trying methanol injection and ending up with mostly liquid methanol instead of vapor. I think if methanol is used it will definatley absorb a lot of the heat and vaporize uniformly as well as keeping the incoming air from being too hot. I think if the intake had a slight taper so that air would have a tendancy to want to flow into the engine more, the effect of the methanol being injected as liquid and then turning to vapor could give some kind of boost, as well as cooling the air charge a little.

With propane it might ignite before you want it to, possibly. Although the conical combustion chamber does seem to do a good job of retaining the heat to the combustion chamber and not transmitting it down the tailpipe like a lockwood, as long as the intake or the tailpipe does not glow red preignition shouldnt be a problem, and with the intake stream flowing over both of these realitivley cool areas the problem shouldnt exist.

If you want, i could probably make the engine you are talking about, in 2.5" x 1.25" ID, just by changing the tailpipe to a shorter lenght of straight tubing.

Eric

[Bruno Ogorelec]

Thanks for the offer Eric. Much appreciated!

I like your engine. It is such a simple layout that it must be investigated. I wonder why it has not been so far. (Well, it has in a few half-hearted attempts I know of, but nothing with a degree of determination.) I',m really looking forward to seeing you build and test one.

However, I thing your layout is different from mine. My concept does not have an intake and an exhaust, but a single port that serves as both. The narrow annular space that I am letting the gas into is closed at the right end.

[Eric]

Ah, yes, the heavy black construction line intersecting the tailpipe..... When I first read the post and heard closed at both ends I for some reason thought you were talking about the entire engine, and then I figured it was a typo. :)

If you want me to make a prototype Id be glad to. I have enough stainless to make an argus v1, well enough to make it out of 24 gauge, which would just explode/implode :(. But anyway let me know what dimensions you think would be best. Preferably if the conical shape was some portion of a FWE cc, as I have a bunch laying around waiting to become jet engines.

Eric

[Bruno Ogorelec]

Eric, I'll be damned if I know.

Yes, the FWE cone should be fine, and a part of the FWE tailpipe of about the same length as the cone. We want the cylindrical part to be roughly the same length as the cone.

The BIG problem is the diameter of the open-ended tailpipe. I have no idea how large the annular space between the inner and the outer pipe should be. I want the amount of propane arriving into the chamber to be about right for a good mixture. The flow rate will depend on the section of the annular space, the length of the annular section, the temperatures of the inner and outer pipe, the initial temperature and pressure of the propane... very, very difficult to work out.

With all the conditions for air and propane being equal, I'd have the annular section be 1/16 od the tailpipe/intake pipe section, to give the 16:1 air-propane ratio, but propane will be heated here and at appreciably higher pressure than atmospheric...

We don't want the mixture too rich, do we? How about having the annular area 1/30 of the intake/exhaust pipe area?

Here's how some other iterations of the same basic concept looked like -- from the one derived straight from the Reynst to the one I posted.

[Bruno Ogorelec]

I was thinking about the bottom one perhaps having the internal diffuser cone, like the Reynst pot.

[Mark]

It dawned on me too that it might be interesting to try a single port breather that would produce substantial thrust. As having tried a lot of streamline shapes, I can say most will take on this scenario. If you mist the shape in question with fresh methanol from a sprayer and then light the tip of the tail, what you get is a very sharp, sudden bark. Often it is quite loud and it hurts your ears. The burn goes so fast, it's going to be tricky to keep it alive, acetylene or hydrogen would be the most forgiving, but the high test fuels also can flame out because they "go" so fast.
I think of the Schmidt tube, perhaps the most slippery shape there is. It's a simple cone and breathes from both ends. It also is more fussy to start, less reliable than the typical V-1 shape. You don't have the capacitor effect of a bulbous chamber and necking exhaust.
If there was a way to induce turbulence and mixing enough to have the next shot ready to go but slow the burn down enough to keep the fire alive while still keeping up the in/out action, you might pull it off. Mixing and breathing are quite fragile in a streamline duct. Everything has to be ten times faster in a streamline duct, adequate mixing is paramount and there is the requirement of a way to sustain ignition in a constantly changing environment.
I've run a straight tube/duct pulsejet style, a single beveled flapper reed valve in the front. It's slippery to pull off, I had to plug the tail end with a cork and impulse start it (spark the "chamber" and blast the cork off), I couldn't get enough sudden evacuation to build feedback otherwise.
Everything sounds easy until you try it. For every one hundred ideas/tries, perhaps one will work, at least that is how it usually goes for me. I think that is the ratio of millionaires in the US too, one out of every one hundred. Who is going to come up with that idea that will put them in the top 1 percent?
Mark

[Eric]

Bruno,
If you have such a small volume of gas flowing over that large of a surface area, you will probably be able to heat it all to the desired temperature in a much shorter distance than the full tailpipe lenght, or a large fraction of it. The fwe sized engines run on a tiny ammount of propane, basically my fuel line is 3/32 ID and low flow. I think if you have an area as large as you described you are going to end up with a blowtorch instead of a pulsejet.

I was thinking maybe a lot of tiny little holes drilled at a slight angle around the entire circumfrence of the tailpipe cc junction, maybe going an inch or so back would probably give the desired effect, heating the fuel and giving a much larger mixing area and a lot more turbulance.

Another idea would to be just to take a lenght of brake line, wrap a tight spiral to the desired length around the hot part of the engine, and have it lead to the desired point for injection. Possibly right at the edge of the intake like my valveless ones, or into a ring somewhere inside with the many holes, or some kind of slot opening. Something that comes to mind would be just to cut a slot about half way through the tailpipe with a band saw just infront of the cc juntion, on both sides, and have a ring or tube over top of that for a coil / pipe pre heater.

Other than the fuel delivery system I basically have the engine sitting here on the desk right now.

Eric

[Mike Everman]

Eric, the annular fuel feed area on Bruno's design is irrelevant. He can still be feeding that volume with some small orifice from the tank, and his large feed annulus <insert joke about Bruno's anatomy here> effectively lowers the pressure of fuel delivery to something like ambient; a very good thing, I think, the fuel vapor being nearly at rest when the inrush slams into it.

[Eric]

I use the blow torch description to describe anything from a bunsen burner to a blow torch, essentially anything thats on fire and not pulsing.

With that much area, and such a low flow of fuel, im not sure how the mixing would be. Maybe you are right that the stationary fuel and the high speed influx will crash and cause turbulence. If it gets enough fuel, but dosnt mix it right, its just going to be a bunsen burner.

Well either way, I have the fwe cone, and a 7.5" long piece of pipe with a nice rounded edge like i have on my lockwood intake. I will weld it together and try starting it with some methanol to see if it even wants to work.

Eric

[Bruno Ogorelec]

Eric, as Mike has pointed out, all will depend on the amount of propane fed into the annulus. I think.

I see that same amount expanding rather a lot and streaming at some speed (but relatively low density) into the chamber, mixing with the partially heated fresh air better than a denser cooler slug of teh same mass would.

All the time I am thinking in terms of very lean mixture, but heated, rather than cold. I want to take some of the combustion heat back into the engine to achieve greater thermal efficiency, which will translate into greater fuel efficiency and, I hope, greater SFC.

Nothing should ignite before the gas speed stagnates -- which should happen as the pressure shock reflected off the end of the tailpipe hits the chamber. That is usually taken to be the trigger for ignition.

But, that's what it looks like to me on paper. Real life is often more difficult.

[Viv]

Yes, Viv, I will freely admit that the arrangement is perilously close to the Collins Collar, but it (a) less sophisticated in terms of fuel delivery and (b) served double duty here as a heat exchanger.
The slit through which it will be coming into the combustion chamber also coincides with the entry of fresh air. Both will expand into the chamber together. I’m hoping for good mixing.

Yes I will give you that one Bruno:-) but what every one is missing is the most important feature and that is the compression space!

Fuel heating is just an added benifit, good mixing is a design criteria but the compression space is the critical thing as it makes the fuel flow pulsate!

A resonant fuel system is what the Collins collar was designed to be so that the fuel was delivered in time with the engines intake, this boosts the engines cyclic nature.

Nice to see you using it but every one is hung up on fuel heating! forget that its small beer compared to getting the fuel delivered in time with the engines cycle.

Treat it as a tube type Helmholtz resonator for tuning dimensions.

Viv

[Mark]

A long time ago I bought some car exhaust pipe, but it's the kind with the choked ribbing or whatever you call it, so that you can bend it to some extent. I thought about introducing a fuel somewhere on the ribbing, over a large surface area, perhaps the fuel could hide in the channels or by experimentation find what part of the wavy region would work best for dispersion. I was hoping to use this straight duct with fuel wicked over a large portion of the duct, so that it might snorkel without a tank, the ribbing aiding in mixing the fuel/air well enough to work.
I had read of a duct in Popular Science with little side channels/dead-end tubes, the whole length of the main tube. I can't even remember what it was used for other than turbulence. Each tubette was 180 degrees opposite another, like the teeth on the nose of a sawfish.
Then too, it might be curious to try some tubing where linear channels run parallel to the lenght of a duct , just to see how that would behave, kind of like fine rifling only without the twist.
It will be interesting to see just how far one can go with a single port breather in the direction of a pulsejet. I wonder if a smaller design would have more advantage over a larger design, getting fresh air and a charge down into a "streamline" duct deep enough is tricky business.
Happy channeling to all,
Mark

[Eric]

I think that duct you were talking about was some part of an earlyish PDE. I remember reading the same thing. I have wanted to build an entire pulsejet out of the wavy wall tubing, but no place around here has it in stock. Some prototype pde's used wavy walls, and so did that one german particle beam accelerator.

Eric

[Mark]

For review, I'm posting this design out of one of my books, "First International Symposium on Pulsating Combustion", 1971. Perhaps there is a way to run the incoming fuel/air along the walls of a snorkel and have ejection for the most part go out the central region, loosely akin to this design. Note the top sketch, everything is going on or "allowed" only from the one region.
Imagine the time and effort toying with the lengths and shapes to make the pictured device work optimally with so many variables.
Mark