FWE Theory - Focused Wave or NOT Focused Wave?

[larry cottrill]

Eric wrote me commenting on his observations from the tail end of the big FWE while trying to start it under apparently cold air conditions, and I responded as follows:

[
It must have been doing somewhat better under warmer ambient conditions - but, basically, it sounds to me like ignition is taking place too early in the cycle - right where the mixture spouts into the chamber. This is definitely going to yield suboptimal power, even if it will run. The only way you have true 'focused wave' operation is when you can get the initial explosion right up against the front dome - PERIOD. Anywhere else in the chamber, and the energy will be divided into a primary blast and a delayed reflected blast - a 'two humped camel' wave. That may not be so bad for the blast phase, but it will not be good for the suction wave that comes back down the pipe, since the minimum pressure value of that wave will be cut in half. The excellent breathing capability of the design will be lost.

...

A similar warning could be given about the plan to use a 'Chinese' style intake. If you want to do this, the thing to do is to make the bend into the chamber much shallower, so that the intake spouts right toward the center of the dome, rather than across the chamber to the opposite cone wall. The dome is where you want the braking of the high speed flow to happen - not somewhere out in the middle of the chamber! It would be more like the Melenric intake, just re-aimed slightly toward the center of the front wall [dome]. If you can do that, I don't see how it could do anything but gain on what we have already. This idea might be good for the large engine, too, since it will shorten the flow path inside the chamber significantly. In fact, that would probably be the easiest place to try it out first. I would still always provide a lot of nozzling of the inward flow, though, by flattening the forward end of the intake pipe significantly. The idea is ALWAYS to spout with high speed flow, then brake it by slamming it up against the front wall; that's how we'll keep the explosions right up front where they belong. I intend to talk about this some on the forums.
]

This needs some emphasis. You're not going to get the power of the Focused Wave design just by perfectly welding up the cone and dome. What you MUST achieve is having each explosion right up against the dome, or just as near to it as you can possibly arrange. It is VERY likely that in a large engine, you won't get that with the penetrating intake that blows air across the chamber, or with a pure Chinese-style geometry, and I think something more like the Melenric Thermojet design will work better. You want the incoming air spout to be too fast to burn practically until it hits the dome.

Steve and Bruno - I think this is where the 'Unicone' version of the engine will inevitably fall short, though you will probably get it to run. I think it will always divide the energy into a two-humped wave, which when reflected back as the suction wave will be suboptimal for breathing.

Something easy to forget in pulsejets is that thrust is driven [mostly] by mass acceleration, but breathing is driven by the wave! And it's the breathing of your engine that most significantly determines the power of the next blast.

L Cottrill

[Mike Everman]

I'm thinking the domed-or-not shape to the end is irrelevant, especially when you've got the intake inserted in the wave path. The conical shape to the cc is the niftiest thing, making the pressure peak at the end swing as high as it can. the over-all layout of the chinese style is particularly great, as the CC sees double duty every cycle. I think it's a very forgiving design because of this.
I'd be inclined to make FW CC unit with Chineezy intake(s) that does not protrude internally.

[Eric]

Mike wrote

I'd be inclined to make FW CC unit with Chineezy intake(s) that does not protrude internally.

I already have, works great, works better than great. I guess supremely excellent would be a good choice of words. But then again I kind of tweaked the overall design a bit and think I hit the "focused wave" ideal right on the head. Ill get some videos and pics tomorrow along with some thrust measurements.

Eric

[Mike Everman]

Buenisimo, Steve! I can't wait to see!

[steve]

Buenisimo, Steve! I can't wait to see!

No, no, I didn't do it , it was Eric. Get your eyes checked
out or something... :-)
can't wait to see those videos!

[Mike Everman]

OOOOOps! Sorry, Eric!

[Eric]

No problem, I have a hard time remembering who wrote what too. The only thing I can be sure about are those fantastically detailed microsoft paint drawings.

I have to remember to clean out the engine before I videotape it tomorrow. I stuck some brass tubing in the tailpipe to see if it would melt and shrapnelize like larrys dynajet. It did to some extent, the majority just melted into little globs and spattered all over. For some reason that just made me wonder what a platnium pulsejet would be like, it would be interesting to see how it catylized the fuel.... ah well you can tell I am tired by how I am jumping from topic to topic... will post pics and vid footage tomorrow... I think prolonged exposure to puslejet noise has had an adverse affect.... must sleep

Eric

[Eric]

I ran the engine this morning, made a little inverted rig to attach to the scale. I was able to throttle the engine from probably a quarter pound of thrust to between 1.75 and 2 pounds. The intake flare is very minimal (even smaller than the first) and I decided to try the engine before I made the major long exhaust flare. When I flare the intake out more and flare the exhaust it should put out over 2 pounds which is quite impressive for a valveless of this size.

Well Im done with lunch, now to go back out and get some video footage....

Eric

[steve]

Wow! two pounds!
if that measurement is accurate then that is pretty exciting. (just for reference my engine produced about 1.1 lbf)

[Eric]

Im going to make a rig for the tripple beam balance so I should be able to get the thrust down to a 10 gram range or so. I have a bunch of video and photos to post, so I will tell you all about the adventures in annoying my neighbors...

Started the engine up again in the afternoon, took some pics and some video, then did the same at night time. The engine is really really responsive to how much fuel you give it. I put it on as low of an idle as I could get and then squeezed the torch handle which bypasses the regulator and gives it full pressure. It goes from idle to full thrust as fast as you can do it. I will post some clips and if anyone can check out the frequency of low power / high power that would be great. It is really great not only can i blast my neighbors with high volume, pulsating sound, but also vary the amplitude to form beats almost like music.

Once I flare the engine intake more (I have it flared barely over an inch dia) and flare the tailpipe I have no doubt that it will put out much much more thrust. The other design has a bigger flare in the intake and when you take the vac nozzle away from it, the output level drops barely any, where as this one it isnt getting quite as much air as it really wants. The current tailpipe has absolutely no flare at all.

Eric

[Eric]

Ok here are a bunch of pictures.... lets see if I can figure out how to put captions on them......... ok .... now I need to edit the 10 minutes of video

[Eric]

So many videos where to start... guess with the smallest one....

I will have to reinstall some programs so I can check the frequency, unless someone is interested in seeing how it changes.

The "afterburner" video is running at a decent idle, I didnt want to put it on the lowest possible setting because sometimes it just decides to quit because it is too low, same with the throttle down video ( I didnt want to go behind it and have the engine quit and me get toasted by big yellow flames).

In the throttle down video you will notice that the combustion is not smooth and is taking place in the bottom part of the pipe and not against the end cap but every once and a while it jumps back to the end cap. It is on the border line of running propperly at that fuel level.

Larry,
It just hit me that the big FWE looked like the small one on low idle. The big one even looks more like it when the fuel is at the bare minimum level for the small one to run. I think that I have to just get more fuel into the engine.


Eric

[larry cottrill]

Larry,
It just hit me that the big FWE looked like the small one on low idle. The big one even looks more like it when the fuel is at the bare minimum level for the small one to run. I think that I have to just get more fuel into the engine.

You're probably spot on - that baby's got to be from Gas Hog City to get up and really howl. Your current setup is probably too restrictive for that kind of demand.

Very nice work, all around!

The frequency in the first tailpipe video is just a hair above the B below middle C [American] so maybe around 250 hz. On the throttling video it starts out at about that frequency, and the lowest it gets to is down a half step to B-flat, approx. 233 hz.

L Cottrill

[larry cottrill]

In the throttle down video you will notice that the combustion is not smooth and is taking place in the bottom part of the pipe and not against the end cap but every once and a while it jumps back to the end cap. It is on the border line of running propperly at that fuel level.

Amazing - what you're describing is that it works as a dual-mode device: true focused-wave running at high power, and a less efficient mid-chamber firing mode at low power. I think that's wonderful.

Of course, I'd like it pulled down just a bit more to see if it would run in a stable fashion in that second mode; maybe it's just not in the cards for it to do that, though, if you really think you're right on the edge of it quitting.

L Cottrill

[Eric]

In the video it was a decent bit above the quitting point, even a tiny hair above quiting point will run from 3-5 minutes before the tank gets cold and the pressure drops just enough for it to quit.

When I flare out the intake more it will probably run a lot lower and smoother.

Eric