Intake into exhaust tube instead of CC

[Ogge]

I was looking thru some of my older designs and came across a design I had forgotten about.

It seems most common pulsejet designs have the both the exhaust tube and the intake directly into the CC. Is this just an estabilshed custom or a requirement Im overlooking?

I have a design that the intake taps off the exhaust tube at 90 degrees about 1/4 L from the CC. The fuel is inserted in the intake. My thinking at the time was that momentum of the exhaust gasses would continue to carry it towards the exhaust instead of out the intake tube. The intake tube then curve towards the front of the engine to allow ram air pressure.

At about 3/4 L the exhaust tube again branches at a shallow angle into 2 side tubes (bent to run parallel to the exhaust) with conical plugs with the main exhaust tube continuing straight to the exit. These 2 branched tubes have the reflective plug at the same distance as the exhaust. The purpose of these tubes where to trap some of the unreacted fuel (since it is deposited from 1/4L inside the exhaust tube) being pushed along by the massflow and trap, compress and ignite them to provide a much stronger (powered) reflective wave back into the CC for ignition of the follow on cycle.

Basicly it should operate similarly to a Reynst with blast compression reflective feedback. It should be very easy to construct and test.

The CC itself was very simple. It is just a length of pipe the same diameter as the exhaust with a conical plug to limit ringing. Therefore I guess you could say the 1st 1/4 of the entire exhaust tube is the CC. Granted this was designed as a PDE where the tubes where fairly long and a .43 BR (blockage ratio) was used to accelerate the waves but a shorter setup without BR should be safe to use as a pulsejet.

Comments, suggestions?

[larry cottrill]

Adam -

That was one principle behind the Fo Mi Chin design - see below. Of course, as is my wont, I extended the intake about halfway down through the stack.

I got this engine to roar pretty well under forced air a few times; I never got it to sustain, though - but then, it does not have good 'Hinote Criteria' proportions. Particularly, the intake pipe is too long for its more rearward location. With a very long tailpipe, as in the Elektra I & II, it probably would run. It would need to be over a metre in total length to get the intake flare at the L/3 point.

L Cottrill

[Ogge]

Adam -

That was one principle behind the Fo Mi Chin design - see below. Of course, as is my wont, I extended the intake about halfway down through the stack.

L Cottrill

Larry,
I will never understand your desire to extend the intake into the wave paths. IMHO it can only harm the performance by reflecting waves that would have propogated down the exhaust tube, back into the CC. Yes the placement of the intake is critical and yes I see that you are trying to prevent direct wave entry into the intake but it appears you are creating more ringing in the CC by doing so.

You will still get mass flow due to the pressure expansion wave which will try to expand in any direction open to it. If you want to prevent energy/heat waves from propogating down the intake tube, make the tube to small to propogate the frequency of the chemical reaction or place the intake somewhere there is a low pressure zone due to wave reflection interaction.

This can be seen in the positioning of the Logan engine intake. Note that the intake appears to be along the outer wall directly above the percieved focal point of the elongated elipsiod. If everything is focus at that point, it will be an anti-node while the outside wall will be at a much lower pressure and the intake is depositing the fuel charge directly at the correct ignition point for optimal performance.

Another method of limiting massflow into intake is to make the front of it into a high frequency resonance chamber. Just inside the intake tube, suddenly increase the diameter for a short distance then reduce it back down to smaller then the original opening. Waves entering this chamber will be bouncing back and forth in the chamber rapidly thus preventing much massflow thru the chamber. Every time a reflection occurs some energy is lost, a small high frequency resonance chamber should disipate rapidly due to the high frequency of oscilation allowing the wave to disapate before the recharge cycle but remaining high impedance during the extreme overpressure time of expansion. This is a trick used in RF waveguides.

Another option is putting a grill over the intake/cc boundary, this will prevent wave entry if the area is to small to admit the wavelength of chemical oscilation though it will not block massflow due to expansion pressure. This is like the screen on a microwave oven that allows you to see inside but prevents the microwaves from bouncing thru the door and cooking you. Dont quote me but I think the size is smaller than 1/4 wavelength to act as a mirror to the frequency due to diffraction properties

The problem with this method is limiting the refresh cycle by generating to much turbulance. In order to do this well, you would have to increase the area of the boundary to get the same flowrate.

As for the 'Hinote Criteria' proportions, I am not even familiar with that term. It appears I have some more research to do.

I am assuming its something you are using to calculation nodes and anti-nodes???? So much new stuff to learn. Sometimes I feel totally ignorant about how some of you guys percieve the operational characteristics. I realize my perceptions are alot different. Hopefully I am helping to provide a fresh way to look at some of the issues.

I havent yet figured out how you are calculating correct lengths. What criteria is used to determine CC volume requirements, where the lengths are calculated from. What harmonic standing waves you are trying to generate.

Most of my designs rely on Blast compression of some form to be more prevelant then any resonance. Due to the fact that each tube is operating at different tempatures, changes in pressures. I dont even begin to see how you can possibly correlate intake length to some multiple of harmonic of the exhaust when you have other resonant frequencies based on CC cross sections and shapes or other factors.

[pezman]

What I don't get is why folks seldom use the fuel to pull in air. If it runs using forced air, then by all means, force the air. A high speed jet of propane shooting into a flared intake will entrain plenty of air and move the mix into the CC at a pretty good clip. In fact a successful design will probably need to choke the incoming air a bit to get the mix to a level that can easily ignited.

I wish that I could find some literature that helps to design such a critter (which is essentially an air amplifier of sorts) but I haven't had any luck so far. If you look at the engineering that goes into a Bernzomatic torch, for example, it is absolutely amazing. Simple, elegant and it seems to carburate the torch to an almost perfect stoichiometric mixture. The swirl tip torches are especially cool (but kind of expensive). I'd be interested if anyone knows how to scale the design.

An alternative that I've ben entertaining is to buy a Red Dragon weed burmer -- basically a scaled up Bernzomatic torch that might make a dandy front end for a valveless pj.

[larry cottrill]

Adam -

Well, after trying to make it work, I never claimed that it was very good.

Yes, you put forth many good points. Eric's experiments with the FWE seem to show that penetration of the chamber doesn't necessarily bring forth any great benefit. All of that [my love of deep intakes] is inherited from my original Elektra I design idea, of course. Perhaps it should just be quietly put to rest ;-)

In its defense, though, the dangling intake will reflect some - but it won't be much. That's why the tube is flattened at the sides and rounded at the front and rear - to keep its interference profile down to a reasonable area, and allow smooth passage of the mass flow around it, as well.

L Cottrill

[Ogge]

Adam -

That's why the tube is flattened at the sides and rounded at the front and rear - to keep its interference profile down to a reasonable area, and allow smooth passage of the mass flow around it, as well.

L Cottrill

Ah, that is not visable from the diagram. So you flattened the intake so its not the same diameter? This may also have the same effect as reducing the size to prevent diffraction into the tube by an easier method.

[larry cottrill]

Adam -

You have to look at where it says "Tube flattened to oval section" ;-)

Here it is in a couple closer up views - the completed intake, and the tube tacked into the tailpipe.

L Cottrill

[jmhdx]

Hi Adam, good to see some fresh ideas on the forum. I thought I'd take the liberty of posting this design again as it has some possibilities. The radial tubes were intended as gas springs, to return some of the blast to aid compression as this design I hope would suck little back up the augmented exhaust at speed. Gas springs might not work but a second firing in the tube ends would certainly aid compression, but I made no provesion for fuel air to enter.
Mike.

[Mark]

Well if you want to melt snow, you can interject the side port deep into the combustion chamber according to this remote Canadian railroad switching station design to de-ice the track. Still, I would not opt for intrusions in the main chamber, you want things to be as slippery as can be and at the same time the most intimate mixing to get the air and fuel to interact.
Mark

[Ogge]

Hi Adam, good to see some fresh ideas on the forum. I thought I'd take the liberty of posting this design again as it has some possibilities. The radial tubes were intended as gas springs, to return some of the blast to aid compression as this design I hope would suck little back up the augmented exhaust at speed. Gas springs might not work but a second firing in the tube ends would certainly aid compression, but I made no provesion for fuel air to enter.
Mike.

Mike,
I just went back and read your original thread. Yes you are thinking along the same lines and the annular designs is interesting.

With that said, I dont like the idea of a gas spring unless it is powered to generate waves that are controlable and focused. You will note I am not only generating a reflected wave but I am directing those waves to intersect as well as using them to help pump in the next mixture charge. It is the intersection of the waves into the mixture that produces the pressure spikes im interested in.

I dont see how the gas spring in your design would do anything but detract from the performance of the combustion pressure generation. You have no means to concentrate the reflection.

But I do see posabilities with the design concept in an annular design. For example if you were to remove the curved front wall of the CC and use the outer wall as a means of concentration into the point. If you also provided some reflective surface from the axis of the point towards the exhaust to help channel exhaust in the designed direction.

You could also provide and intake into the reflection tube to the outside of the engine near the CC and input fuel from that location.