Reynst type propulsive duct

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Mike Everman
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Re: Reynst type propulsive duct

Post by Mike Everman » Mon Nov 29, 2004 5:56 pm

I only know of the oil fed Reynst, but my objection still holds no matter what the fuel. I believe it would have grief if the oil, gas or yak butter were mixing with air in that plenum too soon, as your layout has it. Some would be fine, as long as it is too rich to touch off, right? By definition, since your venturi is the only inlet, you have just the right fuel mix travelling up the hot sides of the CC. IMHO, you must introduce fuel later and mix in the chamber.

I think yours could be fueled by spraying liquid on the hot CC wall just before the slit.

I've been pondering this a lot lately, vis a vis carburetion, and I'm uncomfortable with fuel air mix in any volume between fuel introduction and the cc, that is, without reed valves.
Mike
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Re: Reynst type propulsive duct

Post by Bruno Ogorelec » Mon Nov 29, 2004 6:26 pm

Mike Everman wrote:I only know of the oil fed Reynst
Well, here's the gas-fed one. Remember, the slit is only 'active' on suction. Weird but apparently true.
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Re: Reynst type propulsive duct

Post by Bruno Ogorelec » Mon Nov 29, 2004 6:35 pm

Mike, I actually agree with you; the above was more an exercise than something I'd actually do. It was originally meant to test the possibility within the Dynajet-sized package, which has set some kind of a standard. If I remember right, I came up with it conversing with Larry Cottrill.

If I really wanted to do it, I'd probably do it a bit differently. Liquid feed is hardly a problem, given that you have a strong flow in the intake duct. You put a venturi diffuser wherever it seems right. Just before the entry into the chamber does sound OK.

BTW have you looked at the right-hand side part of the Saunders Roe drawing? It has a neat tubular ring feed for fuel. Reynst also provided similar rings for some of his gas-fired layouts.

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Re: Reynst type propulsive duct

Post by Mike Everman » Mon Nov 29, 2004 7:07 pm

Oh, yeah, that gas feed! The open bottom plenum really is inspired.

I've been thinking along similar lines since you showed me this a while ago. I'd like to pack the bottom of the gas plenum with coarse steel wool to reduce mixing turbulence, but that's a finer point that may not be necessary.

Yes, I do like the ring feed.
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Re: Reynst type propulsive duct

Post by pezman » Tue Nov 30, 2004 5:56 pm

Maybe stating the obvious, but I think that the ring feed works because you can feed in fuel that is too rich and then air coming in the tail gets the mixture down into the combustible zone.

I think that this dictates the kinds of fuel that would work -- anything with too big a spread between the upper and lower limits for combustion (e.g. hydrogen, methanol) would probably backfire. Gasoline, propane, MAPP etc. would probably work nicely (Gasoline LEL = 1.4% UEL = 7.6% Propane LEL = 2.1% UEL = 9.5%, MAPP LEL = 3% UEL = 11%).

For similar reasons, I suspect that methanol can't easily be venturi fed. Anyone know based on experience?

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Re: Reynst type propulsive duct

Post by Nick » Tue Nov 30, 2004 6:36 pm

Just a comment on fuel feeds, i realised something today which im sure is apparnt to most of you but regarding your last comment Bruno about the "....slit duct only being active on suction.."

The slit is in the position of a carb venturi and works like one but, and here is the rub sometimes its a venturi sometimes its not.

What i mean is on a constant velocity or uni-directional duct the fuel is drawn into the airstream as its an area of low pressure, in the case of a piston engine its because of a lower pressure area further down the line ie the induction stroke. The airstream stops when the valve closes and pressure momentarily rises as the air slows, now this doesnt normally matter as its only the kinetic energy contained in the volume of air in that stream and it happens down stream of the venturi so the effects are minimised.
In the pot however the air slows and stops and reverses direction very very quickly and right at the place where the venturi is so the cycle at the location doesnt just go in/out or low pressure exhaust/low pressure induction but in fact LP Exhaust/High pressure static/ LP induction so it does work in both directions not just on induction. Checkout the little pot on my website its spitting as well as inducting, this would not be obvious on a gas fed pot or one where the fuel is introduced lower in the pot where the fuel cannot make it outside.

Food for thought

Nick

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Re: Reynst type propulsive duct

Post by tufty » Tue Nov 30, 2004 8:32 pm

Hi Nick.

For the 'current' crop of pots, what you say is true. Placement of the slit is such that it feeds fuel into a moving airstream, which happens to be the inlet / exhaust phases of the pot. Hence your (and my) lipfeeders spit as well as suck. Hell, this is beginning to sound like porno spam. But hey, let's carry on.

- On the 'inlet' phase, air rushes past the horizontal slit which contains only fuel, and sucks it out in classic carburetter fashion.
- Sadly, on the exhaust phase, the exact same thing happens.

But, if we go back to the reynst drawings Bruno posted a while back, they are from 'Pulsating Combustion' I believe, we see that the ever ingenious Mr Reynst had a way around that, too.

The lipfeed doesn't occur at the lip. It occurs a ways back. Basically, what happens is this, if I'm reading the diagram correctly:

- On the 'inlet' phase, air is sucked into the pot, and passes directly over the fuel 'trough', picking up fuel on it's way.
- On the 'exhaust' phase, what little air is passing in through the inlet slot (and that should be precious little due to his cunning sizing of exhaust), passes higher above the fuel trough, picking up, I believe, much less fuel, if any other than free vapour.

Reynst was very cunning in giving the air only one way to come in, thus meaning he could put his fuel feed anywhere, and therefore only feed fuel when the engine is aspirating. On top of that, he provides a way that the fuel is only fed when the engine is breathing 'in'. I think.

Still, his pots didn't have built in afterburners, so we're ahead of him there ;-)

Simon

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Re: Reynst type propulsive duct

Post by Nick » Tue Nov 30, 2004 9:30 pm

Hi Simon,

well put, much more precisely phrased, thats what i was alluding to with my rather vague and imprecise "..or one where the fuel is introduced lower in the pot where the fuel cannot make it outside."

Reynst rarely missed a trick, did he.

Has anyone managed to get a resonant pipe/cone fixed in position over the top of one of our little pots yet though? thats what you would need to use the reynst system.


Nick

PS i now have a little carb which sits in the "jet stream" of the pot, this also works

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Re: Reynst type propulsive duct

Post by Bruno Ogorelec » Tue Nov 30, 2004 9:43 pm

Nick, here's how I see the events around the intake slit. My bet is that, if it is configured properly, the slit does not work as a venturi at any point. It just looks like one.

When the combusting gases blast their way out of the chamber, what propagates out of the chamber is both a flow and a wave. The two behave like one at first, but the slit offers them a problem. Suddenly they discover they are not one and the same thing.

All the hot gas shoots down the tailpipe, riding the crest of the pressure wave like a mad surfer. What gets into the slit, however, is just a wave, without the flow.

People forget that a wave is not matter. It is a disturbance in a force field.

As it blasts into the slit, it roils the gas that is already there, carrying no gas of its own with it. It cannot, because the momentum of the hot gas is directed towards the tailpipe and it would take a strong force to move it at the right angle towards the slit. The wave has no mass and is not bound by inertia. It can turn any corner with equal ease. It slams into the gas that is in the slit and behind the slit, roils it and even pushes it back a bit.

The same thing happens in a ducted pulsejet. There, hot gas _and_ the wave blast down the duct, but only the wave blasts upstream.

In the suction part of the cycle, the situation is different. There is no blast. Indeed, the thing starts with things almost static. The pressure in the chamber has ebbed, the momentum of the hot gas has been spent, a relatively mild vacuum has been created. It starts sucking air from the slit, which is now the point of the highest pressure. Air and fuel shoot in.

It is at this point – not during expansion -- that some of the fuel-air mixture enters the tailpipe, as the pressure in both the beginning of the tailpipe and the combustion chamber is below atmospheric. (This part of the cycle can also be described with a wave scenario, but the flow scenario explains the events better, I think.) However, in this part of the cycle, the flow in the tailpipe is reversible, but in the chamber it is not. So, eventually, the chamber sucks in what little mixture has entered the tailpipe, too. Practically nothing of the mixture truly goes down the tailpipe.

Nick, you argue that your pot spews fuel down the tailpipe, but I see an easy explanation for that. Fuel gets sucked into the pot. Some of it combusts. What does not combust gets blown back out of the chamber and spews into the tailpipe. Not directly from the intake slit but in a roundabout way, via the chamber.

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Re: Reynst type propulsive duct

Post by Bruno Ogorelec » Tue Nov 30, 2004 9:47 pm

Simon, you saw this trick well, but it does not explain the gas-fired combustor, which has gas inside the slit at all times. I think my explanation fits the known facts better.

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Re: Reynst type propulsive duct

Post by Mike Everman » Tue Nov 30, 2004 10:01 pm

Agreed all around, but I like to boil it down to this: suction wants to enter the chamber omnidirectionally, exhaust is a directed jet.

The pot high up on this thread is has the outer lip of the lip feed a bit larger than the inner. The directed jet of exhaust can't entrain any fuel unless the fuel is forced into the stream. Ingestion has no trouble entraining the fuel because air wants to come from every direction. That is why this pot is such a nice layout, the opening in the bottom of the fuel annulus, I mean; it allows the fuel to be fed continuously, but not be blown into the exhausting jet.
Bruno Ogorelec wrote: Nick, you argue that your pot spews fuel down the tailpipe, but I see an easy explanation for that. Fuel gets sucked into the pot. Some of it combusts. What does not combust gets blown back out of the chamber and spews into the tailpipe. Not directly from the intake slit but in a roundabout way, via the chamber.
I suspect that the washers that form Nick's slit have the same inside diameter, too, or do they? If they are the same then you'll blow fuel out for sure.
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Re: Reynst type propulsive duct

Post by Nick » Wed Dec 01, 2004 10:18 am

I hear what you are saying guys but the earliest iteration of the lip fed pot system on my pot was with the top washer hole larger than the bottom hole, with the same porn star sequences as the later model which featured same size holes and incidentally apart from the smoothed induction path/bell mouth, looks very similar to the gas fed pot further up this thread, including the internal bell mouth duct (mine's not water cooled).
However i think it is impossible to argue just how the reynst pots worked based on theory regarding the ducts, we need someone to build a replica (to scale is fine) and let hard data replace supposition.
Personally I will stick with the two way venturi effect as it is an observed phenomenon.

Nick

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Re: Reynst type propulsive duct

Post by Bruno Ogorelec » Wed Dec 01, 2004 12:37 pm

Sure thing, Nick. Standing by your theory is the thing to do until better evidence comes along.

I am just trying to find a theory that satisfies all the facts observed by the peopel who have run pots, with Reynst foremost among them. His combustors definitely did not spew fuel into exhaust.

But, all theory pales before reality. We'll see.

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Just Another Way to Say It

Post by larry cottrill » Wed Dec 01, 2004 3:15 pm

The remarkable thing about the classic Reynst Pot is that it achieves full breech loading of the cold air piston. Most pulsejet designs muzzle load the piston, at least partially - some designs to a greater and some to a lesser degree.

Imagine trying to design a machine gun that has to be muzzle loaded! And yet, that's exactly what most pulsejet designs are forced to do - even highly successful ones, like the Dynajet.

L Cottrill

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Re: Reynst type propulsive duct

Post by tufty » Wed Dec 01, 2004 3:59 pm

Bruno Ogorelec wrote:I am just trying to find a theory that satisfies all the facts observed by the peopel who have run pots, with Reynst foremost among them. His combustors definitely did not spew fuel into exhaust.
"definitely" is a very big word, Bruno. Even with the 'big boy' pots (as opposed to the gas plenum model pictured above) I can't see any way to completely avoid fuel getting entrained - with the gas plenum chamber, it absolutely _had_ to lose _some_ fuel on the exhaust phase. I can't see any way around it with the fuel feed as pictured.

One should also note that Reynst, although brilliant, was trying to sell these things, he would be rather unlikely to add 'Oh, and by the way, it burns 1/4 it's fuel uselessly in the exhaust' to what was effectively his sales pitch :-)

For what it's worth, my 'spitting' pot has equal-sized washers, so I would expect it to spit.

If Nick's had differing sizes, this gets a lot closer to the reynst 'plenum chamber' design, and if that spits - well.

I've been playing with my 'widemouth' some more today, playing with mouth restrictions to get at least partial sustaining with as big an aperture as possible but without an exhaust before moving on to exhaust and feed designs.

So far :
  • 10mm - no pulsing
  • 12mm - strong pulsing, 'strangles'
  • 14mm - pulses for longer, but weaker
  • unrestricted - 2 or 3 pulses, very weak
I am tempted by an exhaust/feed system a la reynst (i.e. lipfeed, but away from the lip), to see what happens.

Of course, what we _really_ need is to get hold of one of the originals ;-)

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