Proposed Design - 'Reflector Bucket' Valveless Pulsejet

Moderator: Mike Everman

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Proposed Design - 'Reflector Bucket' Valveless Pulsejet

Post by larry cottrill » Fri Oct 10, 2003 8:17 pm

This is a re-submittal, for your comments and criticism, of one of my earliest posts on the old forum [posted a couple of years ago for Bruce's amusement; how time flies] -- only, this time, instead of just describing it, I've finally drawn it up. Part of the explosion gases go forward through the venturi and are focused by its slight taper into the 'bucket', where the charge of hot gas is compressed and 'reflected' back into the front of the venturi, which then acts as an ejector, picking up fresh air [similar to the action of a 'thrust augmentor'. The length and spacing of the bucket is such that the timing ensures that the reflected pulse augments the suction developed in the chamber following the blast [so this is a case of augmented Kadenacy action]. The gap and venturi are designed to maximize fresh air input mass, and thus slow down the intake stream. By making the exact position of the bucket [and thus the gap] variable, throttling would be achieved.

What do you think?

L Cottrill
---------
Attachments
Reflector_bucket.jpg
The 'reflector bucket' valveless engine [drawing Copyright 2003 Larry Cottrill]
(58.03 KiB) Downloaded 1116 times

Andrew Parker
Posts: 63
Joined: Thu Oct 09, 2003 3:04 pm

Post by Andrew Parker » Sat Oct 11, 2003 5:26 am

Larry,

The design is reminiscent of the patent that Bruno mentioned earlier, Patent # 2,731,795, Acoustic Pulse Jet Engine with Acoustic Air Intake System, A. G. Bodine, Jan. 24, 1956. How do you propose to deal with acoustic forces when adjusting the gap? As always, nice artwork!

Andrew Parker

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Post by larry cottrill » Sat Oct 11, 2003 1:32 pm

Andrew -

I've always thought that this is one of the best design ideas I ever had -- so I guess I wouldn't be surprised if it has already been done years earlier. It led logically to the Synchrodyne(TM) engine, which I have so far failed miserably to perfect.

I hadn't even considered an acoustic component to the problem, except in the sense that effectively, the path across the gap and into the bucket and then back again needs to approximate a "half wave" of the basic cycle in the rest of the engine, in order to boost the intake cycle at the right moment. The biggest problem as I see it is that this will vary with the operating temperature, creating the need to gradually adjust the position of the bucket as the whole machine heats up. That is no worse than the case of the Reynst pot, however. The bucket would need to be mounted on a rail and slider so alignment could be maintained as the relative position of the bucket is adjusted -- a rack-and-pinion or lead screw mechanism could be used.

L Cottrill
---------

Ivar
Posts: 23
Joined: Sat Oct 11, 2003 10:34 am

Post by Ivar » Sat Oct 11, 2003 4:19 pm

The bucket is a good idea but is too small. You are not reflecting sound - you are storing pressurised air in there wich hopefully will reverse and ram the pulsejet`s intake.

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Post by larry cottrill » Sat Oct 11, 2003 5:15 pm

Ivar -

Like most of my drawings, this is not meant to be to scale, just intended to get people thinking about the principle. I expect that experimentation would lead to a longer bucket than I have shown. Some of it would depend on how much gap is really effective between the intake and the bucket.

Remember also that I'm relying on the reflected blast to drag in a lot of fresh air from around the gap. This would take less reflected energy than might be apparent, if the 'ejector' intake design is well optimized.

L Cottrill
----------

Bruno Ogorelec
Posts: 3542
Joined: Sat Oct 04, 2003 7:31 am
Antipspambot question: 0
Location: Zagreb, Croatia

Post by Bruno Ogorelec » Sat Oct 11, 2003 9:33 pm

>I expect that experimentation would lead to a longer bucket than I have shown. Some of it would depend on how much gap is really effective between the intake and the bucket.

Remember also that I'm relying on the reflected blast to drag in a lot of fresh air from around the gap.
----------

Larry, I think this is a hopeless task. If you intend to use the rebound of compressed gas from the 'bucket', this bucket will have to be very long to accommodate all the gas spewed out of the engine intake, and to start spewing it back only after the 'normal' intake is over. If the compressed gas spews back too soon, you will have it displace fresh air -- not a good idea. I don't think a bucket size can be found to accommodate those requirements. If it's big enough, the rebound will be feeble. It it's not big enough, you will have hot gas dillute/pollute fresh air.

Bruno

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Never Hopeless

Post by larry cottrill » Mon Oct 13, 2003 3:26 am

brunoogorelec wrote:>Larry, I think this is a hopeless task. If you intend to use the rebound of compressed gas from the 'bucket', this bucket will have to be very long to accommodate all the gas spewed out of the engine intake, and to start spewing it back only after the 'normal' intake is over. If the compressed gas spews back too soon, you will have it displace fresh air -- not a good idea. I don't think a bucket size can be found to accommodate those requirements. If it's big enough, the rebound will be feeble. It it's not big enough, you will have hot gas dillute/pollute fresh air.

Bruno
Bruno -

Well, I respectfully submit that this time, I think you're wrong. Your criticism would be quite reasonable for the Synchrodyne, where the 'bucket' is large and a large gas mass is expected to move. This case is a lot more like the Schubert, where the mass is small and rebounds fairly quickly. In the Schubert, the small puff of useless exhaust gas is pulled back in, with the fresh air charge following along from the front. In my 'bucket' design, the small puff of gas is fired back by the momentary compression, followed by the fresh air charge drawn in through the gap. It's really a slightly different geometry for the same effect. I would expect the whole bucket and gap to be no larger [and probably significantly shorter] than the long snout of the Schubert, and if properly sized and spaced, much more effective.

The Synchrodyne attempted to take this kind of action to an extreme, and probably went so far as to make the mechanism unworkable, partly or perhaps mostly for the reason you describe. But if the mass is kept small, as in this case [or the Schubert, or for that matter, the Lockwood 'front end'] the action should be beneficial, once the geometry is exactly right.

L Cottrill
----------

Bruno Ogorelec
Posts: 3542
Joined: Sat Oct 04, 2003 7:31 am
Antipspambot question: 0
Location: Zagreb, Croatia

'Reflector Bucket' Valveless Pulsejet

Post by Bruno Ogorelec » Mon Oct 13, 2003 1:13 pm

Larry,

I don't get it. I mean, I don't get several things.

If a small amount of gas is supposed to be reflected immediately, what happens with the rest of the gas that blows out of the intake?

If the reflected blow hits back at the chamber _followed_ by fresh air, what is it that gets usefully compressed?

the only useful comperssion I can see is that of the fresh charge. Here, however, compression precedes the intake. What purpose would it serve?

Bruno

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Post by larry cottrill » Mon Oct 13, 2003 3:58 pm

Bruno -

OK. Here's what I mean that should be happening, if the geometry is properly tuned for a particular frequency and operating temperature:

1. The bucket is unpressurized and contains mostly air, because its own Kadenacy action has pulled air in after the last cycle. The mixture in the combustion zone fires.
2. Combustion gas is ejected from the intake, across the gap and into the bucket. This partially mixes with the air in the bucket, and compression in the bucket begins, as does the thrust force captured by the bucket. At the same time, the main charge of combustion gas starts to move down the tailpipe.
3. The combustion zone reaches equilibrium with the outside air, and stops ejecting gas through the intake. The gas mass ejected has filled the bucket, which has reached maximum static pressure [greater than outside air pressure], and the bucket is contributing maximum thrust force to the front end of the engine. The main charge of exhaust gas is still moving down the tailpipe.
4. As pressure drops in the combustion zone, the intake begins pulling air from the gap and the compressed gas starts moving from the bucket across the gap into the intake. This causes an increase of fresh air induction to begin in the intake ejector zone [the front bell part of the intake]. Also, venturi action begins to carburete fuel into the mixture passing through the intake.
5. The bucket reaches equilibrium with all the combustion gases expelled. The last of the reflected gas & air pass through the intake, along with new air picked up by ejector action, and fuel. The combustion zone static pressure is still below outside conditions, however, because the total mass delivered by the bucket and ejector is too low to completely make up for the total ejected mass [forward and rearward].
6. The bucket reaches a lower-than-ambient static pressure, and begins to pull air from the gap. The combustion zone continues to pull air through the intake as the tailpipe charge slows down in its rearward traverse of the tailpipe. Fresh air and fuel continue to be drawn in through the intake.
7. The bucket reaches equilibrium and contains mostly fresh air. The combustion zone is approaching equilibrium, and the tailpipe gas charge is almost at the point of flow reversal. The last of the fresh air charge, no longer assisted by any reflected gas, is pulled in with the remaining fuel for the next cycle.
8. Reversal of the tailpipe flow begins to imact the combustion zone, and firing of the next air/fuel charge occurs. The cycle repeats.

Note the following [which were not clearly stated before]: The bucket will always recover some fresh air from the gap during its own low-pressure part of the cycle. Fresh air will be entrained by the passage of reflected gas through the intake venturi, not just following it. The bucket, due to its own pressurization and reflection of the gas, contributes to the thrust of the engine [this is why this scheme should be superior to the Schubert, straight Lockwood, or other 'open front end' designs] -- I didn't show any connecting struts in the drawing, but I think the need for them was always understood, from my previous description. During the part of the cycle where reflected gas is passing through the intake, there will by MORE fresh air entering per unit time than would be drafted in by Kadenacy-induced aspiration alone, NOT LESS.

L Cottrill
----------

Bruno Ogorelec
Posts: 3542
Joined: Sat Oct 04, 2003 7:31 am
Antipspambot question: 0
Location: Zagreb, Croatia

bucket too small

Post by Bruno Ogorelec » Tue Oct 14, 2003 4:46 am

Larry,

OK, now the intention of the concept is clearer to me.

I still have doubts. One is the establishment of the Kadenacy cycle.

The reason the Kadenacy oscillation takes place is gas inertia. Moving gas gets accelerted so much that for a while, its momentum can fight the pressure differential between the ambient and the vessel.

This means that a vessel that will exhibit teh oscillation has to have means of accelerating the incoming and outgoing gas. This is done with a venturi throat or some close approximation of a venturi, or by a process of combustion.

The bucket has neither.

In order to take in hot combustion gas, its opening has to be fairly broad. This will work against the Kadenacy function, providing too little resistance to the compressed gas swinging back out. The aspiration will be insufficient and the chamber will not refill with fresh air. It will only reduce its pressure down to ambient.

With a cavity of small volume and a large opening, you don't get Kadenacy breathing. You get gas spewing out from the center while at the same time fresh air comes in over the periphery. Because the venturi is too broad, acceleration is small and the process is too gentle and there is not even much in the way of turbulence. Just an evening out of pressures.

As I said in the first posting, for this thing to begin to hope to work, the bucket should be much bigger in proportion. It will have to be big enough to take in all the hot gas that is spewed out in the front, or else the excess of hot gas will stream out of the intake and exert thrust against the direction of movement. It will also have to be big enough to have sufficient volume -- compared to the size of its orifice -- to generate the expansion part of the Kadenacy cycle.

The proportions will be governed by the Helmholtz rules for cavity resonators, I am sure.

Bruno

Mark
Posts: 10807
Joined: Sat Oct 11, 2003 10:14 pm

Re: Never Hopeless

Post by Mark » Tue Oct 14, 2003 12:52 pm

Larry Cottrill wrote:
brunoogorelec wrote:>Larry, I think this is a hopeless task. If you intend to use the rebound of compressed gas from the 'bucket', this bucket will have to be very long to accommodate all the gas spewed out of the engine intake, and to start spewing it back only after the 'normal' intake is over. If the compressed gas spews back too soon, you will have it displace fresh air -- not a good idea. I don't think a bucket size can be found to accommodate those requirements. If it's big enough, the rebound will be feeble. It it's not big enough, you will have hot gas dillute/pollute fresh air.

Bruno
Bruno -

Well, I respectfully submit that this time, I think you're wrong. Your criticism would be quite reasonable for the Synchrodyne, where the 'bucket' is large and a large gas mass is expected to move. This case is a lot more like the Schubert, where the mass is small and rebounds fairly quickly. In the Schubert, the small puff of useless exhaust gas is pulled back in, with the fresh air charge following along from the front. In my 'bucket' design, the small puff of gas is fired back by the momentary compression, followed by the fresh air charge drawn in through the gap. It's really a slightly different geometry for the same effect. I would expect the whole bucket and gap to be no larger [and probably significantly shorter] than the long snout of the Schubert, and if properly sized and spaced, much more effective.

The Synchrodyne attempted to take this kind of action to an extreme, and probably went so far as to make the mechanism unworkable, partly or perhaps mostly for the reason you describe. But if the mass is kept small, as in this case [or the Schubert, or for that matter, the Lockwood 'front end'] the action should be beneficial, once the geometry is exactly right.

L Cottrill
----------
What if you put a petal valve in the bucket too? Might be interesting to toy with staging the bucket reeds with the engine intake reeds. Maybe you could even introduce fuel from the bucket for pre-mixing a charge to the main intake. Well, just a capricious thought, not thought out.
Mark

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Synchrodyne II [Re: bucket too small]

Post by larry cottrill » Tue Oct 14, 2003 9:35 pm

brunoogorelec wrote:As I said in the first posting, for this thing to begin to hope to work, the bucket should be much bigger in proportion. It will have to be big enough to take in all the hot gas that is spewed out in the front, or else the excess of hot gas will stream out of the intake and exert thrust against the direction of movement. It will also have to be big enough to have sufficient volume -- compared to the size of its orifice -- to generate the expansion part of the Kadenacy cycle.

The proportions will be governed by the Helmholtz rules for cavity resonators, I am sure.

Bruno
Well, then ... what you want is the 'Improved Synchrodyne(TM)', another beast I think I described long ago but never drew up until now ...

L Cottrill
----------
Attachments
Synchrodyne_II.jpg
Improved Synchrodyne(TM) - drawing Copyright 2003 Larry Cottrill
(55.81 KiB) Downloaded 956 times

Don.K
Posts: 6
Joined: Fri Oct 03, 2003 11:14 pm
Antipspambot question: 0
Location: Alberta,Canada

Post by Don.K » Wed Oct 15, 2003 3:39 am

I have played with this a bit, using a pulse firing into the same kind of chamber larry proposes. My findings kind of agree with Bruno, the chamber would fill and gasses spill over the sides with little or no rebound effect. I figured exactly what Larry did, logically it should compress and rebound but in my tests it didn't. Man it can get frustrating when you know logically something should work a certain way and it doesn't. I think with an extreme amount of velocity it could work to some extent.

larry cottrill
Posts: 4140
Joined: Sun Oct 05, 2003 1:17 am
Antipspambot question: 0
Location: Mingo, Iowa USA
Contact:

Synchrodyne Early Trials Disappointing

Post by larry cottrill » Wed Oct 15, 2003 4:17 am

Don -

Well, that agrees with the trial firings of the original Synchrodyne prototype. My belief is that this failure is caused by making the throat too narrow, so that the forward flow is essentially choked. Of course, there could be a more basic problem that is not being understood, at least by me [stranger things have happened]. I'll try to post the best shot here of what the prototype 'Sync' looks like.

Thanks very much for trying out some experimentation on the concept. Approxiamately what dimensions did you give the chamber and opening? Do you have some measurement or realistic estimate of the gas temp and velocity?

L Cottrill
----------
Attachments
Synchrodyne intake 2.jpg
Synchrodyne(TM) prototype front end - photo Copyright 2002 Larry Cottrill
Synchrodyne intake 2.jpg (97.34 KiB) Viewed 10725 times

Don.K
Posts: 6
Joined: Fri Oct 03, 2003 11:14 pm
Antipspambot question: 0
Location: Alberta,Canada

Post by Don.K » Wed Oct 15, 2003 6:27 am

I used a 2" Diameter x 6" chamber choked to 3/4" final outlet.This fired into a 1-1/4" Diameter x 4" reflection chamber choked to 1". Both outlet and inlet had 3/8" radius flares on them. Fuel was finesse hairspray (seems to burn faster than propane). The exhaust is rich as well so you can kind of see where the exhaust goes(don't blink though). I fired the shots across 1,2 and 3" gaps and it seemed that the reflection chamber just would not take air. The shot would fire and flames would flow around the reflector inlet. The velocity was subsonic, with no real crack to it, more of a fast whoomp. It really bites- I think the velocity has to go supersonic to have any effect in that kind of reflector setup.The air takes the path of least resistance.

Post Reply