"Thermoacoustic" Stirling - theory of operation

[Tom Booth]

I was mistaken above in stating that the "thermal lag" engine did not work.

What I had in mind was this article, which was in regard to Senft and the "laminar flow" type engine.

https://www.stirlingengines.org.uk/thermo/lamina.html

[Bumpkin]

I think these engines have a lot of potential, but it irritates the heck out of me when folks call the mesh heat exchanger in a thermal lag engine a regenerator. If I’m wrong in not seeing how that’s possible, I wish somebody smarter would come along and enlighten me. I’d seriously welcome what could lead to progress.

Bumpkin

[Tom Booth]

I think these engines have a lot of potential, but it irritates the heck out of me when folks call the mesh heat exchanger in a thermal lag engine a regenerator. If I’m wrong in not seeing how that’s possible, I wish somebody smarter would come along and enlighten me. I’d seriously welcome what could lead to progress.

Bumpkin

I believe, part of the "thermoacoustic" theory, as applied to,... well, what shall we call it?

"Lamina" maybe?

These look pretty much identical to those models marketed as "thermoacoustic". Here they are called "Lamina":

https://www.stirlingengines.org.uk/thermo/lamina2.html

Is this what you mean by "thermal lag"? Because, as far as I know as stated above, Tailer's "thermal lag" did not have a mesh, as a regenerator or heat exchanger. Or did it?

It doesn't look like there a mesh of any kind depicted in the patent drawings, or in the patent descriptions.

Resize_20231206_071249_9599.jpg (62.53 KiB) Viewed 18542 times

https://patents.google.com/patent/US5414997A/en


I may have been mistaken again, about the thermal lag engine running. I read just now, in the patent, that:

Part of the patent text reads:

The engine 100 showed no indications of running.
..., t

...the engine 100 ran in this manner as a free piston engine for half an hour with a piston ...

...The thermal lag engine 100 was operating, but only barely. It could not sustain a flywheel 130 even with a reduced stroke.

Links of chain(not shown) were placed in the heated chamber 140 and in the portion extending through the reservoir 108 to increase heat transfer. The 13/4" nominal pipe heated chamber 140 held about 11 chain links per inch of length and each link had a surface area of about 1 sq in. The chain increased heat transfer surface in chamber 140 from about 4 sg in/in of length to 15 sq in/in of length. This was merely an experimental expedient and not a suggested way to increase heat transfer area.
With the added chain(not shown) engine 100 then drove flywheel 130 at 450 rpm with a 3/4" stroke of piston 110. This indicated that a thermal lag engine 100 could function and that further experimentation could lead to improved power and efficiency.

So, I was right (I think) in that the original thermal lag design did not work, or "only barely" worked, until some "links of chain" were put into the heating chamber as an experiment.

So,...

The thermal lag (with the chain links "to increase heat transfer") begins to resemble the "lamina" engine with its "mesh" in the chamber.

Tailer's thermal lag also does not show or describe any "venturi" or narrow passage, but then, this is not described in association with the lamina engine either, but is depicted as simply a means of holding O ring seals.

Resize_20231206_074423_3565.jpg (49.52 KiB) Viewed 18543 times

Anyway, back to the topic of the "regenerator".

The "mesh' or "stack" when interpreted as an element of a "thermoacoustic" engine functions, or is described as functioning LIKE a regenerator, alternately absorbing and releasing heat, but in a manner different from a counter current heat exchanger.

Instead, when the "standing wave' shifts position within the "stack" the hot and cold "compression waves" move within the "stack" in some way so as to cause the "stack" to either absorb or release heat.

This is, apparently how a "real" thermoacoustic device actually works, in some thermoacoustic devices, (refrigerators or heat pumps).

I do not believe however that such a delicate shift in a "standing wave" can take place in an engine with a piston driving a stream of air at high velocity through this mesh or so-called "stack"

[Tom Booth]

The narrow passage that I'm suggesting acts as a kind of venturi or nozzle to accelerate the air flow improving heat transfer, seems to have developed as a result of trial and error, starting out simply as a means of securing seals in place.

Later, the theory that this (usually metal, often aluminium) passage acts as a "heat sink' developed.

Experimentally, to settle that issue, I replaced the aluminum "O ring holder" with one made of wood.

The wooden "O ring holder" did not alter the operation of the engine at all. Except that the wood eventually started to smoke from the heat and the smoke provided some opportunity to observe the actual air flow through the passage.


https://youtu.be/Yt5CYSXK3A8?si=AbBXa4o0H7G9sPwR


I actually suggested this here (putting smoke in the engine to make the "standing waves" if any visible), as an experiment back in 2010


viewtopic.php?f=1&t=490

[Tom Booth]

Another thing worth taking note of, I think, is that the original patent (of the Thermal Lag engine) most definitely illustrates the power cylinder surrounded by a water cooling jacket.

My recent experiments have indicated that this is not a necessary feature for these engines to run quite well.

At least in my experiments so far, the engines ran just as well with the power cylinder insulated with styrofoam as not, though I have not actually tried submerging the power cylinder in water or cooling it with ice, as typically these model engines do not come with any water cooling and don't seem to need it.

So,.. it remains a question I guess: would one of these models run BETTER with water cooling, or with an ice cooled power cylinder, but some kind of retrofit cooling jacket would be needed.

At any rate, they can at least run without water cooling.


Sometimes in curious ways:


https://youtu.be/wzJCnwubAz4?si=G27DAHwEm1nOf13X


I would argue that that engine is doing MORE actual "work" by constantly reversing the direction of the flywheel, moreso than if it were making the normal full revolutions.

[Bumpkin]

Yeah, that’s the patent drawing, but as with Stirlings, the principal is more important than the machine, which could of course take many forms. Tailer’s premise was that the natural lag in heating/cooling would cause higher average pressure on the power stroke than the compression stroke, and he eventually found a combination of thermal mass and surface area to get a very low friction machine to move on its own power. The point, I believe, of lamina designs is to use flow trickery to delay the heating until closer to tdc, and then rapidly heat, vastly increasing the average pressure difference between the strokes. I would admit some of the different placements of the heater mesh in Lamina engines seems to fly in the face of my reasoning, but Tailer’s engine ran without any at all. Just not very well.

If I made a lamina engine, I believe I would put the heater at the far end of the tube, with the nozzle sized such that a blow would reach the heater at maybe 3/4 of the compression stroke. Ah well, it’s interesting to ponder, but I haven’t even got back to my big Stirling yet. My shop is full of projects I trip over trying to get to other projects. I’m sure nobody else has that problem.

Bumpkin

[Tom Booth]

My main ambition or goal in all this is to build power producing engines of manageable size. Something that could be carried around as a portable generator, put in the trunk of a car to take camping, run from a campfire or the heat from a charcoal grill, maybe take to the beach or to a weekend cabin.

It doesn't need to power a whole house or anything, maybe just a few lights, radio, laptop, phone charger.

Obviously the "toys" found on the market are inadequate and practically the only thing up from there is a NASA Stirling converter that cost millions of taxpayer dollars nobody could ever hope to afford.

But if we can't even get the basic theory of operation right, what hope is there of ever producing anything on a consumer market portable generator scale?

Are we trying to "tune" this thing to the right vibrational harmonic like a pipe organ, get the "thermal lag" just right, size the "regenerator" or is it a heat exchanger or a "stack" or what the heck does this ball of steel wool actually do? Where to begin? What theory do we go with? Is it scalable? Simple, cost effective.

My way of looking at it is when you have eliminated all the non-essentials, what's left is what actually works, then it should be clear how to go about scaling it up a little.

I don't think I exactly agree with your statement: '...as with Stirlings, the principal is more important than the machine".

IMO the alleged "principle" could be crap. What matters is the machine. We could guess all day long theorizing on how an engine works and which theory is correct but in the end the theories could all be wrong, even those of the inventor. Robert Stirling, I believe, based his idea of an "economizer" or regenerator on the theory that heat could be reused, that is; Caloric theory. Can you really store heat in a regenerator, take it out to expand the gas, do some work then put the heat back in the regenerator to cool the gas, then take it out again, etc. etc. Indefinitely? That's a theory or principle that I think time has proven wrong, but the machine itself still goes on working.

[Fool]

As Dr. Senft wrote in one of his books, build one with and without the regenerator. See which gives a greater kick.

If the thermal mesh has a different temperature on both ends and the gas is cyclically flowing back and forth through it, I don't see how it wouldn't give sone regenerator effect. Longer would provide more effect.

That effect might be lost in the conduction of heat directly from hot to cold through the mesh. I wonder what the effect would be to have three thermal meshes with two of Tom's wooden nozzles in between or just full inside diameter thermal breaks.

[Fool]

Human hearing frequency range is 20 to 20,000 Hertz roughly. That equates to 1200 to 1,200,000 rate per minute. An engine running at 200 to 500 rpm will not trigger a human's hearing resonance.

The "ping" heard when hammering a nail is probably the fundamental frequency of the nail and wood dampening combination. The fundamental is the system's lowest resonance frequency. All harmonics will be higher.

I keep thinking an acoustic heat engine requires tuning, similar to tuned exhaust for an ICE. A jam jar jet engine, or valveless pulse jet, comes to mind too. The putt putt boat is an ECE with a rhythmic tune. It uses the Rankin cycle.

The constriction nozzle seems to increase mV so that the expansion goes beyond the buffer pressure, a vacuum. This provides the return force, outside pressure greater. The reverse happens on the compression wave, mV is converted to higher pressure than outside buffer pressure. Hence, an oscillation is invoked. The frequency should depend on the volume, length, construction, piston-crank-flywheel mass, and dampening from the thermal mesh.

The valveless pules jet has a larger chamber and narrower exhaust and intake tubes. I'm wondering if a tube could be welded on a jam jar pulse jet to improve and slow it's operation. Perhaps with a thermal mesh in side as well.

Externally heated air heat engines, piston less and diaphragm less, examples have been built and operated. Seems all they have is a tube nozzle and mesh, plus external heat. The nozzle seems to lower the rpm below the human hearing range.

[Bumpkin]

Tom, thanks for catching my spelling error and correcting it; principal — principle, perhaps without even noticing. Maybe the principle was more important. Like you said, (paraphrased,) it’s probably not the sound of the hammer-strike that’s driving the nail. As in; if you really want to advance science, the principle is more important than the machine. Seems like you’d have to have a natural inclination to argue with agreement to not see that those last two sentences are saying the same thing. It’s entirely irrelevant whether we agree on what the Lamina working principle is, but we obviously agree that understanding it is important for progress. Cool.

Bumpkin

[Tom Booth]

Tom, thanks for catching my spelling error and correcting it; principal — principle, perhaps without even noticing. Maybe the principle was more important. Like you said, (paraphrased,) it’s probably not the sound of the hammer-strike that’s driving the nail. As in; if you really want to advance science, the principle is more important than the machine. Seems like you’d have to have a natural inclination to argue with agreement to not see that those last two sentences are saying the same thing. It’s entirely irrelevant whether we agree on what the Lamina working principle is, but we obviously agree that understanding it is important for progress. Cool.

Bumpkin

Well, perhaps I have a natural inclination to argue, I don't know. Not so much argument as just attempted clarification. Understanding.

I can't say if I agree or disagree with an idea or concept if I don't first at least have a clear understanding of what is actually being proposed.

I'm afraid that sometimes my questions and probing might be taken as argument or an attack or disagreement. Usually if something doesn't make sense to me, it's usually, or I tend to assume it's my own ignorance or lack of understanding at fault.

And no, I didn't notice any spelling "error", I'm an abysmal speller.

Anyway, you wrote: " It’s entirely irrelevant whether we agree on what the Lamina working principle is, but we obviously agree that understanding it is important for progress."

Actually, I don't agree with that.

Well, I kind of do and I don't I guess.

A lot of progress, from what I've seen, is based almost entirely on trial and error. What works in practice actually works, we may never understand why.

Theories and ideas about how something works can lead to experiments; trying new things in a different way, different combinations, but often new unexpected discoveries are made having little if anything to do with the theories that lead to those discoveries.

What really puzzles me though, is how some seem unable to see that something actually works if it does not fit in with their theories. They refuse to acknowledge that such a thing, or such a result exists at all. This seems to be a particularly common trait amongst "scientific" types, who are so convinced of the validity of "established science" even experimental results are "inadmissable".

That is what I mean by the machine is more important than the theory. The bloody thing works. You can see it operating. Maybe it doesn't fit in with anybody's theories, but clinging to past theories over actual current physical evidence that can be observed first hand I don't understand. The refusal to even look or consider the possibility that some "established" way of looking at some phenomenon might be incomplete or even completely wrong.

Thinking you understand something stifles progress.

You make progress when your willing to admit you don't understand something and so start tinkering.

[Fool]

https://en.m.wikipedia.org/wiki/Lamina

https://en.m.wikipedia.org/wiki/Thermal_lag

I don't see how "lamina" or "thermal lag" have anything to to do with these engines.

Laminar, maybe, as laminar flow has less drag. Less drag means more speed and more mV. More mV produces higher over pressure.

Lamina, refers to two flat layers. Hmmm?

Maybe we could call them pressure lag heat engines? Or acoustic over pressure wave heat engines?

My guess is someone misspelled laminar and the name now gets repeated.

And yes they run on heat, temperature difference, not sound. Sound is the output. Subsonic sound.

Sorry, not anyone's fault. It just hurts my brain to read such use of well defined scientific terms. Use whichever you'd like, people will understand either way. Similar to the nuc-le-ar verses nuk-yah-ler versions of nuclear.

[VincentG]

Tom, you have inspired me to order a thermal lag engine for testing. It can't be that hard to get to the bottom of this. There is no doubt that the operation of these engines causes sound waves to form. The question; Is that the consequence of the engine operating, or the fundamental mechanics of operation?

I'm inclined to think it's a mix of both. The engine likely would operate without these sound waves, and so they will either be helpful or hurtful to performance. The nature of my approach would be to slow the engine down as much as possible(start with a heavy flywheel) while making changes to reach peak torque. From there you can get a better understanding of the true mechanics involved.

Thermal lag seems most fitting to me, as without the orifice the heat of expansion would occur too early before TDC.
(Edit) I agree with fool here, as "pressure lag" seems more fitting. But PV=mRT so it's a case of drive v. driven.

The apparent lack of need for external cooling seems to come from what Matt is always on about; An expansion ratio that exceeds the compression ratio. This would be the result of a bias of less air in the engine.

Watch this video in .25x speed starting at 6:45. As the engine cools down and stops, you can see it still effortlessly passes through TDC, but is stopped dead in it's tracks when approaching BDC for the final time. And that's with a cold(ish) engine. At operating temp there is likely even more of a bias.

https://www.youtube.com/watch?v=TEiBScfR4Vk

[VincentG]

In this video, starting at 4:00 in .25x speed, it's pretty easy to see the movement of the hot bulb as it slips on the O-rings. The high pressure pushing the bulb out corresponds to the piston approaching TDC, and the low pressure pulling the bulb back corresponds to the piston moving away from TDC. In other words, the opposite of what you would want in a power producing engine.

So it could be that these engines are nothing more than a self-sustaining air spring, or they need many variables tuned to really come alive.

https://www.youtube.com/watch?v=xCnxsoXtlmY

[Tom Booth]

...
So it could be that these engines are nothing more than a self-sustaining air spring,...

I can agree with that. I've said for a long time in here that these engines remind me of a Ruchardt Experiment.

https://youtu.be/vT6n7VVBvqw?si=AAb9yndzjchf0DVi

And if you'd like some math:
.
https://youtu.be/e1VDAa4ttOc?si=qyUgSSWVNRrS56jn


Except that with a Stirling or hot air heat engine, instead of maintaining the oscillation by adding a little pressurized gas, the oscillation is maintained by adding enough heat to compensate for friction or whatever load is on the engine.

Without a load it is basically, as you said, "a self-sustaining air spring", or nearly so, with just enough heat being taken in to compensate for friction.