"Thermoacoustic" Stirling - theory of operation

[Tom Booth]

First of all, by "thermoacoustic" I mean those engines frequently SOLD or ADVERTISED AS "thermoacoustic"

For example:

Resize_20231204_034959_9508.jpg (120.52 KiB) Viewed 33828 times

https://www.stirlingkit.com/products/st ... experiment


Also, by "theory of operation" I mean MY THEORY, and NOT the prevailing "thermoacoustic" theory after which these engines are typically named.


Examples of the "thermoacoustic" theory explained:

https://youtu.be/xCnxsoXtlmY?si=7Mc3dRLVMBziGyE4

https://youtu.be/wg96lDw7sNw?si=7EMuCxOtjSAF1yjS

IMO, these are probably best described as Laminar Flow type engines, due to the fact that the actual laminar flow of gas in the engine can be observed if some smoke is introduced into the working fluid.


https://youtu.be/Yt5CYSXK3A8?si=9A29VFDA_e92QzHq


Initially these engines are inoperative. That is, if heat is applied externally, by means of a candle flame, nothing much happens other than the piston will GRADUALLY creep outward as the working fluid SLOWLY expands.

It is important to note this initial slow expansion as it is a measure of the speed at which heat can be CONDUCTED through the glass cylinder, into the steel wool and finally from there into the air inside the engine.

As an example, in this video, the initial expansion of the gas can be observed to take about 20 seconds which is typical for this type of engine, generally.


https://youtu.be/OhEUZASZaEI?si=YClnA75PzfFBcnAk

[Tom Booth]

The above two videos from The Action Lab, Grand Illusions, and there are many others, collectively, seem conclusive in demonstrating that the so-called "thermoacoustic" type hot air engines do indeed "run on sound".

I've always been astounded, however, amazed and somewhat skeptical. If true, this seems truly extraordinary.

If you've done a lot of carpentry work, the old fashion way, as I have, with an actual claw hammer, you probably have noticed that steel nails, if struck with a hammer will sometimes ring out and vibrate similar to a tuning fork.

Now if you hit a bell with a hammer it will also ring out.

We could seize upon this correlation and conclude that hammers actually drive nails into wood timbers by the sound waves produced.

Correlation however is not causation, as I've heard, or had pointed out to me occasionally. Not sure where that insight or saying actually originated though, I'll have to look it up sometime.

The problem is, there is actually IMO more evidence that hammers drive nails by sound vibrations than there is that "thermoacoustic" engines run by sound.

I've actually heard a "ping" type ring from a nail myself while driving it into a 2x4, but I've never actually heard that familiar thermal-acoustic humming sound emanate from any "thermoacoustic" Stirling heat engine while in operation.. So there is not really even a clear correlation. At one time, in one manner, a glass tube can be induced to produce sound by the application of heat. In a somewhat different way, at a different time, a similar looking hot air engine can be induced to run by a similar application of heat.

However, I think I can safely say that no one has ever seen a typical "thermoacoustic" engine, like those depicted above produce that familiar loud humming sound while also running as an engine.

If the engine truly does run when just the right "resonant frequency" is somehow arrived at, should not that "resonant frequency" be in the same audible range as when the same glass tube is actually producing a heat induced audible sound? At least sometimes?

Well, not necessarily I guess. Maybe the engine operates at a lower harmonic.

Unable to conclusively resolve the issue through logic and reasoning alone, I've also resorted to experiment.

I've found that many of the suppositions presented in "thermoacoustic" demonstration videos don't actually hold water when tested experimentally.

[Tom Booth]

My investigation of this "thermoacoustic" question is not, or has not been merely for the sake of debunking.

My goal has always been to understand how these engines actually work so that they might be improved, possibly scaled up and built for use as practical power producers. If they do not actually operate as generally supposed, by acoustic waves at a certain "resonant frequency" much time could be wasted in pursuit of a goal that simply does not exist or is actually impossible.

If these engines operate by some entirely different means, than to make any progress in developing these novelty "toys" for practical use of any kind, we would need to discover what the operating principles actually are.

[Tom Booth]

I the Action Lab video above, several matter-of-fact statements are made about how these "thermoacoustic" engines operate.

1) "the mass of the piston changes where you need to place the steel wool or the stack as it's called, if you get it in the right spot, instead of making sound it vibrates this piston which turns this wheel".

I have not ever been able to verify this assertion, that the steel wool needs to be placed in "the right spot".

You could watch 100 different videos and in each case the placement will be different. The amount of steel wool, the location in the tube, which end of the steel wool is heated, or if it is heated in the middle or all over for that matter, how densely it is packed, the grade of steel wool etc. etc. there is no real consistency or pattern to the placement of the steel wool in the tube

The only actual observation I've made regarding the placement is that the engine seems to heat up and become ready to run sooner and seems to run better and more consistently when the steel wool is relatively close to the center orifice and when the end of the steel wool closest to the orifice is heated.

The engine will usually, however, still run eventually regardless of the placement. My impression is, if the far end is heated, the engine may take longer to start up, probably because it will not start until the heat migrates through the steel wool to the end close to the central orifice

What appears to NOT work is removing the center orifice altogether.

In this video for example, where the central orifice with a narrow passageway between the piston and the steel wool is left out, the videographer was not able to get the engine to start up and run at all.


https://youtu.be/ZT0Vi5zmcUA?si=1ZV7XtkkyX-oDs3R


It might be concluded then, that this center orifice plays some pivotal role in the operation of the engine.

[Bumpkin]

For interest in real thermo-acoustic engines I’ve referenced forum-member Tibsim’s site a few times. There are some neat videos at the bottom of this page: https://tibsim-thermoacoustics1488.blogspot.com/

Bumpkin

[Tom Booth]

IMO a "real" thermo-acoustic devices or "engine" is mostly exclusively confined to the area of converting a heat differential into sound waves, or vice-versa, sound waves into a temperature difference, such as a thermo acoustic heat pump or refrigerator, with no moving parts, other than perhaps a loudspeaker or the sound itself, as vibrating air molecules.

The conversion of heat into mechanical power via piston, crankshaft etc , however is, IMO, a whole different category of phenomenon and whatever slight overlap between the two might actually exist has been generally WAY overblown.

The small engines commonly dubbed "thermoacoustic" I believe operate more along the lines of a hammer driving a nail through blunt force. Heat is applied to a working fluid causing expansion which drives a piston, that is all. No vibrations or sound waves involved. There is, of course, generally speaking, a certain rhythm to the working of any engine or mechanical device, just as there is a rhythm to the blows of a hammer driving nails. To imagine, however that a piston in an engine is itself "driven" by the incidental vibrations or sounds or rhythm produced is a kind of compelling romanticism, which tends to miss or overlook the real advantages and unique characteristics of these small model engines and their actual working principles.

[Tom Booth]

In an actual thermoacoustic device, the "stack" is usually a very precisely arranged array of plates of one sort or another, spaced according to the wavelength of sound waves being generated and positioned at specific nodal points in the resonant chamber. As an example:

Resize_20231204_165715_5295.jpg (70.77 KiB) Viewed 33777 times

But perhaps a simple wad of steel wool can serve the purpose to some extent. The "stack" however plays an important role in that it absorbs and releases heat in conjunction with the spacing determined by the wavelength of sound used.

In a so-called "thermoacoustic" Stirling hot air engine, there are no sound waves of any known wavelength being produced. No means of determining proper spacing of the plates in the "stack". Generally a wad of steel wool is jammed into a test tube more or less at random.

The "stack" in a thermoacoustic device also must have a thermal gradient.

Experimenting, I've found that a hot air engine of the type under discussion will run more or less the same regardless of where or to what extent the steel wool plug is heated. There need not be any temperature gradient at all.

2. Another assertion made in the Action Lab video was that the central metal orifice served as a "sink".

I'm not picking on Action Lab, in particular, this is a not uncommon assumption.

Experimentally I've found that this orifice can be made of any material, capable of conducting heat or NOT.

While the central narrow passage seems to be necessary and plays a key role it is apparently NOT as a sink for heat. So what is its role?

Apparently it's function or role, previously known or not, is mostly just to accelerate the air flow for adequate heat exchange between the steel wool heating element and the air or working fluid.

As seen at startup, the initial heating of the working fluid is a relatively slow process taking several second, if not minutes. For the engine to run at a reasonably high speed the exchange of heat with the working fluid must be quite rapid.

After each power stroke, where heat is utilized, the heat must be fully replenished, within a fraction of a second.

I believe that the jet like laminar flow imparted to the working fluid air stream by being forced through a kind of narrow "venturi" as the piston approaches Top Dead Center creates the necessary RAPID heat exchange needed for the engine to run at high speed. The air, being accelerated to high speed makes thorough penetrating contact with the large surface area provided by the steel wool.

If true, knowing this could lead to various new methods for accomplishing this goal and optimizing the heat exchange as well as possibly eliminating some of the unnecessary or superfluous elements.

[Tom Booth]

Just for example, if there is no "resonance" involved, perhaps there is no real need for a long "resonance tube".

Does this long tube with a mass of steel wool inside serve any real purpose or is it mostly just "dead air space"? Maybe there would be no harm done by shortening the tube to increase the compression ratio and instead of some steel wool or makeshift "stack" in the middle of the tube somewhere, maybe some more effective heat exchange surface could be fashioned.

Perhaps the "venturi" that was believed to be nothing more than a heat sink could be optimized as a real high velocity nozzle.

Instead of this:

Resize_20231204_203824_4153.jpg (101.25 KiB) Viewed 33769 times

Maybe something like this:

Resize_20231204_203825_5081.jpg (84.49 KiB) Viewed 33769 times

[matt brown]

NightHawkInLight channel covers these buggers with a series of videos, but in this one, the dancing styrofoam balls is almost a slam dunk:

https://www.youtube.com/watch?v=kkBBkQ8jFRY

I don't worry about optics, acoustics, electricity, etc...my dance is full with the basic thermo.

[Tom Booth]

NightHawkInLight channel covers these buggers with a series of videos, but in this one, the dancing styrofoam balls is almost a slam dunk:

https://www.youtube.com/watch?v=kkBBkQ8jFRY

I don't worry about optics, acoustics, electricity, etc...my dance is full with the basic thermo.

A "slam dunk" in what respect?

IMO the first part of the video is an excellent demonstration of actual acoustic phenomenon which is almost entirely unrelated to the later demonstration in the same video, which demonstrates an application of brute force to compress air in a cylinder with a piston, as actually takes place in an engine.

Relating the two phenomenon pointing out the similarity AND DIFFERENCES is fine and can lead to a better understanding of both, but efforts to fuse the two together and identify an actual reciprocating engine that uses brute force compression as an "acoustic" device is ungrounded, as I stated previously:


viewtopic.php?f=1&t=5594&sid=698f9f4a89 ... cdd#p20718

[Tom Booth]

To summarize, my observations and experiments over the years has led me to the conclusion that the popular so-called thermo-acoustic Stirling hot air engines, such as depicted at the start of this thread, do NOT infact operate by means of sound, sound waves, standing waves, traveling waves or "vibrations" of any kind.

Yes, engines generally produce both sound and vibrations, but those are essentially byproducts or WASTED energy NOT in any way what actually POWERS any reciprocating piston type engine.

Be that as it may, to conclude, I'll give a brief description of how, according to my own investigations, observations and experiments, how these little engines REALLY work.

First of all, as previously mentioned, heat CONDUCTED from a flame outside the engine, through a glass cylinder, into a metal mesh and finally to the air inside the engine is a relatively SLOW process that takes several seconds if not minutes. The engines, when running, however, complete their cyclic revolution and reciprocating compression and expansion cycles in a fraction of that time.

The "thermoacoustic" explanation for this apparent discrepancy is that the steel wool or "stack" very rapidly absorbs and releases heat along its "gradient" or within its "nodal points" or "zones" where acoustical compression and expansion are taking place simultaneously so that a slight shift of the working fluid in one direction or the other in the tube results in the release or alternatively the sequestering of heat so as to effect the necessary RAPID heating and cooling of the gas.

With a laminar flow type jet stream of air flowing into and through the "stack" this is clearly impossible.

What IMO actually happens is that the steel wool, having high thermal conductivity throughout its interconnected mass of fibers, when heated, rather quickly heats up to a more or less uniform temperature from end to end.

When the piston approaches TDC forcing a jet stream of cold air into this mass of hot metal fibers the fibrous mass of metal provides the abundant surface to air interface for a rapid heating up and expansion of the working fluid.

Due to the rather slow progression of heat into the fibers from the external heat source, the adiabatic expansion and cooling of the working fluid very quickly outpaces the speed at which the heat in the fibrous metal mass can be replenished with heat from the external source.

As a consequence of the disparity between rapid heat utilization and lagging supply, the working fluid, during the expansion/power stroke has opportunity to largely use up the heat that has been supplied to it and cooling begins to contract.

Simultaneously, as the working fluid is expanding and cooling, outputting work, and then contracting, (or being compressed by outside atmospheric pressure) the metal mass is being reheated.

Due to the generally very poor heat conductivity between metal and still air, the metal mass mostly retains the heat supplied to it, and does not impart that heat to the working gas until the piston again approaches TDC forcing another jet stream of cold air into the now re-heated metal fiber.

The high speed blast of air being forced through the narrow orifice provides the brute force impact of high velocity air into the high surface area metal mesh so that the heat that was gradually replenished in the metal mesh can be very quickly released all at once and imparted to the working fluid.

Other contributing factors aside, such as the heat of compression generated on the return stroke, the effects or contributions or transfers between atmospheric pressure, energy stored in a flywheel etc. this constitutes a full and complete explanation of how these engines operate without any recourse to or reliance upon acoustics or some subtle or mysterious power of sound.

[Bumpkin]

Tom, that was a long ride on a horse I thought we beat to death years ago. You were there. I agree those sample videos you posted at the top are “Lamina” type, because of the nozzle that’s used to produce a (sort-of) laminar flow. They’re definitely not acoustic and definitely not Stirling. So I wish folks who don’t understand them would just call them heat engines or something. I could well be one of those folks, but in my own reckoning they are thermal lag engines, and the lamina version nozzle is a way to assist the lag. I’m sure there are other ways, but it all comes down to getting closer to tdc before the air (or as much of it,) hits the heater, hopefully with enough remaining inertia to complete heating before the power-stroke is too far along. Anyway that’s my story and I’m sticking to it. (So far.)

Bumpkin

[Tom Booth]

To further drive home the point,

Take a common space heater:

Resize_20231205_121043_3914.jpg (161.71 KiB) Viewed 33587 times

Two basic elements work together to transfer heat to the air, the heating element consisting of a long coil of metal wire to provide ample metal to air surface area, and a fan to provide forced air flow into and through this hot metal matrix for rapid heat transfer:

Resize_20231205_121044_4098.jpg (114.17 KiB) Viewed 33587 times

The same basic elements are incorporated into these engines

Metal heating element in the form of some stainless steel wool, usually from a common kitchen dish scrubber:

Resize_20231205_122636_6396.jpg (161.82 KiB) Viewed 33587 times

And a kind of nozzle to provide adequate forced air flow through this mass of metal fibers

Resize_20231205_122636_6592.jpg (96.88 KiB) Viewed 33587 times

[Tom Booth]

... I thought we beat to death years ago. ...

Bumpkin

Well, it's been a topic of discussion here, however, the combined viewership of the two videos posted at the start is well over 4 Million and there are many many other similar videos and they continue to be produced, so I don't think the issue is dead.

Anyway, the complete detailed picture of how these engines actually work as described above has only recently been formulated in my mind, though "laminar flow" and "thermal lag" are theories or descriptions that have been around for a long time, the actual details of what "thermal lag" actually means or how it works has always been rather vague, at least in my mind, up until now

[Tom Booth]

I think it should also be noted that Peter Tailer's original patent for a "thermal lag" engine did not include a metal mesh for facilitating heat exchange. Infact, the absence of any such "regenerator" is one of the claims made in the patent that is supposed to distinguish his "thermal lag" engine from a "Stirling":

Part of the patent text reads: "This is exactly opposite to the function of a displacer in a Stirling engine (not shown) where fine wires or screens provide a very large heat exchange area so that the regenerator captures heat from the hot working fluid flowing through it one way to give it up on flow through it the other way".

From what I remember in researching this, a "thermal lag" engine as described in the patent did not work in practice and later trial and error experimentation led to the type of engine that is very common on the market today and advertised as "thermoacoustic".

I'm not sure anyone has ever attempted a serious analysis of how these engines actually work before now, beyond, or as an alternative to the "thermoacoustic" hypothesis, which in actuality is almost universally promulgated as well established FACT.

There is no hesitancy in the various websites and videos describing these model engines as "thermoacoustic" going into great detail about the function of the "stack", the "heat sink" and the "power of sound" as if all this is proven fact tracing back to Lord Rayleigh in 1877.