"Thermoacoustic" Stirling - theory of operation

[Tom Booth]

Consider "translate information" modern buzz for effect change.

So, can you provide a "modern" reference for (or with) such a usage.

I'm also a little curious why you and VincentG take turns answering for each other. It makes it a bit difficult to keep a conversation straight, as far as who thinks what.

Note in the following that the translational speed for pure hydrogen or pure nitrogen is ~1.5x the speed of sound for each.


gas speed.png


Also note the last line: "When we measure the temperature of a gas, we are measuring the average translational kinetic energy of its molecules."

original article: http://teacher.pas.rochester.edu/phy121 ... ter18.html

So?

If you have a thermometer surrounded by a gas, the "average translational kinetic energy" of the gas is, or becomes the same as the "average translational kinetic energy" of the glass tube filled with mercury, though mercury isn't used in thermometers these days, call it "fluid".

The fluid in the thermometer also has the same "average translational kinetic energy", and so with anything and everything that exists in the world, let's say, at the same "ambient" temperature.

The thermometer does not go shooting off and smash into the wall at the speed of sound due to contact with the air, whatever it's "average translational kinetic energy" may be.

Likewise with the piston in a Stirling engine in contact with the gas inside and outside the engine.

....
If everyone hasn't got it by now...isn't it possible that these "thermoacoustic" engines are little more than the variation of gas speed between 2 temperatures where the 'speed' of the piston approaches the difference between the gas speed of these 2 temperatures.

Leaving out pistons traveling at the speed of sound or 99,000 RPM.... Sort of maybe the "difference", but not really.

Your talking on a scale of,... To draw a comparison, of gas particles colliding with a piston, how fast does a semi - tractor trailer truck move up hill while having the paint on the back doors sand blasted.

Now, turn up the pressure on the sand blaster a little. The sand is hitting the truck a little bit faster so now the truck will move proportionately a little bit faster also...

But the speed of the individual sand particles is hardly comparable to the speed of the truck, which is basically not moving at all, because, there is another guy with a sand blaster removing paint from the front of the truck as well.

So basically, no. I think your off in the weeds somewhere with this. Still not sure where "information" fits in here.

Of course, if one guy in back of the truck turns his sand blaster up really high...

The truck may move some percentage of the difference of the speed of the sand particles between low and high pressure. The difference, or 1/1,000,000 of the difference or something maybe.

[VincentG]

Sound, vibration, pressure, are all forms of information. The speed of sound is a mechanical vibration and is closely linked to molecular speed. A piston or turbine all react to information carried by the gas and impart their own information on the gas based on rpm.

[Tom Booth]

Sound, vibration, pressure, are all forms of information. The speed of sound is a mechanical vibration and is closely linked to molecular speed. A piston or turbine all react to information carried by the gas and impart their own information on the gas based on rpm.

Let's unpack that a little, we can start with the first statement:

Sound, vibration, pressure, are all forms of information.

I have no clue what on earth you're trying to say here. How do you define these terms. Particularly "information".

What are your references? Google is no help in this case:

iMarkup_20231219_105909.jpg (42.84 KiB) Viewed 15083 times

[Tom Booth]

Well, while waiting for a response, I've also tried "A piston or turbine all react to information"

And variations, such as "A piston reacts to information", and "A turbine reacts to information" with the same null results, which is actually quite unusual. More often than not I can type any nonsensical thing into the search bar and it will turn up somewhere on the internet.

[VincentG]

https://www.sciencedirect.com/topics/ch ... ectroscopy

Maybe I'm guilty of getting a bit too esoteric, but I think it's all applicable to what we are trying to do here. We communicate every day with sound, we use radar, we have seismographs, all ways of gathering information from our environment. An engine, at the end of the day is just a microcosm of an environment in which we try to extract power. Just like wind turbines extract power from wind driven by the engine that is the world. Heat from one end of the world interacts with the lower heat from another, just as does a thermoacoustic engine, albeit on a far smaller scale. An ICE harnesses a controlled explosion, which is quite a bit less subtle than what we are trying to harness in an ECE. And even at that, most of the energy from an ICE goes wasted. I could provide many links to try and demonstrate what I was getting at, but I'm not trying to prove anything so much as understand it better myself.

[Tom Booth]

Well, references or not, everybody's entitled to have an original idea or opinion. Can't say I understand what, if anything you and Matt are driving at. You say "I think it's all applicable to what we are trying to do here" but what is "it"?

All I see is what looks to me like a complete and total misunderstanding of what "average translational kinetic energy" of a gas means.

It doesn't have anything to do with translating information., or somehow allowing pistons to travel at 99,000 RPM.

[Tom Booth]

How, or in what way is the power "ruined"?

There is not much heat energy in a fixed volume of hot air, so between expansion and cooling from work output the pressure may reduce too fast. That's why I favor heat input for as long as possible. Even with my completely ideal 4:1 volume ratio isothermal model, the pressure is down to ambient at 55 degrees BBDC. How quickly would the pressure reach ambient without continuous heat input after compression? I think too early to make useful power.

It may work with a much larger starting hot space volume like a stock LTD(or similar), but these thermoacoustics(or similar) have much less gas to work with.

Personally, I think less gas is not necessarily a disadvantage. An IC engine has only what gas is in the power cylinder. It does not have this enormous displacer chamber that you see in an LTD nor does it have the relatively large heating chamber of a "thermoacoustic" which is often 3 or 4 times or more the volume of the power cylinder.

In an IC engine the gas in the power cylinder is reduced to about 1/10 the volume. Double that for a diesel engine.or halve that rather to 1/20th

Compression in a Stirling engine, due to this large extra volume is negligible by comparison.

I think little if any attention has been given to the possibility of reducing the size of the hot space in these "thermoacoustic" engines, but if there is not some "magic" to having a long "acoustic" tube and the wire wool is really nothing more than a kind of temporary holding area for heat, then probably all the "extra" dead air space is not actually needed.

That was at least part of the idea in the comparison images I posted earlier:

Your normal "thermoacoustic"

Resize_20231204_203824_4153.jpg (101.25 KiB) Viewed 14693 times

Similar engine but with greatly reduced heating space:

Resize_20231204_203825_5081.jpg (84.49 KiB) Viewed 14693 times

That orangish colored plug in the second image is intended to represent a solid copper cylinder with some heat exchange vanes or groves on the end facing the power cylinder where a high velocity jet of air would be forced through the center nozzle.

The compression ratio would be comparable to an IC engine and total volume considerably reduced by the elimination of the likely unnecessary acoustic "resonance chamber".

Now, granted, I have yet to build and test such a design so this is mostly theoretical, but if a "thermoacoustic" type Hot Air Engine of the type under discussion here does not actually "run on sound waves" then why not? If not a "resonance chamber" what purpose could this large "dead air space" actually serve?

[VincentG]

Well, references or not, everybody's entitled to have an original idea or opinion. Can't say I understand what, if anything you and Matt are driving at. You say "I think it's all applicable to what we are trying to do here" but what is "it"?

All I see is what looks to me like a complete and total misunderstanding of what "average translational kinetic energy" of a gas means.

It doesn't have anything to do with translating information., or somehow allowing pistons to travel at 99,000 RPM.

If we were building an engine to run on photons, would not the speed of light play a role in engine design?

I think little if any attention has been given to the possibility of reducing the size of the hot space in these "thermoacoustic" engines, but if there is not some "magic" to having a long "acoustic" tube and the wire wool is really nothing more than a kind of temporary holding area for heat, then probably all the "extra" dead air space is not actually needed.

Only one way to find out. Starts to look a lot like a hot bulb engine at that point. When I tried this with a chainsaw engine, it turned over differently when the bulb was heated. Easier I think, like it was trying to do something. Maybe the orifice will create the lag needed to run.

[Tom Booth]

[...
If we were building an engine to run on photons, would not the speed of light play a role in engine design?
...

Maybe.

A heat engine does, arguably, run on infrared.

A weatherman reports on wind speed or "velocity" and direction. He's not generally concerned with kinetic theory, ideal gas equations or the "average translational kinetic energy" of individual air molecules.

Matt, is or was IMO erroneously relating "velocity" on a molecular level, at the speed of sound, to mass air flow or expansion in a cylinder and engine RPM.

If you want to show that there is some relationship between molecular "vibration" or whatever and RPM, that might be interesting but claiming that the molecular speed IS piston velocity or RPM and actually equating the two is just nonsense.

[VincentG]

I took it more like the piston is trying to respond to the motion of the gas, so if there were a "perfect" engine the piston would be as responsive as the gas. So that may manifest itself as 99krpm. As I see it and bringing it back to the topic of this thread, there are too many uncontrolled events in a thermoacoustic to make a useful engine without tons and tons of trial and error.

[Tom Booth]

OK, talking in broad generalities.

Maybe the speed of individual gas molecules on a molecular level is not entirely irrelevant.

It may even be of some considerable importance, but I think it needs to be kept in mind that the molecular speed of the individual gas molecules is not the velocity of the gas as a whole as it expands.

To illustrate, think of a swarm of bees.

When bees swarm, they all fly around at random, more or less circling the queen.


https://youtu.be/NOGzPYUs910?si=Uk5oCTFWsnh5QZVF


Don't confuse the average speed of individual bees with the speed at which the swarm as a whole might (or might not) be moving or possibly "expanding", if for example, they start taking off from a branch. or "contracting" if they gather on a branch.

If you stuck a thermometer into the swarm to measure it's "temperature", according to kinetic theory, the temperature is related to how hard individual bees are bumping into the thermometer, or their average speed.

Increasing or decreasing the average speed of individual bees (temperature) does not have any influence or bearing on the "velocity" of the swarm as a whole. Increasing the temperature does not increase the RPM.

Now, as much as I reject Matt's whole analysis about a temperature increase resulting in an engine running at 99,000 RPM or whatever, I'm kind of glad the subject came up, as in a round about way it illustrates the point I've been trying to make.

Bees swarm because they are looking for a home, inside a hollow tree or whatever.

When they finally locate a suitable place where they can all live, they take off en masse, fly up clear of the trees and other obstacles and the swarm takes off like the wind, all the bees flying in the same direction as fast as their wings can carry them. Which could be up to about 20 miles per hour.

Now, the average speed of individual bees takes on some significance.

My general argument has been that heating a gas is not very effective, it is akin to throwing rocks at a swarm of bees.disturbing them a bit.

Using a venturi or nozzle forces the gas molecules to all "fly in the same direction" like a swarm of bees that just found a new home.

The speed of a gas passing through a nozzle can infact exceed the speed of sound. Just raising the temperature however, by itself is not going to result in that kind of unified motion. If you heat a container of gas, the "velocity" of the gas is still zero. Let the heated gas escape through a nozzle converting the "pressure" into velocity, then maybe you have something.

[Fool]

There's something buried in this innocent post. Vincent quickly nailed that fact that KE is linear temp (deg K), but once you realize that 400k gas speed is 2x faster than 100k gas speed, it should become apparent that 1200k gas speed is 2x faster than 300k gas speed. Soooo, since the speed of sound at STP is 1100 ft/sec and approximates 1 bar air at 300k, then 1 bar air at 1200k is moving at 2200 ft/sec.

Now, if we consider a piston engine with 4" stroke (100mm), 1100 ft/sec piston motion (the speed of sound at STP) is 99,000 rpm as in 1100 x 3 x 60 / 2 = 99,000. Just so everyone gets this, that's 1100 ft/sec x 3 strokes/ft x 60 sec/min and divided by 2 strokes/rpm equals 99,000rpm.

I used 4" stroke since it's close to a common metric value, but 2" stroke (50mm) would double this rpm. However, the elephant in the room is that increasing temp from 300k to 1200k also doubles this "rpm". If everyone hasn't got it by now...isn't it possible that these "thermoacoustic" engines are little more than the variation of gas speed between 2 temperatures where the 'speed' of the piston approaches the difference between the gas speed of these 2 temperatures.

Three things you nailed. Very good. Speed of sound increases with speed. The piston can't be 'pushed' if moving faster than sound speed. And 99,000 would be that maximum speed, good calculation.

Case in point: John Ericsson's ship engine. Bore 14', stroke 6'. Speed limited to sound speed (1100x60)/(6x2)=5500 Rpm. It allegedly ran a 6.5 rpm. Slow enough to have a table and chairs on top of the piston and eat dinner while the engine was running. Nowhere near the limit imposed by sound speed.

The speed of sound is the point where the expansion is fully converted from isentropic to irreversible adiabatic free expansion. Adiabatic with work becomes fully irreversible/without-work. It means, at that speed no work can be output because the piston is going outward as fast as the pressure wave.

The degradation of power-out actually is progressive and why engines seldom run anywhere near that speed.

Another point that I'd like to make is, for all our engines-here, 250-500 rpm is no where near that limit. So, all expansions in the engines-here is 'with work'. That leads me to the thought that when a load is taken off and the engine tends to overheat is isn't from lack of work output. It's from lack of heat input. The hot plate isn't getting cooled by imputing heat into the engine, however, heat into the hotplate continues. Hence the hot plate melts.

Speed of information, good point. If you have a long tube held up to your ear and someone blows into the other end, when you hear the air begin to come out the other end, perhaps several seconds later, you have been informed of them blowing. This travels through the tube at the speed of sound. The air blown in doesn't come out of the your end untill much much later, and only if they keep blowing and blowing and blowing. Speed of information verses speed of mass delivery.

Bees flying around a treed cluster are flying at 20 mph, center of bee mass isn't moving. Bees in a moving swarm are flying at 20 mph and the center of mass is moving at 20 mph. If the weather is cold. Bees will fly around a treed cluster more slowly, say 15 mph. If that cluster begins to fly in unison somewhere, I wouldn't expect them to fly any faster than 15.
Kinetic energy pressure temperature, verses, kinetic energy mechanical. Same bees. Same speed. Each one burns the same energy. The second produces more work. Kind of a Carnot effect, irreversible verses reversible.

If one has a completely open tube it's resonance is a factor of it's length and gas properties, example: Ryke Tube. Speed of sound and 4 inches would be 99,000 or 1650 Hz. Kind of makes you sing.

If a venturi is inserted, a nozzle, it will significantly change the frequency. That is thoughts for how a piston-less heat engine operates.

[Tom Booth]

Thinking about it, a power cylinder is a kind of nozzle directing the flow of air molecules down a corridor towards the piston.

But as such, how effective is it?

Being rather wide and spacious, the expanding gas is probably not persuaded to stay on a direct course to impact the piston.

The advantage, or what I've been looking at as an advantage of rapid "adiabatic" expansion may involve this tendency for moving molecules to align.

That is, the faster the general mass of molecules can be induced to travel as a "swarm" in a straight line towards the piston, the more energy can be delivered to the piston/time interval.

Of course, the greater the energy transfer during the power stroke, the greater the subsequent "jam jar" type contraction to follow.

What about "isothermal" expansion? IMO it is inherently slow, allowing time for the particles to meander. It is not as effective as rapid adiabatic expansion, but perhaps that is not so much a matter of adiabatic vs isothermal or rapid vs slow as it is general directionality.

Anyway, with "lamina flow" from gas expansion through a nozzle like orifice, you get, probably, the greatest possible laser-like unified, directional flow possible in a reciprocating type engine.

How much of an influence does a "nozzle" really have on power output? I don't know. This is admittedly just theory, but I do believe there is strong evidence that a "thermoacoustic" engine or whatever one might choose to call it, won't run without that "choke".

Is it an actual "choke" that causes a time delay? (Thermal Lag) Is it a heat sink? (Thermal acoustic?) Or is it's function more of a nozzle (lamina flow)?

Experimentally, I think we can at least cross off heat sink. A wooden orifice seems to work just as well as aluminum and does not prevent the engine from running.

Thermal Lag? Eh? Not sure.

Venturi-like nozzle?

Well, that can be observed with a little smoke in the cylinder, the stream of air from the orifice can be clearly seen, so if nothing else, it does exist.

I think the role of the "choke" as a kind of nozzle helping to direct the expanding air molecules into a higher speed more uniform "jet" has been under investigated.

With all the conflicting theories being promulgated as to how these engines actually work, I'm basically just trying to narrow things down.

I don't think they "runs on sound".

[VincentG]

If one has a completely open tube it's resonance is a factor of it's length and gas properties, example: Ryke Tube. Speed of sound and 4 inches would be 99,000 or 1650 Hz. Kind of makes you sing.

This is exactly what I was trying to drive at, and why harnessing this energy (thermoacoustic) with a piston is probably a daunting task.

[Fool]

I think they run on the train-chain theory. The nozzle forces the train to speed up promoting an inertial over evacuation until reversal and that then provides an inertial over pressure. Heat is absorbed by the working gas at the high pressure end and removed at the low pressure end

Heat only goes into and out of these engines through a delta T.

Delta T between the hot plate and inside gas.

Delta T between the inside gas and the cold plate.

If the delta T stays constant during the expansion or compression it would be isothermal.

Isothermal but colder than the hot plate.

Hotter than the cold plate.

The faster the expansion the larger the delta T would be. The flame temperature would also need to be hotter than the hot plate. And room would have to be colder than the cold plate.

My point is, it would still be isothermal with those extra delta It's.