Thermoacoustic temperature gradient?

[Tom Booth]

The first time I saw a thermoacoustic engine running without any flywheel, I thought something strange was going on.

These engines run at such a very high frequency, I also wondered how in the world heat could dissipate to a "sink" so quickly that the piston could be "pulled" back by the gas cooling and contracting, in a fraction of a second.

I also wondered which end of the regenerator was supposed to be the cold end acting as the "sink".

Well I've been doing experiments for years with LTD engines, and have come to the conclusion that LTD engines don't seem to really require any "sink" for cooling, rather, the gas heats and expands and cools as a result of work output (at or through the power piston)

Supposedly that violates some law of thermodynamics or the other.

I've been told over and over that a little LTD is so inefficient and uses such a small amount of heat the temperature difference is just too small to measure.. yada yada yada...

OK, so this evening I've been testing my "theory" or observation with a higher temperature thermoacoustic engine.

I've always heard, read and basically assumed myself that only one end of the "regenerator" could be heated. That there needed to be a "temperature gradient" because one end of the regenerator needed to be hot and the other end cold, or at least not heated.

But which end needs to be hot?

Usually the end closest to the power piston is heated, but I've also seen it done the other way around where the very tip of the engine, or test tube is heated, and even sometimes in the middle.

So which works better?

Also if a cold side is not needed in an LTD, is a cold END really necessary in a thermoacoustic engine (or "laminar flow", or "thermal lag",... different names for the same thing?)

Anyway, I found one of my high temperature engines that runs rather well without a flywheel and did some experimenting.


https://youtu.be/i0Gudw8KQiU?si=03rHNqkakSFTVBCq


https://youtu.be/H3fV27BJs_A?si=JaH98fhNJ8DoV4JW


As far as I can tell, it doesn't matter which end of the regenerator is heated. I basically tried to heat one end, then the other, then tried to keep both ends the same temperature.

Nothing really made much, if any difference, in fact, heating the aluminum body in the middle and/or the power piston hardly made a difference.

Some observations, though.

Once the engine was running, sometimes the temperature of one end or the other of the regenerator seemed to shoot up way high, like over 1000°F (without applying additional heat)

Also applying heat directly to the power piston did not seem to elevate the temperature of the power cylinder much (as if it was absorbing the heat?)

In fact there were times when the temperature of the power piston kept falling even while heat was still being applied to the engine.

Some of this may just be due to the thermocouples not staying in place with all the vibration. I need to do more testing.

So where exactly is the "sink"????

Maybe the aluminum body in the middle?

So next I'm going to be turning a wooden piece to replace the aluminum and see what that does.

There were however, as far as I could tell, clearly moments when the regenerator tube was very nearly the same temperature at both ends and this did not effect or hinder the running of the engine at all, though there was no temperature gradient across the "regenerator".

[Tom Booth]

Someone on YouTube commented that the central aluminum piece is the regenerator.

I guess maybe that could make sense, if the test tube side is the hot cylinder and the power piston is the cold cylinder.

I replied, though, the orifice through the aluminum is a hole only about 1/8 inch.

Resize_20230902_042020_0952.jpg (176.23 KiB) Viewed 8033 times

Not a lot of surface area for heat exchange in there I don't think.


Anyway, I'm done making the wooden, non-heat conducting engine body.


https://youtu.be/bf1GKvrvaig?si=iEAPDMMK8EHhEHgT


That should make a pretty lousy heat sink or lousy regenerator, either way.

I always suspected that a non-heat conducting/insulating engine body would probably perform better and be more efficient than a metal engine. Particularly copper or aluminum as they conduct heat out of the engine.

You want the hot air to expand and push the piston right? So why force the hot air through a narrow metal orifice that is going to take all the heat away?

I guess the Second Law of Thermodynamics, right? Somewhere to dump all that supposed "waste heat".

But a heat engine runs on heat. Cold is the absence of heat.

Anyway I guess we shall see.

Is anyone taking bets?

I bet, if anything, the engine should run a little better with a wooden body. Less heat loss, better efficiency.

[Tom Booth]

This video shows a thermoacoustic engine made from a single test tube with a wooden dowel inserted into the middle for an orifice.

It runs.


https://youtu.be/seIHO5XO5kM?si=Da8GJuZ-PWQB3wal


What we really want though is ginormous cooling fins like this:


https://youtu.be/RHFKvevqQnI?si=Dcl5KNFJ2JbGVBhc

That runs too.

[Tom Booth]

I suspect the gas passes through the orifice at such a high velocity there is no time for any heat exchange between the high velocity gas and the walls of the passage or orifice as either a "sink" or a "regenerator".

So, generally, IMO, the engine with huge cooling fins runs in spite of the enormous heat sink rather than because of it.

Perhaps it could make some slight difference, or have some effect at the end of each stroke, during the "dwell" when the piston is changing direction, one way or the other and the gas is relatively motionless.

Anyway the non-conductive orifice should pretty much eliminate the aluminum orifice as a potential "sink" or other major heat exchange element. Or show, by comparison, what, if any difference it actually makes.

[Tom Booth]

It's a bit anticlimactic at this point but:


https://youtu.be/RODm1LCqwuk?si=hsTCs7FiCJshL1rm

[Tom Booth]

So, what to make of this?

Here is, I suppose, the generally accepted explanation of how a thermoacoustic engine works:

The thermo-acoustic engine works by converting sound waves into motion. The sound waves are generated by heating one end of a 'stack' of coiled material and allowing the other end to remain cool.

https://www.enginediy.com/blogs/engined ... -principle

From Engine DIY. They should know I guess, they sell these things.

Then there is frequent mention of Lord Rayleigh:

https://youtu.be/wg96lDw7sNw?si=cphb3xURBYkV3SbQ

The narrator of this video doesn't really manage to actually strike a note when blowing on the bottle.

Personally I'd find this "standing wave" theory entirely laughable. A Stirling engine is not anything like a flute, but then we have this apparent confirmation and demonstration from Blade Attila, certainly a well respected and recognized expert on Stirling engines:

https://youtu.be/Qqk36IZqIgY?si=Q15n8g3ikcBai7mE

For anyone who may be interested there is also a build video showing how this "thermoacoustic flute" was made: https://youtu.be/t7mBOcmEADc?si=Hfq37jqaXcuyU7AU

This video again mentions standing waves that are clearly supposed to be inside the long hollow glass tube with the small piece of steel wool or "stack" which must be carefully heated at one end only while keeping the other end cold.

Sorry, but it is necessary to actually read the text.

https://youtu.be/ErlvMZI0tlA?si=wPIfOMyd4Nxxg3ed

And this theory has even more elaborate and spectacular proofs and demonstrations, flaming tubes with nodes and antibodies and it goes on and on.

https://youtu.be/xCnxsoXtlmY?si=AU1-WYXnKro9OpeD

And then there are more mundane explanations of simple heating/expansion and cooling, with cooling by the large block of aluminum and/or the cold power cylinder walls.

https://youtu.be/TEiBScfR4Vk?si=c8lx3sRVNNDEBnGu


All interesting food for thought.

But, apparently, not actually true.

[Tom Booth]

One last thing I can think of to try.

I made this styrofoam sleeve to go over the power piston.

Resize_20230904_110437_7504.jpg (148.03 KiB) Viewed 7962 times

[Tom Booth]

Well, still keeps on running.

https://youtu.be/LG09AXAjpio?si=3X3o7ZlAZB4T5EiI

[Fool]

I'm wondering what would happen if it were put in an oven? Would it keep running on the heat of the oven? Low enough temperature so the foam doesn't melt, of course. High enough to keep running, too.

[Tom Booth]

I'm wondering what would happen if it were put in an oven? Would it keep running on the heat of the oven? Low enough temperature so the foam doesn't melt, of course. High enough to keep running, too.

There is still the back of the piston that does work against the relatively cold external atmosphere that is a bit of a wild card.

When the piston moves down the cylinder displacing atmosphere it does "work" and presumably warms the atmosphere slightly. Since the atmosphere is a kind of infinite sink, not all of that heat is returned, or is it?

In an oven, assuming the engine were started outside the oven, the only variable remaining would be the back, exposed end of the piston interfacing with the atmosphere. Outside the oven the ambient atmosphere is relatively cold (relative to the propane torch) on the back of the piston. Inside the oven, the back of the piston would be subject to the same heat as the hot end.

I couldn't predict if that would have a beneficial or detrimental effect, or no effect at all.

If the piston were non-conductive, the additional energy provided by the oven's hot "atmosphere" might actually put more heat into the engine during compression, eventually causing the engine to overheat, unless maybe work output could be increased.

Or perhaps the back side of the piston could be enclosed in a "buffer space" so it too could be insulated.

Basically, the power cylinder is insulated in a "cold hole". Which can only stay cold if very well insulated and the engine is exporting energy to some external load.

In other words, the engine, if equiped with a generator and wires from the generator pass out of the oven to some external electrical load outside of the oven.

I suppose now I'll have to construct some sort of buffer chamber for this thing

Probably some more heat resistant insulation would need to be used, like KAO ceramic wool because these engines need high heat, and the styrofoam would not survive in a hot oven.

Relatively speaking we are already in an oven. The ambient atmosphere itself. The sun being the radiant "heating element" heating the atmosphere.

A long time ago I posted this little gif as a possible or theoretical thermoacoustic ambient heat engine.

ezgif.com-optimize.gif (527.32 KiB) Viewed 7921 times

That isn't an oven. That's a cold box to go inside the "oven".

In theory, the gas is heated and expanded driving the piston resulting in cooling and lowering the pressure.

There is a fluid pressure "valve" so atmospheric pressure can return the piston, possibly not needed with an appropriate size buffer space.

The "work" output to keep the piston moving continually lowers the energy level in the gas, keeping it relatively cool. Heat that enters gets converted to additional work output through the external linear generator.

[Tom Booth]

Here is my somewhat recently revised idea of how something of this sort could work.

"Revised" mainly do to my recent observation of a laminar jet through the nozzle in a thermoacoustic engine.

The gas is heated in the hot chamber which forces air through the nozzle which converts heat into a high velocity stream that impacts the piston doing work.

So "heat" is not what drives the engine, it is the velocity of the laminar gas stream.

Since the gas stream is narrow the pressure in the power cylinder doesn't actually rise much at all. Velocity is converted to work, so "heat" never really leaves the hot chamber. The power cylinder maintains a relative vacuum. The high velocity jet stream shoots through this vacuum driving the piston, not by "expansion" of the gas in the power cylinder, but rather due to the high velocity air stream. The "pressure" is narrowly focused on the piston, leaving the rest of the power cylinder in a relatively low pressure state.

I've considered such a possibility before, but it seems that much more plausible now, after actually seeing this laminar stream due to the smoke in the engine.

Then, the gas cools and the vacuum (relatively low pressure) pulls (allows the piston to be pushed) the piston back and the air stream goes the other way through the nozzle picking up heat in the hot chamber.

The "cold hole" probably only needs to be relatively cool not cryogenically cold as previously imagined as temperature really has little to do with the driving force of the engine, which is velocity.