Boundary layer heat exchanger - theory

[Tom Booth]

I've been re-studying the TK Motors engine in comparison with various other similar engines, with the intention of trying to duplicate it, and at the same time trying to figure out what makes it appear to run with so much energy.

One thing I noticed, that seems different from other designs, is that he has the inner can, apparently pressed down quite tight against the outer, heated can.

I tended to think that this very tight fit would restrict air flow, so a possible "improvement" might be to raise the inner can above the hot "floor" some distance.

The bottom of the inner can also has a rim or ridge, and it seemed to me this ridge would nearly block the air flow almost entirely. So I continued rather puzzled how this engine could run at all, never mind, with such seemingly exceptional power.

I say, "apparently" because I don't know that this engine has ever been hooked up to a load.

Anyway, it occured to me this morning that perhaps what I considered a restriction is actually what is responsible for the high energy output.

Thinking about the "boundary layer", air tends to cling to surfaces. In other words, there is a layer of rather stagnant air clinging to the surface of the heat exchanger.

So, possibly the very tight fitting bottom lip of the inner can, pretty much in direct contact with the bottom of the outer can, right where air would be forced to flow back and forth across the bottom hot plate, causes a breaking up of this insulating boundary layer. A great deal of air being forced back and forth through such an extremely narrow passage, bringing the air into direct contact with the hot metal, rather than only glancing off a thin, insulating layer of stagnant air clinging to the metal surface.

Then, I thought, if one close fitting ring produced such an effect, what if there were several such bottom rings, one inside the other ?

Resize_20220828_075534_4511.jpg (179.3 KiB) Viewed 1406 times

Of course, this is only a theory, but it makes some sense to me.


Video showing the construction of the TK Motors engine

https://youtu.be/CXsFNPmjluo

And running:

https://youtu.be/r9lYsW0Df08

[skyofcolorado]

I love the simplicity and elegance of this engine.

I don't suppose there's a rule of thumb on the various parameters when building these? I've seen so many variations and almost no general rules on dimensions, displacement, stroke, diameter, etc. It's as if they just build them randomly (or intuitively) and post the ones that work with no math to support the design.

[Tom Booth]

After rewatching the video a few more times, I noticed that, not only is the inner can concave on the bottom, but the outer can is also hammered in, concave on the bottom to match.

So my assumption that the bottom edge of the inner can formed a "pinch point" is incorrect. The bottoms of the two cans appear to be nestled together like spoons.

The steel wool on the sides, I would think, almost entirely, prevents the inner displacer (?) from moving, but the steel wool is a little loose and springy, so possibly the displacer does move very slightly, but the "fact" (I think, probably) remains that the air has to pass through, or actually squeeze through, what appears to be a very narrow gap between the two cans at the bottom.

This seems quite different from the typical displacer that gets lifted up away, quite a distance from the bottom.

Also, in another video about a similar earlier version of the engine, the inner can is apparently fixed in place completely, held down by some metal clips.

It was this earlier "free piston" version that he decided to attach a crankshaft to, just on an impulse, or a hunch.


https://youtu.be/iFXiaO8iwhk

In that engine, there are tightly packed ballbearings in the gap between the cans, creating a network of narrow passages.

It seems odd, in a way, that "the same" engine will run either with, or without a flywheel. With either a crank or "free piston" but at different speeds.

I did a few experiments. Even a store bought toy model thermoacoustic will still operate without the flywheel, but at a much higher frequency, with no clunky crank and flywheel to slow it down.


https://youtu.be/iOs3BADFeKI

As far as "rules". After watching so many videos of so many engines built from all kinds of random tin cans, beer/soda cans, pipe, glass jars, tuna cans, cookie tins, etc. etc. I have to conclude that the engine will have or find its own particular operating temperature and frequency.

I saw a video where someone demonstrated that their free piston engine would run, or settle into different running modes or frequencies or power levels, depending on how hard they banged the piston to get it going.

A light tap and the engine would run one way. A stronger tap and it would run at a higher power level or different frequency.

There seems to be no end to the possible variations and configurations.

On the other hand, some go to considerable lengths to measure the length of tube to "tune" the engine, and as you pointed out, who knows how many iterations we're tried prior to making a video of "the one that worked".

In most cases, IMO, the "one that worked" was more often than not, the first. That is, however the engine is put together, it will run at some given natural frequency that is discovered rather than planned.

In most cases a failure, is more likely to be some fatal flaw, such as a bad seal not holding air rather than a failure to follow some mathematical rule or formula.

[tibsim]

The biggest mystery is why the membrane doesn't burn.

[Tom Booth]

The biggest mystery is why the membrane doesn't burn.

Exactly!

But personally, I no longer consider it a mystery. I've been doing a lot of experiments to test Carnot (all or most of the heat as a kind of material fluid, goes through a heat engine) vs. Tesla (Heat is just a form of energy that is converted to work, a different form of energy so is no longer heat, so the "heat" does not necessarily pass through, in a very efficient heat engine).

The heat is consumed by the heat engine before it reaches the membrane. It indicates that the engine is very efficient at converting heat into mechanical motion or work.

I think Tesla was right.

On the other hand, he has what looks like a wet rag wrapped around the engine to keep it cool, shakes his hands, while changing hands to hold the can, as if his fingers are getting unbearably hot, dispute the wet rag, and while glueing the crank to the membrane, there is already a rainbow of colored material that looks like it is probably previously melted balloons.

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So, the engine is not 100% efficient. Regardless, it runs for quite a long time with a very hot looking gas flame before apparently failing at the end of the run.

The engine seems to peter out before he turns off the burner. I'm guessing the membrane failed (again). But then it is apparently, just a balloon. Hot or not, remarkable it could last that long IMO.