Absurdly simple thermoacoustic-steam "rice" engine – What's going on here?

[tenbitcomb]

Like many who learn about Stirling engines, I came across the concept of "thermoacoustic" and "thermal lag" engines and became fascinated by their simplicity. I haven't built one yet, but have been addicted to watching YouTube videos on them.

Last night I came across some videos that are particularly interesting. They feature what appears to be a heat engine with a single free piston (diaphragm), upright configuration, *no displacer*, and no fluidyne or pressure wave reciprocating force. Although the guy in the videos adds a small amount of water, he claims that you don't actually need water but that it performs a lot better with water.

This appears to be how it's composed:

- Single cylinder
- Steel wool regenerator (unattached)
- Spacer sitting on bottom of cylinder holding up the regenerator by maybe 1/2 inch.
- Tiny hole at bottom of cylinder for adding water
- A layer of dry rice placed on top of the regenerator
- Small pressure release hole near top of cylinder
- Balloon diaphragm with an attached magnet

His first video showing how it's built:
https://www.youtube.com/watch?v=WwvOM8Sfzts

Video where he demonstrates that there is a thermoacoustic property to this:
https://www.youtube.com/watch?v=m-HlURJBLcw

Subsequent video where his engine goes nuts when producing steam:
https://www.youtube.com/watch?v=Lyhylj5kHjw

His latest video where he talks about replacing the rice with glass beads:
https://www.youtube.com/watch?v=eSLsXvNivUI


Now for the things I'm confused about:

- If a displacer isn't needed for this type of engine, why are they supposedly needed with other "metronome" engines?
- Is the rice acting in a similar way to a restrictor in a thermal lag or lamina flow engine?
- Other thermal lag engines seem sensitive to where the source of heat is placed near/on the regenerator, so why does that seem to not be the case here?
- Is there reason to believe a long tube acting as an air piston could work in place of the steam? Or filling the cylinder with helium?

I understand that a typical thermoacoustic/thermal lag engine is effectively as simple in terms of moving parts, but it's the configuration of this engine that defies my understanding as to why other similar engines need a displacer and only operate well when the heat source is hitting the regenerator on the end facing the piston. Maybe it just comes down to having some steam in there, but the purpose of the steam seems to only enhance the thermoacoustic property of the engine as opposed to being an analog of a traditional steam engine. Pretty much every example of vertically-oriented thermoacoustic engine I've seen has insisted on configurations that are absent here.

This is really interesting because the performance appears to be impressive for being the least complicated engine I think I've seen up to this point. Even if it doesn't provide as much kinetic power as a Stirling, it appears to provide enough that it would be worth building and using to trickle charge something like a phone. With the steam it looks like there's a lot of potential for practical use.

[Tom Booth]

I guess it's not the rice absorbing steam, if it can be replaced by glass beads.

So many unusual variations

The only essentials are apparently heat supplied to expand the air and work to cool it.


https://youtu.be/t6cGw1scLvc

[tenbitcomb]

I did a test last night, albeit imperfect, and it indeed produces an audible and quite loud tone. I couldn't totally sustain it, and I'm guessing it has something to do with not having the kind of hole/vent configuration this guy has in his videos. The vessel I used was a steel camping mug thing and so I didn't have a drilled hole either above or below the regenerator. I did poke some holes in the balloon, which I think messes things up because those holes expand or get water in them over time. The $1.98 balloons from Wallmart I used were also too small. In any case, something interesting is definitely going on, and I will be doing some better tests in the coming days. I assumed getting a thermoacoustic effect would take more forethought, but it worked on the first try without really measuring anything.

The rice (or whatever it's meant to be) does seem to be needed. I tried without the rice and didn't succeed in producing a resonant tone. Doesn't mean that it can't happen without it, but it *seemed* to make a difference.

What most impressed me about my test is it seemed to demonstrate how the work performed keeps the diaphragm from getting too hot. During the minutes that the "engine" was producing a frequency, the diaphragm was relatively cool to the touch. Only when the resonance was lost did it seem to heat up more.

The rice is kind of a mystery to me still. This engine seems like it might share some qualities with a "heat pipe", and maybe a similar effect of a wick in a heat pipe takes place if the rice or glass beads act similar to the packing material in a reflux alcohol still. The author of the videos says rice actually works a bit better than the glass beads, so maybe absorption is playing a role. I don't really know.

[Tom Booth]

What I'm wondering is what in the world motivated someone to use rice in a hot air engine in the first place, and was there some actual theory behind it.

Maybe someone was cooking rice and noticed how the lid on the pot kept jumping up and down and had an Aha! moment.

[Tom Booth]

I have a thought.

A very long time ago, though I've been unable to locate the post again, maybe it was a different forum, but someone made the statement that their Stirling engine ran better if they put a little water along with some smoke in the engine

Apparently with changes in pressure, the water vapor was alternately, and very rapidly condensing and then evaporating and condensing again on the smoke particles, the same way clouds form.

Of course, the phase change between the liquid and gas states results in a change in volume, greater than heating and cooling dry air.

So, what does rice have in common with glass beads?

A lot of surface area for the water vapor to condense on ? A low surface tension coating ?

Under higher pressure the microscopic drops of water all over the rice are heated enough to evaporate causing expansion -> work -> cooling -> pressure drop and the microscopic water droplets condense once again.

That would be my theory, anyway.

Instant steam, and instant condensation alternating back and forth with the alternation in pressure during expansion and contraction.

If that is the case, what would help facilitate condensation ? Of course water vapor condenses readily on glass. Is there anything better ?

Metal maybe? Copper BB's like for a BB gun? Aluminum ?

I think some lubricants are sometimes used to coat metal tubes to promote condensation in refrigeration systems etc.

Maybe BB's coated with silicone ?

Now I'm wondering if a typical "regenerator" consisting of a wad of fine steel wool might not be providing the surface area for the condensation of water vapor.

Hmmm...

Rock salt maybe?

I spilled some rock salt in the (humid) basement and the salt crystals just attracts and condenses so much water vapor it forms little puddles of water around each salt crystal after a while.

And of course, people put rice in salt shakers to keep the salt from clumping, because...

[Tom Booth]

Aha!

I had to look up what a reflux still is and what the packing is for:

https://www.clawhammersupply.com/blogs/ ... gher-proof

Wow! that is almost like confirmation.

The packing provides surface area to condense water vapor.

So my theory was that heat of compression and cooling from expansion causes rapid condensation and evaporation, directly augmenting the ordinary expansion and contraction with an element of phase change.

So maybe just use some standard reflux still packing material

But since that is for a consumable product they would not use a lubricant that could end up as a contaminant.

Rice has a coating that causes it to foam (like soap bubbles) so it boils over easily.

I see refrigeration condenser systems sometimes coat the condenser piping with a surfactant to produce more rapid condensation.

[Tom Booth]

BTW, as I recall, the person in that forum who suggested adding water and smoke to the displacer chamber received a good reprimand. "No! The last thing you want is water in the engine!"

[tenbitcomb]

Under higher pressure the microscopic drops of water all over the rice are heated enough to evaporate causing expansion -> work -> cooling -> pressure drop and the microscopic water droplets condense once again.

Yes, I think yours is the best theory yet.

Perhaps what we are looking at is a phase-change ("wet") thermoacoustic engine?

https://www.sciencedirect.com/science/a ... 4222015687

Unfortunately, I don't have access to the full article, and I didn't find it on Sci-Hub. Still, what the abstract describes seems in line with what we're thinking.

[tenbitcomb]

Rock salt maybe?

Maybe. I'm tempted to try calcium chloride salt, which is extremely hygroscopic but easily drops out water once it reaches its saturation point. Only thing is the saturation process is exothermic, so I'm unsure whether that actually helps or hinders the process. Only one way to find out!

However, that definitely wouldn't be a sustainable engine by any means since the calcium chloride would melt and fall into solution.

[tenbitcomb]

So far I'm of the opinion that the steel wool is not necessarily a regenerator per se but a stack merely meant to cool the air/vapor. The rice may be serving two purposes. The primary purpose being to merely restrict the flow of vapor so it remains in the stack rather than overheat the diaphragm or escape elsewhere. The secondary being that its absorptive capability somehow helps the phase transition This may explain why the rice can be replaced by something like glass beads, but allegedly performs slightly better.

I don't have much time to perform more experiments today, but I made another attempt and have failed. My first few attempts with the camping mug absolutely produced a tone. It was unmistakably singing, indicating a definite acoustic property. I think it was thermoacoustic because the diaphragm definitely maintained a reasonable temperature as long as the engine was singing. If only I'd taken video. I made some attempts last night and this morning with some steel cans and haven't had any luck whether I added a weight to the diaphragm or not. Hopefully my initial success wasn't just a miracle.

[tenbitcomb]

BREAKING! I stumbled on an article called "Do It Yourself: make your own thermoacoustic engine with steel wool or rice."

https://hal.archives-ouvertes.fr/hal-01397360/document

Video demonstration:
https://www.youtube.com/watch?v=dBqa5t8nefo

So now this makes me think I had it backwards. The rice is the primary component. While it's possible that steel wool (absent in the demo) is having a regenerative effect, but judging by the paper, it's being used in place of the reducer.

[Tom Booth]

Under higher pressure the microscopic drops of water all over the rice are heated enough to evaporate causing expansion -> work -> cooling -> pressure drop and the microscopic water droplets condense once again.

Yes, I think yours is the best theory yet.

Perhaps what we are looking at is a phase-change ("wet") thermoacoustic engine?

https://www.sciencedirect.com/science/a ... 4222015687

Unfortunately, I don't have access to the full article, and I didn't find it on Sci-Hub. Still, what the abstract describes seems in line with what we're thinking.

Seems a bit strange that the publication is dated in the future?

Resize_20221009_195712_2475.jpg (66.12 KiB) Viewed 13372 times

[Tom Booth]

Rock salt maybe?

Maybe. I'm tempted to try calcium chloride salt, which is extremely hygroscopic but easily drops out water once it reaches its saturation point. Only thing is the saturation process is exothermic, so I'm unsure whether that actually helps or hinders the process. Only one way to find out!

However, that definitely wouldn't be a sustainable engine by any means since the calcium chloride would melt and fall into solution.

I wouldn't be 100% sure about the melting.

We are talking about a basically almost microscopic absorption and release of a minute possibly invisible quantity of HUMIDITY. At most a bit of visible "fog" in a can. (Probably).

It might be interesting to use a clear container. Is there any visible steam or a "cloud" formation ?

https://youtu.be/z4M1Rr3SWIg

Of course, if the rice or whatever is taking on the role of the smoke particles as a condensation platform, there is likely nothing to see, without a high speed microscopic camera that could capture the tiny droplet formation.

But at say 500 RPM, the droplets would appear and disappear like 8 times / second, on the surface of the glass beads or whatever.

Rice in a salt shaker never absorbs enough humidity to get soggy. It absorbs some on humid days and releases it on dry days, staying relatively dry on average.

Same idea, but each "day" is 1/2 engine cycle.

[Tom Booth]

As far as the audible sound, I may be wrong, but my view or opinion is it is apple's to oranges, or unrelated to engine performance. Many, who have gone much deeper into thermoacoustic would disagree I'm sure.

But a pipe in a pipe organ makes a sound, but that doesn't mean the knocking in my pipes is what brings the water into my sink.

The plumber doesn't need to consider the musical properties inherent in the lengths of pipes he solders together to ensure the transportation of the water through them.

IMO, in most Stirling engines, any potential acoustic properties inherent in the "pipe" or whatever used to build the engine play no role in the heat, expansion, cooling, contraction cycle that gives the engine the ability to produce power output

The birds singing in the morning are not what actually cause the sunrise, though the birds may think so.

[tenbitcomb]

In terms of the calcium salt, I have to somewhat disagree because I've worked with it quite a bit. If you put it in an enclosed container with a one way valve and blow through it, it won't take long at all for the vapor in your breath to be absorbed and eventually fall out (with the salt in solution). Rice and table salt may not get soggy in general if left exposed, but calcium chloride definitely will. Damprid dehumidifiers wouldn't work if it didn't.

IMO, in most Stirling engines, any potential acoustic properties inherent in the "pipe" or whatever used to build the engine play no role in the heat, expansion, cooling, contraction cycle that gives the engine the ability to produce power output

When it comes to things like "metronome" engines, I definitely agree. As much as they are very interesting, calling them thermoacoustic isn't accurate from what I can tell. They are really glorified gamma engines with an air-piston in place of the power piston. The air piston may put it into a gray area of acoustics, but it's not exactly the kind of thermoacoustic resonance occurring because of air moving through a pipe. Nothing is necessarily resonating there. Add a solid piston and a flywheel to a metronome engine and you've got a LTD.

In this case, I'm unsure if the rice engine is not thermoacoustic. Maybe. The effect of the rice might be doing what a displacer is meant to do, and it seems a displacer is superior because the rice cannot sustain the temperature gradient, which is why the tone diminishes. I'm still willing to buy that the power output is indeed thermoacoustic, but it does also seem possible that the thermoacoustic properties are incidental to the actual kinetic output when weight is attached to the diaphragm. Still, the only cohesive theory seems to be thermoacoustics since that property is observable. I'm not sure how the engine can be tied to Stirling principles outside of conjecture, only because I don't have the knowledge or capability to test for those principles here.

As an aside, I've become much less confident that the rice engine is as practical as other thermoacoustic and Stirling engines. That is if it's even practical at all. The temperature gradient is momentary and it does require water, albeit small amounts at a time. Maybe it could run continuously if the cold side was water-chilled, but at that point enough complexity has been added that you might as well have built a more stable engine in the first place. The best thing about the rice engine, I think, is that it may be the simplest way to demonstrate thermoacoustics with everyday items. No need for pipes, glass tubes, or even steel wool if the rice can be suspended by some other means.