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

[Tom Booth]

Maybe cellulose fiber (paper) could substitute for the rice. Might be worth experimenting with:

A stack made of cellulose paper strips was designed to ensure sufficient liquid supply and rapid liquid absorption during the phase-change thermoacoustic conversion

Don't have full access, but just the abstract alone is interesting.

https://www.sciencedirect.com/science/a ... via%3Dihub

There seems to be quite a bit of current research into these "wet" so-called "thermoacoustic" engines.

I get the idea of compression and expansion with phase change, but the significance of any "acoustic" element escapes me.

Sometimes any engine of just about any kind will "resonate" at some audible frequency, but most of the time even so-called thermoacoustic Stirling engines do not produce any tone when actually running.

Anyway, the implication of using paper is that absorption does play some roll, if only as a means of making the water readily available with a lot of surface to air interface.

It would seem just about anything that provides a lot of surface area for condensation and holds water in a way that promotes rapid evaporation will do.

Obviously, I think, just having a puddle of water in the bottom of the can is at best inefficient. Having many small (microscopic?) water droplets suspended on some substrate where both rapid condensation and evaporation may transpire is likely optimal.

Maybe instead of cellulose/paper fiber, which I think has some undesirable characteristics, being an organic substance like rice, subject to getting mushy or moldy or even burning if it dries out, maybe something like the ceramic fiber I've been testing for displacer material would work.

Having a damp fibrous material might make a more compact substrate with more surface area in less space, unless perhaps the added thermal mass or heat transfer capacity of the pebbles, glass beads or copper BB's and the like proves to be essential.

[Tom Booth]

I'm not sure if anyone mentioned or noticed this before, but research is apparently so prevalent lately (looks like since about 2017 ) a standard-ish acronym has developed: "Wet thermoacoustic engine" or "WTE" which can be of assistance when searching on the internet for information on the subject as just using "rice engine" only seems to turn up this forum and what has already been covered.

Mostly PDF's behind various paywalls, but something can often be gleaned from the abstracts.

For example, I've seen it mentioned several times that a "wet" thermoacoustic engine may realize a power output increase by as much as 8X to (15X ?) over a "dry" engine. I'm not entirely sure about the 15 number as I can't locate the reference at the moment, but that number seems to stand out in my memory from browsing around last night.

Also, the temperature differential required is lower, by as much as 200° and the start up temperature at which the engine is able to run is lower, making the use of "low grade" heat more feasible.

Further improvements are seen when fluids with a lower boiling point than water are used (mostly ethanol, with a boiling point 40° lower (F) than water).

How much of this information might actually be applicable or have a bearing on a simple DIY "rice" type engine or Stirling/hot air engines generally, I don't know, but from what has been accomplished here so far with the various kitchen stove and tin can experiments, I'd say the results are fantastic and it looks like further improvements are possible.

I can hardly wait to try it myself. Unfortunately I've been pretty tied up with other things that need attending to and my Stirling engine project catch up list is already a mile long.

[tenbitcomb]

I'd share a link to the full version of that paper, but I hesitate to do so since it sounds like Daemon's original account and posts were wiped, and he was the one who originally shared it. I don't want to suddenly disappear because of sharing something like that in case that's what happened. However, if you do a web search for "1-s20-s0360544222015687-main-pdf", you might come across a Docdroid page with it.

My guess is a lot of this information applies to our engine because it's about as far from being an isolated thermodynamic system as one could get. Every component has a direct effect on how it performs, including your body if you choose to grasp the engine with your hand. So far I struggle to think of anything that doesn't affect it in some way. Even rice seems somewhat different in contrast to glass beads.

What that paper indirectly highlights it he lack of an actual cold exchanger with our engine so far. Every paper I've seen on thermoacoustic engines suggests an explicit cold exchanger. The closest I've seen on any TA engine on YouTube is having heat sinks. Improved cold exchanging should be another area to be explored here.

Speaking of, I think I remember you (Tom) questioning the need for space between the bottom of the hot end and the stack. That's an interesting thought because while academic examples of TA engines exhibit a "resonator" on the hot side, resembling the hot end on our engine, it's unclear to me whether two resonating sections are needed. My perception is that the entire engine is a resonator, so it shouldn't matter if the stack is spaced above where the water is boiling. The water in our engine is effectively the heat exchanger (HHX), and I don't think it necessarily needs to have dead space between it and the stack unless it's an obscene amount of water. I've been very busy, but I think I'll have a moment to try this out tonight and see if I can find an answer. If the engine works just fine without the hassle of making a divider for the stack, that would really blow me away further. The engine is already stupidly simple, but a can with pebbles at the bottom is even simpler than that.

> Also, the temperature differential required is lower, by as much as 200° and the start up temperature at which the engine is able to run is lower, making the use of "low grade" heat more feasible.

There's the factor of the working fluid, but then there's also the matter of tuning the rest of the engine. I have an inkling that the engine could operate at lower temperatures by finding the right balance between the side of the stack, the length of the cylinder, and the inner diameter. But I don't really know for sure because I've stuck mainly with two cans of the same size.

> Obviously, I think, just having a puddle of water in the bottom of the can is at best inefficient. Having many small (microscopic?) water droplets suspended on some substrate where both rapid condensation and evaporation may transpire is likely optimal.

Sounds an awful lot like a heat pipe:

https://en.wikipedia.org/wiki/Heat_pipe

Makes me wonder the best way this can be applied to our engine. Could even be a good idea for cooling the cold section.

[tibsim]

My old steam thermoacoustic engine:
,

an experiment alcohol:
https://www.youtube.com/watch?v=hJV7HzZVr5A
A big engine. ThiS is not mine:
https://www.youtube.com/watch?v=hUOwnrnv8ak
The main problem with these machines is that the temperature difference is small! Carnot h = 1 - (Tc/Th) ...
My webpages:
https://tibsim-thermoacoustics1488.blogspot.com/
URL removed because of it's "anti-Semitic" views. Do not re-post.

[Tom Booth]

Is that "steam Stirling" page dated 2013 ? You've been at this a long time.

The main problem with these machines is that the temperature difference is small! Carnot h = 1 - (Tc/Th) .

.
LOL

At least I hope you're joking.

As far as I'm concerned it's just a couple more nails in the coffin.

Carnot said the working fluid used could have no influence, only the emperature difference.

IMO, it is quite obvious a phase change fluid makes an incredible difference. Your generating power from a pot of warm water with that thing running on alcohol with no apparent difficulty.

The temperature difference between water vapor as vapor and water as liquid is, or can be ZERO!

Obviously expansion ratio has a much greater significance in terms of power output than temperature difference.

[tenbitcomb]

I get the idea of compression and expansion with phase change, but the significance of any "acoustic" element escapes me.

The term "acoustics" is somewhat of a misdirection. Acoustics are merely kinetic wave dynamics, and that may or may not have anything to do with audible sound a human being is capable of hearing. Audible sound, driving pure air at standard pressure, is just the simplest target of work of this class of heat engine. Whether it's a volume of atmosphere or a solid piston on a flywheel, they both have weight and inertia such that they can reciprocate against a pressure change from hot air cooling, contributing to a net resonant frequency that allows oscillating expansion and contraction to occur.

With an open-ended TA engine, you can think of the air surrounding the cold end of the tube as the piston or diaphragm. Hot air flows up the tube to the cold end where it contracts and slows down. Because air has inertia, there's a delay while the pressure drops, creating a vacuum that pulls surrounding air at standard pressure down the cold end of the tube, thus completing the wave cycle. Placing a piston or diaphragm on the end means there is greater than standard pressure that the engine must work against, and that's basically the only difference.

Tom, remember how you said that more weight added demands more heat to be converted to work? This is precisely it! Give the engine something to work against, give it enough heat, and it will do it.

Sometimes any engine of just about any kind will "resonate" at some audible frequency

Right, and that's reason why expecting what a human would consider an audible tone is a poor metric for judging whether a thermoacoustic phenomenon is taking place. Not only can any form of engine potentially generate secondary harmonics that resonate with some unrelated facet of said engine, but a thermoacoustic engine may not even generate a recognizable tone depending on how it's configured (or what it's driving).

, but most of the time even so-called thermoacoustic Stirling engines do not produce any tone when actually running.

It may or may not depend on what you're referring to. There are "hybrid" thermoacoustic Stirling engines like the so-called "metronome" engine, and those are unlikely to produce a smooth sine wave that can be considered a tone. The metronome engine is effectively a Ringbom engine with an air piston (which I've realized is indeed thermoacoustic), and Ringbom engines generate a square-sawtooth wave most of the time. But I'm off in the weeds here.

The expectation shouldn't be that a TA engine produce an audible tone *unless* it's been configured that way. A TA engine's frequency is dictated by its components, the piston and crankshaft being one of them. It will not produce a standing wave that isn't supported by the the engine components including the moving part because that would be paradoxical.

In the case of most TAs on Youtube, their frequency is determined by variables such as the volume of the cylinder at any given time, the radius of crankshaft movement, and the rate of heat energy provided. The engine will only operate if the energy available can be converted to work in the time needed to move the piston one cycle in relation to the circumference of crankshaft movement (to kind of oversimplify things a bit).

I think the reason TA engines are perplexing is that they are truly the sum of their parts. A 2-stroke gas engine will still run mostly the same without any muffler or tailpipe connected, albeit loudly. There's no question that it's working on the exact same principles as before because it's not doing anything remarkably different. But so much as grasp a TA with your hand and it may either stop immediately or actually start!

Hypothetically, a TA with a piston could operate in a way that it would make a tone, in which case the piston would be operating a lot like the cone of an audio speaker. Yet we don't refer to speakers as electromagnetoacoustic engines! Not that would be funny.

This is why I consider a "preferred frequency" to be one of the deciding factors as to whether an engine is thermoacoustic. A TA is going to operate at the frequency that it is capable of supporting. A Stirling engine, because it doesn't need to rely on resonance to both move hot and cold air back and forth (but instead opposing pistons with offset crankshafts), can operate at variable frequencies depending on how much heat can be delivered to it. Stirling engines are known for being capable of operating quietly, relatively speaking, but I think it's rather obvious that thermoacoustic engines are anything but quiet when doing the job expected of them.

I'm still leaving the door open for some other explanation for how our engine works, but I consider thermoacoustics to be the one that best fits by far. If there's any necessity for boiling or for water droplets to condense, or for vapor to condense from pressure changes, it's neither obvious nor demonstrated so far. There are already plenty of examples of similar engines that work without the presence of water, just with less convenient components and configurations.

[tenbitcomb]

My old steam thermoacoustic engine:
,

an experiment alcohol:
https://www.youtube.com/watch?v=hJV7HzZVr5A
A big engine. ThiS is not mine:
https://www.youtube.com/watch?v=hUOwnrnv8ak
The main problem with these machines is that the temperature difference is small! Carnot h = 1 - (Tc/Th) ...
My webpages:
https://tibsim-thermoacoustics1488.blogspot.com/
URL removed because of it's "anti-Semitic" views. Do not re-post.

Really cool! I wonder what's going on with the inner tube. Maybe it's acting as a restrictor given the limited space for air to flow around it? Very impressive that you got it working that well with a small flame using the alcohol.

[Tom Booth]

... If there's any necessity for boiling or for water droplets to condense, or for vapor to condense from pressure changes, it's neither obvious nor demonstrated so far.

Wow, really?

I'm pretty much absolutely convinced evaporation (expansion/phase change from liquid to gas) and condensation (contraction/phase change from gas to liquid) of the saturated vapor, induced by pressure changes is what's happening. The pressure changes induced by heat.

There may be a kind of reverberation, but IMO that would be a secondary and largely incidental effect and in no way causative.

Not to say that thermoacoustic devices like heat pumps don't exist where sound or vibration is actually induced with a speaker-like membrane driven by an electromagnet. In that case the sound does produce compression waves. But I think that is the opposite of a heat engine. The chain of cause and effect is reversed. You have heat induced expansion and contraction driving the engine with (or without) a resultant vibration.

In an engine though, sound/vibration is generally undesirable and indicates energy loss. It is not what makes the engine run.

There are already plenty of examples of similar engines that work without the presence of water, just with less convenient components and configurations.

Not without pressure changes. Expansion and contraction. There are already a lot of engines that people are, IMO, mistakenly calling "thermoacoustic" that IMO are no such thing. They are heat engines driven by heat.

In a heat engine you have heat induced expansion and contraction that drives the engine which may or may not result in incidental sound or vibrations.

With actual thermoacoustics, on the other hand, you have sound waves inducing pressure changes that result in a temperature differences. The cause-effect relationship if any is reversed.

I think the whole idea I often see of people thinking they have to "tune" the engine like a pipe organ to resonate at a certain frequency, and that there is some significance to various specific measurements and length and diameters somehow relating to the tonal frequency is just misdirection. A kind of red herring and mostly a non-productive waste of time and effort. Much like another myth: the non existent so called "Carnot limit" where efficiency is measured against a non existent completely impossible engine as a standard using a fictitious mathematical measure based on the temperature difference which has little if any actual bearing on what makes the engine run. Not that HEAT can be dispensed with, but heat is something different from temperature. Phase change requires heat but not necessarily a change in temperature. You can get expansion and contraction without a temperature change.

Of course, I suppose I could be wrong and we can agree to disagree.

However changing the fluid from water to alcohol seems to have a profound effect having everything to do with the change in the boiling/condensation point of the liquid and nothing whatsoever, as far as I can see, with any sound, vibrations or acoustics.

So from a scientific point of view, when we have a number of different variables, how do we know which are relevant or causative?

I don't know but personally, I've done a lot of experimenting with Stirling engines and I've never seen an engine suddenly quit running because I touched it or picked it up, dampening the sound or supposed vibrations.

We could eliminate pressure as a variable. Leave off some seal somewhere, anywhere. Will the engine run at all with anything more than a very tiny pinhole pressure leak? No. So we can be pretty confident that pressure is essential. Pressure changes play an essential role. Even a pin hole is better plugged or kept covered once pressure has been equalized and it is no longer needed.

I've never seen anything to persuade me that "acoustics" play an essential role, in this "rice"or any other heat or hot air engine.

But I'm certainly wide open to any evidence to the contrary. If acoustics actually make some difference in any way, what evidence is there of that?

I know about pipe organs and flutes and all kinds of musical instruments. My mother was a music teacher.

No acoustic instrument I've ever seen can resonate when plugged up with anything like a piston.

Sound waves don't drive heavy machinery.

[dlaliberte]

Maybe the way to think about thermoacoustics is to observe that the random motion of particles holding a certain amount of heat energy is basically equivalent to noise, and vice versa. So if we can organize the noise by amplifying any resonance of motion of the particles, then we can take advantage of the pressure changes in a more uniform sound vibration.

Vibrating a surface is perhaps not a very efficient way of extracting the energy. I wonder if there might be some advantage in vibrating a statically charged medium, and then extracting electrical energy by doing so in a magnetic field.

[Tom Booth]

I certainly don't want to discourage research in any direction.

Sound waves, acoustics, vibrations can and do have profound effects. Sometimes large effects.

I've been mesmerized by Chladni figures for a vary long time.

https://youtu.be/wvJAgrUBF4w

https://youtu.be/YedgubRZva8

Endlessly fascinating.

https://youtu.be/2awbKQ2DLRE


Does this play a major role in a tin can engine rilled with random quantities of rice, or glass beads and a little water or alcohol ?

I'll give it a maybe. It's a rather captivating idea, but as yet I'm not convinced it plays a significant role, but like I said I'd be happy to be proven wrong.

[Tom Booth]

Going back to the first post:

(...)
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. ...

As far as least complicated engine...

I think this is possibly the least complicated:

https://youtu.be/vT6n7VVBvqw

Not exactly an engine but it has what I think may be all the essential elements aside from some added heat.

I've often wondered what would happen if the bottle were heated on the bottom, would the "piston" continue bobbing up and down in the tube?

[Tom Booth]

My old steam thermoacoustic engine:
,

an experiment alcohol:
https://www.youtube.com/watch?v=hJV7HzZVr5A
A big engine. ThiS is not mine:
https://www.youtube.com/watch?v=hUOwnrnv8ak
The main problem with these machines is that the temperature difference is small! Carnot h = 1 - (Tc/Th) ...
My webpages:
https://tibsim-thermoacoustics1488.blogspot.com/
URL removed because of it's "anti-Semitic" views. Do not re-post.

Interesting spring arangement for keeping the weight on the diaphragm centered. (A different video from the channel you linked to for "a big engine)

https://youtu.be/rS3biwzLWKs

I see so many "thermoacoustic" and/or "free piston" type engines where the weight just flops around everywhere out of control. This looks like an easy method for keeping the weight /magnet/ linear generator or whatever centered without the necessity of machining some sort of planar bearings.

I don't know Russian, but did translate the descriptions. So I was wondering how you know that his engine is "wet". As far as I can find, he only describes his engines as "Stirling".

I'm interested in the possibility of a "wet" or rice type engine being built that large!!

He has another video of what looks like an even bigger engine running on a wood fired steam boiler (?) of some sort or other.

I haven't had time to go through and translate all the comments.

Looks like the diaphragm is weighted down with a huge concrete block or stone. Must weigh what, about 30 pounds?

https://youtu.be/Aitz1mj_GV4

[Tom Booth]

In this abstract we can read:

We present the thermodynamic cycle of the conversion in a wet, standing wave, thermoacoustic engine

Step(1) The mixture parcel moves towards the low pressure (also low temperature) region while being expanded adiabatically.

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

I see a few problems with this, first of all, the pressure in an enclosed volume of gas is pretty much always uniform I believe, or so I've frequently read in the literature on the behavior of gasses.

Think maybe, blowing up a car tire with a bicycle pump. The pressure, as the air is introduced, does not increase just around the valve or on one side of a tire, the pressure increases (or decreases if the air is let out) all through the entire volume of air inside the tire simultaneously. Same thing when inflating a balloon, etc. etc.

I mostly became aware of this while studying about the Vuilleumier heat pump, which takes advantage of this property.

https://en.m.wikipedia.org/wiki/Vuilleumier_cycle

When a chamber of gas is heated, the temperature and pressure of an adjoining chamber increases simultaneously.

Does something like a bunch of glass beads create enough of a restriction to maintain separate regions of high and low pressure? I very much doubt it.

Maybe some slight delay in pressure changes between the gas on the hot side and the gas on the cold side of the beads (or whatever medium) can be assumed, but personally I would not count on it or consider it significant when factoring together all the elements that comprise a running engine.

Be that as it may, the other issue I have with this description is the use of the term "adiabatic".

The gas moves to the cold region "while being expanded adiabatically".

I agree with the statement itself. What I have a problem with is the next step.

Next, in step (2) The compression makes the vapor pressure higher than the local equilibrium, resulting in condensation, which is accompanied by the heat rejection to the wall.

Were we not just told that the expansion takes place adiabatically?

Also, this is described as being "expanded adiabatically" but also as "the compression makes the vapor pressure higher".

I had to just double check and make sure that was not describing compression (or contraction) where the gas moves back down through the beads or mesh or whatever. No, we haven't gotten that far. That is described later in steps 3 and 4.

So the "adiabatic expansion" is the equivalent of "the compression".

Do the authors of this paper have any clear idea what they are talking about or what is actually going on? I don't really think so. $40 to read more? I don't think so.

Adiabatic expansion results in cooling and condensation all by itself. "accompanied by the heat rejection to the wall" - that I think is a contradiction, considering the definition of adiabatic.

Anyway, I think a more consistent and not self contradictory explanation would be that as a result of adiabatic expansion (along with cooling due to any work output, like lifting a big block of concrete) the gas cools and condenses on any available surface.

Generally under most ordinary circumstances condensation takes place due to hot humid air or water vapor giving up heat to something like a cold glass of lemonade on a hot summer day.

The dynamics inside a running engine, however are not such an ordinary circumstance. More like the expansion and cooling of rising hot humid air with cloud formation. In that case the air is not giving up heat to any surfaces. Well, dust particles ? I suspect dust particles in the air don't have much if any capacity to absorb heat.

I don't think that there is any necessity to postulate any "heat rejection to the wall" other than a perceived need to conform with the Carnot hypothesis.

[tibsim]

Hi Tom!
You link very interesting things, thanks!
My English is very weak, and I may often have misunderstand many things or expressed myself poorly, etc., but I also have a habit of writing silly thingsso first I'm sorry!
In a thermoacoustic half-pipe, the pressure fluctuates. You have to imagine it like a piston that goes through its resonance. The processes maintain and operate this resonance. When a gas moves inward, like a piston, the steam compresses, when it moves outward, the steam expands and condenses in the meantime. Condensation of the steam further creates a greater vacuum in the half-tube (cylinder), so the atmosphere pushes the gaspiston back harder. So I think the description is good, but I may have misunderstood something. First of all, increasing the temperature difference could be done in this similar way,

but we also dealt with it a long time ago [with Blade (Kovács) Attila ], if I remember correctly, with a porous surface, such as in vacuum tubes. In this case, it might be enough to just bottom of the pipe to up high. Although I don't think it's really good ...The bottom of the pipe can dry out, and there the steam is superheated with high temperatures... The videographer wrote to my friend Blade Attila about what kind of the big machine this is.

[tibsim]

Also due to the language difficulties, I don't fully understand what you write, and I haven't read the link thoroughly yet. But I have one thing in mind, just in case it's relevant. One is that when the steam piston is pushed in and the pressure increases, the moisture at the higher pressure becomes at a higher temperature. Steam, on the other hand, is mostly formed when the pressure drops and the steam condenses in the upper part. This is good and bad qualities of the engine. In a cold place, however, it condenses, the lower the pressure, the better. It condenses at a lower presure at a lower temperature... Thus, a larger delta-T can be created.