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

[Tom Booth]

I made an observation due to following a recommendation from one of the youtubers and welded my top and bottom sections together. When I did, I noticed that I reach a thermal limit in the engine way faster.

(...)

I'm not sure if what you've related after the above takes this into consideration or not, but as a rule of thumb, In any heat engine, as far as possible all heat transfer should be made to travel into the air, or in this case, water vapor. Heat conducted through the engine body, the metal can, is wasted heat.

So, soldering the top and bottom cans together would probably greatly increase conductive heat transfer and reduce air or steam transfer.

That could be an advantage of a transparent (glass) engine, as glass is a poor heat conductor. Stainless steel would be less conductive than tin, but possibly a middle or top section made of PVC pipe could block conductive heat loss almost entirely. Ceramic would probably also work well.

The conductive heat traveling through the can also quickly reduces the temperature difference because no work is taken from conducted heat through the engine, so the top warms quickly, whereas the air/steam looses energy/heat quickly by expanding and doing work.

[tenbitcomb]

I've now confirmed that it's absolutely feasible to run the engine continuously for at least an hour, and likely much more so.

As I think I've already mentioned, the key is really about maintaining a balance between the input and the output. An external combustion engine shouldn't really be that different from an internal combustion engine in this regard. IC engines need the addition of fuel to be precise and controlled and must have a rate of cooling necessary to carry away waste heat. If something fails, the engine may be able to run for a while with one of these factors out of balance (or even absent) before it fails. Having gone from struggling to even get it to start to being able to make it run pretty consistently, I am now pretty confident that these same principles need to be applied to the external combustion engine.

Just now I ran the engine nearly continuously for around an hour and 10 minutes. I could have gone longer but I got tired of holding it. The reason I say nearly is because I sometimes didn't control the process enough so the engine would stop and I would have to restart it, but I could always get it back up to speed in seconds. This only happened maybe 3 times during the run. My process was to keep the engine on my stove at an angle (holding it with my hand), pull it away or towards the flame depending on the overall convexity of the diaphragm, and periodically spritz some water on to the paper towel cooling the resonator/stack section. Now that I'm using bigger washers to sandwich the diaphragm with a nut and bolts, I didn't get any sudden failures because of holed developing or the main hole getting stretched and exposed.

Good control is the answer. We should definitely work on ways to optimize the engine, but heat management will always be a factor. It seems to me that this should also be true of other heat engines that can operate on high amounts of heat, but few seem to talk about that in detail.

[Tom Booth]

I've been reading up a bit about H2O transitioning from liquid to gas/vapor.

There is a 1:1700 expansion ratio between water at 100°C and steam at 100°C

If the steam is hotter than 100°C, like if a drop of water falls into hot metal at say 500°C the expansion can be greater.

Also, the water molecules tend to form "blobs" or droplets that when heated transition to gas all at once.

In other words, it takes a group of water molecules in close proximity to create enough attraction between them to change to liquid, and the transition back works in the same way, a large group of molecules transition from liquid back to gas simultaneously.

I imagine that is why sometimes boiling water suddenly forms really huge bubbles.

But it takes much more heat or a much greater temperature change to expand a gas, like air, without phase change anything like 1700 times.

Imagine a drop of water the size of a pea. In a can. Then imagine the can is suddenly filled with 1700 peas

o <- 1 drop


X 1700 ->
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And that is with NO temperature change.

[Tom Booth]

I'm wondering about the purpose or usefulness of having some kind of screen or divider between the water on bottom and the rice and if that is necessary, or serves any real purpose when using pebbles or beads.

My original thought or impression was that the screen was to keep the rice up out of the water to help prevent it from becoming soggy right away.

With other materials that shouldn't be a problem, so maybe with glass beads or gravel it might not be needed, and depending on what's really going on, which I don't assume I know, the extra wicking, from having the gravel or whatever directly on the bottom in contact with the water might allow for greater and/or more rapid evaporation, and/or smoother, more controllable and/or more balanced phase change / transition between liquid to vapor and back.

In other words, no dripping water causing abrupt surges of steam or rapid changes in the rate of condensation.

Also, if that results in anything interesting, what about some actual wick-like material ?

Like fiberglass batting, carbon fiber board, possibly just some window screen tightly rolled up into a cylinder, ceramic fiber board, pumice stone or vertical layered sheets of glass partly submerged in the water, bundles of wire, glass or metal rods, wooden dowels, straws, ice cream sticks.

Possibly something like PTFE beads might be too hydrophobic so there is no wicking.

I'm just going by the observation that materials like rice and gravel which seem to work well, do have the quality of being "wettable", creating a lot of damp surface area

So, maybe something that has a balance between making a good wettable wick to draw up some hot water at the bottom for evaporation but also somewhat hydrophobic to shed water as it condenses at the top, or possibly two different layers of material. Something "wettable" that wicks water up in the bottom and a second layer of something hydrophobic that can condense and rapidly shed cold water on top.

Maybe the earlier design using steel wool in the bottom worked well because the steel wool acted like a wick of sorts. Perhaps stainless steel wool dish scrubber on the bottom, would "wick" both water and conduct heat for rapid evaporation with the hydrophobic beads on top of that.

3:00 in the morning I wake up thinking about this stuff. I might forget it by the time I wake up in the morning.

[Tom Booth]

I confirmed a suspition: cooling the condensation is important and is one of the most significant factors to achieving consistent and long term performance of the engine.
(...)
- I welded the top can to the bottom can, increasing thermal conduction between the cans and therefore providing conductive heat to the beads
- I encountered thermal buildup issues as the beads cold no longer maintain thermal equilibrium with that much applied heat
- I replaced the glass beads with .177 mm copper BBs since copper is a better conductor of heat and will transfer excess from the center to side walls 300X faster than glass beads
- No more thermal overload

Did you undo the weld?

I would have thought that if having the parts welded together, eliminating the slight air gap that would prevent conduction, resulted in thermal buildup, it would be better to undo that. If the copper is conducting away "excess" heat, wouldn't that reduce the efficiency?

(...)

2. As the power piston moves up, the steam begins to move through the condensation medium and starts to cool and condense

I agree with this, but would ask why?

True, the copper beads, if kept cool will take away heat, but I think what isn't mentioned is the heat taken away by work output. What heat the copper beads conduct away would be at the expense of power output (I think).

most of heat energy that was in the vapor gets transported into the condensation medium and out of the cooling system.

There is this difference between a heat engine and a heat pipe.

A heat pipe merely moves heat from point A to point B.

A heat engine transforms heat which goes out as work output rather than heat.

It is, or would be a violation of physics if the heat could do both, producing work AND also going out through the cooling system.

Otherwise, I mostly agree with everything else.

I would keep the air gap, or use some other thermal barrier so as to conserve heat for work production so the heat is not wasted.

[Tom Booth]

And add a generator!

Otherwise we end up with another low efficiency heat engine that goes through some motions but can't actually do any work.

[Tom Booth]

@Tom Booth

You can't convert heat 100% to work with a heat engine, that would violate the carnot equation.

The maximum heat-to-work of a perfect heat engine is determine by Q=((max temp - min temp)/max temp) X 100% where temperature is in Kelvin or 0 is absolute 0.

Think of it like flowing water: there isn't a hydroelectric dam in the world that converters water to electricity, water flows through the turbine and some of the kinetic energy is stolen to do work.

Same with a heat engine, you can only steal some kinetic energy from osmotic flow heat from high concentration to low concentration.

(...)

I don't adhere to the Carnot theory of heat.

The Carnot "efficiency equation" is unsupported by experimental evidence.

I pull on the piston. Doing so drops the pressure in the chamber, causing the water to rapidly boil to match the new, lower boiling point due to the loss of vapor pressure. Now the chamber is filled with steam and the pressure from steam

(?????)

steam increases the boiling point. Now I push the piston down.

There is a crucial step, in an actual running engine being left out in that description. the expansion of the water from the liquid to vapor drives the piston doing work, which takes energy out, which results in cooling, so the water vapor condenses.

There is no "law" that prevents the 100% conversion of heat into work up to that point.

I don't believe that there is any intrinsic need for the transfer of heat to the condensation medium.

At this point the "piston" or weighted diaphragm has been fully expanded and ALL the heat that was taken in has been converted to work, so the vapor condenses leaving a vacuum.

Atmospheric pressure, then, does the work of driving the piston back in.

When a liquid condensed it adds heat to the surface it condenses on. Therefore the condensation medium must be cooled, otherwise the medium will become too hot, it will eventually be heated to boiling point and no longer condense water.

When you have a weight on the membrane, you have momentum, that momentum, when the weight flies up, pulls the membrane further back than that boiling rate wanted to move it naturally, this overcools the boiling water,

I agree about the momentum of the weight pulling the diaphragm and causing overcooling.

However, again, this leaves out the primary source of cooling that results from the work/energy output.

Carnot was completely wrong about everything, and the efficiency equation attributed to him has no foundation.

Edit: I seem to have replied to a post that was deleted ?

[Daemon]

@Tom Booth

I accidentally double posted, there is no delete option, I reported the double post because I can't clean it up myself, I am banned. I don't understand why. I am autistic and I struggle enough with online interactions without being permanently banned for trying to correct my own errors.

Any ways, back on topic.

So I agree that it should be possible to convert nearly 100% of heat.energy to kinetic energy, however you need more than a single engine to do it for this reason:

When you heat up a gas 10deg c you add roughly 1 PSI to the pressure of the container it is stored in.

If you have a piston on one end and that piston weights 1 G and it lifts 1 meter in one second, you'll have spent 1 joule of energy lifting the weight. If you did that by heating a gas, the gas first has to be heated to develop pressure, as it does so, it eventually overcomes static friction and then the piston begins to move, as it does so heat energy is converted to kinetic energy.

If the chamber had no piston, it would heat with however much energy is required to heat the volume of air, however the addition of the piston means that the energy required to heat that volume of air is equal to the system with no piston plus the kinetic energy being removed by the piston.

Now, the piston is at full stroke, in order to return the piston to its base position, we need to relieve vapor pressure in the chamber so that the internal pressure is lower than atmosphere.

Currently the piston is sitting on top of hot expanded gas which is at the correct heat level to have 1 ATM of pressure at that volume for its mass. Since the vapor pressure is being created by heat, if we remove heat, we remove vapor pressure.

So we cool the gas down and the piston returns to base position.

If we don't "lose" the energy that creates the vapor pressure that holds the piston against atmospheric pressure.

How can the piston return to base position?

[Tom Booth]

@Tom Booth ... I am banned. I don't understand why.

Usually there would be several warnings by Private message before anyone would be banned. I'm guessing it may have been accidental.

Any ways, back on topic.

So I agree that it should be possible to convert nearly 100% of heat.energy to kinetic energy, however you need more than a single engine to do it for this reason:

When you heat up a gas 10deg c you add roughly 1 PSI to the pressure of the container it is stored in.

If you have a piston on one end and that piston weights 1 G and it lifts 1 meter in one second, you'll have spent 1 joule of energy lifting the weight. If you did that by heating a gas, the gas first has to be heated to develop pressure, as it does so, it eventually overcomes static friction and then the piston begins to move, as it does so heat energy is converted to kinetic energy.

If the chamber had no piston, it would heat with however much energy is required to heat the volume of air, however the addition of the piston means that the energy required to heat that volume of air is equal to the system with no piston plus the kinetic energy being removed by the piston.

Now, the piston is at full stroke, in order to return the piston to its base position, we need to relieve vapor pressure in the chamber so that the internal pressure is lower than atmosphere.

Currently the piston is sitting on top of hot expanded gas which is at the correct heat level to have 1 ATM of pressure at that volume for its mass. Since the vapor pressure is being created by heat, if we remove heat, we remove vapor pressure.

So we cool the gas down and the piston returns to base position.

If we don't "lose" the energy that creates the vapor pressure that holds the piston against atmospheric pressure.

How can the piston return to base position?

There are at least two assumptions (bolded & underlined) that I would question.

If a gas is expanding it will expand regardless of there being a piston or not. Either it will push out and do work against atmospheric pressure directly or it will do so using the piston or diaphragm as intermediary, especially if the piston is already in motion.

If the atmosphere were likened to a large truck. The piston is the back bumper. If I push the back of the truck
directly or use the bumper, the difference is negligible I would think.

Also the piston creates a seal, so pushing it out results in a vacuum or pressure imbalance. If I do work pushing on the truck to move the truck (or piston) the "work" is already done. My energy (heat) has been spent so is already gone, used up in the process. What remains is a pressure imbalance, which will push the truck (piston) back. The atmosphere that was displaced will return as soon as I stop pushing.

It is like stretching a spring. If I put effort into stretching a spring, I'm not going to need to "remove" the effort I put into that action for the spring to return, it will return as soon as I "let go".

Or, like pushing the truck up an incline. If I stop pushing, then the truck will roll back down the hill (and likely keep going when it reaches the bottom). In that case gravity is likened to pressure, or to spring tension.

So, the piston at the end of the power stroke is not "sitting on top of hot expanded gas which is at the correct heat level to have 1 ATM of pressure". On the contrary the piston was driven out by a build up of pressure, like a cork in a bottle popping. The pressure was converted to velocity. As you had said in the deleted post, the piston, or weight on the membrane has momentum and stretches the membrane. It is also "stretching" (expanding and cooling) the gas that expanded enough to set the piston in motion.

So at the end of the power stroke, the "heat" has already been converted, it has already been utilized and used up putting the piston into the position where the membrane is stretched like a rubber band, and there is a strong pressure imbalance between atmospheric pressure outside and the vacuum that has been created inside.

That would be my analysis anyway.

We are however, trying to analyse a dynamic process that is repeating, maybe 10X per second in a blur of motion, but that would be my best guess.

So as far as:

Now, the piston is at full stroke, in order to return the piston to its base position, we need to relieve vapor pressure in the chamber so that the internal pressure is lower than atmosphere.

I'd say, no we don't. The internal pressure and temperature has already been made lower than atmosphere. The expanding gas has already done it's work to put the piston into that position, and "let go".

[Tom Booth]

In other words, observing the actual operation of this engine, as in the screenshot I posted earlier:

Resize_20221011_172514_4257.jpg (72.19 KiB) Viewed 7745 times

It appears obvious to me that the weight on the diaphragm has become a projectile, like a cork popping out of a bottle.

As I said when I first posted that image, that is obviously NOT isothermal expansion, which by definition is a very slow process.

This engine, probably more than a "normal" Stirling engine demonstrates that the engine depends for it's operation on a very sudden expansion, akin to a ball hit by a baseball bat. The piston, that is, is "knocked" out by a build up of heat and pressure.

At full compression, the pressure suddenly drives the piston back out. This is anything but a slow gradual process. The heat and pressure are converted to velocity.

That is something overlooked or ignored by Carnot. Real pistons have actual weight and momentum. The heat is converted to velocity very suddenly and the piston/ weight flies outward in a fraction of a second, expanding the gas and stretching out the diaphragm until the weight can go no further. By that time the heat that drove the weight out is already used up and the pressure has become a cold vacuum.

Basically the "cloud in a bottle" repeating 10x per second.

https://youtu.be/MgRcQxUZ8iw

[Daemon]

I'd say, no we don't. The internal pressure and temperature has already been made lower than atmosphere. The expanding gas has already done it's work to put the piston into that position, and "let go".

Ah, I see where the misunderstanding lies now. You may want to research the "ideal gas law"

Coles notes:
Basically, if the pressure external to an expandable container is constant, I.e. if it's exposed to atmosphere, them the volume of said container will be known and calculable if one know the mass of the gas in the container and it's temperature.

If the container is rigid, then you use the mass, volume and temperature of the container to determine the internal pressure of the container.

These are constant and is considered to be fla fundamental law in physics and engineering when dealing with gasses.

In other words it's industry proven and we'll worth the read.

[Daemon]

Let's put it this way, just because I extracted 10 joules of energy from the gas as it heated and expanded, doesn't mean I don't have to heat the gas to the same temperature to achieve the same volume. It means hearing the gas is going to use 10 joules more energy than if I was exclusively heating the gas.

Had I no piston and added 10 extra joules, I would simply heat the gas 10 joules hotter and store 10 joules of potential energy extra in the gas.

It means I need to add in the extra energy in order to achieve the same volume when work is being done.

From a relativistic perspective, one could say the gas is being cooked as it is being heated by the work being done.

But you can only cool it by the work being done, and one way or another, that had must be at a certain heat level to be the desired volume and pressure.

So you are technically correct.

The engine does in fact cool the gas relative to a gas that is not being used in an engine but is merely contained in a pressure vessel for the same addition of energy.

However, that is because you are heating x mass of gass to achieve y volume and z pressure and that requires a preset amount of total energy to do.

The harder you make it to move that piston, the more heat energy as a proportion of total energy goes up.

You can never escape that you must heat the internal gas to a given temperature to achieve a desired pressure and volume.

For a piston to be at a given position due to the action of the gas the gas in question must be at a specific temperature to be that volume and pressure.

This is the ideal gas law.

I'm an instrumentation and controls engineer I use this principle on a daily basis to control factories.

[Daemon]

Not being facetious, the factory I worked in was really old and all the automation was still driven with pneumatics.

Most of the controllers and sensors I worked on are basically pneumatic analog computers.

[Tom Booth]

Sorry but I'm not going to debate your interpretation of what may or may not be stated in a student handbook.

I've provided my observations and opinions, take it or leave it.

Just FYI, the "IDEAL" gas law is just that "IDEAL".

"Ideal" in this context is defined as: "existing only in the imagination; desirable or perfect but not likely to become a reality."

There is no such thing as an "ideal gas" in reality.

The ideal gas law can serve as a general rule of thumb under ordinary known stable circumstances but things can deviate far from "ideal" in a dynamic system with rapidly changing volumes, pressure, temperature and work inputs and outputs, and other extreme conditions where the basic assumptions of the ideal gas law don't even apply.

Real gases DO have forces of attraction and repulsion. Real pistons have actual weight, velocity and momentum, which are not accounted for in the ideal gas law. I could go on and on but I have no inclination at the moment.

I might suggest, if not already familiar with it, that you look into the history of the liquefaction of gases and the various methods employed, in particular, the rapid expansion of a gas through a reciprocating expansion engine or turbine. Claude's method, cryogenic cooling, air/gas cycle refrigeration etc. (All in the phase change category)

The "cloud in a bottle" phenomenon has much in common with these kind of compression/expansion phase change type phenomenon, condensation and evaporation. A vast gray area where gases transition to liquid and back again with instantaneous changes in pressure and temperature and between states.

The Ideal gas law particularly breaks down at such pressures and temperatures near the phase change pressure and temperature when the gas, infact, transitions away from being in a gaseous state at all, at that point things like molecular attraction and repulsion become much more relevant.

In this case we are talking about precisely that kind of circumstance where water is transitioning between a gaseous and liquid state. Expanding or contracting to a factor of 1700x in less than the blink of an eye. The ideal gas law is hardly applicable under such circumstances.

[Daemon]

I feel the same way about debating you about efficiency in energy conversion processes.

I'm hear to talk about a closed cycle, piston driven steam engine. I'd like to return to that.

I have no interest in speculating about achieving 100% thermal efficiency when I could get a nobel price for getting 80% Carnot efficiency lol.

I already know how to make a heat trap air conditioner with no hot side exhaust that runs itself. No fire necessary, I can take you right down to absolute 0.

But even then, I wouldn't call my process 100% efficient.