Lets beat up Carnot

[VincentG]

I look at heat as a continuous flow of energy. Trying to stop, convert, or alter it is like fighting the flow of a river. Instead, extracting energy while maintaining the flow is how I see it. In that respect, heat rejection to the sink is part of maintaining that flow.

[Tom Booth]

I look at heat as a continuous flow of energy. Trying to stop, convert, or alter it is like fighting the flow of a river. Instead, extracting energy while maintaining the flow is how I see it. In that respect, heat rejection to the sink is part of maintaining that flow.

Well, you've stated the argument for Caloric theory.

However, taking a thermocline as an example.

In this video, there is virtually no mixing between the hot and cold water. Far from being unstoppable, the "flow" between the hot fluid on top and the cold fluid on the bottom is, well, non existent.

https://youtu.be/wqtFeAvDOwk?si=-WTRV3tBW-Q6Ee7V

Infact if you heat a water bath, or the air in a room, the hot air will rise and the cold air will fall, separating into layers.

Separating!

There is no intrinsic "flow" between "hot" and "cold" to maintain or intercept to extract energy.

Generally a Stirling engine will FORCE air against a hot surface using a displacer to get the air to heat up and expand.

As the air is FORCED to heat up and expand, the expansion drives the power piston. No "flow" of energy is being harnessed.

Some of the energy of this expansion of air is used to move the displacer to drive the cold air that just gave up its energy back into the hot plate again, to repeat the process of FORCED heating.

I've done a lot of experiments trying to run a Stirling engine on ice, with ice on the bottom.

I was very surprised and puzzled to find that this did not work very well.

It is better to have the heat on the bottom, then natural convection will do some of the work of the displacer.

If an engine is left with ice on the bottom too long it will become very difficult to even get it started.

The tendency for heat to "naturally flow" from hot to cold just does not exist. It's complete mythology.

Heat will travel through certain materials at some rate, fast or slow, depending on the material, but in general heat dispersal is random in all directions, or often, as in a thermocline, hot and cold materials may actually separate and remain isolated.

This is what happens when a Stirling engine is placed on top of ice. The displacer has to work HARDER to push cold air UP. The cold air always wants to sink back down or STAY down, and the engine will have to labor harder and it will run more slowly.

So, in practice, heat engines are almost always heated from the bottom so the difference in air density, resulting in convection can help with the FORCED heating of the air.

Sorry Tesla, just "digging a cold hole" to let the heat "flow in" and then intercepting that flow doesn't actually work.

Not only is heat not a fluid, it doesn't "flow" like a fluid to a "cold reservoir" either.

It mostly just sits there.

Getting the air to heat up and expand actually involves an expenditure of some energy. THEN as the air expands, some additional energy can be harvested from the resulting molecular repulsion.

We are harvesting energy from the molecular attraction and repulsion that results as a consequence of the FORCED heating and cooling.

The whole idea that a "heat engine" works by intercepting a natural "flow" of heat, like a water wheel in a river is poppycock.

[Goofy]

Quote :
"I've done a lot of experiments trying to run a Stirling engine on ice, with ice on the bottom.
I was very surprised and puzzled to find that this did not work very well."

I am sure, that I have read somewhere, that Sterling constructed his engine with the power-piston on the cold side.
That make sense, as the expansion makes the medium even colder. So I think by running you engine so to say opposite, the
temperatures are perhaps working "against" each other.

It is obvious that the cold on the bottom and heat on top should be best practice, but then you better turn your engine
upside down and do the experiment again . . .

Perhaps not so easy, but you are creative ! ;-)

BR
Petter

[Tom Booth]

Quote :
"I've done a lot of experiments trying to run a Stirling engine on ice, with ice on the bottom.
I was very surprised and puzzled to find that this did not work very well."

I am sure, that I have read somewhere, that Sterling constructed his engine with the power-piston on the cold side.
That make sense, as the expansion makes the medium even colder. So I think by running you engine so to say opposite, the
temperatures are perhaps working "against" each other.

It is obvious that the cold on the bottom and heat on top should be best practice, but then you better turn your engine
upside down and do the experiment again . . .

Perhaps not so easy, but you are creative ! ;-)

BR
Petter

You make a good point.

I tried something like that before but the engine mysteriously "froze", after running quite well initially.

I never tried that again, and don't want to speculate. I should try that arrangement again.

I had put the ice on top of the engine wrapped up in an aluminum pouch in contact with the top plate and then the entire top of the engine covered with Styrofoam, so the ice was insulated UNDER the Styrofoam but on top of the engine.

The ambient heat could enter through the bottom "hot" plate, and rise up into the engine aided by convection.

The engine ran remarkably well for a while and I was very excited at the apparent success, but then I was showing my wife the engine running vigorously when it quite abruptly stopped. The piston seemed to suddenly jam. Then it became free again a moment later, but the engine would not start back up

My wife, observing this said, "maybe it froze again". This sort of thing had happened before.

I took the engine back downstairs to examine and try to figure out what might have actually happened.

Everything seemed wet under the insulation, including the power piston and cylinder, even INSIDE, which seemed very strange at the time.

I had to take the piston out and dry off the piston and cylinder inside with a paper towel.

At the time I thought the ice had melted and got everything wet, though I had wrapped up the ice very carefully in a way I thought that could not happen.

Now I'm thinking that all that moisture may have been due to condensation.

[Fool]

I look at heat as a continuous flow of energy. Trying to stop, convert, or alter it is like fighting the flow of a river. Instead, extracting energy while maintaining the flow is how I see it. In that respect, heat rejection to the sink is part of maintaining that flow.

It's kind of like the tide. It can be slowed down a little, but over time it will find a way to become cold.

It's not so much that the flow of heat pushes the engine. It's more like it must travel away from hot. Away from hot is cold. It dissipates and becomes cold and useless.

It's similar to water and appears to follow the path of least resistance. Easily demonstrated by an infrared photo of a house in the winter showing most of the heat going out through the window glass. It's also seen coming out everywhere else, but more slowly.

Insulation doesn't stop heat and in some cases it speeds it up.

Heat flows into a cold engine warming the internal gas, pushing a free piston out until pressure equalizes with the outside. Heat flows into a displacer regenerator combination.

Compressing the gas on the return stroke raises the Temperature allowing heat to flow out of the engine.

Heat flows out of the displacer/regenerator into the internal gas. The cycle continues.

It's all about pressure change from heat transfer and the temperature difference is what instigates heat flow.

[Tom Booth]

.... and the temperature difference is what instigates heat flow.

Well that's the question.

On a particle scale, the individual gas particles carry their own "kinetic energy" and will travel any which way in a straight line, indefinitely.

Sooner or later there is a collision with another particle, also going any which way.

What happens then depends on the trajectory of each particle, it's mass etc.

Does "temperature difference" INSTIGATE anything?

Like I said, in my experiments with operating Stirling engines on ICE I was rather disappointed that the ambient heat did NOT aggressively transfer itself into the cold engine.

If I let the engine sit on the ice without running the engine would not start, submerged in a well of cold, though the top was fully exposed to the ambient heat, nothing instigated any "flow".

https://youtu.be/41d6kIHLK7M?si=Kff2Gh1ttODHpZ1f

I was not trying to "prove" anything. I was counting on the "heat flow" as the means for extracting ambient heat. But after repeated experiments with this same sort of result I realized the heat would have to be coaxed. Or some method had to be engineered to persuade the ambient energy to exchange heat with the engine. The natural buoyancy that keeps hot air and cold air separated was a much stronger determinant of where the heat would go, or not go, than temperature difference. Disappointingly.

Is "kinetic theory" itself true?

I tend to doubt it. Or why would the gas particles above the engine not keep the top of the engine warm? Constantly bombarded by the hot particles above traveling at the speed of sound.

[VincentG]

The difference between ice water and room temperature is only 35 degrees Fahrenheit, not enough to notice a huge flow of energy. It just seems like a big difference because at 70 degrees we are comfortable and at 35 degrees we would freeze to death without proper clothing.

[Tom Booth]

The difference between ice water and room temperature is only 35 degrees Fahrenheit, not enough to notice a huge flow of energy. It just seems like a big difference because at 70 degrees we are comfortable and at 35 degrees we would freeze to death without proper clothing.

That ∆T was plenty enough to run the engine on ice for 33 hours nonstop until the ice .melted completely.

It was the "control" non-operating test where the engine sat idle. After sitting for several hours it would no longer start. The ice, however melted 5 hours more quickly.

[matt brown]

Is "kinetic theory" itself true?

I tend to doubt it. Or why would the gas particles above the engine not keep the top of the engine warm? Constantly bombarded by the hot particles above traveling at the speed of sound.

An old thermo adage is that cold gas pools (and forms a boundary layer). Some 'forced heating' via a small fan would likely alter this.

[matt brown]

The difference between ice water and room temperature is only 35 degrees Fahrenheit, not enough to notice a huge flow of energy. It just seems like a big difference because at 70 degrees we are comfortable and at 35 degrees we would freeze to death without proper clothing.

Now there's a thought...the line between life and death is less than 25K !!!

[Tom Booth]

Is "kinetic theory" itself true?

I tend to doubt it. Or why would the gas particles above the engine not keep the top of the engine warm? Constantly bombarded by the hot particles above traveling at the speed of sound.

An old thermo adage is that cold gas pools (and forms a boundary layer). Some 'forced heating' via a small fan would likely alter this.

With the engine running the flywheel serves as a very ineffective fan, as evidenced by the line of condensation in direct line with the flywheel.

flywheel_condensation.jpg (273.3 KiB) Viewed 8080 times

https://youtu.be/-7zntz8kwIk?si=fb-qT3x_2FD3L-hK


Far from the heat being unstoppable.

Some paddles along the edge of the flywheel to act as more effective fan blades to keep the air in circulation would help.

It doesn't require much "engineering" but some of the engines energy output needs to go into some kind of "forced heating". Or so it seems.

Still air is excellent insulation, but how could the air ever really be still if the gas particles are really zipping around carrying all that "kinetic energy"?

[VincentG]

In the past I've thought a fan within a Gamma displacer chamber would improve heating and cooling response. Now I'm not so sure it's necessary.

The phrase "air is a good insulator" never sat well with me. It should be more like "air confined to many relatively small pockets is a good insulator". Otherwise, free to move, it's a pretty bad insulator. It's a bit like the molecules are fast acting messangers, but if corralled in a cage they can't get the information out.

[Tom Booth]

It's quite difficult to find a demonstration using air, but same principle.

Heat under a Stirling engine vs. above.


https://youtu.be/tR_Oc2Ky2vA?si=21gxEMgHqRU7QvFk


With the ice under the engine, the natural buoyancy of the air tends to prevent the heat from moving down into the working fluid. Even with the engine running the displacer has to do more work than with the heat on the bottom.

The hot and cold air layers tend to remain separated.

Even individual hot air molecules are more buoyant than cold and will tend to separate out and "float" up like microscopic helium balloons.

Cold gas from a refrigerator can be poured out of a container onto the floor. It will mostly just lay in a puddle on the floor like water.

Like the dry ice fog:

https://youtu.be/BsO1B-FWD6I?si=iv_dPQbvNQcA6AQV

[Fool]

Dry ice fog always seems to 'dry' up and go away eventually.

The term 'temperature inversion', comes to mind.

It seems anytime something is below the dew point, condensation happens.

Isn't the original proverb, 'dead airspace is a good insulation'?

[VincentG]

Isn't the original proverb, 'dead airspace is a good insulation'?

Could be, tried searching but no good results.