Forces of attraction and repulsion of gas molecules in a Stirling engine.

[Tom Booth]

Some time back I requested any references in support of the repeated assertion that molecules in gaseous form are NEVER attracted to each other anywhere above the liquefaction point.

Bellow is just a potion of my response to one post where this assertion was made.

There has been no response to my request for any citation or reference.

Nevertheless, I did say; "Perhaps you could cite some credible sources to that effect, (in reference to REAL rather than "Ideal" gases), because I believe I can easily site several seemingly credible sources that say exactly what I have stated."

As this seems to be an ongoing and as yet unresolved issue, I would like to go ahead and provide my own references in support of my contention that gas molecules do infact attract each other and that this attraction does have a significant influence on the operation of a Stirling engine.

...

Tom Booth wrote:
"Real gases are always influenced by forces of molecular attraction and repulsion."

No. Not above the liquid/vapor temperature and below the critical pressure. They act like gases, always with positive pressures. And temperatures for that matter.

...

Perhaps you could cite some credible sources to that effect, (in reference to REAL rather than "Ideal" gases), because I believe I can easily site several seemingly credible sources that say exactly what I have stated.

The attractive forces of atoms and molecules come into play whenever those atoms or molecules are in close enough proximity, and I would venture a guess that in a closed sealed chamber such as a Stirling engine, the gas molecules are always in close enough proximity to influence each other, especially when compressed at all above 1atm. Above 1 ATM gases generally no longer behave "ideally".
(...)

[Fool]

Since I have no clue as to what sources you would accept as "credible", and I've posted several that prove nothing more than someone else's hearsay, it is pointless for me to Google bash for you. Please do your own research. Try not to just look for verification of your point.

Logically, if you completely fill a cylinder with a liquid, at any temperature and expand that volume so is larger than the volume of liquid you will have a volume above the liquid and a volume of that liquid. This expansion can be done by either a piston, or a very long vertical tube where the weight of the liquid pulls a void against the ambient, atmospheric or buffer pressure.

Now you must ask yourself, what is in the volume above the liquid?

1: Perfect vacuum? Nope.
2: Partial vacuum? Yes.

Since it is only a partial vacuum that leaves a partial pressure behind that is higher than zero pressure. Zero pressure would be a prefect vacuum. That is called the vapor pressure of a liquid at a specific temperature. It gets higher as the temperature gets higher.

Your question here seems to imply that even in that partial pressure molecules attract each other, and they do, somewhat.

Two molecules approaching each other slowly enough will connect into a two molecule liquid. Their attraction will bind them until being knocked loose by another faster moving molecule. At higher temperatures, especially above the critical temperature, this knocking loose happens to a greater extent.

One more thing occurred to me, it's usually a bad idea to mix microscopic properties with macroscopic properties. Molecular attraction is a microscopic property. Pressure is a macroscopic/bulk property. Yes microscopic properties lead to the bulk properties, but leaving even one out, like kinetic energy, can lead to errors. Kinetic energy is what produces the pressure and is part of internal energy.

ST_diagram_of_N2_01.jpg (115.3 KiB) Viewed 6599 times

That diagram shows that even at very low temperatures there is always a positive pressure, lower right corner. Meaning gas phase "vapor pressure" above any liquid. Compressing that gas requires work and produces temperature rise, also called an increase in internal energy. Or as Matt Brown would call it, "back work". It is the reason why thermodynamically 100% efficiency is not realizable. A Stirling engine doesn't operate in the low pressure right corner. They operate in the middle around 1 to 100 bars and 300 to 1000 K. Plenty of pressure to compress and have a temperature rise.

Do molecules have both repulsion and attraction? Yes. Do they also have kinetic energy? Yes. Does the kinetic energy provide the pressure phenomenon? Yes. Is there an area where the ideal gas law is accurate? Yes. Do we need to use the ideal gas law? No, we have real indicator PV diagrams that show how the real engine with real gas works.

Trust those diagrams, to a point. Add more pressure sensors and diagrams. Verify, verify, verify. Doctor use twelve leads just to measure heart electrical signals.

Some other links you may want to visit:

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

Look in the models section.

https://en.m.wikipedia.org/wiki/Critica ... odynamics)

From that last link:

660px-Phase-diag2.svg.png (34.15 KiB) Viewed 6599 times

It is well within that lower right hand area, Gaseous Phase, where real gases can be modeled as an ideal gas, sorta kinda. It is above critical temperature and below critical pressure. It is still best to use tables, phase diagrams, and indicator diagrams of real gases and engines.

[Tom Booth]

(..)Try not to just look for verification of your point.

You've thrown a lot of stuff against the wall. Not sure what point(s) if any you are trying to make with any of that.

My "point" is simply the observable, measurable, verifiable FACTS that:

1) In a thermal lag or thermoacoustic type Stirling engine in particular as well as other types (to be determined, Andrew Hall demonstrated several) possibly others, the piston can return fully to TDC with no involvement from stored energy in a flywheel, and no apparent means for any RAPID external cooling.

Certainly anyone can imagine that the engine is SOMEHOW "rejecting" relatively enormous amounts of heat instantaneously across a highly insulative barrier by some kind of magic, or just because 200 years ago some natural philosopher considered it necessary, and that's fine. Think whatever you like.

My assumption, for whomever may be interested is simple.

As the gas does it's expansion work driving the engine, between TDC and BDC it ends up in a low energy state which results in a temperature drop and low pressure.

The low pressure pretty obviously must have something to do with an increase in molecular attraction and/or decrease in molecular repulsion of the "working fluid".

This drop in pressure is rather dramatic in that it allows the piston to return, in a thermal lag type engine, even when heat is still being continuously applied and this heat is not blocked by any displacer, as such engines have no displacer.

There are innumerable sources I could cite immediately that confirm attractive forces between molecules actually exist and have an influence.

That has always been good enough for me. The number and complexity of molecular forces that could potentially be involved is a bit overwhelming, so I'm pretty content with the experimental observations, I don't really need to know the details.

However, now I'm taking some time to research the question a little more in depth.

Something I'm finding of considerable interest at the moment is dipole or dipole-dipole attraction type forces which are apparently capable of causing a cascading effect.

That is, dipole-dipole molecules may confer polarizing attractive force to neighboring molecules which would not normally be attracted. Non-dipole molecules may also become temporarily attracted when they come close together and cause a temporary "distortion" or their electron fields or some such thing.

Water vapor is highly suspect as a contributing factor.

Whan I come across anything that seems particularly relevant or especially significant I'll post it here, but it is bound to be a slow process.

In the mean time I found this video particularly interesting, mostly just as a general introduction to the subject.


https://youtu.be/sTufBlv9e20?si=40qkFd-6m2CHdTZz

[Fool]

Tom,

"the piston can return fully to TDC with no involvement from stored energy in a flywheel, and no apparent means for any RAPID external cooling."

Another interesting question is: If a volume of gas in a cylinder is heated by a temperature difference the entire length and time it is being expanded, will it stop expanding merely from external atmospheric pressure without a flywheel, crank, connecting rod, or even piston?

The answer is: Yes. The pressure at any temperature, will decrease until at or below atmospheric pressure at which point atmospheric pressure will slow, stop, and reverse the expansion motion.

[Tom Booth]

Tom,

"the piston can return fully to TDC with no involvement from stored energy in a flywheel, and no apparent means for any RAPID external cooling."

Another interesting question is: If a volume of gas in a cylinder is heated by a temperature difference the entire length and time it is being expanded, will it stop expanding merely from external atmospheric pressure without a flywheel, crank, connecting rod, or even piston?

The answer is: Yes. The pressure at any temperature, will decrease until at or below atmospheric pressure at which point atmospheric pressure will slow, stop, and reverse the expansion motion.

Have you ever actually seen this or tried it?

Generally speaking, this can be observed (with the piston in the cylinder) whenever a Stirling engine is started.

The gas very gradually expands and the piston is pushed out very slowly.

Eventually, the piston will stop, due to friction, or reaching the end of its stroke and the gas will continue to expand and "leak" past the piston, or the piston will extend so far that the gas at the extreme end of the power cylinder cools from the cylinder walls as much as it is being heated at the other end and the piston stops.

The piston stops.

No, it does not return. Not unless the heat is removed so the gas can gradually cool. Or, the engine is started by QUICKLY (adiabatically) applying some external force to alter the energy balance.

Without a piston, in some engines, I think the gas may begin to oscillate spontaneously, very gradually at first, but this is rare, and I suspect some initial impulse comes from some cooling draft or vibration or possibly even "Brownian motion" since the oscillations are at first microscopic.

Honestly though "fool" I have better things to do, but if you are going to insist on following me around making false and misleading assertions I feel obligated to respond. I don't particularly want one of the threads I've started full of false and misleading statements, but I don't particularly want them filled with reams of tiresome nonsense either.

[Tom Booth]

I thought this was interesting in this context:

https://youtu.be/e-7DFp_B0y4?si=BEzU7LUQcHL_INIG


The metal refrigerator magnet paper clamp thing weighed in at 30 grams

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