Stirlingengineforum.com

Already done, here's an old Vertasium video. 55gal toward the end. 25mS to crush. Still seems completely consistent with combined gas law and the expected phase changes. Further, it might not be as impressive at my altitude of 1700 meters. Better at sea level where the pressure is higher.

https://www.youtube.com/watch?v=xmYss_hNF4Q

Though I'm not suggesting this is at play, "negative pressure" is most certainly being talked about in various fields now. I've heard it in passing at least once.

https://www.google.com/amp/s/phys.org/n ... essure.amp

I read that too, or at least skimmed it. My impression was that it was one of those intermediate theoretical things like irrational numbers that is conjured to solve some other problem. Could be wrong though, didn't spend much time on it.

Even so, everything he has mentioned so far is explained by conventional physics, so Occam's Razor and all that..

skyofcolorado wrote: ↑Tue Feb 06, 2024 12:50 pm ...everything he has mentioned so far is explained by conventional physics,...

I'm not necessarily saying it isn't.

However, "conventional physics" covers a lot of Territory and is not always consistent.

Probably 99% of any issue I have with physics. (I'm assuming you mean me and not the Veritasium guy) is with the 2nd Law of thermodynamics and whatever issue I have with that boils down to the Carnot efficiency limit equation, which IMO is not supported at all by the rest of conventional physics. It does not agree with the first law, with conservation of energy or anything.

Thanks for posting that video BTW, lots of interesting stuff that IMO seems to support my intuitions or theories.

Talking about "negative pressure", did anyone else catch the brief mention of that in the video?

"You can actually get negative pressure, if the fluid is inside a tree" or something like that he mentioned.

There seems to be a lot of information about this.

One interesting article I just found:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4824372/

I've only just begun reading through this very lengthy article, but a few points jumped out already:

Motivated by the rising of sap in trees, we built a 15.4 m siphon that shows that absolute negative pressures are not prohibited

Ahhh .. Absolute negative pressure ?

I've only been arguing that it is OK to use the term "negative pressure" in reference to a RELATIVE "vacuum". I've. basically agreed there is obviously no such thing as literal negative pressure ABSOLUTE. But I have thought that perhaps there is molecular attraction that could exert some "pull". I don't think I would associate that with "pressure" necessarily. A "pulling" force could be gravity, magnetism, Brownian motion, cohesion, adhesion, whatever. But, in this article they call it "negative pressure". "It" being, whatever force pulls water up a tree higher than it should be possible. The article goes on:

Capillary forces produced at the pores in the leaves are transmitted by cohesive forces between water molecules and can sustain the weight of water columns greater than one hundred meters. Water inside the capillaries in the xylem is pumped due to transpiration at the leaves in a process described by the cohesion tension theory [3–5]. Water under absolute negative pressures should boil at ambient temperature forming bubbles that would eliminate the cohesive forces. However, nature provides trees with some not completely well understood mechanisms that forbid bubble nucleation, and that even make possible to refill the capillaries after an embolism [6, 7].

The effects of forces produced by pressure differences on fluids are typically explained using only pushing forces because many people do not believe that fluids are capable of pulling. Some years ago a report in Discovery Magazine about the mechanism used by trees to raise water mentioned the existence of negative pressures and produced criticisms of several scientists against the journalist. Fortunately, she wrote a second column [8] after asking the opinion of respected scientists in the fields of physics and botany, resulting in a story full of contradictions between experts. The main conclusion was that negative pressures do exist, and that the physics community still has problems in accepting the existence of negative pressures that are a well established fact for botanists.

The idea of a fluid that can only push follows naturally from the “ideal gas model”, where the only mechanism available for momentum transfer is by means of collisions between atoms or molecules. In this scenario it is possible to diminish the pressure by lowering the density of the fluid, up to the point where there is no longer any fluid left, or by lowering the kinetic energy of its molecules, up to the point where all molecules stop and there is no exchange of momentum. In this model both cases correspond to a situation of zero pressure, which cannot be further lowered. It is in this context where pulling forces or negative pressures are absurd. However, if there are attractive forces pulling (instead of pushing) molecules or atoms, then negative pressures are perfectly well defined. Because attractive intermolecular forces do exist, negative pressures are expected to appear when liquids are pulled. It has to be noted that, although the topic is controversial, negative pressures are frequently measured to catalog properties of liquids [4, 9–12].

What I've had mulling around in the back of my mind for a long time, and may have mentioned previously is...

If a gas, in the process of "contracting" exerts some "pulling" force that draws the piston in, then it would seem in "contracting" the gas would be continuing to do "work", so losing energy and cooling and further contracting

I only came up with such a theory in an effort to explain actual PV diagrams traced in real time from real Stirling engines that indicate a continued drop in pressure during "compression" while the piston is moving towards TDC.

How in hell is that possible?

How is it possible for the working fluid to cool BELOW the "sink" temperature:
So this is my own annotations, but a real PV tracing:
The pressure and temperature plummet during "compression" ????

How could a "sink" be a heat sink or "cold reservoir" if it is actually warmer than the working fluid and how did the working fluid get colder than the "cold reservoir"???

Conventional wisdom does not explain such real world experimental measurements and observations.

Well, Tom Booth is just a careless experiment and a perpetual motion crank.

Well OK, but that's not me. Those measurements are from other sources, I'm just looking for some rational explanation.

If there is "attractive" or "pulling" force of "molecular cohesion" between cooling and "contracting" gas particles and the piston/container, could this constitute "work" resulting in an energy loss due to "pulling" the piston, similar to how a tree "pulls" water higher than it should be possible by so-called "negative pressure ?

skyofcolorado wrote: ↑Mon Feb 05, 2024 11:20 am

Or, what if we were to do the steam condensing in a can experiment, but before the can can cool and collapse, put it inside a vacuum chamber.

Would the "attractive" force of the condensing steam collapse the can without the external atmospheric pressure?

No. No collapse if done under vacuum. It would collapse after the vacuum is released.

combined gas law.jpg
...

That's assuming no molecular cohesive force influencing the collapse from the inside as the water molecules condense into a liquid, which is what we would be testing for.

Inside the can would not be a true or complete vacuum as outside the can. Inside the can is initially full of steam. We want to know, as the steam condenses does it "pull" the can inward? Even under vacuum conditions

What about cooling down a steam filled plastic bag?

I was going to say balloon, but a balloon has its own elastic force that would cause it to shrink when cooled.

Something like a steam inflated zip lock bag put in a vacuum chamber.

It should still cool down gradually even in a vacuum due to infrared radiation.

Would the bag grow under vacuum?

Cool but stay inflated?

Or:

Cool and get "sucked" inward due to the "molecular cohesion" of the water molecules as they condense?

Edit: "...Well, Tom Booth is just a careless experiment..." LOL...

Experimenter of course.

Or...

Maybe the ordinary old egg in a bottle trick.

Put some water in a milk bottle. Boil and peel an egg.

Put the bottle into a pan of boiling water until the water inside the bottle starts to boil filling the bottle with steam.

Put the hard boiled egg on top and tansfer the pan and bottle into the vacuum chamber and turn it on before everything has a chance to cool down.

Allow the bottle with the steam inside and the egg on top to cool inside the vacuum chamber.

If there is no "cohesive pulling force" from inside the bottle as the water vapor condenses, the egg should just sit there.

I don't think the egg would explode as it has already been hard boiled.

A vacuum inside and outside the bottle should be equal and have no effect on the egg.

But as mentioned, inside the bottle is not a vacuum until the steam condenses. Perhaps it could exert some "cohesive pull" on the egg as condensation takes place, though...

Will the steam remain in the bottle under the egg or be sucked out by the vacuum?

Will the steam condense at all in the bottle under a vacuum?

Will the steam/water in the bottle "cold boil" and so refuse to condense?

If the egg is not sucked into the bottle, will it do so once the vacuum is released?

Something else interesting I came across while researching and browsing around.

When a bottle of carbonated drink is frozen it will build up pressure inside the bottle and potentially explode.

Why is that? You (or I anyway) would think that frozen = cold = shrinkage = less pressure.

So why would freezing soda pop build up so much pressure?

https://youtu.be/rRdJopyVYsk?si=fo2c6pE424dvNQy1

https://youtu.be/WFyaL6iozKY?si=-rZrBJTt2vIm-g-M

https://youtu.be/TYKFdJSDjEs?si=QZuRQpvZAAoeirBa

Well, yes, the ice swells compromising the integrity of the container, but the pressure is way higher than normal.

I'm guessing that freezing causes the CO2 to dissociate from the liquid ? Kind of like how ammonia dissociates from water when the water is heated resulting in ammonia gas at high pressure in an absorption refrigeration system?

Maybe instead of an egg. That would probably be too difficult to keep the steam in the bottle from being sucked out by the vacuum pump.

Instead put a screw top on the bottle, maybe use a canning jar.

On the lid of the canning jar have a simple pressure gauge.

Now boil some water in the bottle filling the bottle with steam, screw on the lid with the pressure gauge and pop it in the vacuum chamber.

There will be a vacuum in the vacuum chamber but the steam cannot possibly escape the bottle. Eventually the steam should condense and create a vacuum inside the bottle.

But as there is a vacuum both inside and out, the gauge should not register any pressure either way.

But, probably the dial on the gauge should be video recorded, because there may be some sudden fluctuation in pressure when the steam finally condenses in the bottle, that could be missed if someone blinks.

The gauge needs to be able to read "negative pressure" such as the type used by HVAC technicians maybe?

A sudden jump to "negative pressure" as the water vapor condenses in the bottle.

What would actually happen?

Will the canning jar implode? Shattered glass inside the vacuum chamber? BAM!!! Like when the 55 gallon drum suddenly gets crushed by "atmospheric pressure" but NOT from OUTSIDE atmospheric pressure, because that has been eliminated.

The implosion would be, then, from this theoretical? Molecular cohesion of molecules or "negative pressure" such as what "sucks" water up an impossible 200 foot "capillary tube" inside a tree trunk??

Or, maybe nothing.

Maybe a simple "pressure gauge" could be just a rubber diaphragm, like a piece cut from a heavy latex glove.

Instead of a pressure gauge just cover the jar with a section of rubber gloves or perhaps just a balloon held on with a rubber band? Or held on by a canning jar lid rim, without the center piece


The balloon or whatever should provide some visual indicator of the relative pressure between vacuum chamber and steam filled jar.

Eventually the steam should cool and condense and either "suck" the balloon into the jar,

...or not

Tom,
What I see a few posts up, is a quote about a process seen in Botany , where sap gets lifted up trees beyond what 'positive pressure' can explain.
I can't pretend to provide a full explanation of how capillary action works, but I do know that it has at least something to do with "surface tension" which is a phenomenon that (as far as I know) only applies to liquids, not to gasses.
Incidentally, my take on the egg video that you shared at the beginning of this thread is that the egg was too big for the bottle, and so there was only a pressure differential across _some_ of the egg, with the remainder getting shaved off by the lip of the bottle.

As far as Hot Air engines are concerned, I think the proof lies in careful experimentation - far too much of what YouTube has to show is subject to many competing variables being involved at indeterminate times. The experiment you have just suggested, seems good to me.

Or...

Since we really want to know about hot air engines, rather than steam...

Just warm the jar, put on the lid with the balloon, or tie the balloon on with the rubber band.

Put that into the vacuum chamber.

If gas NEVER "sucks" by molecular adhesion or whatever, the balloon should never become concave, drawn down into the jar while inside the vacuum chamber.

Ummm... The ∆T should be,... Well, something, so...

After heating the bottle and capping it off, have an ice bath in the vacuum chamber ?

Put the hot capped bottle in the ice bath?

Could be some variation but the air in the bottle should be made to cool somehow, as much as possible.

Maybe a peltier refrigeration chip or something under the bottle, or just one of those extra cold reusable freez pack things:

skyofcolorado wrote: ↑Mon Feb 05, 2024 2:32 pm

I'm thinking of a piston returning to TDC "by atmospheric pressure" without any apparent means for heat removal.

I think these are two completely and totally unrelated matters, which may be part of the confusion. The power piston in a gamma engine has zero role in heat transfer/removal. Its motion is the result of heat transfer changing the system pressure during the cycle. ...

Well, that is the mystery.

I said, "without any apparent means for heat removal".

For example my dozens of experiments insulating the "heat sink".

The piston still returns, hundreds, thousands of times, over and over again for hours and hours with continuous, unrelenting heat input from boiling water or even a propane torch.

If the heat cannot be removed to the "heat sink" and as you said: "The power piston in a gamma engine has zero role in heat transfer/removal. Its motion is the result of heat transfer changing the system pressure during the cycle. ..."

Well, I think the power piston DOES have a central role in heat removal. It is the mechanism through which heat is CONVERTED into mechanical WORK. It does work, causing the heat to "disappear".

Or rather, the working fluid does work driving the piston, which causes the reduction in the temperature of the working fluid.

But is that reduction in temperature enough by itself to cause the working fluid to "contract" and "suck" the piston back inward by "molecular adhesion" or whatever? Or maybe it's just atmospheric pressure. Whatever, the gas cools enough to make it very easy to compress and for the piston to return to TDC (full compression) in spite of continuous heating, zero active cooling (such as a water jacket or ice) and no apparent place for the heat to go because the engine is buried under insulation!


Just a couple, low temp and high temp

https://youtu.be/zEqg1TgLqXI?si=DU4AL_8N1CbuaRuK

https://youtu.be/LG09AXAjpio?si=nO79tFPKeWIOdI5-

skyofcolorado wrote: ↑Mon Feb 05, 2024 2:32 pm

I'm thinking of a piston returning to TDC "by atmospheric pressure" without any apparent means for heat removal.

I think these are two completely and totally unrelated matters, which may be part of the confusion. The power piston in a gamma engine has zero role in heat transfer/removal. Its motion is the result of heat transfer changing the system pressure during the cycle. The power piston and power cylinder could be made of a perfectly insulating aerogel that holds and transfers nearly zero heat energy, and the engine would work the same.

I would imagine the piston in an "acoustic" or whatever kind of engine would be the same since the motion (the traveling wave) is apparently what acts as the "displacer" and the resulting contact of the fluid mass with the hot or cold parts (far removed from the power piston) are what change the system pressure and move the power piston. Didn't you experiment with a wood piston once? Isn't that largely the same idea? What possible (practical) role could a piece of wood have in heat transfer, yet the engine works?

Perhaps in an alpha config the piston might have a role in this because the heat differential is based on the two cylinders being at opposite ends of the flow, but not a gamma or acoustic. At least that's how it registers in my mind anyway, but I don't know if I'd put money on it though.

I've used wood displacers, but that's not relevant to the power piston

The only time I attempted anything with a non-heat-conductive power piston was when doing ICE experiments, trying to see if the engine had a "refrigerating effect" on the ice it was running on.

I learned that graphite is HIGHLY heat conductive, but epoxy conducts almost no heat, and I knew some engine builders have fashioned epoxy pistons successfully, so I tried to make an epoxy piston to replace the graphite piston in order to reduce heat entering the engine through the graphite piston, which would be uncontrolled, out of phase heat input that might destroy any slight cooling during expansion.

If there was cooling from adiabatic expansion a potentially HOT or relatively hot piston would be letting in heat as the working fluid expanded and cooled due to conversion of heat into work.

Anyway, the epoxy piston I made came out way too tight in the cylinder, so I tried honing it down with some lapping compound. Just in case anyone doesn't know, the lapping grease contains micro-fine silicon carbide grit that basically acts like very fine sand paper to get metal parts like engine valves to fit together perfectly.

Well, being lazy, I smeared the compound on the piston and got the engine running ON ICE, just because I already had this setup for running my experiments

But technically, this was not an experiment, I was just trying to make a piston in preparation for an experiment.

The piston and cylinder were actually ruined because I left the engine running too long and the cylinder got so wallowed out and couldn't hold pressure and then the engine would not longer run. It stopped in the middle of the night and could not be restarted. I'd have to start over from scratch, but never got around to it.

But,...

This event had very interesting results anyway. Before the engine got completely ruined.

As the engine ran, for the first 45 minutes or so, the partially melted ice kept re-freezing so that the engine got "stuck" firmly to the surface of the ice.

I thought that this was very strange and potentially significant, as I had done several experiments of this sort already with the same setup, but with the normal graphite pistons that came with the model engine, and this re-freezing of the ice the engine was running on had never happened before, (or since).

Without ever following through with a formal experiment, I was already satisfied that I had seen some apparent "refrigeration" or at least a reduction of heat input through the piston and the change in material had an observable effect on heat transfer.

Personally I was utterly flabbergasted when unable to lift the engine from the ice. HEAT (ambient heat) should be flowing THROUGH the running engine into the ice.

I think the room at the time was about 80° F

The ice had already been exposed to the air for some time and had started melting on the surface. It seemed inexplicable that the melt water on the surface kept refreezing when I ran the engine. According to the Carnot formula, the heat rejected by the engine into the ice would be hardly any less hot than exposing the ice directly to the ambient air. If the engine, for example took in 10 joules per cycle, maybe 1 or 2 at most would be converted to work, the other 8 or 9 would be "rejected" to the ice.

So the engine should have been, to the ice, like a heater, if only a heater of some fractional wattage, a heater nonetheless. The ice should continue to melt, not re-freeze over and over again for 45 minutes or more.

So after I had to forcefully pry the engine loose from the ice a couple times over the course of about 15 or 20 minutes, I thought this was so jaw dropping that I went upstairs to the bedroom to get my phone to record a video just in case it happened a third time. The engine was left to run on the ice as I went and got the phone.

I waited a while until I though that perhaps sufficient time had passed to give the ice time to re-freeze AGAIN. I was skeptical that would happen, but if it did I wanted concrete video evidence.

So,... I let some time pass, about another five or ten minutes, then tried to lift the engine off the ice once again, but this time shot a video:


https://youtu.be/2b2dIR8Eql8?si=2JHu3AVEAcNjgsPr

So, is that relevant to your question?

Anyway, the piston is what the "working fluid" does "work" on. The expanding gas does work pushing the piston. So IMO, the piston is a significant focal point for thermodynamics transformation of heat into work. It's thermal properties and heat transfer capabilities are of critical importance.

If the "working fluid" is not only doing work while expanding and pushing the piston, but also doing work while contracting and pulling the piston, like a stretched rubber band might pull an object as it "relaxes", that might result in an additional temperature drop of the working fluid during the. "compression" stroke, which might also help explain such "weird" re-freezing and other similar more formal experimental observations.

skyofcolorado wrote: ↑Tue Feb 06, 2024 8:09 am ...I've had a global shutter high-speed video camera for a lot of years, ...

Add that asset to my offer of a vacuum chamber experiment. ...

That would be great.

Something like the hot air (or steam or both) in a jar with a balloon tightly sealed over the mouth of the jar with a rubber band, put into the vacuum chamber and allowed to cool/condense would, I think, be easy to do and relevant.

Expected result?

If there is some actual "suction" due to molecular attraction inside the jar, the balloon should become concave as it is drawn inward by whatever "attractive" molecular cohesion, adhesion, whatever, inside the jar "pulling" the balloon inward as the gas cools and the attractive molecular forces begin to dominate and create an "absolute negative vacuum"

Otherwise, a simple vacuum will form which would be balanced by the external vacuum and the balloon should remain level or swell outward rather than inward due to some residual internal pressure.

Probably just a normal video recording should be able to capture something.

But I suppose there is some small chance of something happening very quickly, like a sudden "implosion" bursting the balloon.

But it's an easy repeatable experiment. The high speed camera could be used on a repeat experiment if necessary, or if it seems there is some reason to do so. Up to you.

I usually record experiments the first run just in case something of historical importance happens, it would be nice to have the "discovery" on film.

MikeB wrote: ↑Wed Feb 07, 2024 4:04 am Tom,
What I see a few posts up, is a quote about a process seen in Botany , where sap gets lifted up trees beyond what 'positive pressure' can explain.
I can't pretend to provide a full explanation of how capillary action works, but I do know that it has at least something to do with "surface tension" which is a phenomenon that (as far as I know) only applies to liquids, not to gasses.

Yes, though steam contains some actual water vapor droplets, though not likely significant.

However "surface tension" would, I think, be a subset of molecular attractive forces generally.

What exactly the "attractive force" if any, of a gas rapidly cooling in an engine cylinder might be, I don't know. There are, I think, a number of possible candidates.

Generally, the umbrella term is "Van der Walls".

Van der Waals Forces

Van der Waals forces' is a general term used to define the attraction of intermolecular forces between molecules. There are two kinds of Van der Waals forces: weak London Dispersion Forces and stronger dipole-dipole forces

https://chem.libretexts.org/Bookshelves ... lar_Forces

Incidentally, my take on the egg video that you shared at the beginning of this thread is that the egg was too big for the bottle, and so there was only a pressure differential across _some_ of the egg, with the remainder getting shaved off by the lip of the bottle.

One possible explanation.

As far as Hot Air engines are concerned, I think the proof lies in careful experimentation - far too much of what YouTube has to show is subject to many competing variables being involved at indeterminate times. The experiment you have just suggested, seems good to me.

I could likely do that myself. I do have a vacuum pump, but I would have to build a chamber. But there has been an offer, so for the sake of expedience, if skyofcolorado is willing, I'd love to take him up on the offer.