Rarefication

[LES_Thermodynamics]

Yes, the expansion and compression of a gas inherently come with temperature changes, even in the absence of direct heat transfer with external heat sinks. This phenomenon is a fundamental principle of thermodynamics and can be explained using the ideal gas laws and the concepts of adiabatic expansion and compression.

Adiabatic Expansion: When a gas expands without exchanging heat with its surroundings, its temperature decreases. This is an adiabatic process. In simpler terms, as the gas molecules spread out (expand), they do work on their surroundings and lose internal energy, which results in a decrease in temperature.

Adiabatic Compression: Conversely, when a gas is compressed without heat exchange, its temperature rises. As the gas molecules are pushed closer together (compressed), work is done on the gas, increasing its internal energy and, consequently, its temperature.
These principles are evident in several real-world scenarios:

Diesel Engines: These rely on the adiabatic compression of air to increase its temperature to the point where diesel fuel injected into the combustion chamber ignites spontaneously.
Bicycle Pumps: When you pump air rapidly into a bicycle tire, the pump gets warm due to the adiabatic compression of the air inside.

[MikeB]

Les,
The trouble with that description of Adiabatic processes, is twofold:

1. In the real world, and especially in hot-air engines, there is ALWAYS some heat transfer in/out of the walls of the engine.

2. Timing. At low speed, or in step-by-step analysis, all of the working-fluid changes temp/pressure instantaneously. In a real engine, it takes time for heat to propagate from one part of the engine to another.

[Tom Booth]

Les,
The trouble with that description of Adiabatic processes, is twofold:

1. In the real world, and especially in hot-air engines, there is ALWAYS some heat transfer in/out of the walls of the engine.

2. Timing. At low speed, or in step-by-step analysis, all of the working-fluid changes temp/pressure instantaneously. In a real engine, it takes time for heat to propagate from one part of the engine to another.

What description MikeB?

Can you please point out specifically where Les made either of those absolute assertions.

[LES_Thermodynamics]

Les,
The trouble with that description of Adiabatic processes, is twofold:

1. In the real world, and especially in hot-air engines, there is ALWAYS some heat transfer in/out of the walls of the engine.

2. Timing. At low speed, or in step-by-step analysis, all of the working-fluid changes temp/pressure instantaneously. In a real engine, it takes time for heat to propagate from one part of the engine to another.

You're right in pointing out the practical challenges and nuances of adiabatic processes, especially when it comes to real-world applications such as hot-air engines. It’s good conversation so let’s dive into it.

Heat Transfer Realities:
Imperfect Insulation: No material offers perfect insulation, meaning there will always be some level of heat transfer in and out of the engine's walls. This stray heat transfer can deviate the processes from being truly adiabatic. In my engine I specially chose 304 ss for its thermal conductivity.


Heat Transfer Rate: The rate of this heat transfer depends on factors such as the material of the engine walls, the temperature gradient between the working fluid and the surroundings, and the design of the engine. These considerations directly impact the engine's performance and efficiency. During my prototype phase I tried several wall thicknesses and found this out the hard way.

Timing and Propagation:
Heat Propagation Delays: In dynamic systems like hot air engines, there's a lag between when a portion of the engine is heated (or cooled) and when adjacent parts experience this change. This is due to the finite speed of heat conduction and the inherent thermal inertia of materials. You’ll notice that why NASA uses helium or hydrogen instead of air. The molecular structure of both those gasses is thinner than air and can cool down or heat up quicker. That “X” factor is exponentially increased with a vacuum. There is some really interested research on triple points and I highly recommend some follow-up for some quality late-night reading.

Thermal Waves: At high operating speeds, these delays can lead to the formation of thermal waves or temperature gradients within the working fluid and engine components. These gradients can cause non-uniform expansion or compression, affecting the engine's efficiency and stability. With thermal acoustics, which is all I will talk about, you can adjust the frequency during the construction of the engine. We can talk more about this if you’d like. You can’t control it, but you can make this factor consistent - which means you can engineer around it/leverage it.

Resonance and Harmonics: In some engines, especially those operating at high frequencies, these waves can lead to resonance phenomena, where certain parts of the engine may experience amplified oscillations. This can be both an advantage or a challenge, depending on the design and desired operation. In my engine I have used this to my advantage.

To the point that I think you’re making, it's essential to move beyond the idealized models when designing or analyzing real-world engines. Computational tools, such as Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA), can help model and predict these non-ideal behaviors. However, empirical testing and iterative design play crucial roles in optimizing engine performance.

Good ol R&D.

[Tom Booth]

LES/Travis, thanks for joining the forum and sharing your insights.

I found your comment in one of your video demonstrations (mentioned/posted above) about the air escaping until it becomes nearly a vacuum quite interesting. ("Less air more work").

In that context I found this interesting:

https://www.tec-science.com/thermodynam ... -of-gases/

Apparently as a gas heats up and gets "thinner" it's conductivity actually increases, which is kind of counterintuitive. Apparently this is due to the increased freedom of movement or "mean free path".

At higher concentration/density, (lower temperature) the gas molecules are, in effect, in each other's way and so do not conduct heat as well as when the molecules are more dispersed and have greater freedom of movement.

The conductivity of 304 Stainless Steel is relatively low, but in comparison with air is still very high.

The conductivity of air is lower at colder temperatures, for example, at the temperature nearing that of liquid nitrogen, the conductivity of air is cut to less than 1/4 of what it is at room temperature, but conductivity of air is doubled at about 600° F

At some point the heated air can be so thin that the size of the container (distance between heat source and piston) matters.

Either helium or hydrogen are about 6X more heat conductive than air, and their conductivity also increases at higher temperatures.

Still, even at high temperature the conductivity of air is like 0.03 while that of Stainless steel is 14.4 (W/mK)

So even with SS we are trying to get heat to go into and expand the air inside a container that is 480 times more heat conductive than our "working fluid".

To me, that is comparable with trying to get electricity to flow through wood that is covered with metal. Most of the energy will be lost to the container.

This presents a difficulty when trying to construct a heat engine that has both high strength and low heat conductivity.

Locally here, I happen to have a source of good quality brick clay I'm just begining to experiment with.

Ideally, or logically, the working fluid should, I assume, be more heat conductive than the engine body and other engine parts, other than the regenerator.

Many small model engines are made of acrylic or glass (between the hot and cold ends). Due to their high insulating properties, with these, some heat at least has a chance of getting into the working fluid without being conducted away through the engine body.

[Tom Booth]

Of course, between the heat source and the piston at least, if it is imagined the air particles actually travel through the engine, a very rapid (adiabatic) transfer can effectively mitigate heat loss to one degree or another, otherwise the old cast iron engines could never have run at all.

But do the gas particles actually travel through the engine or just the "pressure waves"?

[Tom Booth]

One thing I'm finding a bit puzzling in researching all of this heat conductivity of gases, particularly under pressure, is that the consensus seems to be that pressurization increases power.

There also seems to be a general consensus that pressure has little or no influence on the heat conductivity of a gas

Perhaps pressurization allows "pressure waves" to travel faster, though the molecules would theoretically be hindered in their movement by being packed closer together.

[Bumpkin]

My two cents: I’ve always thought of the “heat is molecular motion” theory to mean internal atomic motion in the molecules actually makes them bigger so they push out on their neighbors a bit, but then stop until they get fatter or skinnier. Such as a Gamma displacer could run a piston through a tube voluminous enough that none of the heated/cooled air ever even gets to the cylinder. (Though obviously the extra volume would diminish the pressure difference of the pulses.) But the pressure difference of the pulses would still be higher with a higher internal pressure average. A non-sealed engine without a proper snifter would naturally leak off enough pressure to average half of its internal pressure difference below atmospheric, but I don’t think less pressure would ever mean more power without other things going on. Less weight and thermal mass though; would have the advantage of heating and cooling faster. So a lighter gas that fills the same space with the same pressure/temperature is needed. As I understand it helium is not only lighter, but also has the advantage of being monatomic so there are no atomic bonds and thus less thermal mass for its weight. That last is sorta beyond my ken, but I can accept it. So helium could mean more power, but it wouldn’t necessarily mean more thermal efficiency, just that it could process more heat.

I think.

Bumpkin

[Tom Booth]

I don't know. I'm not particularly a "believer" in the kinetic, or any other theory. Do the gases literally shrink and grow or expand and contract, or just move faster so need more "elbow room" so to speak ?

What seems apparent is I've seen several videos of Kontex and others putting some helium in a model engine and it runs better, faster and measurably puts out more power without pressurization.

One example:

https://youtu.be/dMH0I5HiitI?si=W07khOc2h30fuXeE

So, how much better might some model run with a little pressurization? And why?

Like so many other things about Stirling engines there are a number of different theories.

"Real" gas behavior is rather complex and unpredictable. I said gas is less conductive at lower temperature but apparently not necessarily or not always for REAL gases. At high pressure and low temperature the conductivity goes back up.

https://www.engineeringtoolbox.com/air- ... _1509.html

Not sure if gas conductivity has anything to do with power in a Stirling but is it just coincidence helium and hydrogen are much more conductive than other gases and that's what NASA uses in their stirling converters?

I just try to clear my mind of all theory and preconception and just do my experiments and watch what happens, then maybe try to figure out what caused whatever transpires.

[MikeB]

Tom,
I don't think you need to be a believer in Kinetic Theory, or any other specific theory, to see that ultimately, the component molecules of your working fluid push on the power piston individually, though it is always easier to think of them "as a whole".
All things being equal (which is never true), having twice as many molecules doing the pushing seems like it _should_ provide twice as much power?

[Tom Booth]

Tom,
I don't think you need to be a believer in Kinetic Theory, or any other specific theory, to see that ultimately, the component molecules of your working fluid push on the power piston individually, though it is always easier to think of them "as a whole".
All things being equal (which is never true), having twice as many molecules doing the pushing seems like it _should_ provide twice as much power?

That would make sense.

But there are other ways of looking at it.

Imagine a waiter trying to get hot food to the table in a crowded restaurant. The kitchen is the heat source, the table the piston the waiter the air molecule.

If the restaurant is nearly empty the food can get to the table quickly. If extremely crowded and the table a long way from the kitchen, it might be difficult to get the food to the table while it's still hot without bumping into another waiterr or customer.

But if there are waiters packed together side by side so they can't even move, maybe they can set up a relay system all the way from the kitchen to the table, and still get food to the table while it's hot.

That's a lame example, but there could be more than one factor in play

In an open space, movement is easy, but when objects, people or whatever are close packed together, movement can still be transmitted.

But in a partial crowded room, you can't move freely OR transmit energy any distance.

I'm just trying to imagine the possible reason for the curves in those graphs.

Resize_20231009_133901_1966.jpg (80.22 KiB) Viewed 6502 times

[MikeB]

... there could be more than one factor in play

Ah, but that's the thing, isn't it? Some other types of engine it might be easy/relevant to use a simple model, but for Stirling/Hot-air engines there is ALWAYS more factors at play, the difficulty is in working out which ones have a significant effect and which ones don't.

On which note, that graph is certainly useful/interesting.

[Tom Booth]

... there could be more than one factor in play

Ah, but that's the thing, isn't it? Some other types of engine it might be easy/relevant to use a simple model, but for Stirling/Hot-air engines there is ALWAYS more factors at play, the difficulty is in working out which ones have a significant effect and which ones don't.

On which note, that graph is certainly useful/interesting.

Like what type of engine are you talking about that is easy/simple compared with Stirling/hot air?

Aside from that this topic is largely speculation, but if true would certainly be relevant to other engines.

[LES_Thermodynamics]

Hi all, I’ve been meaning to reply but have been caught up in projects. To Tom’s point - yes it would be ideal to have a container that is not conductive IF you wanted power only when heated. When you use something like 304ss it holds heat like a phase change material, which will allow the engine to run longer without heat.

I have a planned test coming up with Methylene chloride inside the engine. The boiling point is around 100 degrees and should answer a lot of questions, especially when it comes to if SOUND is causing the power or if it’s waves pushing vapor, or if it’s just rapid cooling and heating on the inside of the engine (the Methylene chloride will boil at the bottom, vaporize in the middle and condense at the top).

[LES_Thermodynamics]

I have this plan someday to fill a tri-clamp pipe to the top with salt, with one end welded closed. Then heat that up- which would heat up the bottom of the engine once attached. Molten salt, which is then combined with water to create steam - then pumped through a turbine is how most people receive power on the grid. It does an incredible job as a phase change material and allows for 12 hours+ of heat retention.

There was a really cool project in Arizona, USA using molten salt and mirrors. Unfortunately the place was shutdown due to maintenance costs, since the idea worked so well on a small scale that they made is huge. Unfortunately, the cost of maintenance on gigantic pipes transporting and storing molten salt made the whole project cost prohibitive. I think now they are just sitting on it until the cost of electricity goes up and the whole thing becomes profitable again.

Anyways - I’ve never seen a Stirling engine powered by molten salt. If someone beats me to it, or already has, please share!