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Is Carnot efficiency valid?


Tom Booth
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I've done nearly everything possible I can think of to block the "flow" of heat out of a Stirling engine, with the idea that if the "heat rejection" could be blocked, the engine would quickly overheat and stop running, or be unable to start running in the first place.

To that end, I recently sent away for a silica Aerogel blanket, which is supposed to be very good at blocking heat. I was able to apply a propane torch to one side and not feel heat through the blanket with my hand on the other side.

I also used a glass globe from a Coleman lantern to block drafts.

As in previous experiments just using styrofoam or house insulation, the engine started and continued running.

This result seems contrary to the "efficiency formula" or "Carnot limit" which, would calculate that some 80% or more of the heat supplied to or entering into the engine would need to be rejected to the sink or cold side back to atmosphere for the engine to continue operating.

So why does the engine not stop from overheating when the cold side is insulated, blocking the flow of heat out of the engine to the "cold reservoir" ?

 

 

 

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2 hours ago, Tom Booth said:

I've done nearly everything possible I can think of to block the "flow" of heat out of a Stirling engine, with the idea that if the "heat rejection" could be blocked, the engine would quickly overheat and stop running, or be unable to start running in the first place.

To that end, I recently sent away for a silica Aerogel blanket, which is supposed to be very good at blocking heat. I was able to apply a propane torch to one side and not feel heat through the blanket with my hand on the other side.

I also used a glass globe from a Coleman lantern to block drafts.

As in previous experiments just using styrofoam or house insulation, the engine started and continued running.

This result seems contrary to the "efficiency formula" or "Carnot limit" which, would calculate that some 80% or more of the heat supplied to or entering into the engine would need to be rejected to the sink or cold side back to atmosphere for the engine to continue operating.

So why does the engine not stop from overheating when the cold side is insulated, blocking the flow of heat out of the engine to the "cold reservoir" ?

 

 

 

Because you evidently still have a temperature difference, in spite of your efforts. What you really need to do is attach thermometer probes to the engine's contact with the hot source and its contact with the cold sink, to see what the input and output temperatures experienced by the engine actually are.. If the temperature of both becomes equal the engine will stop. If it does not become equal then you have a heat leak - or else the engine uses so little heat that the cold side takes ages to heat up and stop.

As is usual with videos, the video shows us nothing worthy of comment.     

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55 minutes ago, exchemist said:

- or else the engine uses so little heat that the cold side takes ages to heat up and stop.

It is my understanding that the less heat that is used by the engine the greater would be the amount of heat that would need to be rejected to the cold side so as to cool the engine each cycle so the piston could complete a cycle.

In other words 2% used, 98% rejected = 100% or 20% used 80% rejected to the sink.

So the less efficient the engine is at utilizing the heat, the more heat rejected, the faster the cold side should heat up. Is that incorrect?

Also the bottom of the engine body or hot side is aluminum. The rest of the engine body or cold side is acrylic. Aluminum is an excellent heat conductor. Acrylic is a very poor heat conductor. Even without the added insulation it is difficult to understand how acrylic could serve very well as a heat sink.

 

55 minutes ago, exchemist said:

...What you really need to do is attach thermometer probes to the engine's contact with the hot source and its contact with the cold sink, to see what the input and output temperatures experienced by the engine actually are..

That could certainly be done. I'll run the experiment again with temperature probes 

However, logic would seem to dictate that the hot bottom would be near 212°F and the top cold side no less than the room temperature which was about 71°F give or take a few degrees. That is enough information for an estimate of the maximum Carnot efficiency and minimum percentage of heat rejection required for the engine to complete a single revolution.

The engine ran a total of 3 hours before I concluded the trial with no indication of slowing down. (After I stopped the insulation from rubbing on the flywheel too much, that is)

 

BTW, I'm not suggesting that the engine could run without a temperature difference, necessarily.

It seems certain, if the engine is running there is a temperature difference.

The question is, how is the temperature difference maintained?

There are two means of reducing the temperature of the cold side, or maintaining the relatively cold temperature of the cold side.

1) heat rejection

2) heat conversion to mechanical motion, vibration, sound, or thermodynamic "work".

I would suggest that if #1 could be largely eliminated then it could be assumed that #2, heat conversion to other forms of energy would be the primary source of cooling.

Edited by Tom Booth
Added about the insulation rubbing edit2 added the word "largely" on the flywheel.
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Quote

As is usual with videos, the video shows us nothing worthy of comment. 

!

Moderator Note

One of the reasons that we require text descriptions to be posted, rather than relying on videos

 
41 minutes ago, Tom Booth said:

However, logic would seem to dictate that the hot bottom would be near 212°F and the top cold side no less than the room temperature which was about 71°F give or take a few degrees. That is enough information for an estimate of the maximum Carnot efficiency and minimum percentage of heat rejection required for the engine to complete a single revolution.

The temperatures (using an absolute scale) give the efficiency, but I don’t see how that gives you the heat rejection to complete a revolution.

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3 minutes ago, swansont said:
!

Moderator Note

One of the reasons that we require text descriptions to be posted, rather than relying on videos

 

Thanks.

There is a rather complete and thorough text description before the video. If not adequate I'd be happy to post any additional information requested if anything isn't clear. I'm not relying on video but I assume video is not forbidden as a supplement to a description, is it?

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Just now, Tom Booth said:

Thanks.

There is a rather complete and thorough text description before the video. If not adequate I'd be happy to post any additional information requested if anything isn't clear. I'm not relying on video but I assume video is not forbidden as a supplement to a description, is it?

I don’t consider it to be complete or thorough. You don’t have the temperatures that exchemist asked for, for starters. You don’t give the rotation rate of the wheel. You need to describe the experiment such that it could be replicated, with all the pertinent data.

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3 minutes ago, swansont said:

I don’t consider it to be complete or thorough. You don’t have the temperatures that exchemist asked for, for starters. You don’t give the rotation rate of the wheel. You need to describe the experiment such that it could be replicated, with all the pertinent data.

OK, I'll do my best to supply the additional information requested ASAP. Thanks!

17 minutes ago, swansont said:
!

Moderator Note

One of the reasons that we require text descriptions to be posted, rather than relying on videos

 

The temperatures (using an absolute scale) give the efficiency, but I don’t see how that gives you the heat rejection to complete a revolution.

212°F = 373.15 K

71°F = 294.81 K

Therefore Carnot efficiency = 20.994%

Heat rejection should then be no less than 79.006%

At least that is how the heat rejection is typically calculated in textbook examples.

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What design of Stirling engine is it? Alpha? Beta? Gamma? Something else?

Manufacturer/Model number?

Is there a schematic diagram available so we can see the piston arrangement, heat exchangers locations, regerator (if any) just to give us some idea of what you're asking us to comment on.

What's the process gas? Hydrogen? Helium? Air?

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44 minutes ago, Tom Booth said:

OK, I'll do my best to supply the additional information requested ASAP. Thanks!

212°F = 373.15 K

71°F = 294.81 K

Therefore Carnot efficiency = 20.994%

Heat rejection should then be no less than 79.006%

At least that is how the heat rejection is typically calculated in textbook examples.

That’s percentage. It doesn’t tell you the amount of heat rejected so you don’t know the energy it takes to rotate the wheel

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52 minutes ago, sethoflagos said:

What design of Stirling engine is it? Alpha? Beta? Gamma? Something else?

Manufacturer/Model number?

Is there a schematic diagram available so we can see the piston arrangement, heat exchangers locations, regerator (if any) just to give us some idea of what you're asking us to comment on.

What's the process gas? Hydrogen? Helium? Air?

The particular engine used in this experiment is available from eBay or Walmart.

https://www.ebay.com/itm/254879289942

The type of engine does not seem to be particularly relevant though. I've done many similar experiments using various types and brands of Stirling heat engines with similar results. This one however is a "gamma" type "low temperature difference" engine with a magnetically actuated displacer.

26 minutes ago, swansont said:

That’s percentage. It doesn’t tell you the amount of heat rejected so you don’t know the energy it takes to rotate the wheel

Carnot efficiency is what's at issue. Carnot efficiency is a percentage.

The heat source (facial steamer "sauna") is rated 100 watts, if that's of any help. According to online sources that would be 100 joules per second, assuming all of the that wattage actually went into the engine which is obviously not the case. Enough steam was escaping to fog up the glass going around rather than into the engine.

Logically though, I would think that if the temperature difference is 88K and more than 80% of the heat applied passes through to the sink but has nowhere to go, the cold heat exchanger should quickly heat up to nearly the same temperature as the hot side heat exchanger.

The engine displacer is constantly transferring the working gas (air) from the hot to the cold side and back. There is no regenerator.

The cold side should quickly heat up, excluding conversion of the heat to "work" output that is.

Edited by Tom Booth
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45 minutes ago, Tom Booth said:

Carnot efficiency is what's at issue. Carnot efficiency is a percentage.

The heat source (facial steamer "sauna") is rated 100 watts, if that's of any help. According to online sources that would be 100 joules per second, assuming all of the that wattage actually went into the engine which is obviously not the case. Enough steam was escaping to fog up the glass going around rather than into the engine.

You spoke of “minimum percentage of heat rejection required for the engine to complete a single revolution.” and I was pointing out that this is an energy (the mechanical work that it does) and not something you get from the efficiency.

Using a 100 W source means that’s the maximum, but the actual QH is likely much less

Quote

 

Logically though, I would think that if the temperature difference is 88K and more than 80% of the heat applied passes through to the sink but has nowhere to go, the cold heat exchanger should quickly heat up to nearly the same temperature as the hot side heat exchanger.

The engine displacer is constantly transferring the working gas (air) from the hot to the cold side and back. There is no regenerator.

 

But you’ve insulated the device. Which means the hot side can be hotter and stay hot. Before insulation the heat differential is likely smaller.

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5 hours ago, Tom Booth said:

So why does the engine not stop from overheating when the cold side is insulated, blocking the flow of heat out of the engine to the "cold reservoir" ?

You are not reducing the cold side overall heat transfer coefficient as much as you think you are. The machine as supplied is designed to run despite the cold side heat rejection passing through two 'insulators': acrylic and air. Adding a layer of aerogel only retards convection. The machine clearly is able to run on 'conduction only' mode. So aerogel is just a different type of air as far as the machine is concerned.

Designs of this type do not approach the Carnot limit in any way shape or form so trying to draw any conclusions about the validity of Carnot efficiency is kind of crass. 

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2 hours ago, swansont said:

You spoke of “minimum percentage of heat rejection required for the engine to complete a single revolution.” and I was pointing out that this is an energy (the mechanical work that it does) and not something you get from the efficiency.

The gas/air in the engine is heated and expands. The expanding air drives the piston outward doing work. A certain percentage of the heat, (a form of molecular kinetic energy of the air molecules) is transfered to put the piston in motion, (a different form of mass kinetic energy). As a result the air loses energy. The loss of energy results in a reduction in temperature (of the gas/working fluid).

A reduction in temperature of the gas is necessary in order for the piston to return (recompressing the heated and expanded gas) If the gas is not cooled back down in some way, the air cannot be re-compressed and the cycle would end there, the engine coming to a halt, unable to fully recompress the heated and expanded air.

Carnot efficiency is the ratio (percentages) of working fluid cooled by the above described conversion to work output and the remaining heat which needs to be gotten rid of by conductive transfer to the sink.

If 10% of the kinetic energy is transfered to the piston, the other 90% must be gotten rid of by transfer to the sink.

So to complete a cycle, if the engine is 20% efficient at transferring energy to the piston, the other 80% must be wasted or "rejected" through the engine to the sink or cold side.

So the work the engine does IS what you get from the 20% or whatever efficiency. No?

That is my understanding anyway. The remaining unconverted "waste heat" must be transfered to the sink so that the working gas (air) can cool back down and contract. Otherwise the piston has to expend as much energy to compress the gas as it received by the gas while it was being heated and expanded.

The efficiency is supposed to be a measure of the work output of the engine. Whatever heat does not go towards driving the piston out has to be gotten rid of.

So there is a "minimum percentage of heat rejection required for the engine to complete a single revolution."

If the engine is 20% efficient at converting heat to mechanical "work" the remaining 80% of the heat would be that percentage of heat "rejected" to the sink.

2 hours ago, swansont said:

...But you’ve insulated the device. Which means the hot side can be hotter and stay hot. Before insulation the heat differential is likely smaller.

I don't see how that follows.

If I have a heating element in a box (like an oven) and there is no insulation, the walls of the oven exposed to outside air, the walls will be relatively cool. Insulate the outside of the oven and everything inside would heat up to a uniform temperature eventually.

2 hours ago, sethoflagos said:

... Adding a layer of aerogel only retards convection. The machine clearly is able to run on 'conduction only' mode. So aerogel is just a different type of air as far as the machine is concerned...

I can see your point to a degree, but I believe convective cooling is a huge factor. Otherwise what use would any insulation be anywhere? Why do people insulated their homes, attics, wear coats, use blankets etc. except to reduce convective cooling.

2 hours ago, sethoflagos said:

Designs of this type do not approach the Carnot limit in any way shape or form so trying to draw any conclusions about the validity of Carnot efficiency is kind of crass. 

On what basis do you draw the conclusion that engines of this type do not approach "the carnot limit"?

In what way is that anything more than your personal opinion?

How could this "Carnot limit" ever be tested objectively by actual experiment?

As far as I've been able to determine it (the Carnot limit) is nothing more than an unverified supposition. Where and when and how has it ever been empirically demonstrated?

I can find no historical record of any such thing, only that Sadi Carnot thought heat was like water "falling" from a high to a low temperature so considered that the "distance" of this "fall" somehow determined efficiency.

He, BTW abandoned that hypothesis when it was discovered that heat is a kind of motion rather than a fluid, and recommended others abandon it as well, but died before these views were ever published.

 

Edited by Tom Booth
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8 hours ago, Tom Booth said:

OK, I'll do my best to supply the additional information requested ASAP. Thanks!

212°F = 373.15 K

71°F = 294.81 K

Therefore Carnot efficiency = 20.994%

Heat rejection should then be no less than 79.006%

At least that is how the heat rejection is typically calculated in textbook examples.

That's far too glib. You do not know the temperatures I said you would need to know. It is the temperature of the two heat exchangers (the nature of which you have not so far specified) at the hot and cold ends that you need to know. You cannot just presume they will have the temperature of the medium conveying heat to and from them. (Strictly speaking it's not even that: it is the hot and cold temperatures reached by the working fluid, during the cycle, that determine the thermodynamics.)

Apparently one of these media is boiling water, though you did not say so. (This illustrates why videos are not a substitute for a written description of the setup.) But the hot end is transferring heat into the engine, so the temperature of the heat exchanger will not be quite 100C (212F). 

If you are trying to insulate the cold end, the presumption must be that its temperature is not the ambient temperature of the room but significantly higher. If the insulation is perfect, then what will happen is that the waste heat rejected by the engine will progressively raise its temperature, eventually reaching that of the input temperature. As this takes place, the engine will run progressively slower and eventually stop.

What is for sure is that for the cold end heat exchanger to reject heat into the room, its temperature must be quite a lot higher than the ambient temperature. Recall Newton's law of cooling, according to which the rate of heat transfer is proportional to the temperature difference. So, no temperature difference, no heat transfer.

You need to know the real temperatures of hot and cold ends before trying to do a thermodynamic calculation. 

 

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52 minutes ago, exchemist said:

...

You need to know the real temperatures of hot and cold ends before trying to do a thermodynamic calculation. 

 

I agree, but you misunderstood. I did not intend the various known facts about the experiment as an answer to your question. You said:

"What you really need to do is attach thermometer probes to the engine's contact with the hot source and its contact with the cold sink, to see what the input and output temperatures experienced by the engine actually are.."

I said:

"That could certainly be done. I'll run the experiment again with temperature probes"

That will though, of course, take some time.

The infrared thermometer, I had thought would be adequate as it provides spot,as well as maximum and minimum readings, but unfortunately, as I only discovered in the course of the experiment, cannot penetrate either the pyrex? Coleman lantern glass or the acrylic.

I don't think there is a need for anyone to take an adversarial stance. The suggestion is a good one. As I said, I'll repeat the experiment using temperature probes ASAP. That is not likely to be today or tomorrow. Maybe within a week.

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7 hours ago, Tom Booth said:

The gas/air in the engine is heated and expands. The expanding air drives the piston outward doing work. A certain percentage of the heat, (a form of molecular kinetic energy of the air molecules) is transfered to put the piston in motion, (a different form of mass kinetic energy). As a result the air loses energy. The loss of energy results in a reduction in temperature (of the gas/working fluid).

A reduction in temperature of the gas is necessary in order for the piston to return (recompressing the heated and expanded gas) If the gas is not cooled back down in some way, the air cannot be re-compressed and the cycle would end there, the engine coming to a halt, unable to fully recompress the heated and expanded air.

Carnot efficiency is the ratio (percentages) of working fluid cooled by the above described conversion to work output and the remaining heat which needs to be gotten rid of by conductive transfer to the sink.

If 10% of the kinetic energy is transfered to the piston, the other 90% must be gotten rid of by transfer to the sink.

So to complete a cycle, if the engine is 20% efficient at transferring energy to the piston, the other 80% must be wasted or "rejected" through the engine to the sink or cold side.

So the work the engine does IS what you get from the 20% or whatever efficiency. No?

That is my understanding anyway. The remaining unconverted "waste heat" must be transfered to the sink so that the working gas (air) can cool back down and contract. Otherwise the piston has to expend as much energy to compress the gas as it received by the gas while it was being heated and expanded.

The efficiency is supposed to be a measure of the work output of the engine. Whatever heat does not go towards driving the piston out has to be gotten rid of.

So there is a "minimum percentage of heat rejection required for the engine to complete a single revolution."

If the engine is 20% efficient at converting heat to mechanical "work" the remaining 80% of the heat would be that percentage of heat "rejected" to the sink.

I don't see how that follows.

If I have a heating element in a box (like an oven) and there is no insulation, the walls of the oven exposed to outside air, the walls will be relatively cool. Insulate the outside of the oven and everything inside would heat up to a uniform temperature eventually.

I can see your point to a degree, but I believe convective cooling is a huge factor. Otherwise what use would any insulation be anywhere? Why do people insulated their homes, attics, wear coats, use blankets etc. except to reduce convective cooling.

On what basis do you draw the conclusion that engines of this type do not approach "the carnot limit"?

In what way is that anything more than your personal opinion?

How could this "Carnot limit" ever be tested objectively by actual experiment?

As far as I've been able to determine it (the Carnot limit) is nothing more than an unverified supposition. Where and when and how has it ever been empirically demonstrated?

I can find no historical record of any such thing, only that Sadi Carnot thought heat was like water "falling" from a high to a low temperature so considered that the "distance" of this "fall" somehow determined efficiency.

He, BTW abandoned that hypothesis when it was discovered that heat is a kind of motion rather than a fluid, and recommended others abandon it as well, but died before these views were ever published.

 

The Carnot cycle is a theoretical, ideal thermodynamic cycle, with no losses, that enables the maximum efficiency of any heat engine cycle to be determined. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html

All real heat engines are less efficient than this, small ones usually much less, as the incidental losses are large compared to the heat throughput.

Any designer or operator of a power plant engine or turbine will be well aware of how far short of Carnot efficiency even such huge machines are in reality. The value of it lies in what it tells you about the gain in efficiency to be had from maximising the temperature difference between hot source and cold sink. 

 

Edited by exchemist
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1 hour ago, exchemist said:

 

The Carnot cycle is a theoretical, ideal thermodynamic cycle, with no losses, that enables the maximum efficiency of any heat engine cycle to be determined. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html

All real heat engines are less efficient than this, small ones usually much less, as the incidental losses are large compared to the heat throughput.

Any designer or operator of a power plant engine or turbine will be well aware of how far short of Carnot efficiency even such huge machines are in reality. The value of it lies in what it tells you about the gain in efficiency to be had from maximising the temperature difference between hot source and cold sink. 

 

My question relates specifically to the carnot efficiency limit equation (rather than "cycle") though related, two different things.

That equation calculates a specific ratio of heat utilization (maximum possible conversion of heat to mechanical work output by a heat engine) and waste heat rejection of the remainder to a sink. (Neglecting other factors or potential loses: friction, vibration etc.) All based exclusively on the temperature differential.

To my mind, as a life long engine mechanic and repairman, this appears completely ridiculous on its face. There are dozens of factors that go into engine efficiency aside from the supposed distance of "the fall of caloric" from a high to a lower temperature "reservoir".

There is an interesting article on the subject that reads:

"Carnot's result was that if the maximum hot temperature reached by the gas is TH, and the coldest temperature during the cycle is TC, (degrees kelvin, or rather just kelvin, of course) the fraction of heat energy input that comes out as mechanical work , called the efficiency, is

Efficiency  = THTC/TH

This was an amazing result, because it was exactly correct, despite being based on a complete misunderstanding of the nature of heat! "

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7 hours ago, Tom Booth said:

I can see your point to a degree, but I believe convective cooling is a huge factor. Otherwise what use would any insulation be anywhere? Why do people insulated their homes, attics, wear coats, use blankets etc. except to reduce convective cooling.

And yet your engine still runs. Therefore there is still a significant thermal gradient across the device.

7 hours ago, Tom Booth said:

On what basis do you draw the conclusion that engines of this type do not approach "the carnot limit"?

Because they have significant losses over an ideal isentropic process. In addition to the usual friction losses, they feature approximately isothermal expansion and compression stages that are a lot less efficient than the corresponding approximately adiabatic stages employed in turbines for example; mixing of warm and cold working fluid occurs around and through the displacer piston; and the kinetic energy of your aforementioned convection currents has to come from somewhere.

8 hours ago, Tom Booth said:

In what way is that anything more than your personal opinion?

How could this "Carnot limit" ever be tested objectively by actual experiment?

As far as I've been able to determine it (the Carnot limit) is nothing more than an unverified supposition. Where and when and how has it ever been empirically demonstrated?

710163850_testcurves.thumb.jpg.7e0290d31d40d340bfda8465be84556a.jpg

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https://galileo.phys.virginia.edu/classes/152.mf1i.spring02/CarnotEngine.htm

 

My question is; how was it ever determined, experimentally or otherwise that this equation, admittedly based on "a complete misunderstanding of the nature of heat" happened to be "exactly correct" ?

I could find absolutely no record whatsoever of any such experimental verification or even rational justification for such a conclusion.

The equation itself is nothing other than the calculation of the temperature difference. A reiteration of Carnot's "fall of Caloric" nonsense in mathematical form, slightly obfuscated by being put into the form of a percentage of the absolute temperature scale.

At any rate, encountering such paucity of any concrete empirical verification for this preposterous equation and conclusions derived therefrom, I decided to conduct a few simple experiments myself.

So far, I do not see any evidence of the tremendous amount of "waste heat" from any of my heat engines that is supposed to be there.

In fact, taking away the means of disposal of waste heat to a sink appears to have no detrimental effect on the engine. How can that be explained?

Insulators are really conductors?

13 minutes ago, sethoflagos said:

 

710163850_testcurves.thumb.jpg.7e0290d31d40d340bfda8465be84556a.jpg

Turbines are not actual heat engines.

Turbines require some actual fluid mass to pass through them. A Stirling heat engine is sealed so no fluid of any kind passes into or through the engine. The only thing crossing through is heat or "work".

Your pasting turbine performance charts from a Google image search says and proves nothing.

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14 minutes ago, Tom Booth said:

My question relates specifically to the carnot efficiency limit equation (rather than "cycle") though related, two different things.

That equation calculates a specific ratio of heat utilization (maximum possible conversion of heat to mechanical work output by a heat engine) and waste heat rejection of the remainder to a sink. (Neglecting other factors or potential loses: friction, vibration etc.) All based exclusively on the temperature differential.

To my mind, as a life long engine mechanic and repairman, this appears completely ridiculous on its face. There are dozens of factors that go into engine efficiency aside from the supposed distance of "the fall of caloric" from a high to a lower temperature "reservoir".

There is an interesting article on the subject that reads:

"Carnot's result was that if the maximum hot temperature reached by the gas is TH, and the coldest temperature during the cycle is TC, (degrees kelvin, or rather just kelvin, of course) the fraction of heat energy input that comes out as mechanical work , called the efficiency, is

Efficiency  = THTC/TH

This was an amazing result, because it was exactly correct, despite being based on a complete misunderstanding of the nature of heat! "

It only seems ridiculous to you, because you don't understand the science behind it. You and I have been down this road before. You are a person who seems incapable of distinguishing between a basic principle of operation and the fiddly incidental details involved in constructing particular machines.

In fact, you are now arguing against yourself here. Yes of course there are many incidental factors that go into engine efficiency. That is the whole point of having a theory that tells you what the basic principle is, so that you can set these sources of confusion to one side, determine the theoretical result first - and then look at the confounding factors, as things that cause deviations from the ideal.

If you can't do that, you will never see the underlying pattern in anything. You will trap yourself in a medieval world in which all you can do is appeal to God to explain how nature works. To do science, you have to be able to simplify things to their essence, so you can see the pattern, in spite of the complicating factors.

As far as Carnot goes, he got it right using the wrong model for what heat is. That can happen. As far as heat flows go, it doesn't affect the result if you think heat is a substance like calorific, or just an energy flow.  

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3 minutes ago, exchemist said:

It only seems ridiculous to you, because you don't understand the science behind it. You and I have been down this road before. You are a person who seems incapable of distinguishing between a basic principle of operation and the fiddly incidental details involved in constructing particular machines.

In fact, you are now arguing against yourself here. Yes of course there are many incidental factors that go into engine efficiency. That is the whole point of having a theory that tells you what the basic principle is, so that you can set these sources of confusion to one side, determine the theoretical result first - and then look at the confounding factors, as things that cause deviations from the ideal.

If you can't do that, you will never see the underlying pattern in anything. You will trap yourself in a medieval world in which all you can do is appeal to God to explain how nature works. To do science, you have to be able to simplify things to their essence, so you can see the pattern, in spite of the complicating factors.

As far as Carnot goes, he got it right using the wrong model for what heat is. That can happen. As far as heat flows go, it doesn't affect the result if you think heat is a substance like calorific, or just an energy flow.  

It certainly does affect the result.

A flow of "caloric", comparable to the flow of water through a turbine passes through unaltered. The fluid that goes in one side of a turbine comes out the other side.

Heat, as a form of energy on the other hand can be converted to other forms of energy, such as mechanical motion.

With "work" output, there is a corresponding reduction in heat in the working fluid.

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48 minutes ago, Tom Booth said:

Turbines are not actual heat engines.

Some are some are not.

I think Seth was referring to those that are.

 

You should listen to him and answer his questions properly.
He is/was a pretty good engineer.

I don't any response to his question about the working fluid.

The term 'working fluid' has a particular meaning to an engineer and may be imaginary or representational depending upon the configuration of the machine.

This is because Carnot and the 2nd Law refer to a working fluid undergoing a cyclic process.

They do not apply directly to non cyclic processes

This is of fundamental importance and would be asked by any engineer working on a heat engine.

Edited by studiot
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37 minutes ago, Tom Booth said:

Turbines are not actual heat engines.

Refresh your memory of what a heat engine is at https://en.wikipedia.org/wiki/Heat_engine

44 minutes ago, Tom Booth said:

Your pasting turbine performance charts from a Google image search says and proves nothing.

Other than demonstrate that industrial machines are tested by professionals up to their thermodynamic limits on a daily basis contrary to your claims.

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10 hours ago, Tom Booth said:

The gas/air in the engine is heated and expands. The expanding air drives the piston outward doing work. A certain percentage of the heat, (a form of molecular kinetic energy of the air molecules) is transfered to put the piston in motion, (a different form of mass kinetic energy). As a result the air loses energy. The loss of energy results in a reduction in temperature (of the gas/working fluid).

A reduction in temperature of the gas is necessary in order for the piston to return (recompressing the heated and expanded gas) If the gas is not cooled back down in some way, the air cannot be re-compressed and the cycle would end there, the engine coming to a halt, unable to fully recompress the heated and expanded air.

Carnot efficiency is the ratio (percentages) of working fluid cooled by the above described conversion to work output and the remaining heat which needs to be gotten rid of by conductive transfer to the sink.

If 10% of the kinetic energy is transfered to the piston, the other 90% must be gotten rid of by transfer to the sink.

So to complete a cycle, if the engine is 20% efficient at transferring energy to the piston, the other 80% must be wasted or "rejected" through the engine to the sink or cold side.

So the work the engine does IS what you get from the 20% or whatever efficiency. No?

My point is this: If the reservoirs are at 20 C and 100 C, tell me from that information alone how many watts of mechanical power are being extracted.

You suggested that you could.

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How is insulation different from a low temperature?

According to the Carnot formula, an engine with a sink at absolute zero would be 100% efficient.

At 100% efficiency, all the heat would be converted to work output so no heat would flow into or out from the sink.

Likewise. Insulating the cold side of the engine mimics the condition of having the cold side at a lower temperature. Heat exchange on the cold side is reduced, the same result that would be achieved by reducing the cold side temperature.

A perfect insulation would, or should have the same effect as having a sink at absolute zero.

How would the engine know the difference?

If there is heat flowing from the engine into a sink at absolute zero, then there is not 100% efficiency (conversion of the heat to mechanical work output).

There are, IMO, unacceptable consequences to this whole Carnot limit equation nonsense.

In Carnot's mind, a 100% efficient engine would be an engine that transfers 100% of the heat to the sink the complete opposite of reality.

 

5 minutes ago, swansont said:

My point is this: If the reservoirs are at 20 C and 100 C, tell me from that information alone how many watts of mechanical power are being extracted.

You suggested that you could.

No, 

It is the Carnot limit equation that purportedly tells us the maximum  efficiency of any heat engine with no more information than the temperature difference.

Typically, in textbook examples, some actual quantity of heat in Joules is arbitrarily postulated.

Example:

If Carnot efficiency is 20% based on the temperature difference, and 100,000 Joules are supplied to the engine, how many Joules of heat will be transfered to the cold reservoir?

80,000 joules.

Supposing 100 Joules/second as the theoretical maximum that could be delivered by the steamer, 80 Joules/second would be transfered through the engine to the cold side per second, as long as the engine is running. 20 Joules as an absolute maximum could be converted to work output. (According to the Carnot limitation).

In simple terms, nearly all the heat needs to pass directly through the engine to the sink for the engine to operate. (According to the Carnot limit equation).

 

 

 

Simply running one of these engines it can be very easily seen or felt that no such massive heat transfer to the sink takes place.

The cold side of the engine remains relatively cold as long as the engine is running. Just touch it. The hot side is scalding hot. The cold side feels cool to the touch.

Not very "objective", but rather convincing.

It seems to me, in my experience that a Stirling engine is literally a kind of heat pump. Not a heat pump that transfered heat from the hot side to the cold side, but just the opposite. The engine is always transferring heat from the cold side and concentrating it back at the hot side, then using that concentration of heat to expand the gas and drive the piston.

Contrary to Carnot's theory that a heat engine operates by "transporting" the heat between a hot and a cold "reservoir", the truth is almost the direct opposite.

 

2 hours ago, sethoflagos said:

... and the kinetic energy of your aforementioned convection currents has to come from somewhere.

710163850_testcurves.thumb.jpg.7e0290d31d40d340bfda8465be84556a.jpg

There would certainly be convective currents IF heat were actually being transported to the sink.

Evidently, however, no such heat is transported and therefore no convective cooling is necessary.

1 hour ago, studiot said:

Some are some are not.

I think Seth was referring to those that are.

 

You should listen to him and answer his questions properly.
He is/was a pretty good engineer.

I don't any response to his question about the working fluid.

The term 'working fluid' has a particular meaning to an engineer and may be imaginary or representational depending upon the configuration of the machine.

This is because Carnot and the 2nd Law refer to a working fluid undergoing a cyclic process.

They do not apply directly to non cyclic processes

This is of fundamental importance and would be asked by any engineer working on a heat engine.

The charts from the Google image search are, of course, all over the map. It is not clear what exactly he might be referring to that related in any way to the verification of the Carnot efficiency equation, historically or otherwise.

The working fluid in the model engine in the experiment, if that has not already been made clear, is just air.

 

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