Re: mod II stirling engine report

Discussion on Stirling or "hot air" engines (all types)
Aviator168
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Re: mod II stirling engine report

Post by Aviator168 » Mon Dec 12, 2016 11:10 am

Read through the Mod II report a few times. I still can't wrap around my head that it needs 15MPa to get 56kw. The only things I can think about are 1) internal heater contact area is too small and 2) combustion temperature is not high enough. Any other thoughts?

Ian S C
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Re: Re: mod II stirling engine report

Post by Ian S C » Tue Dec 13, 2016 12:59 am

You'v got to count in the cooling system too, getting the heat out is just as important as getting it in.
Ian S C

Aviator168
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Re: Re: mod II stirling engine report

Post by Aviator168 » Tue Dec 13, 2016 4:09 am

That is absolutely more important than the hot side and there is no information on the report.

Ian S C
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Re: Re: mod II stirling engine report

Post by Ian S C » Wed Dec 14, 2016 2:28 am

A lot of designs almost ignore the importance of the cooling. There is a story in "Model Engineer" back in the 1990s, one chap had a problem with the small gas cylinder he was using on his motor, so he put it partly in the cooling water that was getting too hot, the cooling of the gas cylinder balanced the heating of the cooling water and the power out put rose dramatically.
Ian S C

Aviator168
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Re: Re: mod II stirling engine report

Post by Aviator168 » Wed Dec 14, 2016 7:01 am

People in general cannot wrap their heads around the idea of "Cold is Power". There is a lot of heat energy flowing around in the atmosphere if the other side is absolution 0. Cooling is a lot more difficult to do than heating. On the hot end, you can create a larger temperature gradient by burning more fuel and there is nothing you can do on the cold end since the lowest you can go is atmospheric temperature. On top of that, you are removing heat with 1 bar of air.

Ian S C
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Re: Re: mod II stirling engine report

Post by Ian S C » Thu Dec 15, 2016 1:48 am

It would be possible to use a refrigerant as a cooling system, And use a fluid that freezes below 0*C.
The simplest way of keeping the temperature down is to use a positive loss system, hook it up to a tap, and stick the outlet down the drain/or water the lawn. If you have a swimming pool, circulate the water from there.
Ian S C

Trevor
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Re: Re: mod II stirling engine report

Post by Trevor » Thu Dec 15, 2016 3:06 am

Hello Aviator 168,
Can one use a good efficient radiator here with a fan blow if you have enough power source to spare. On the small models they work very well.
Trevor

Aviator168
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Re: Re: mod II stirling engine report

Post by Aviator168 » Thu Dec 15, 2016 8:28 am

@Trevor. I don't see why not. You need to do the calculation so the minimum amount of power for the fan and pump is used. You might want to keep the water temperature as high as possible to increase efficiency, and that means the temperature of the fluid in the cold cylinder is above (way above) that.
hook it up to a tap, and stick the outlet down the drain/or water the lawn.
That would be perfect on a boat. Here comes the Gotland.

ulther
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Re: Re: mod II stirling engine report

Post by ulther » Thu Dec 15, 2016 11:02 am

When talking about cooling and heating you have not to forget about flow friction. The idea in powerful engines is to have small tubes so that contact area for heating/cooling would be big, and to do that heating/cooling is done by air going through narrow pipes. Now this is where problem occur. Narrow pipes mean higher surface area and faster cooling/heating, but at the same time higher friction for fluid flowing through and a lot of energy is lost because of that. When combined with high pressure, density of working fluid is even higher and loses because of friction increase.

Aviator168
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Re: mod II stirling engine report

Post by Aviator168 » Thu Dec 15, 2016 11:38 am

Small tubes are fine as long as you have many and short. One of things most stirling builders don't concern about is the way heater and cooler are layout. Remember, the working fluid is only heated up/cooled down during the time when it is in the tubes. Once it is out of the tubes, it no longer gets/releases any more heat, so to speak (don't get me started using Hydrogen). Now, if you know the dead volume and it's layout, using a targeted rpm, you can figure out how much time any individual molecule spend in the tube, and from there, you can calculate the rise in temperature in degrees. The goal to just minimizing dead volume is not ideal. By the same token, knowing the dead space configuration, you can calculate the amount of energy needed to push the working fluid between the cylinders for each cycle. So there you have it. For the same amount of dead volume, you can have one very long tube or many short tubes. Which would you rather have?

Trevor
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Re: mod II stirling engine report

Post by Trevor » Thu Dec 15, 2016 2:44 pm

Wonderful discussion - I learnt a bit in that discussion and is well appreciated by me :big smile: Trevor

Aviator168
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Re: Re: mod II stirling engine report

Post by Aviator168 » Thu Dec 15, 2016 3:38 pm

I can't add to my previous post; but here is.......
Narrow pipes mean higher surface area and faster cooling/heating,
Actually, small pipes have very little contact area since it is limited by the ID. The best way is to force air through many very arrow gaps of heating plates.

ulther
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Re: Re: mod II stirling engine report

Post by ulther » Fri Dec 16, 2016 2:50 am

Aviator168 wrote:I can't add to my previous post; but here is.......
Narrow pipes mean higher surface area and faster cooling/heating,
Actually, small pipes have very little contact area since it is limited by the ID. The best way is to force air through many very arrow gaps of heating plates.
I had in mind that you increase surface area by having more narrow pipes of smaller diameter, than less pipes with wider diameter. Overall volume of cooler/heater should be the same.

Also regarding to your previous reply. Flow friction is not something you should ignore. We are not talking about hobby/toy models. We talk about engine with 150 bar pressure and high rpm. Density of the gas is dramatically increased and it has to travel on high velocities. Making thermodynamic model as real as possible it is easy to see that flow friction causes major efficiency losses and highly influences maximum engine power.

Ian S C
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Re: Re: mod II stirling engine report

Post by Ian S C » Fri Dec 16, 2016 4:55 am

Here is an old one of my motors based on James G. Rizzo's Dyna from vol 1 of "the Stirling Engine Manual". a small DC motor is used as a generator that drives the cooling fan on the radiator, there is surplus power to drive a 9V radio.
Ian S C
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Aviator168
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Re: Re: mod II stirling engine report

Post by Aviator168 » Fri Dec 16, 2016 6:59 am

Flow friction is not something you should ignore. We are not talking about hobby/toy models. We talk about engine with 150 bar pressure and high rpm. Density of the gas is dramatically increased and it has to travel on high velocities. Making thermodynamic model as real as possible it is easy to see that flow friction causes major efficiency losses and highly influences maximum engine power.
Let's settle on one thing here. We are not talking about toys models. I understand the parasitic drag induced by the connector; but that's a compromise I am willing to make, and why not. It is in direct proportion to heat exchange area. Now a couple of question. Why do you need 150 bar and what rpm and temperatures are you talking about? Have you consider the efficiency losses just moving the air back and forth (regardless of friction losses). For instances, 1 liter of air weights about 1 gram at 1 bar. At 150 bars, it will weight 150 grams. Regardless of rpm, it will take a lot of energy to move 150 grams of working fluid back and forth.

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