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LTD Stirling engine with vertical shaft - perhaps
reflad:
Picclock,
I have seen designs that have steel wool and or metal screen material placed inside holes in the styrofoam displacer, and the steel wool acts as the regenerator. Not sure how this would react with a small size, but a person could certainly do some trial run to see if it would pay off.
I think the gift idea for the kids is a great idea! Good luck with the build, and keep us and the HMEM crew filled in. Take plenty of pics!
Ronald
picclock:
@ reflad
Well I think I have enough time (13 months or so), but as I only plan to do this in my spare time, past experience suggests that this will fly all too quickly. Once I have all the bits together I will start a build log to expose my triumphs and disasters to all, and possbly encourage someone else to have a go.
For the initial version I will try out all of my crackpot ideas and document the results. I figure that balancing the displacer by weighting the bellcrank should be a lot easier than the flywheel method, and allow changes to be made quite easily. My main concern is getting the graphite piston fit correct.
I'm very much a newbie to all of this and am still finding my way, thanks to the help of forums like this.
Best Regards
picclock
picclock:
This is my current state of LTD thinking. I think I've got a bit more figured out now.
First a nice simple optimum ratio equation:-
optimum temperature differential = 273 / (power cylinder volume/displacer volume)
In Jan's design (latest LTD coffee cup) with a ratio of 51-1 the temperature change for optimum work is 273 / 51 or only 5.4C.
So why do Jan and others use such a high ratio, and optimise for such a low temperature differential ?. The answer is that during the time that the displacer is in a heating or cooling position, the maximum change of temperature that can be made to the working fluid is limited. These limitations are caused by the low thermal conductivity of the working fluid, and the small surface area available to heat/cool the fluid.
So despite a fairly large temperature differential, I'm guessing 35C, only 5.4C makes it to the gas in the time of 1 revolution, so anything which improves this will help a great deal.
The problem with the current design is that a large amount of gas must be heated and cooled for only a small amunt of gas used. In the above case 50 times the volume of gas is heated and cooled for the small expansion/contraction of the power cylinder.
@ John, Billtodd
I think the 'economiser' idea is spot on. A hollow displacer filled with fine aluminium wool (if there is such a thing), with two diametrically opposed holes in the top and bottom surface would improve things dramatically. The constructor would then have the choice of leaving the expansion ratio's the same with the engine working at a far lower differential temperature, or decreasing the ratio and extracting more power from the device at the current teperature differential.
Currently the gap around the displacer works out to 314 mm2, so the two holes would need a diameters of 20 mm. Smaller may be OK because the 1mm cylinder gap exposes a large volume of gas to the boundary layer effect. On the other hand for a heat exchanger to work well would involve high boundary layer contact.
The only downsides to this I can see is that resistance to airflow will be slightly higher, and some additional driven mass will be added to the displacer.
Many thanks for the helpful comments and insights, now I just have to hone my newly learned engineering skills ( ::) to produce something.
Best Regards
picclock
kwackers:
My (albeit limited) experience of Stirling engines suggest that the biggest issue is often the cold side warms up too much and the engine stops running.
So it's not just a case of heating one side, it's also a question of how to cool the other. It's much easier with 'hot' Stirling's driven by direct heat, since the high temperatures increase the rate heat can be dumped on the far side, but low temperature engines tend to fail fairly quickly.
The importance of this is related to the fact that if you increase the amount of heat transferred then the problem of dumping it on the far side becomes more pronounced.
picclock:
Hi Kwackers
Using the regenerator means that less heat and less cooling is needed. The hot air is cooled by warming up the regenerator, effectively storing the heat in it, and pre cooling it for the cold part of the cycle. When the cold air blows through the regenerator, it warms from the heat stored, pre heating it.
Overall there is less heat transfer from the hot and cold ends.
Best Regards
picclock
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