Heat Exchanger Design

[derwood]

It could be a characteristic of the all aluminum displacer cylinder.

[Ian S C]

My experience has been, aluminium displacer, motor slows on heating, pull off hot cap, displacer melted down. The problem with aluminium (apart from low melting temperature), it conducts heat too well, and soon reaches an over all even temperature. Stop for a short time, it cools, and away we go again. Using a coke can for the cylinder/hot cap has a similar effect, a good candle flame could actually cause a burn out. Ian S C

[derwood]

Last night I cut the end cap off of the heating tube to expose the heat exchanger. I made some ss coils/springs just the right size to fit snuggly between the inner and outer tube. I made rings out of them and stacked them the entire length of the heat exchanger (3.5 in.). I stretched them out a little so that there was plenty of space between the coils. I did this so as not to restrict air flow. I heated the exchanger before I welded the end back on and the coils glowed red hot. When I tried to start the engine I noticed response was quick but it wanted to run at a shorter stroke and took longer to finally run at the current 4in. stroke. I had to spin it very hard to get it to run. but when it did run, it ran very fast and strong but for only about 15 seconds at a time. water cooling did not help. It has shown these characteristics before and decreasing the inner tube O.D. just one eighth inch made it go from not running to running good. It seems that the increased heating may be causing premature heating, (less delay). Decreasing the inner tube O.D. seems to delay heating due to the added volume between the exchanger and cold cylinder. Too many variables! A person could go crazy trying to fine tune these engines. I do believe that this could possibly be a good way to improve heating for a stirling hot cap. If you made the coil rings a little larger than the hot cap I.D. then you could compress them into the hot cap and the whole hot cap would be lined with them. They make good contact with the hot cap walls. I guess the only question would be, will they stay in place over time with continued use? I chose the thermal lag engine because I can keep the heat source far away from the cold side and thought it would be easier to mount to a wood burning stove. I think it is time to go to a different type of stirling and stop wasting my time trying to solve the thermal lag mysteries.

[bladeattila]

Can you make a drawing from this? I think the air flow was prevented. Thermal lag and the lamina flow is a similar engine, just the thermal lag is a folded lamina flow.
If that flow can't flowing easily laminar, then the air isn't going enough deep cross the wires!

[Hawke]

Too many variables! A person could go crazy trying to fine tune these engines.

Ditto!!!! ...especially with my low end stirlings. Its all good tho, superb challenges.

[derwood]

I noticed that the ss wool inside the inner tube was burnt up in the area where the coils surround it. Not completely burnt. Perhaps the coils are conducting heat to the inner tube.

[bladeattila]

OH, I see!
The inner tube's wool is too dense!!! Also that spring is change the air flow currently.
But If you can change the inner tubes SS wool's dense (half approx), then it should to run better.
My opinion is: Better if you use this spring only on the end. The gap around your inner tube should be empty. The flow will be laminar and the end heat exchanger will be enough.
And after heat up the end. But not the last 3 inch, just 1 on the end! That is not a problem if the inner SS tube will burn a little bit, because the stack first section should be hot. But the cylinder tube other side should be cold, really cold.

[derwood]

I came to the exact same conclusion. I did finally realize that I intentionally packed the wool tighter. Also ss wool at the end does much better than the spring. I also realized that the compression was too high for the large displacement volume. It seems that I can get more power and rpm if I go to a shorter stroke with the same compression ratio. Same compression but less displacement volume.

[Ian S C]

With this type of engine I like to (at least while in development), make a crank that can be adjusted. I make the motor with an over hung crank for these motors, so that it's easy to change the stroke. You could make yours adjustable in 1/4" stages, from 1" up, you know how it runs at 4", but I think 2" or a bit more might work better. While experimenting, don't worry about the dead space at first, space the crankshaft to operate at its maximum stroke/ minimum clearance at top dead centre. As you shorten the stroke the dead space will increase, when you find the best stroke, then adjust the crankshaft distance. Ian S C

[Bumpkin]

Here's something relevant to exchanger design. It also shows an adjustable crank and rod.

http://www.redalyc.org/pdf/2251/225125743004.pdf

Bumpkin

[janemcready]

What can we use instead of stainless steel? I have heard most of those who do these kinds of experiment use stainless steel as it is a good conductor. An alloy is usually not as good a heat conductor as the pure metal.




__________________________________________
Kooba sale

[Ian S C]

The reasons why SS is used are that it is reasonably heat resistant/ doesn't oxidize like carbon steel. It has a fairly high resistance to heat conduction. It's cheap compared to some of the more exotic metals.
Ian S C