Materials question, thermal insulation, Gamma proposal

[spinningmagnets]

Before I start researching a wild goose chase, I was thinking that there must be members here who have used (or read about) various materials that were used for the different parts of a desktop Stirling engine. I am asking about the black part in-between the hot and cold ends of the Gamma displacer cylinder shown in the graphic.

A

StirlingGammaProposal1.png (4.55 KiB) Viewed 2212 times

To recap the obvious for any new casual readers...

The hot-end of the displacer cylinder will likely be a stainless steel cylinder, possibly a kitchen utensil holder (or the old reliable water bottle)
https://cdnimg.webstaurantstore.com/ima ... 272072.jpg

The cold-end of the displacer cylinder would be more easily and affordably made to match the purchased hot-end, constructed of aluminum with the ID of the cylinder being chosen to match the hot end.

The gray power-piston is easily made, especially because it would be loose-fitting and sealed by a "rolling diaphragm" seal made from high-temp silicone (the green part). The precise size of the power-piston and its cylinder would be constructed to match a readily-available seal.
download/file.php?id=3197

The actual cylinder for the power-piston could be aluminum (or any other suitable material), so the "black part" could be made from a flat thick bar of a suggested material. Or, the entire black part could be 3D-printed as one piece. But what material to use?

I've often thought that drawing the air from the middle of the displacer-cylinder would be acceptable, with the added benefit of possibly using a material that would place a thermal barrier between the hot and cold ends to reduce "thermal creep". I "could" place an insulating high-temp gasket between the hot-end cylinder and the central "black part", but the gasses would still be hot when passing through.

If it's 3D-printed, which material would be heat-resistant in the temp range we would see in a small Gamma with candle-heat? Thanks in advance...

[matt brown]

In theory, that "black" part can be made from anything, but in practice it will have to dissipate heat, so expect metal. The temp spread on the hot side will be far greater than the temp spread on the cold side, so cooling will always be an issue even with low cycle frequency. The transfer port can be anywhere, and one guy in past (can't recall his name) was famous for middle location. Dead space is really only an issue in cold space/s (which complicates cooling) but no issue in hot space. Another thing to remember is that lacking any regenerator, "all that heat" that would have been recycled each cycle now adds to the cooling burden of the cold space (hence previous black part aiding cooling). No regenerator also burdens the heater, but this is minor for any model. Your rolling sock seal is best for larger stuff since at small scale it will be quite delicate and drastically increase cold side dead volume. For a simple model, I favor a common cylindrical bellows with central 'alignment' cylinder, but this is even worse for dead space.

Dare I ask, why a gamma instead of a rotary 'Gloy' of some type ?

[matt brown]

What type of drivetrain do you have in mind ?

[Tom Booth]

...
possibly using a material that would place a thermal barrier between the hot and cold ends to reduce "thermal creep". I "could" place an insulating ...

I agree. Something that would form a thermal barrier would be best, for what I think should be obvious reasons.

You want as little heat as possible transferring from the hot cylinder to the cold cylinder through whatever the engine body is made of.

I'll be working on similar projects soon. My personal choice is to make everything but the heater end and possibly the power cylinder out of ceramic, if possible, the power cylinder and piston as well.

https://m.youtube.com/playlist?list=PLp ... MSE41kA_Ag

[VincentG]

viewtopic.php?t=5570&start=60

Towards the end of page 5 in this thread shows the epoxy filled 3d printed forms I based this engine on.

I'd recommend the cooler have similar or less internal heat transfer rate than the heater, as far as the working gas is concerned in order to balance the flow of energy. Many designs need more cooling due to thermal bridging of the engine body. This was a non issue with the epoxy design.

If you are only 3d printing, nylon has a pretty high temperature range for a plastic, but peek is probably the best assuming a printer that can run it.

[Tom Booth]

Stainless steel is pretty non-conductive to begin with, so if the SS utensil pot is mostly heated on the bottom there might not be much thermal creep to worry about up that high.

I've sometimes thought with SS it might actually be a good idea to electroplate the outside with copper, like a copper bottom cooking pot to help distribute the heat more evenly.

[spinningmagnets]

matt brown, Tom Booth, and VincentG, thank you all for responding. I appreciate every advice given.

Matt, metal is always a possibility, with a thermal gasket between the hot-end and the central body to "slow" heat transfer. Aluminum and brass are easy to work with and polish up nice.

I have seen displacers that were made from a cage and filled with metal mesh to also act as a regenerator, and the displacers can be swapped out for different types to see which operates better. I've seen youtubes that compare a solid displacer to a foamy regenerator/displacer, and the regen displacer ran faster with the same heat input.

The rolling diaphragm may be worse than a pancake diaphragm, I'd like to test them one after the other to see if the performance difference is large or small.

As far as making a Gamma, its just a fun test bed, to show my grandkids that I made something that can actually spin. I do plan to make a rotary displacer at some point, just for fun.

Tom, I had not even considered ceramic before, and since it sounds like ceramics might work for me, that's a great option. I will have to research ceramics now.

Vince, epoxy is definitely something that should be an option to compare. I will search the link you provided, and and see what I can learn from it. I don't need a perfect insulating material, just something to slow heat transfer to a tolerable level. I have made gaskets from raw stock before, so its possible I may just use some thermal gasket material to improve whatever ends up working.

Peek as a material sounds intriguing. I hadn't heard of it before, but I do have acquaintances with 3D printers. I may have to try that.

I remain open to any additional suggestions, but you gentlemen have given me lots of leads to research, so I am already very happy with the result of this thread. Thanks again, and I shall check for more comment every few days.

[Jack]

I doubt peek is printable. It'll have to be machined.
A 3d printer works by melting plastic, if it doesn't melt very readily it won't print well.
Peek is great though. Best bang for the insulating buck.
For a structural piece that is.

[Fool]

PEEK is a new one for me too. I did a search on "PEEK material 3D print" and found out it is a thermoplastic and there are printers and filaments to do so. There are videos out there claiming to explain it. You may have to replace your hot section and feeder? The filament is expensive, $300 to $400 for 2 lbs, 1 kg, roughly.

If you 3D print with "wax", you could use investment casting and the lost wax process. Opening up more possible materials.

Also it is possible to get sinterable filaments of metals for regular 3D printers. The print needs to be slightly larger for shrinkage during the firing/sintering process.

Lots of desktop models use glass displacer cylinders for visual effects. Test tubes and marbles come to mind as an example.

Of course the old standby stainless steel, especially 316 stainless, is great for the whole engine. Can be made very thin for good thermal conduction into the gas. And long for reducing thermal condition longitudinally, helping to separate hot, regenerator, cold sections.

Note: Thin, broad and flat, pancake engines don't do so well. That is why aluminum is used often in LTD Stirling Engines. Thickness is necessary in flat plate heat exchangers. It is why the bottom of pop cans are concave, and the tops slightly domed.

SS is better for High Delta T's, which by nature are thin longer smaller diameter tubes.

A glass, or ceramic middle section would be interesting. Graphite piston in glass cylinder is common. "Air Pot" makes them too. Maybe they could be glued to a glass midsection using sodium silicate and a firing.

"Tim Tinker" has some formulas for making your own refractories.

https://timtinker.com/category/refractories/

https://timtinker.com/diy-refractory/

[spinningmagnets]

I can't thank you all enough. This is all very useful information.

If PEEK ends up being a good choice, and it must be machined, then I suppose ill have to try that. I may make a prototype from another material, and save the machined PEEK for the V2.

A glass cylinder for the cold end would be fabulous, perhaps with an aluminum finned cap. If I could pull that off, it would be an interesting visual element, even if it didn't perform quite as well...although it would complicate finding a hot-end with the same ID...

[Fool]

Glass hot end too.

All glass Beta.

Make it out of a large test tube and epoxy piston.