aquarium air stone for regenerator or displacer piston?

[vile_fly]

According to my research, a regenerator must have a low thermal conductivity for a higher efficiency. I was wondering anyone has use aquarium air stones for either a displacer piston or a regenerator.
"The matrix of the regenerator can be made from several
materials. The performance of the engine depends on the
matrix material. To increase heat exchange of the
regenerator and to reduce internal losses by conductivity,
a material with high heat capacity and low conductivity
must be chosen. Stainless steel and ordinary steel are the
most suitable materials to make the regenerator matrix."

If air stones work, it may be a cheap form of regenerator. Sintered stainless steel filters would be ideal, but they are so expensive and smaller than what I would need. Might work very well for model-sized stirling, though.

Porosity is a problem, too. It can increase dead air space. Technically, air stones and sintered metal filters are less porous than homemade regenerators, so it looks like a nice alternative.

[jimlarsen]

I may be in the minority, but I have always disagreed with the theory that the displacer must be of "low thermal conductivity". The argument for low conductivity is to prevent it from conducting heat through the body of the regenerator and creating a "thermal short". For this purpose, perhaps the body of a regenerator should be made of a "low thermal conductivity" material. But the function of a regenerator will be enhanced if it is able to quickly conduct heat to and from the working fluid of the motor. It will work better if it contains a material that can quickly absorb heat. There are methods of construction that can be employed so that a high conductivity material can be used, and thermal shorting avoided.

Consider for a moment that a motor is rotating at 2000 rpm. The air passes through the regenerator twice with each revolution, which means an exchange occurs 4000 times per minute. Any transfer of heat from the fluid to the fabric of the regenerator must happen within 0.015 seconds. A material of low thermal conductivity will absorb less heat, and therefor be less effective.

As for your stone, it may be worth a try. If it creates too much drag it may not be worth it.

[vile_fly]

I think that they might be referring to an "annular" design regenerator. I don't think this rule would apply for my V-twin compressor engine, due to the fact that the plumbing is external. Then again, I have not put this to the test. Heat capacity will still be a factor.
A thermal short in an annular design regenerator would be very problematic indeed. A thermal short circuit in external plumbing is highly unlikely in my opinion.

[Ian S C]

I'm not sure if it would hold together, but I have thought of using pumice, its a light weight porous volcanic rock, its easy to carve it to shape, and it withstands heat. You'll find it in the bathroom, if you have'nt already. If you live near a volcano you'll have unlimited supply, its good for lapping pistons to cylinders if you crush it finely. Ian S C

[vile_fly]

ah! Found something better, Ian. Ceramic soldering board. I didn't know it existed before today. Was looking for honeycomb ceramics that were not catalysts. Found it on ebay. Call it blind luck.

http://cgi.ebay.com/SOLDERING-BOARD-CER ... 2eb2497cc2


Refractory "filters" look promising, too. I just bought a pack of 10. (60mm x 10mm thick) I will leave the rest for anyone who wants it.

http://cgi.ebay.com/10-New-Ceramic-Hone ... 255ea3bcac

[Aviator168]

I may be in the minority, but I have always disagreed with the theory that the displacer must be of "low thermal conductivity". The argument for low conductivity is to prevent it from conducting heat through the body of the regenerator and creating a "thermal short".

That makes us two. If there really is a "thermal short" when the engine is running even at sub thousand RPM, there would be no automobiles running on the streets. ICs have the biggest "thermal short".

As for your stone, it may be worth a try. If it creates too much drag it may not be worth it.

I have the same feeling that the stone will impede air flow.

[vile_fly]

Yep, I suspect it will clog with graphite dust, too. So I got a hold of these refractory filters for $10.00.

[Ian S C]

Whisper tec, the makers of the Whispergen even tryed autmotive catalytic converters as regenerators. I mentioned using them once when I was at their workshop, and Don Clucas directed me to a shelf full of them, they worked but not ideal. I can't remember what they finaly used, whether it was stainless foil, or stainless wire. Ian S C

[vile_fly]

Well, looks like I might have to start knitting stainless chain maile. Maybe I can use the refractory filters as displacer pistons or something. Looks like they might be sitting on a shelf, too. Perhaps using 14-18 gauge wire to knit with will keep the heat capacity high.
Having rubbed elbows with Dr. Lucas is fairly impressive, Ian. I wonder if he enjoyed his work developing stirling engines. It might have drove him to distraction at times. (heh)

[Ian S C]

Dr Don Clucas has been building hot air engines for years before going to university, and doing his engineering doctrate on hot air engines, it was this that led to the Whisper Gen.
If your going to use SS wire, some thing between 22g - 26g is easier to handle, I'm not sure but I think SS foil may be better than wire. Ian S C

[vile_fly]

22-26 gauge is a bit too fine for me to knit. I did it once to make earrings.......once. My hands hurt like crazy afterwards and my nerves were shot. I will have to use commercially available stainless mesh...for my hands' sake. Thanks for the tip, though.
This is a picture of 26 gauge wire chainmaile earrings that I made. 100 rings per earring. ouch.

[Ian S C]

I forgot about wire gauze, the only hing I use it for at the moment is on the flame guard on my home made LPG burners on my motors.
Bet you look good in your ear rings, or were they for some one else. Ian S C

[vile_fly]

Nope, I made it for someone else. Their toddler got a hold of it, and they corroded away as well as ripped apart. No warranties were ever offered. Besides, they look silly on a guy.