displacer insulation

[Bumpkin]

Hi. New guy here, glad to find folks of like interest. Does anybody insulate the inside of hollow displacers? I think the air flinging about in there would carry a lot of heat to the cold end. Convection inside a long narrow displacer wouldn't be as severe, but mine is 12" dia. X 7". Are there any practical (as in cheap) heat-resistant lightweight coatings that could be painted, (perhaps a thin coat of rtv?) or even better, poured in through a little hole and just sloshed around? Thanks for any input.

[Ian S C]

Hi Bumkin, not insulation , but included in the method I use to atach the rod to the displacer. I fit a thin disc on the end of the rod, and this gets inserted about half way along the inside of the displacer, and the plug goes in the cold end, usually held with Loktite high temp shaft lock. The idea is (a) to stablise the rod, and (b) to cut the flow of air from end to end. Yours seems a funny shape to me, but a similar system could perhaps work, maybe two , could we call them air dams might be better, one thing, don't take the rod right to the hot end, or it to will conduct heat directly to the cold end. One thing is that air its self is not a good conducter, its just that its swerling around. Ian S C

[Bumpkin]

Ian, thanks for responding. My list of compromises is already long and I don't want to add more than I have to.

Isolation instead of insulation is a good idea, it's just that I already welded up my displacer before realizing the problem. It's a small air-compressor tank cut to length, with one end inverted from convex to concave. It will stroke vertically about 2.5", with about .125" clearance inside a chamber cut from a five gallon propane tank. The convex bottom of the displacer matches the interior of the chamber, and the convex top matches the shape of a beta diaphragm. The displacer is fairly light-gauge metal, but the convex shape of the end should be stable enough to just weld the shaft on with maybe a washer for a gusset. I can cut and re-weld if I have to, but I arc-weld and get my power from an inverter and some beat-up batteries, so I try to keep welding to a minimum - it'd be nice if I could just pour some coating in through a hole.

The "funny" shape you refer to is to get maximum heating/cooling area at the ends and to get the most flow from the least reciprocating mass. (A 12'' piston will be lighter to accelerate than for instance, a 4" piston moving the same flow.) I cut a diaphragm from a wheelbarrow-tire to tightly slip over a water-cooling ring made of 1.5" pipe (bending that was a bugger) which I welded to the top exterior of the displacer chamber. It's all just backyard scrap-pile obtanium to hopefully get some power from a wood burner without fancy materials or machining. Sorry for rambling, but anyway, that's the plan. If it all ends up in the recycling at least I'll have learned something. Bumpkin

[Ian S C]

Bumkin, the ideal shape for a displacer is about three times longer than the diameter, except in the Robinson type. The idea is that a certain amount of regeneration takes place along its length, the Robinson motor has a regenerator built into the displacer, a short fat displacer should work at a lower temperature than the thinner one, but to get power you need heat. Ian S C

[Bumpkin]

Ian, I admit I have little experience and a lot to prove, but I don't think there could ever be one ideal displacer shape. If I were to scale down to a 7" long by 3" dia. displacer, it would roughly match your ratio, but would still only have the same regenerating length as what I already have, and only 1/4 as much circumference to work in. Granted, assuming similar rpm, it would gain some thermal efficiency with only 1/4 as much flow per area. But there are other kinds of efficiency and everything's a compromise. I think having 16 X the heating/cooling area at the ends is more important for my application. I want to optimize what I can, but it bears mentioning that I fire a woodstove for heat about ten months of the year anyway, and any power from that would be 100% efficient, even without any regeneration at all.

[Ian S C]

Of interest, there's a photo of a stove top motor in my gallery, it's a GAMMA motor with both the displacer, and power piston of 35 mm dia., and the crankshaft made to suit, to give the 1.5 : 1 ratio of swept volume. Ian S C

[Ferraccio]

Excuse me, I would intervene.
Bump, the displacer length/diameter ratio (said by Jian) has been defined, most likely, also to give away the hot from the cold parts, if the displacer is wide and short the hot part (including the cylinder, ...and not only the displacer) ends up to be quite cooled, and heat is still lost.

The experience has also shown that, with large volumes displaced, and desire to have a significant energy (and not only "see turn"), the regeneration in separate devices with well-made and appropriate dimensions, is essential.

Is absolutely correct than in Stirling the compromise is important.
The problem is to know the weight of the elements putted in compromise, otherwise (without a correct weight) you go quickly to loose the good to acquire the bad.
OK, all is good to learn.
Ferraccio

[Bumpkin]

Thank-you both. I've taken my own thread off topic. I'd still like to hear anyone's other suggestions for insulation. I thought about expanding foam, but that's probably not a good idea in a hot chamber. Currently, the partition idea seems most likely, but difficult.

[Ian S C]

bumkin, not too difficult, you need a disc that fits firmly in the displacer, and attach it to the end of the displacer rod, it can be either held with a screwin the end, or two nuts, or braze it on, screwed on fitting is proberbly easiest, and held with loctite, the plug in the cold end I find is best held in with loctite. Yours with a large diameter might need some extra bracing at the cold end, I have not thought too much about it.
For a comercial design 12" dia., I'd make both the dam, and the cold end plug from stainless steel. The dam with a nut welded in the middle, and the dam spot welded to the displacer, half way up, then the end plug with a bush in the middle, TIG welded to the end, the displacer rod pushed through the bush, and screwed into the nut, all held and sealed with loctite. Ian S C