Reusing loss heat

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Reusing loss heat

Postby trgl317 » Mon Aug 11, 2008 1:20 am

Hey, I'm trying to build a Stirling engine, and I've ran into some
problem that I hope someone more experienced can give me some advice
on. In building a Stirling engine, inevitably the cold cylinder wall
would eventually get hotter. I'm looking for a solution to this
problem that would hopefully raised the efficiency of my project. What
would be a good way to reroute the heat back to the original power
source? Since I'm still in the planning stage, I'm still quite unclear
as to how much heat will eventually migrate through despite the insulator.
Thanks for your time.
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Response to Reusing loss heat

Postby johnemack » Mon Aug 11, 2008 12:00 pm

Hello John,

You ask >how much heat will eventually migrate through<. Calculating that is rather tedious – and what is the use of knowing it is x Watts? (I find it best to design for minimum conduction in every detail.)

So, go for a long cylinder of the thinnest low-conductivity material. For LTD that will be of plastic (babies’ bottles can withstand over 100C). Or, since it does not have to fit the displacer closely, it can be of wrapped film with a lapped joint. (On miniature LTD’s I’m using 12u Mylar, half-thou thick, but it buckles a bit under negative pressure.) For MTD’s it will probably be of glass, carefully cut from labware. For HTD’s it will be of stainless foil for low conductivity or of (machinable?) ceramic. Keep to the thinnest wall.

To reduce heat loss from the wall and so preserve the temperature gradient for the regeneration, the cylinder can usefully be lagged, but not so thickly as to cause longitudinal conduction. That also protects the thin wall from handling damage.

Clamp bolts should be as thin and long as you can make them, and of stainless for low conductivity. On small engines use snap stainless wire clips, instead, because they are thin and make poor thermal (ie, point) contact with the hot and cold plates. (Recently, on miniature LTD’s, I eliminate them by bonding the cylinder as a sealed unit. There’s nothing to service in there, anyway). Alternatively, use clamp rings with short bolts at each end.

Your >despite the insulator< might refer to the cylinder gaskets. Keep these as thick and as narrow (radially) as possible. (I’m now using silicone rubber tubes, slit along their length and snapped round the ends of the cylinder wall. They are clamped by the end plates and require surprisingly small end load to seal.)

You refer to > reroute the heat back to the original power
source?< Heat will flow only from hot to cold! However, some waste heat can be recovered by using it to pre-heat the burner air before recuperating from the flue.

There is much scope for ingenuity in maximizing efficiency (but I still haven’t done my sums).

Another John.
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Response to Reusing loss heat

Postby longboy4 » Mon Sep 01, 2008 2:56 pm

..........John, You seem to be introducing complications and conditions to your model that just may not exist. Also not sure of your type of model you are planning. Assuming a traditional Stirling model heated with an open flame. If your model is a horizontal configuration,it is true that the heated air pathway to the power cylinder eventually warms up the components around that cylinder. Also true is that will not prevent your motor from running for extended periods. Solutions to the migrating heat through the body of a model are relativly strait forward. Some are 1) Radiator up! Air cooled.....more fins, larger diameter fins. Liquid cooled.....greater capacity water jacket around the displacer cyl. Or 2) Heat sink it into suppression. The block you bolt the displacer cyl. to......thicker, broader, larger. Mill in some grooves to displace that waste heat as it builds up. 3) using a non-conductive air pathway, vinyl or silicon tubing conecting the displacer & power cyl...........If your motor is a verticle configuration such as a Stirling fan motor, then John Mack's answer comes into play. After all, your whole model is sitting above a alcohol flame and thin wall, non-conducting, insulating materials are needed to deflect, reflect and disperse all this heat rising into the body of the model. I'm not hip to this waste heat reroute. Don't know why this is neccesary. Are you building a competition engine, or as just a running novelty of your own handywork? Don't get lost in the math.............up and running beats efficiency anytime! Dave.
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