My observation on home built strings.

Discussion on Stirling or "hot air" engines (all types)
Aviator168
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My observation on home built strings.

Post by Aviator168 » Mon Dec 29, 2014 12:57 pm

Here is my observation why almost all the home built stirlings don't have much power, and most of them can't even run with a small load. One needs to look no further than the small heat exchange area on the gas side and the path the gas goes through the heater. It is ironic that no one can give a good estimate of the amount of heat that can be injected into the engine in a give amount of time. HIRAO's engine gives a good hint what a heater supposed to be and given the flame temperature and the material used in the cylinder, it is not difficult to determine the amount of heat that gets transferred into the engine (OK. a good estimate). Also keep in mind since majority of the home builts are not pressurized and gases have very low thermal conductivity, builders have think more about heat transfer through convection instead of conduction. So very large internal heater area helps a lot; but one has to be careful not to increase dead space while doing so. You will be surprised, if it is done right, it is not impossible to built a small, non-pressurized 1kwh engine.

just my 2c

Ian S C
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Re: My observation on home built strings.

Post by Ian S C » Tue Dec 30, 2014 2:46 am

A good start is internal fins in the hot cap, see Andy Ross's book, but this is not exactly the most simple thing to do, all right if you have plenty of spare loot. It is just as important to get rid of unwanted heat at the cold end.
Ian S C

Aviator168
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Re: My observation on home built strings.

Post by Aviator168 » Tue Dec 30, 2014 8:50 am

Exactly. Ian. I also what to added that internal fins have to be very close to each other. The closer the better (get the gaps down to 100um). Another very important aspect is that to make sure the gas to go through the gaps of the fins. Almost all home builts failed all those three.

Rog Tallbloke
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Re: My observation on home built strings.

Post by Rog Tallbloke » Wed Jan 21, 2015 12:32 pm

Hi guys, I'm a newbie here, so forgive me if it's been covered. Has anyone had the hot cap off a sunpower free piston unit to see what it's like inside? One aspect of heat transfer is material thickness. Won't there be a trade-off at some point between finning and mass? Clearly the better the thermal conductivity of the material, the better it'll shift heat from outside to inside, but most hot caps seem to be stainless steel to cope with high temperature.

Corrugating thin sheet is doable, but how to sweat this to the cap without leaving too many voids?
Has anyone tried making a corrugated cap with an exo-skeletal support frame rather than trying to joint it to a cap?
Last edited by Rog Tallbloke on Wed Jan 21, 2015 1:18 pm, edited 1 time in total.

Aviator168
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Re: My observation on home built strings.

Post by Aviator168 » Wed Jan 21, 2015 12:55 pm

There are THREE stages of heat transfer in heater (same in the cooler).
1. From the hot burning gas to the outer surface of the hot cap.
2. From the outer surface of the hot cap to the inner surface.
3. From the inner surface of the hot cap to the working gas.
Reverse the above for the cooler except that the outer surface of the cooler will either have contact to liquid or air, and if it is liquid, you then have 2 more stages of heat transfer before it gets to the environment which is air.

There MUST be a temperature gradient on each stage of heat transfer. You can look up thermal conductivity for different materials (including gases). Using the area and distance, for a certain temperature gradient, you can calculate the amount of heat is transfer. On the same token, given the amount of heat required to be transferred, you can calculate the temperature gradient. From there, you can figure out if your design works or not.

Ian S C
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Re: My observation on home built strings.

Post by Ian S C » Thu Jan 22, 2015 2:15 am

Andy Ross made his internal fins by machining them on a flat piece of metal, then rolling it into a cylinder, and having it brazed into the stainless steel hot cap. Another way to do it would be a lost wax casting of the complete hot cap, using 316 stainless steel, a good casting and there would be almost no extra machining, except the mating surface between hot and cold ends.
Ian S C

Rog Tallbloke
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Re: My observation on home built strings.

Post by Rog Tallbloke » Thu Jan 22, 2015 5:42 am

Ian, would I be right in thinking that there will always be positive pressure in the hot cyclinder, even in an unpressurised stirling engine? I ask because I'm wondering about a much cheaper solution for a small (5W) machine involving pre-corrugated copper sheet. i.e. If an external support lattice could stop the thin material ballooning outwards, would it also need to be soldered to the corrugated copper to stop it being sucked inwards, fouling the displacer? Has anyone played with copper hot end cups?

Aviator168
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Re: My observation on home built strings.

Post by Aviator168 » Thu Jan 22, 2015 8:17 am

Andy Ross made his internal fins by machining them on a flat piece of metal................
Wow. That's a lot of work. I am attempting to print the parts on a 3D printer with conductive plastic. Then have them go through electroforming. I am experimenting to see how small a void I can produce. Hope this works.

Ian S C
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Re: My observation on home built strings.

Post by Ian S C » Fri Jan 23, 2015 2:11 am

Rog, the corrugated metal is a good idea, but stainless steel shim/foil rather than copper. Stainless foil is the preferred material for regenerators, this part of the subject is covered in Andy Ross's book "Making Stirling Engines" (free down load, 68 pages)

Rog Tallbloke
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Re: My observation on home built strings.

Post by Rog Tallbloke » Fri Jan 23, 2015 5:21 am

Thermal Conductivity of stainless 8.1 (Btu/(hr-ft-F))
Thermal Conductivity of copper 231 (Btu/(hr-ft-F))

So if rigidity is the issue, an exoskeleton of titanium wire supporting a corrugated copper cup has to be worth a try, no?

Aviator168
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Re: My observation on home built strings.

Post by Aviator168 » Fri Jan 23, 2015 7:01 am

Rog, I don't think it's worth the effort for a 5w engine. If 0.5mm thick ss is used, your burner side area requirement is only 1.67 cm^2 assuming a 1 degree C temperature gradient. So you just have to work out the working gas side area and just use that.

Rog Tallbloke
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Re: My observation on home built strings.

Post by Rog Tallbloke » Fri Jan 23, 2015 8:50 am

Most of the prototypes I see on youtube use quite shocking amounts of gas to get rpms. This because at higher rpms, lots of heat is needed to pump energy into the displacer cyclinder fast enough. As well as wasting expensive propane, it makes a racket. I'm going to try to make something which runs on a nice quiet US army white-gasoline cookstove I own. Copper transfers energy 30 times faster than stainless steel, so that's what I'll try to use.

Aviator168
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Re: My observation on home built strings.

Post by Aviator168 » Fri Jan 23, 2015 9:04 am

People on those videos you see are not heating the right place either and I doubt those engines have enough internal surface to transfer the heat to/from the working gas. Keep in mind that heat gets transferred to gas is a lot easier than from it, so the internal surface of the cooler needs to be much bigger than the heater. Most of the engines on utube have tiny coolers. Even for a small 5W engine you want to build, if the temperature gradient is 30C and the heating gap is 0.5mm, you still need at least 28cm^2 of internal heating surface (regardless if you are using ss or copper), and that's provided your engine have the correct displacement. Good luck with your build.

Tom Booth
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Re: My observation on home built strings.

Post by Tom Booth » Fri Jan 23, 2015 3:44 pm

I've been thinking about doing something like this to increase surface area and heat transfer between the displacer chamber and the air in the chamber.

The regenerator channels are just tubes. There are holes in the displacer, which would have to be rather close fitting to force the air through the regenerator tubes. Also, unlike an Alpha type stirling where the regenerator is sitting in the middle of a pipe between the hot and cold cylinders, here the regenerator is in direct contact with the hot and cold sides of the engine's displacer chamber.



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Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
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Re: My observation on home built strings.

Post by Aviator168 » Fri Jan 23, 2015 5:00 pm

You have 3 displacers here? You might also want to switch cold and hot ends.
Suggestion. If you get rid of the two side displacers and encase the regenerator section with insulation, it would be a good beta heat exchanger. Keep in mind, stirling engine cylinder surface does not scale well with volume. Increase of surface area is not proportional to increase in volume.

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