Regenerative displacer-copper

[jon james]

Hi all,

I am working on a hi temp Gamma stirling engine. Would it make sense to make the displacer from copper pipe with copper end caps to achieve the regenerative effect? I would probably knurl the surface or wrap a few layers of copper screen for greater surface area.

What are your thoughts?

Thanks in advance.

Jon

[Ian S C]

Jon, no, copper is too conductive, and is very soon the same temperature over all its length, where as it should be cold at the cold end, and hot at the hot end. stainless steel is the best of the readily available materials, and mild steel is a usable second best. Its also hard to make a copper artical light enough, and when heated it softens, and easily deforms. With a steel displacer you have a built in thermometer,ie., starting at the hot end it goes blue, and you can find heat treatment charts that will give you a fairly good indication of temp. You would be better to use stainless gauze if any, and this could line the inside of he displacer cylinder, and be stationary rather than moving, easier to keep it in place.
I'v tried a few things, I'm working on number 15 at the moment. Ian S C

[jimlarsen]

I think copper is a good choice. Because it is very conductive, it also cools faster than other metals. And that is what you need a regenerator to do. It must heat up and cool down quickly. One of the limits to Stirling engine speed and efficiency is the rate at which heat can pass through the machine. Metals which need to conduct heat should do so quickly. This will increase the amount of energy you can capture from the heat as it passes through your engine.

[Ian S C]

Hi Jim, I'm only going on practical experiance, and that of people like James G. Rizzo, and others such as Andy Ross. Ian S C

[Ferraccio]

I do not agree,
the displacer for obligation (for copper) must have a discrete thickness and therefore a good mass, the displacer heat pass between hot to cold parts, remember gamma shape, IN FACT displacer will eventually to warm up almost completely uniformly, being its internal conductivity, and then directly transferring the heat from the hot to the chilled part. This is actually a heat by-pass to escape, cutting off the energy use of the engine.
My statement is based on considerations that are common experience carried out in various forums, and several references, that suggest the use of stainless steel, (for metal displacer) which is quite robust, but less heat conductor of common steel, and much, much less than copper.
To function as a displacer "regenerative" in stainless steel is more than enough, especially if the displacer is long enough, but as I said: is a function of compromise.
The best is to separate the regenerator where the transfer functions of the heat going in the direction indicated by Jan, but with exchanging surfaces (large) and mass (low) that would have an easy gradient of temperature, and for so with an excellent exchange.
And overall: separate regenerator do NOT function as heat by-pass.
This arguments is valid for good performance motor, that is justified for a quite complex shape as gamma, (coaxial or parallel).

May be interesting know previous experiences with copper displacer, with excellent performances.

[Ferraccio]

More clair: do not agree to use copper.

[jimlarsen]

There is more to consider than just the material it is made of. Design is a major consideration and has an implact on material choice. Your design may create a thermal short, or it may add a regenerative effect, or both. If the regeneration is of more benefit than the impact of the thermal short, then you come out ahead.

Ian's point is valid. Thermal shorts will cause a motor to equalize in temperature and stop working, or work less effectively. We are saying similar things, but with different goals in mind. If you want a displacer that insulates well, you must use a poor conductor. If you want a displacer that regenerates well, you will want to use a good conductor. But if you use a good conductor and you don't engineer it well, you will create a thermal short and you won't experience much added impact from regeneration.

Using a displacer as a regenerator should include some engineering solutions that would prevent heat from being conducted along the length of the displacer. So I will amend my advice a little to say that rapid conductive materials such as copper make good material for regenrators, but they must be engineered with caution to prevent creating a thermal short.

Another thing to consider is that there is not really a lot of difference in the conductivity of these structural metals. They look significant on paper, but the difference is not all that great. If your parts are not designed well, the motor will overheat, regardless of the materials used.

[Longboy]

Another thing to consider is that there is not really a lot of difference in the conductivity of these structural metals. They look significant on paper, but the difference is not all that great. If your parts are not designed well, the motor will overheat, regardless of the materials used.

Agreeing with jimlarsen, that is the bottom line...... My thoughts are if you are building an engine, the goal is to have one that runs without having it cook itself into a coma. It is unlikely that a copper displacer will turn into a molten pool inside the hotcap. That any increase in performance through a regenative effect will not be witnessed, and that a thermal stall is a result of not enough radiator, distance/insulation between cylinders and combinations
of these faults in design of your engine. Material experimenting is fine. Will the design accept the material?

[Ian S C]

My motors with stainless hot ends run on LPG at a dull red colour, and will go like that all day if needed, the latest of the higher powered ones is fitted with a water jacket, but this is not used as the bottom end of the hot cylinder is cool enough to put your finger on, its a Ross Yoke type Alpha motor, producing 5 watts, I intend to pressurise it, it then may need the water jacket.
Stainless steel has much higher strength to weight ratio, particulary at high temperatures, and a lower rate of corrosion. Ian S C

[Ferraccio]

Copper does not melt, except with very high temperatures, but undergoes on to the so-called "softening", for elimination of all internal hardenings, in a material very plastic, makes the minimum of stress resistance.
You can make yourself the experiment by heating a copper washer to red heat, and then cool it fast enough: so treated a washer may be fold easily with simple finger pressure.
So if you're OK on low temperatures, but above 400-650 ° F. ....., Please note that the parties are subject to stresses (also if not many heavy), hundreds or thousands cycles per minute.

For the regenerator the mass of material is very important, the regenerator works well if you have little mass and a lot of surface. In this way the temperature change easily, the difference in temperature promotes the exchange of heat, together with the large exchange surface.
Use a displacer to work fine (but not very well if the temperature is constant) as regenerator, ....and very badly properly as displacer, I think it is not balanced. Experience has shown that hybrid function the stainless steel is better.
As said by others, and also above said in this forum (Ross etc.).
Architecture of the motor is important (as interpose insulating material between cylinders cold and hot....).
But it is also important to decide to hive off the displacer (finally poor conductor) from the regenerator (of low-mass, high value of thermal conductivity and high surface area of contact with the gas in transit), the latter typically in thin wires, thin plates or very thin net, in Copper, of course, to make it run better at first, but then in the air, it oxidizes.

Regenerator: The gain using little material with very high surface of exchange was more important than the conductivity of the material, with material very, very thin there is no heat trapped in the layers deep to pull out, and even here the difference with the stainless steel becomes negligible, and said steel does not form insulating oxide layers.
All this is to think in principles; if for someone is enough that the engine is moving is another matter.

[jimlarsen]

I admire any conversation that changes my mind! I always forget that you all run your engines red hot. My motors are all made from aluminum, plastic, and tin and run at much lower temperatures. I think it is a safe bet that my motors never exceed 400 degrees F. My solar designs stay under 120 degrees F. If I put a propane torch on any of my motors they would be destroyed.

With that in mind, I agree with the last couple of posts. In your designs, stainless is the way to go.

[Ferraccio]

Jim, I've never built "red hot" engines, but I refer to my experiences, years ago, to treat copper in so-said "negative hardening"; this for job in a General Electric workshop, ....and the copper, till now, is not changed.
If this may be useful to talk about the copper.
Worked copper has a "good" mechanical strength, in fact, when rolled or drawn, is rather mechanically well hardened. When heated and then cooled abruptly becomes soft as to be bent with your fingers.

[Ian S C]

Copper seems to be the traditional material for building model boilers, not being into steam I'm not sure about some of the does and do'nts about these things, but after the boiler has been silver soldered it must be in an annealed state, I suppose that it rehardens after pressurising and depressurising a few times.
I think I'll stick to stainless when I can, it works well for me. Last year I upped the power of my free piston motor, and promtly collapsed two aluminium displacers in about as many weeks, I changed to a stainless displacer, and it goes well. Ian S C

[Ferraccio]

Copper is the usual material for (domestic) boilers, when soft welded, with, usually special low temperature melting alloy, is taken care to slow cooling, for not practice the "negative hardening (softening of the material), all other parts are of course work-hardened as raw material.

Connecting welded points are however robust "inserted sleeve", in order to substantially strengthen the connection point, by simple addition of supporting material. No one will pay for copper pipe "butt welded".

The temperatures of pipes (in fuction) do not exceeds the boiling temperature of water at low pressure, .....and the pressure, and so stress, are strictly controlled.
Copper tube, if placed on a burner flame of a boiler (without water inside) collect very well heat, and may be switches easily to plastic pre melting, and do not bear yet internal pressure and stresses.