Displacer and hot end tolerance

[ruizfa]

Hi im making right now a gamma type stirling... the hot end have an inner diameter= 65 mm. I was thinking make the displacer = 63 mm external diameter. You guys think this is fine? or need to change something. And, if my hot end have 65 mm of inner diameter, what should be the thickness? i was thinking something like 5~6 mm due to dificult to machining the aluminum. Thanks, and sorry for my english.

[Ian S C]

First thing, ruizfa, what are you heating the motor with? What ever sort of motor you are making, aluminium is not the best material to make the displacer. If the motor is to be a high temperature one, then stainless steel is best, although you can get away with mild steel. If the motor is to be a low temperature one then ridgid foam plastic is the easiest, or a hollow structure made of Balsa wood is another material.
I think you may need a little bigger gap, it would be best at 2 mm or even a fraction greater, which is double your proposed gap.
Ian S C

[ruizfa]

I think it wasnt well explained, my bad. The hot end have an inner diameter = 65 mm (aluminium) and i was thinking to make the displacer (mild steel) with a diameter about 63 mm.. With that diameters, the tolerance between hot end and displacer is 1 mm. Som Ian, u think i need more space? something like 2 mm or more? could be 3 mm?..

The engine will be heated by a parabolic dish.. 1200 mm diameter.. i think to use mylar.. something like this..

http://www.ebay.com/itm/Lot-of-12-Emerg ... 1e701d63be

Will be good? or i need to cover the dish with another material?

[Ian S C]

With a solar reflector you stand a good chance of melting the aluminium. Even mild steel does not do too well at the temperature that can be achieved with a solar heater. The 1 mm gap would be ok in a small motor, say 20 mm diameter, the air must be able to transfer hot to cold with the minimum of resistance.
Ian S C

[Chriske]

1200 mm...! Thats is one dish...!

I'm in the middle of the same project with a smaller dish/engine.
I've tested that dish (600 mm) holding it towards the sun, held a piece of paper at the focal point. An instance later the paper got brown and less than two seconds later that piece of paper was on fire...! A 1200 mm dish is going to generate a LOT of heat... be careful with it....

Chris

[Ian S C]

The displacer cylinder on a motor that size could be about 1 mm thick, the displacer the same, or even less.
My trials with solar heating were with a freznel lens about 600 x 400 mm, the heat was enough to etch a line on the stainless steel displacer cylinder, the power of the motor was up 20% over its normal gas burner.
Ian S C

[ruizfa]

oh well.. wich other material i can make the hot end?? the alumium can melt with a solar dish with that diameter right? (1200mm).. i was thinking in cooper.. it has a melting point 1084.62 °C... and a Thermal Conductivity 4001 k - W/(m.K).. what do you think?

[theropod2]

Couldn't one "play" with the focal point of such a dish collector and somewhat govern the heat input? I mean just because there is potential to generate a pile of heat at a pin point doesn't mean one couldn't adjust the focal length to spread that heat over a larger area. Why run at full tilt? Few mechanical things can take full power for very long. I think this is why the large commercial /experimental array in Arizona suffered early failures.

RS

[Ian S C]

Copper has a number of problems when used as a displacer, it has little strength at high temperature, and it tend to form scales of oxide. To get enough strength, the weight becomes a problem. The heat conductiom is much higher than stainless, or even mild steel, so it conducts the heat from the hot end to the cold end far too quickly. Ian S C