Geometry of pancake medium temperature stirling

[staska]

Hi everybody. I had build some simple stirling engines, fluidynes, in the past, they worked, but they was not power producing. Just a toy. Now i do try to design and build power producing engine in 100-300 watt range. Atmospheric. With rubber power piston as in japan NAS 1 / NAS 2 engines.

The point, there i stuck now is - what is correct speed of air in heater / cooler passage ? By examining other, low speed atmospheric engines it looks to be 10-15m/s ? That is the maximum "thick" of passage ? 2-3 mm according to Peter Lynn or 0.2 mm according to Andy Ross. But - Andy Ross engines are much faster, in 2000-3000 rpm range, Lynn engines are slower, max 300 rpm. How i could calculate needed area off heater?

Geometry in general is borrowed from NAS 2 : Displacer bore x stroke 250x52, working piston 200x52, annular heater at the top, displacer and body of engines from vermiculite plate or ceramic plate sandwiched between two stainless-stell thick plates. And cable driven displacer as in US4330992 by J. Senft, Fig 4.

Link to US4330992 http://www.google.com/patents/US4330992

Link to NAS 2 https://docs.google.com/file/d/0ByfGlz- ... sp=sharing

Stanislav

[theropod2]

Don't have a clue, staska, how to help you, but this sounds like a very interesting project.

I want to also build a 200 watt range engine, but I am thinking of a big, slow, simple gamma. I have a forest full of trash wood and debris I can use as heat input, so that part isn't an issue. Building a 6" bore power piston/cylinder as smooth as glass is where I freak out.

Good luck, and rest assured Ian, or one of the many others more knowledgeable, will come along and comment.

R

[staska]

Building a 6" bore power piston/cylinder as smooth as glass is where I freak out.

Please read a document called NAS - 2. Japanese fixed this problem by using rubber diaphragm as power piston. Same could be used for displacer, if made from silicone rubber. But i would try to make it from turned and polished 215 pipe with ptfe ring. It should not be super-sealed it is a displacer anyway...

[Ian S C]

The diaphragm is ok instead of a piston, but it would need to be properly designed, so you would need to make a mould/die to make it with concentric corrigations, you then have to get the right type of rubber, and the cooking time for vulcanizing it, its a wee bit more than a bit of rubber balloon on a tin can motor.
No a diaphragm will not replace the displacer. Ian S C

[staska]

The diaphragm is ok instead of a piston, but it would need to be properly designed, so you would need to make a mould/die to make it with concentric corrigations, you then have to get the right type of rubber, and the cooking time for vulcanizing it, its a wee bit more than a bit of rubber balloon on a tin can motor.

Yes, it would be better, but i will try to stay on NAS 2 idea on flat fat diaphragm from soft rubber. And buffered from below, i.e lowest engine pressure same as atmospheric.

No a diaphragm will not replace the displacer. Ian S C

Why ? On alfa it is easy.

[Ian S C]

The ALPHA does not have a displacer, but a piston on the hot side, you might have problems with a rubber diaphragm at any temperature above about 80*C for extended hours of running, maybe you could design a metal one, but that may have fatigue problems, a piston with a Heylandt Crown attached is a far easier way to go.
Ian S C

[staska]

The ALPHA does not have a displacer, but a piston on the hot side, you might have problems with a rubber diaphragm at any temperature above about 80*C for extended hours of running, maybe you could design a metal one, but that may have fatigue problems, a piston with a Heylandt Crown attached is a far easier way to go.
Ian S C

Yes, my fault. What about high temperature silicone rubber ? Is good for more thane 250*C continous use..

[Ian S C]

It would depend on how you are going to heat your motor as to whether the rubber diaphragm would stand the temperature. I'v looked up silicone rubber. Low strength properties limit silicone to static seal applications, typically not good for dynamic(moving)seals due to friction properties and poor abrasion resistance. Highly permeable to gases. Temperature range -75*F to +450*F.
Have a bit of a look at google.
NASA (I think) developed the roll sock seal for pistons in hot air engines. Ian S C

[Hawke]

I use "Viton" o-rings in Scrpto Vu Lighters I rebuild. I found it to be perfect for o-rings and chemical resistant to lighter fluid which is naphtha. Further I have a sample 3x4 sheet that's 1/16 thick(guessing) that I used to manually stamp o-rings out of. Its supposed to be resistance to temp of 675 degrees F I think. I was planning on using this in my next SS cylinder stirling engine myself. Would this be a suitable diaphragm for what you guys are speaking of. Im sure its beyond adequate for my plans.

http://en.wikipedia.org/wiki/Viton
Note the "Competing materials" for other possibilities maybe.

[staska]

It would depend on how you are going to heat your motor as to whether the rubber diaphragm would stand the temperature. I'v looked up silicone rubber. Low strength properties limit silicone to static seal applications, typically not good for dynamic(moving)seals due to friction properties and poor abrasion resistance. Highly permeable to gases. Temperature range -75*F to +450*F.
Have a bit of a look at google.
NASA (I think) developed the roll sock seal for pistons in hot air engines. Ian S C

Thanks. Yes, you are correct. Tensile stregth of silicone rubber is 3 to 5 times less than normal rubber. Btw - i had managed how to make extra simple ptfe compression ring from ptfe tube with 6 mm outside and 4 mm inside diameter, by putting spring inside. It takes shape of cylinder beautifully. For displacer of course.

Dd you have any ideas of my question regarding to heater geometry ?

[Ian S C]

staska, although my first engines had horizontal displacers, but now I prefer the displacer vertical, in most cases with the hot end at the top, and a ring shaped burner for LPG. if the hot end is at the bottom, an ordinary camp stove burner is ideal, or it can be used with a spirit burner. With this orientation there is less friction in the gland / bushing for the displacer rod. Ian S C

[staska]

staska, although my first engines had horizontal displacers, but now I prefer the displacer vertical, in most cases with the hot end at the top, and a ring shaped burner for LPG. if the hot end is at the bottom, an ordinary camp stove burner is ideal, or it can be used with a spirit burner. With this orientation there is less friction in the gland / bushing for the displacer rod. Ian S C

Thanks, as far as i can see your engines, they are atmospheric. I am interesteg, how wide i should make a gap in a displacer ?

[zhivko]

Gap in displacer should be best compromise - on one side you want big heat transfer so small gap would be best but on the other hand if you have too small gap you have big pressure drop and therefore you loose power.

[staska]

Gap in displacer should be best compromise - on one side you want big heat transfer so small gap would be best but on the other hand if you have too small gap you have big pressure drop and therefore you loose power.

Yes, it is correct from logic side. But - are there any figures of at least maximum gas speed in gap versus gap depth ? I work in hvac industry, for heat recovery unit i use depth of gap 3 to 5 mm and speed are 3 m/s at maximum, for air. But i have 30 degree delta t.

[zhivko]

In my stirling engine (diameter 250mm) I used 3.5 mm gap and it worked OK. Check slotted heat exchanger to maximize heat area. Can you post some pictures?