Stirling Cryocooler

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Stirling Cryocooler

Postby ibmace » Tue Jun 23, 2009 9:49 am

Hi All,

I am no expert with Stirling Engines and I will be glad to get some
expert help from the senior members here. Please help. Thank You

A week ago I completed setting up a Alpha Stirling engine using a
friends lathe machine. I actually wish to run it as a cryocooler or
heatpump. The engine consists of the following parts.

a) Two copper cylinders with epoxy-graphite pistons - Maximum
cylinder capacity is 20 ML of air volume at full stroke per cylinder

b) Regenerator - A Stainless Steel cylinder (2 inch by 2.5 inch)
tightly packed with steel wool

c) Heat Exchangers - The hot and cold heat exchangers are made out of
two very thin aluminium tubes (1inch * 2.5 inch)

d) The crankshaft is connected to the cam such that one piston moves
90 degrees ahead of the other piston.

e) The flywheel is made of cast iron which is connected to a 120 watt
8000 RPM motor.


When I connect power to the motor, I expect the cold heat exchanger
to absorb heat and the hot heat exchanger to release heat and
therefore function as a heat pump. But the trouble is, both the heat
exchangers are getting pretty hot. The heat exchanger and cylinder
connected to the follower piston gets much more hotter than the heat
exchanger and cylinder connected to the leading piston. The
regenerator feels slightly warm to the touch.

Could someone please tell me what I'm doing wrong. Why is'nt the cold
end cooling down? I'm at the end of my wits now. Every ounce of help
I can get will be greatfully accepted. Thanks a ton in advance

I thought I got everything figured out theorectically. But when I put
my ideas into practise, boy was I wrong.

Basically I started out wanting to make my own Stirling cryocooler. I
have been working very hard spending long hours learning every little
information I could find on the Internet. I learnt that the Alpha
configuration gives the best performance and so I thought that I had
better utilize that for my Stirling Cooler.
ibmace
 
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Stirling Cryocooler

Postby stan.hornbaker » Tue Jun 23, 2009 11:16 am

Obvious first change would be to reverse direction of rotation of flywheel thus effectively changing the lead angle by 180 deg.

IF it will run as a standard Stirling engine by applying heat then it should be possible to drive it with a motor and achieve the desired cooling effect. Design may be inadequate and a cooler expert could be of more assistance than I.
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Response to Stirling Cryocooler

Postby ibmace » Tue Jun 23, 2009 11:20 pm

Thanks a lot for answering my question Mr. William S. Hornbaker.

So reversing the motor direction will effectively change the phase angle to 180 degrees. I would have never thought of that.

I'll give it a try and get back to you with the results.

When not connected to the two pistons, the cam can be easily rotated by hand. But once its connected to the pistons, it kind of becomes difficult to rotate by hand because I have ensured that a good amount of compression and expansion is produced by the piston movement.
ibmace
 
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Response to Stirling Cryocooler

Postby ibmace » Wed Jun 24, 2009 6:41 am

Mr.William,

Could you kindly explain to me how it is when I reverse the direction of rotation the lead angle changes to 180 degrees.

In an alpha configuration engine would'nt reversing the rotation direction just switch the leading piston with the one that was lagging earlier.

I have racked my brains with this idea but just could'nt figure how that can happen. Thanks a lot for helping out : )
ibmace
 
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Response to Stirling Cryocooler

Postby stan.hornbaker » Wed Jun 24, 2009 8:54 am

Do you have a source of heat applied at one cylinder and cooling at the other? See the animations of an Alpha Stirling engine at:

http://www.animatedengines.com/vstirling.shtml

Will the engine at room temperature turn over easily by hand? If it does NOT excess friction is present in the sliding piston surfaces and bearings etc. Friction is the enemy in many cases. DO NOT OIL a Stirling engine.
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Response to Stirling Cryocooler

Postby ibmace » Wed Jun 24, 2009 9:58 am

Mr.William, you had asked me

> Do you have a source of heat applied at one cylinder and cooling at the other?

Since I propose to use this engine as a cryocooler, I do not intend to apply external heat. But I do cool the heat exchanger that's connected to the follower piston using running water.

You had also asked me

> Will the engine at room temperature turn over easily by hand? If it does NOT excess friction is present in the sliding piston surfaces and bearings etc. Friction is the enemy in many cases. DO NOT OIL a Stirling engine.

To minimize friction, I have used epoxy graphite pistons and brand new sealed ball bearings at all moving joints. So I'm certain that friction is a non issue here.

The trouble is, I decided to use a longer stroke length for the pistons so that I can get good compression and expansion of the working fluid (atmospheric pressure air). I learnt from Mr.Brumman http://www.youtube.com/watch?v=TBEkdKBL1TA that its a good thing to have high compression ratios for achieving sub zero temperatures. The high compression and expantion makes my engine slightly difficult to rotate the flywheel by hand.

Mr.William, I'm quite eager to know how the lead angle changes to 180 degrees when the rotation direction is reversed and how it helps to increase the cooling effect. Kindly let me know where I might be going wrong. Thank you for sticking on with me.
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Response to Stirling Cryocooler

Postby stan.hornbaker » Wed Jun 24, 2009 11:57 am

The Stirling engine in the linked video is a single cylinder Beta Stirling engine using a Rhombic drive.
As I understand your engine is an Alpha engine with the cylinders oriented at 90 degrees with the top end of the cylinders connected through a heat exchanger/regenerator.

With the engine vented to atmosphere it should turn over very freely by hand, if not there is far too much friction in the system.

For a complete college level course go to the following web site:

http://www.ent.ohiou.edu/~urieli/Stirling/me422.html

PS Don't rely too much on videos posted on YouTube and similar sites.
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Response to Stirling Cryocooler

Postby ibmace » Thu Jun 25, 2009 11:42 pm

I'll take a look at the site that you have suggested. Thank you Mr.William
ibmace
 
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Response to Stirling Cryocooler

Postby shigin.ks » Fri Oct 23, 2009 1:16 am

hey,please give me your email.i am also on some project like you.i need your design aspects .please send it to shigin.ks@gmail.com
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