Stirling Cryocooler - Please help

[ibmace]

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]

Hi RS,

Thank you for replying Really appreciate it.

R.S have I made a mistake of choosing to build the Alpha configuration Stirling for a cryocooler. Would I have been better off if I had chosen a beta or gamma configuration instead.

Anyways I was highly thrilled to read your post because you have addressed my question from all the possible angles.

You had mentioned:

> Try less material in your heat exchangers. Sometimes the air flow is better served with less resistance.

To tell you the truth I have jam packed the regenerator with steel wool. At high speeds it can certainly constrict free flow of air.

You have mentioned:

> If you have everything right there should be a large heat radiator on the end you want to dump heat. Unless you release off this heat somewhere the heat will creep throughout. Isolating the cool end in a chamber might help too. Isolation of the two ends should be as good as possible. Maybe a fan on the hot end?

Since I am using a simple metal heat sink to cool the hot end of the heat exchanger, the heat exchanger at the hot end is probably not dumping enough heat that gets generated during the compression cycle. So like you have suspected I too feel that probably the heat is creeping throughout the engine. Since a part of the heat remains within the engine on each cycle, it might also be compounding on each cycle. That could explain why the cold end is also getting hot. I have noticed the follower piston's heat exchanger considerably hotter than the leading piston's heat exchanger.

I am really having a problem choosing the right heat exchanger for the hot end. I'm worried that I might end up with too much dead volume.

You have mentioned:
> Also see if you can find a variable speed motor. Maybe the speed is a factor. It would be interesting to see if there was a high RPM point where no more cooling effect is reached, much like the HP/torque curve of an engine.

Hmmmm. That's brilliant I would have never thought of that myself. With some crazy logic, I have been thinking that if I put in more RPM I can obtain more cooling. I'll take up your advice and buy myself a motor speed controller.

You have also mentioned:
> Also check to see if rotating the cooler the other direction will work. I've read that a Stirling run BACKWARDS will create such a cooler. Couldn't hurt, could it?

Well I did try running the engine backwards. All that happened by doing so is it caused the hot heat exchanger to become the cold heat exchanger and vice versa.

In effect I've learnt a lot from you. I thank you for lending me a helping hand.

Signing off hoping to keep corresponding with you likewise in the future as well. God Bless


P.S. When you get some time, could you please suggest the best way for me to get rid of the heat without introducing excess dead volume. Maybe by using several small coiled steel tubes placed within water or something might do a good job ?

[ibmace]

Hi RS,

As you had suggested earlier, I'm entirely convinced that the high speed of the motor is really at the base of the heating problem that I'm facing. If it were'nt for you pointing it out to me, I would have completely overlooked it. Thanks a million

So here's my current strategy. No (1) Reduce the speed of the motor. (2) Get a better heat sink. So today evening I'm all set to go buy myself an electronic motor speed controller.

As for the heat exchanger, maybe I'll get my friend with the lathe to turn a metal cylinder into fins which will end up looking somewhat like the harddrive platter fins you have suggested to me. Luckily I also have a non working computer SMPS fan that I can use to cool the fins. Hopefully those two together should dissipate enough heat.

Regarding posting pictures of the engine, I'm afraid the engine looks like an ugly duckling or more like a 19th century box car. So I'm much too shy to post it onto the Internet :???: Anyway I have already started replacing the cam mechanism with a Ross Yoke. It should get ready in about a weeks time. Probably then it might look pleasing enough to post pictures of it.

You had asked me whether I can measure the input power that goes into the engine. I have a small digital multimeter. If the engine works as expected and produces usefull amount of cooling then I must be able to find out the maximum current drawn by the engine.

I'll post you the results as soon as I put everything together again. Thanks.

By the way, have you see the following videos posted by Mr. Barumman on Youtube. Its a single cylinder beta stirling engine which he had designed by reverse engineering one of the original Philips MP1002CA Stirling generators. To tell you the truth I got the courage and motivation to design my own stirling after watching these videos.

http://www.youtube.com/watch?v=ssvHGUlFokA
http://www.youtube.com/watch?v=TBEkdKBL1TA
http://www.youtube.com/watch?v=MzqbK7p9EmU
http://www.youtube.com/watch?v=F9oQeq54NP8
http://www.youtube.com/watch?v=cc3KCoFnZTQ
http://www.youtube.com/watch?v=M9UKu-AP02k

In the mean time I was able to find a wonderfull resource for different heat exchangers on good ol wikipedia. For my future project I think I'll try to make one of the simpler ones shown in there.

Link : http://en.wikipedia.org/wiki/Heat_exchanger

[ibmace]

Yesterday evening after work I went to an electronic parts store and purchased one electronic motor speed controller. All the ESC's they had were rated more than 1000 watts. Anyway I decided to buy one. It set me back by Rs.215/- (Only about 5 US Dollars ).

I returned home and hooked the motor to the ESC and I was very happy to find that it worked perfectly. I could lower the motor's speed from 8000 RPM's right down to a few 100 RPM's.

Later on I connected the motor to the Stirling engine's flywheel at a pretty low speed. Trouble is, just like they did before both the heat exchangers are still getting hot. But as a result of the lower rotation speed they were not getting as hot as I had mentioned earlier but still hot nevertheless

The next thing thats left for me to do now is to make a good heat exchanger to cool off the hot side heat exchanger.

When I was trying out the new ESC on the Stirling Engine, one question occurred to me. Air passage within the engine is achieved via narrow tubes connected between the expansion cylinder, regenerator and the compression cylinder. So when the air gets pushed back and forth through those narrow tubes it can have a squeezing or compressing effect on the air every time air passes through those tubes. Could the narrow tubes that I had used for connecting the two cylinders to their corresponding heat exchangers be contributing to the heat generated in the cold side heat exchanger ? If yes, tubes with slightly bigger inner diameters may reduce that effect.

Any ideas on this will be greatly appreciated. Thank You