I am working on GM type pulse tube cryocooler and I have found that pressure ratio decreases along with the temperature reduction. I do not know what is the reason. As the temperatures goes down by refrigeration process inside the pulse tube cryocooler, PL increases and PH decreases. Can you tell me the reason?
Can you provide an order of pressure ratio decreasion? What temperatures doyou get? In my opinion the main reason should be the density change of your working fluid and thus the gas spring influencing the piston compressor. Your compressor (I expect you are using a dual piston type) consists of a mass-spring-damper-system. In addition to the hardware spring (flexure or coil type) your fluid dampens and adds a gas spring to you system (as well on the cold side of your expander). This causes a shift of the working point towards non optimal conditions.
If you take a look at the stirling cryocooler schematics at wikipedia (pulse tube is more or less the same with less moving parts) it should become clear. Same volume --> density increases with temperature decreasion --> gas spring gets stiffer & damping increases --> more input power needed for same movement and thus the same pressure ratio.
http://en.wikipedia.org/wiki/Cryocooler
We normally use GM type compressor i.e. scroll compressor. This compressor is used for GM type Pulse Tube Cryocooler. Can you apply your above answer to this?
The explanation should also apply on the pulse tube part. The gas piston compressing and expanding your gas within your pulse tube is also a function of density and thus temperature (gradient).
I find it easier to understand with compressor design. The application of scroll compressors doesnt allow the mass-spring-damper analogy.
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