Coaxial pulse tube cryocoolers are the configuration of choice as they allow better access to the cold head. Hence, a previously built and tested inline pulse tube cryocooler which uses an active displacer for phase control has been modified into a coaxial configuration. The active displacer allows the mass flow and the pressure pulse at the cold end of the pulse tube to be easily adjusted for optimum performance. The displacer also allows the expansion power at the warm end of the pulse tube to be recovered in order to operate more efficiently. A numerical Sage model is used to demonstrate this by examining the work flows throughout the cryocooler and it is shown that more than 6% of the power required to drive the cryocooler comes from the warm end of the pulse tube via the displacer. When using an inertance tube or orifice, this expansion power is dissipated as heat which is why using a displacer can lead to a more efficient cryocooler. Moreover, the effect of changing the displacer phase and stroke on cryocooler performance and pressure characteristics is examined both experimentally and numerically.

A numerical model for a coaxial Stirling pulse tube cryocooler with an active displacer has been developed. An active displacer, in place of an inertance tube, has already demonstrated good efficiency in an in-line pulse tube, but incorporating this design into a coaxial configuration permits better access to the cold head. A model of the coaxial cold head was developed to include the radial flow and cooling in this region. The subassembly models were validated with flow testing of the physical sub-units of the cryocooler. The performance has been predicted for the cryocooler as a function of: fill pressure, operating frequency, and phase angle between the position of the linear compressor and displacer. The projected difference in performance and efficiency of the coaxial configuration was compared to the in-line design. The coaxial cryocooler numerically simulates 6 W of cooling at 80 K with an input power of 85 W, at a fill pressure of 28 bar, an operating frequency of 60 Hz, and a compressor-displacer phase angle of 41º. Overall, the coaxial cryocooler outperforms the in-line design in terms of cooling power, but not in terms of efficiency.

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