Stirling pulse tube cryocooler using an active displacer

•An active displacer allows cryocooler performance to be optimised during operation.•Both cooling power and relative Carnot efficiency have optimum displacer phase values.•Varying displacer phase angle boosts performance by increasing mass flow at cold end.•An active displacer cryocooler operates more efficiently than an inertance tube one. An in-line pulse tube cryocooler with an active displacer has been built and its performance has been examined in detail by experimental measurements and numerical modelling. An active displacer allows the mass flow at the cold end to be easily adjusted for optimum performance. It is demonstrated that both cooling power and relative Carnot efficiency have optimum phase values that are different. It is also shown that the phase optimisations are not critical – good performance is achieved over a significant range. The pulse tube cryocooler can deliver up to 3.8 W of cooling at 80 K with an input power of 88 W (shaft power of 69 W) when operating at optimal phase. Moreover, a numerical Sage model is used to enhance our understanding behind the trends observed by examining the mass flow and pressure pulse at the cold end. It is shown that the variation in phase of the active displacer helps boost the cryocooler performance by increasing the amplitude of the mass flow at the cold end and not by adjusting the phase of the mass flow. The Sage model is also used to demonstrate that using a displacer in place of an inertance tube can result in a more efficient cryocooler.