Numerical investigation of a rotary-integral miniature Stirling cryocooler with variable cylinder angle

This paper investigates the performance of a miniature rotary-integral Stirling cryocooler with variable cylinder angle using a validated thermodynamic model. Unlike conventional designs that fix the cylinder angle at 90°, the present study considers it as a tunable geometric parameter that governs the displacement phase angle between piston and displacer. The model accounts for transient temperature evolution, pressure losses, and heat transfer in each chamber, with thermophysical properties evaluated as functions of pressure and temperature. Simulations are conducted at a charged pressure of 30 bar and rotation speed of 3000 rpm over cylinder angle ranging from 50° to 120°, under heat loads of 0-0.3 W. Results show that the cylinder angle strongly affects the phasing of compression and expansion volumes, thereby altering compression ratio, cold head temperature, and coefficient of performance (COP). An optimal cylinder angle range of 68°-70° yields steady-state cold head temperatures 6-10 K lower than the 90° baseline, with COP improvements up to 0.9% at 0.3 W load. Carnot efficiency can be increased from 17.3% to 18.6% as the cylinder angle is varied from 90° to 70°. These findings demonstrate that geometric phase tuning provides an effective means of enhancing the cooling performance of Stirling cryocoolers.