Performance Parameters Prediction for FPFD Miniature Stirling Cryocooler Considering Polytropic Processes

Abstract

The performance of an actual free piston free displacer (FPFD) Stirling miniature cryocooler has been simulated based on the parameters affecting the performance of the cryocooler. The processes of compression and expansion in the cryocooler have been assumed to be polytropic with variable ratio of specific heats for the working fluid, i.e., helium. The effect of design parameters like piston spring stiffness, displacer spring stiffness, mass of the piston, displacer mass and frequency of operation on the cooling power of the cryocooler has been calculated. The parameters thus obtained with polytropic processes have been compared with the parameters obtained by assuming the compression and expansion processes to be isothermal. The amplitude of the piston and the displacer is not constant in the case of free piston free displacer cryocooler with linear motors. Linear motor drives have been used for eliminating the side forces on the cylinder walls and wear and tear of the reciprocating parts of the cryocoolers giving them a very long life. A computer program named CRYOJIN which was developed for the isothermal case has been modified for analyzing the real-time performance of the cryocooler for polytropic processes. The physical dimensions of Philips 1 W FPFD cryocooler have been taken for study and comparison.