Performance simulation of a Stirling cryocooler using CFD

Abstract

Small Stirling cryocoolers, with refrigeration capacities in the range of 0.5W to 5W, at cooling temperatures as low as 60K have found their way into the fields of aerospace, medical and energy. The present work is aimed at performance simulation of a gamma type, split-Stirling cryocooler using CFD. Simulation of the reciprocating flow inside a 2-D model of the cryocooler has been accomplished using the dynamic mesh technique in ANSYS Fluent. For this purpose, two separate User-Defined Functions (UDFs) have been compiled for the movement of piston and displacer. In order to operate the cryocooler based on the Stirling cycle an appropriate phase difference has been maintained between the piston and displacer movement. Linearly elastic solid model of the dynamic mesh technique has been adopted for mesh motion inside the displacer. This model is time intensive but proves to be appropriate for cyclic simulations. The mesh inside the working spaces is compressed and expanded proportionally avoiding the negative volume error that is often encountered in moving mesh problems. Temperature-dependent properties of the working fluid and matrix material have been incorporated by using appropriate coefficients of polynomial functions as a function of temperature. Area weighted temperature and pressure inside the compression and expansion spaces have been reported for the ANSYS Fluent simulations. It is shown that the modelled cryocooler can achieve a refrigeration temperature of 60K with a heat load of 0.75W. The results thus obtained have been compared with the experimental result and found to be in close conformance. The methodology adopted in the current work can be utilized to simulate the performance of a cryocooler and can be helpful in finding the optimum values of its operational parameters.