In this thesis, a system consisting of two combined cryocoolers that use 3 He as a working medium is simulated with the use of Computational Fluid Dynamics (CFD) and ANSYS Fluent software. The cryocoolers operate with frequency of 1Hz and temperature below 1K. Each of them employs a recuperator and the two recuperators exchange heat with each other, combining thus the two machines into one system referred to as Superfluid Stirling Refrigerator (SSR). These machines will operate with a phase

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... erence. The phase difference is determined by the mass flow, hot or cold, in the recuperator. The aim is to determine the optimal phase difference that provides the better efficiency when is applied to the machine. Afterwards, we will compare it with the 1D and Schmidt models. In order to determine the optimum phase difference, we have to initially determine the operating range. We will achieve this by setting multiple cases and gradually increase the phase difference that the machine operates in each case. Αs it will be detailly explained later, the SSR cooling capabilities are eliminated and starts operating out of this range when the parallel flow towards the system cold boxes is appeared. For every case that the machine operates inside the range that provides cooling power, we will also calculate the Coefficient of Performance (COP). Finally, using a polynomial regression we will correlate the COP with the phase difference resulting us to conclude the optimum and worst design points.