The aim of study is to predict the performance of the optimal thermal-lag engine and study the effect of some geometrical and operating parameters such as the charged pressure, strokes, number of heater fins, number of cooler fins, heater temperature, cooler temperature, frequencies, porosity, and materials of working fluids on the performance of the optimal thermal-lag engine. Moreover, the dependence of transport properties of working fluids on temperature and pressure, and of thermo-physical properties of the solid materials on temperature are reckoned. In order to implement this dependence, solution initialization, and the motion of the piston with fixed rotation speed, User-Defined Functions (UDF) based on the frame of a software package, ANSYS Fluent, are written and compiled. Furthermore, the thermodynamic model in this study is supplemented by adding the solid annular locating between the regenerative heater and the impulse tube instead of the gap between these chambers in the optimal thermal-lag model. Hence, heat transfer in the solid material and the working fluids are combined with the conjugate heat transfer.

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