Study on the influence of real gas compressibility on the performance of free-piston Stirling engines

Free-piston Stirling engine (FPSE) is a promising micro or small-scale energy conversion technology. In recent years, attention has been drawn to the thermodynamic and dynamic behaviour of actual gas compressibility in FPSEs. However, the complex interactions between gas compressibility and engine performance remain insufficiently understood. The main reason is the current inability to directly introduce and manipulate the gas compressibility factor in computational fluid dynamics (CFD) models. To address this, this paper proposes a computational method that indirectly alters gas compressibility by adjusting the molar mass of the working fluid in a two-dimensional CFD model. This method overcomes the limitations of traditional simulations that fix the real gas compressibility. Furthermore, we systematically analyse the impact of gas compressibility on FPSE based on this method. The results indicate that increasing the compressibility factor from 0.6 to 1.2 enhances the acoustic power in the compression space (CS) by 16 % and improves the thermal-to-acoustic efficiency by 2.5 %, attributed to more effective gas compression and a reduction in irreversible losses. Additionally, through the analysis of the distribution of key parameters along the flow path, we find that the increase in acoustic power also stems from the rise in the pressure ratio in the CS and the improvement of the thermoacoustic effect in the regenerator (REG). However, further increasing the compressibility factor to 1.4 leads to a 1.43 s delay in dynamic thermal response time, while the gas density within the FPSE decreases. This does not significantly affect the amplitude of gas velocity fluctuations. These findings provide new insights into the role of gas compressibility in the operation of FPSE.