Improving operational robustness of a free-piston Stirling generator by a clearance dead-space structure on the hot-end side of the expansion chamber

Free-piston Stirling generators (FPSGs) hold great promise for future applications in deep-space and deep-sea exploration. However, the displacer and piston are not rigidly connected in an FPSG; instead, their motion trajectories are determined by the dynamics of a multi-degree-of-freedom vibrating system of which they are a part, making the FPSG prone to tipping into sub-stable operation. In this study, a clearance dead-space structure was configured on the hot-end side of the expansion chamber. To capture the self-sustained oscillations of the modified FPSG driven by the intricate coupling of thermal-fluid dynamics and external excitations, a comprehensive multi-degree-of-freedom dynamic model coupled with flow resistance models, working chamber pressure models, and a clearance dead-space model was developed. The unconditionally stable Newmark-β algorithm then numerically solved the multi-degree-of-freedom thermodynamic-dynamic coupled differential equations. As a result, the clearance dead-space structure plays a crucial role in stabilizing the FPSG. It prevents self-excited oscillations from diverging during sudden surges in the hot-end temperature, abrupt load changes, or external excitation shocks, thereby keeping the displacer from colliding with either the hot or cold end of the cylinder block. Moreover, the clearance dead-space structure was also shown to effectively suppress divergence in the piston's self-excited oscillations through the FPSG's intrinsic energy-conversion mechanism. The clearance dead-space structure broadens the FPSG's permissible operating range and significantly enhances operational robustness in challenging deployment scenarios.