Influence of clearance leakage flow on entropy production and flow non-uniformity characteristics of a supercritical CO₂ co-rotating scroll expander

Supercritical CO₂ (SCO₂) scroll expanders are important components in SCO₂ power generation system applied in SOFC waste heat recovery, exhibiting strong sensitivity to axial clearance size. This study investigates the internal leakage-flow characteristics of an SCO₂ co-rotating scroll expander using a three-dimensional unsteady compressible computational fluid dynamics (CFD) model that incorporates realistic axial clearance passages, dynamic mesh morphing, and real-gas thermophysical properties. Entropy-production analysis and the Omega vortex-identification method are further employed to quantify irreversible losses and elucidate leakage-induced vortex structures. The mass flow rate, isentropic efficiency, filling factor and entropy production of SCO₂ co-rotating scroll expander when AC= 0 μm, 10 μm and 20 μm are quantitatively compared. Additionally, the flow field analysis and axial clearance leakage flow characterization are conducted to reveal the effect of leakage flow on the pressure, temperature, velocity, and entropy production of working chamber. The results demonstrate that the isentropic efficiency decreases from 79.1% to 67.8%, the total average entropy production of SCO₂ co-rotating scroll expander increases from 2.8 W/K to 4.2 W/K when the axial clearance increases from 0 μm to 20 μm. Additionally, SCO₂ axial clearance leakage flow leads to pronounced pressure gradient, non-uniformity temperature distribution and strip-shaped high local velocity and entropy production. Meanwhile, SCO₂ axial clearance leakage flow induces separation vortex, primary leakage vortex and secondary vortex, and vortex evolution intensifies turbulence production and viscous dissipation, resulting in high turbulent kinetic energy dissipation and entropy production. The research results provide a theoretical basis for clearance design and leakage control of SCO₂ co-rotating scroll expanders for SOFC waste heat recovery.