Numerical investigation of unsteady flow and thermodynamic characteristics in asymmetric scroll expander for automotive engine waste heat recovery

• Asymmetric scroll expander supplies high expansion ratio for waste heat recovery. • Asynchronous suction and discharge cause the disturbance of internal flow. • Isentropic efficiency shows greater filling factor sensitivity than total efficiency. • Low pressure difference during expansion results in large exergy destruction. Automotives with internal combustion engines inherently experience thermal losses. Waste heat recovery systems incorporating scroll expanders improve efficiency by converting thermal energy into output power, yet face challenges from variable heat sources. To address this, this study proposed a theoretical model for asymmetric scroll expanders to improve the expansion ratio and adapt to fluctuations in heat source temperature and mass flow rate. The influence of profile variables on the geometric performance was investigated, and a generalized numerical framework for the expansion process was established, analyzing unsteady flow in the working chambers. Under different input parameters, shaft power and filling factor were calculated, with variations in isentropic efficiency and total efficiency examined. Exergy analysis further assessed energy conversion efficiency during the expansion process. The results indicate that decreasing the initial angle optimizes modified area coefficient and increases the built-in volume ratio. Asynchronous discharge from working chambers induces localized high temperatures within the discharge region. Filling factor is more sensitive to rotational speed variations than to suction pressure. Efficiency increases with pressure ratio, reaching a peak of 78.46% for isentropic efficiency and 19.51% for total efficiency before declining. Exergy efficiency drops by approximately 11.61% due to greater flow losses and mechanical friction at higher rotational speeds.