Research on the turn of scroll expanders for organic rankine cycle power generation

The organic Rankine cycle (ORC) power generation system has an interesting perspective for utilizing low-to-medium temperature thermal energy. The scroll expander, known for its compact structure and stable operation, is one of the optimal choices for expansion machinery for small-scale ORC power generation systems. This paper built a three-dimensional CFD model of the scroll expander for an ORC power generation system. Five types of scroll turns, including 2.00, 2.50, 2.75, 3.00, and 3.50 turns, were presented and designed in this study. Transient simulations across various speeds assessed key parameters, including pressure, velocity fields, torque, and efficiency. The results indicate that more scroll turns significantly attenuate the pressure differential between adjacent expansion chambers. This reduction effectively minimizes inter-scroll leakage, leading to an enhancement in volumetric efficiency by up to 26.4%. However, this modification improved volumetric performance at the expense of isentropic efficiency, limiting the isentropic efficiency to a maximum of 32.3%. Furthermore, both the torque and the inlet mass flow rate exhibited non-monotonic relationships with the scroll turns, peaking at 4.55 N m and 0.333 kg/s, respectively, at an optimal configuration of 2.75 turns. As the rotational speed ranges from 3000 to 4500 RPM, the isentropic efficiency first increased and then decreased, peaking at 68.2% at 4000 RPM. The volumetric efficiency improved by 14.4% from 50.6% to 65%. The maximum torque decreases to 2.92 N m at 4500 RPM. The performance of the scroll expander achieves optimal balance at a 2.75 scroll turns, with isentropic efficiency and volumetric efficiency reaching 49% and 63.9%, respectively.