Performance variation and loss mechanism of a radial inflow turbine under off-design conditions

Abstract

Radial inflow turbines (RITs) are key expansion devices in organic Rankine cycle (ORC) systems for low-grade waste heat recovery. Under practical operation, variations in heat source conditions and system loads cause deviations in inlet temperature, pressure ratio, and rotational speed, affecting turbine performance and irreversible losses. This study investigates a pentane-based ORC RIT using three-dimensional numerical simulations and entropy generation analysis. The effects of inlet temperature, pressure ratio, and rotational speed on performance, flow characteristics, and loss distributions are systematically evaluated. The results show that turbine power increases while isentropic efficiency decreases with increasing inlet temperature and pressure ratio. Both efficiency and power exhibit an inverted “U-shaped” trend with rotational speed. Extreme rotational speeds induce different incidence mismatch patterns, leading to changes in flow separation and high-entropy-generation regions. Entropy generation analysis identifies the rotor as the dominant loss source, with turbulent dissipation contributing the largest proportion. The intermediate passage accounts for approximately 67.7%–74.5% of the rotor local entropy generation, indicating that inlet mismatch triggers loss development, while irreversible losses mainly accumulate during downstream evolution of separation vortices, shear layers, and secondary flows. This study provides a region-resolved, entropy-based quantitative characterization of loss evolution and redistribution in the investigated pentane ORC RIT under off-design conditions.