Unsteady energy variation and performance analysis of radial inflow turbines in Organic Rankine cycle systems using enthalpy-gradient evaluation

Unsteady flow in high-speed radial inflow turbines strongly influences internal energy transport and performance, particularly in compact energy systems such as organic Rankine cycle (ORC) units. However, the spatial non-uniformity and temporal variability of energy distribution under unsteady conditions remain difficult to quantify using conventional dissipation-based approaches. In this study, an enthalpy gradient magnitude (EGM)-based indicator is applied to characterize energy variation in unsteady turbine flows. Unsteady CFD simulations are performed for multiple radial inflow turbine rotors operating under identical conditions, enabling a comparative, case-based analysis of internal flow behavior and performance. Instantaneous, time-averaged, and fluctuation components of EGM are evaluated to capture both spatial and temporal features of energy variation. The results show that regions of elevated EGM correspond to strong flow non-uniformity associated with velocity gradients, curvature effects, and vortex structures. A clear inverse relationship is observed between the time-averaged volumetric EGM and turbine isentropic efficiency. Under identical operating conditions, a rotor with approximately twice the averaged EGM exhibits an efficiency reduction of about 13%, indicating that increased spatio-temporal non-uniformity of energy transport is directly linked to performance degradation. The proposed approach provides a practical and physically interpretable indicator for diagnosing energy transport characteristics in unsteady turbomachinery flows. The findings offer useful guidance for performance evaluation and design-oriented analysis of radial inflow turbines in engineering applications. The main contributions of this work are the extension of the EGM method to unsteady flows, the introduction of a remapping-based statistical framework, and the establishment of a quantitative link between EGM and turbine efficiency.