Mixture-based loss evaluation and critical superheat determination in transonic steam flows for flexible turbine operation: An experimentally validated OpenFOAM approach
Guojie Zhang, Qianhao Zhang, Yifan Yang, Zunlong JIN, Sławomir Dykas, Mirosław Majkut, Krystian Smołka
Abstract
Deep peak regulation and flexible operation of steam turbines are imperative for integrating renewable energy into modern power grids. However, operation under low-load conditions frequently drives the last-stage expansion into the unstable non-equilibrium condensation zone, risking significant efficiency penalties and blade erosion. Current loss evaluation methods often rely on simplified single-phase gas assumptions, failing to accurately quantify the thermodynamic irreversibility inherent in these transient two-phase flows. To address this, this study develops a thermodynamically consistent two-phase framework implemented in OpenFOAM. The solver couples non-equilibrium nucleation and droplet growth kinetics and is validated against IWSEP nozzle and transonic stator cascade experiments. Statistical analysis confirms high model fidelity, achieving a coefficient of determination ($R^2$) exceeding 0.98 for static pressure distributions across all configurations and wetness evolution in nozzle benchmarks. Using a reproducible inlet-temperature sweep procedure, a configuration- and operating-condition-specific critical superheat boundary is identified, separating dry expansion from condensation-prone regimes for the examined cases. The results show that increasing inlet superheat shifts the Wilson point downstream, thereby mitigating condensation-induced pressure variations. Furthermore, a mixture-based loss evaluation method is introduced to correct the bias in traditional assessments. Comparative analysis demonstrates that conventional gas-phase formulas systematically overestimate entropy generation by neglecting latent-heat effects, whereas the proposed mixture-based approach remains consistent with the two-phase thermodynamic state. Overall, the proposed framework enables case-specific condensation-risk screening for flexible-operation planning and provides a refined, thermodynamically consistent basis for aerodynamic loss assessment of wet-steam components.