Multi-scale cavitation evolution and pressure pulsation in multistage cryogenic LNG turbine: PBM–thermo analysis

A full-flow-passage numerical model of an LNG multistage hydraulic turbine is developed using a validated population balance–thermodynamic coupled cavitation model (PMZGB). Multi-scale cavitation evolution, vorticity generation, and transient pressure pulsation under cryogenic conditions are systematically investigated. Results show that as outlet pressure decreases, cavitation transitions from local attached bubbles to severe sheet clouds, with significant radial non-uniformity: the cavitation core concentrates near the hub (0.1 Span), while the shroud (0.9 Span) shows weaker, dispersed cavitation due to centrifugal effects. Cavitation is far more severe in the diffuser than in the impeller, making the diffuser the key performance-limiting component. The baroclinic term is identified as a dominant vorticity source in cavitation regions, driving vortex structures from discrete clusters to globally chaotic morphology. Cavitation-induced pressure pulsations are most intense in the diffuser wake, with a dominant frequency of 13.66 times the rotational frequency and amplitudes far exceeding those in the impeller, primarily excited by bubble collapse and vortex–cavitation coupling. Bubble size analysis reveals that the last-stage impeller is dominated by stable attached bubbles (2–2.5 mm), while bubbles in the diffuser are larger and exhibit dynamic evolution, with complementary variations between large (>2.5 mm) and small (<2 mm) groups, confirming coalescence and breakup as the governing mechanisms. This study elucidates multi-scale cavitation and pressure pulsation mechanisms, providing theoretical guidance for anti-cavitation design of LNG turbines.