Full-3D Compressible Pressure Pulsations Analysis of a Huge Francis-Type Pump-Turbine Unit Coupled with Its Water Conveyance System: Modeling Implications for Flexible Hydropower Operation

Abstract

The growing demand for flexible hydropower operation requires a clearer understanding of pressure pulsations generated within hydraulic units and their connected water conveyance systems. A full three-dimensional (3D), weakly compressible computational fluid dynamics model coupling a prototype-scale ultra-high-head pump-turbine with its water conveyance system is used to investigate the rated-head turbine mode and compare it with the previously studied maximum-head pump mode. The turbine mode exhibits stronger rotor–stator interaction inside the unit but faster peak-to-peak attenuation toward the spiral casing, reaching 92.7% compared with 85.57% in pump mode. Smoother flow through the bifurcation and branch pipe is accompanied by less prominent pipeline-related low-frequency content relative to the dominant unit excitation. The corresponding relative spectral index is 3.399% in turbine mode and 6.099% in pump mode. Low-frequency components nevertheless propagate and reflect within the water conveyance system, with reflection-related interference near the closed branch end. These results reveal the operating-condition dependence of unit-water conveyance system interaction and support full-3D analysis when local pipeline flow and pressure propagation must be resolved together. The study provides a methodological basis for subsequent hydraulic-excitation analysis of flexible hydropower units.