Cavitation and associated entropy production characteristic of a pump turbine in pumping mode based on a modified Zwart-Gerber-Belamri model

Abstract

This study develops a modified Zwart-Gerber-Belamri (MZGB) cavitation model in which the saturation vapor pressure is defined dynamically. The proposed model is then applied to investigate the cavitation evolution and entropy production characteristics of a pump turbine under multiple pumping conditions. Numerical predictions based on the MZGB model show closer agreement with experimental data than those from the ZGB model, with an accuracy of 96.17% for pressure variation. With decreasing flow rate and cavitation number, the cavitation region extends along the blade suction surface, and coupled cavitation-vortex structures form within the runner, increasing both cavitation and vortex volume. The precipitation of cavitation bubbles, accompanied by energy absorption, weakens the pressure pulsations within the cavitation region. By contrast, in the non-cavitation region at 0.8 times of the flow rate at the best efficiency point (0.8Q_BEP), the maximum amplitude is 12 times that at 1.0Q_BEP and 1.9 times that at 0.6Q_BEP. Due to vortex development and flow separation in the runner, the dominant frequency of pressure pulsations corresponds to the runner rotating frequency (f_n) at 1.0Q_BEP, while that at 0.8Q_BEP is dominated by 3f_n. In contrast, 0.6Q_BEP exhibits multiple pressure pulsation peaks within f/f_n ≤ 5. Furthermore, as C_σ and flow rate decrease, the primary entropy production region extends to both the runner and guide vane domains, while the dominant mechanism of entropy production transfers from the wall shear dissipation to the turbulent dissipation. These findings provide a theoretical guidance for the cavitation risk assessment and the energy loss mitigation in pump turbines.