Thermodynamic analysis of hydraulic losses in ultra-low specific speed PAT under multiphase conditions

To address the growing needs of the changing electrical grid, Pump as Turbine (PAT) must rapidly switch between operational modes and operate efficiently under deep partial load circumstances, frequently affected by substantial fluid-dynamic instabilities. Due to the escalating demand for energy conservation and emission reduction, there is a heightened emphasis on improving the energy efficiency of ultra-low specific speed pumps as turbines (USSPAT). The emergence of gas bubbles in the liquid medium is significantly impacted by the internal flow structures, which subsequently influence flow stability. Therefore, it is important to investigate the process through which the gas phase influences internal PAT flow and energy conversion. The gas–liquid two-phase entropy production theory is employed to examine the distribution of energy losses across each hydraulic component at varying gas inlet Gas Void Fraction (IGVF) using the k-ε model. Entropy production analysis at 0%, 1%, and 3% IGVF under 0.8Qd, 1.0Qd, and 1.2Qd conditions reveals that energy dissipation patterns vary with gas content, where entropy generation is maximum in the volute (1.3820 kg m2 s−3 K−1), followed by the impeller (1.0370 kgm2·s-3·K). The results showed that flow energy dissipation is concentrated near the volute tongue, while increasing pressure pulsations at higher IGVF reduce USSPAT efficiency under over- and part-load conditions due to unsteady flow dynamics and rotor–stator interactions. Transient simulation calculations were validated with experimental results. This study demonstrates the importance of USSPAT in renewable energy systems and concludes that secondary flow and rotor–stator interaction under gas–liquid conditions are the primary causes of increased entropy generation and energy losses.