Influence of the Gas Model on the Performance and Flow Field Prediction of a Gas–Liquid Two-Phase Hydraulic Turbine

A two-phase hydraulic turbine’s performance and flow field were predicted under different Inlet Gas Volume Fractions (IGVF) with incompressible and compressible models, respectively. The calculation equation of equivalent head, hydraulic efficiency, and flow loss considering the expanding work of compressible gas were deduced based on the energy conservation equations. Then, the incompressible and compressible results, including the output power and flow fields, are compared and analyzed. The compressible gas model’s equivalent head, output power, and flow loss are higher than the incompressible model, but the hydraulic efficiency is lower. As the IGVF increases, the gas gradually diffuses from the blade’s working surface to its suction surface. The gas–liquid separation happens at the runner outlet in the compressible results due to the gas expansion. The area of the low-pressure zone in the incompressible results increases with the IGVF. However, it decreases with the IGVF in the compressible results. As the gas expands in the blade passage, it takes up more flow area, causing the high liquid velocity in the same passage. The runner’s inlet gas distribution affects the liquid flow angle, causing the inlet shock and high TKE areas, especially in the blade passage near the volute tongue. The high TKE area in the compressible results is larger than the incompressible results because the inlet impact loss and the liquid velocity in the blade passage are higher. This paper provides a reference for selecting gas models in the numerical simulation of two-phase hydraulic turbines