Numerical simulation of two-phase flow in an energy recovery micro-hydraulic turbine based on Francis hydraulic model

Abstract

In industrial processing, a centrifugal pump is mostly used reversely as the micro-hydraulic turbine for its lower cost. In some application situation, the working fluid will contain a part of gas which will reduce the efficiency, stability, and the power output of the machine. Due to the lower efficiency and stability of the pump as turbine (PAT) working with liquid flow, the working characteristic will deteriorate furtherly when the working fluid contains gas. Therefore, a micro-hydraulic turbine based on Francis turbine model is designed and proposed to except to obtain good performance under two-phase flow working condition. In this study, the three-dimensional transient two-phase flow model which adapts the k-ω model as turbulence model and Eulerian-Eulerian model as two-phase flow model with the commercial code ANSYS-CFX was established and solved to study the two-phase flow characteristics of the micro-hydraulic turbine with fixed guide vane openings under different gas volume fraction (GVF). The numerical simulation results show that the power, efficiency and torque of the turbine decrease with GVF. Due to the expansion of the gas in working fluid, the velocity of fluid increases with the gas, which leads to the bigger lower pressure area near the exit of the impeller. The vortex at the back of the blade is also intensified when the gas fraction increase. When the GVF is larger than 10%, the gas distribution is inhomogeneous, and most of them gather at the back side of the blade near the exit of the impeller where the GVF can reach 90%. The efficiency of the micro-hydraulic turbine is about 60% when the GVF is 20%, which needs to be improved in the future.