Numerical investigation of the energy evaluation in a Francis turbine based on an advanced entropy production model

Abstract

The Hydraulic turbine efficiency is traditionally calculated by the pressure drop between the inlet and outlet. But this method cannot determine the detailed energy loss distributions along the flow passage. Entropy production theory can predict and exhibit the irreversible energy loss distribution intuitively. An advanced entropy production diagnostic model (EPDM) with phase transition (EPDMS) is introduced in this study to evaluate the irreversible energy loss in cavitating flows. The EPDM method is based on homogeneous equilibrium model (HEM) without interface slip velocity while EPDMS is based on two fluid model (TFM) with the consideration of slip velocity and interface forces. At operating point guide vane opening (GVO) 20.07, the relative error of EPDM for efficiency calculation is 6.87% and the relative error of EPDM is 3.05%. In EPDMS, the interaction between cavitation and entropy production rate (EPR) is defined and interface EPR (IntE) has a considerable proportion of total EPR in EPDMS. With the HEM method, the underprediction of the turbulent eddy viscosity results in the underestimation of irreversible energy loss. In essence, the intensive mass transfer process in cavitation flow induces the generation of IntE and corresponding irreversible energy loss directly.