Investigation on hydraulic loss component and distribution in hydraulic machinery: A case study of pump-turbine in pump mode

Abstract

The sudden increase of hydraulic loss in hydraulic machinery and the attendant unsteady flow characteristics like rotating stall and secondary flow seriously threaten the security and stability of operation. The entropy production method is widely used to estimate the hydraulic loss distribution in hydraulic machinery even if it is a thermodynamic concept expressing the irreversible energy transformation from kinetic energy into internal energy. In this study, the conception and the form of hydraulic loss is derived from the kinetic energy equation under incompressibility assumption according to the conception of hydraulic energy. Then, the conception of local hydraulic loss is proposed and defined as the combined action of dissipation effect and transportation effect. By contrast, the entropy production method can only reflect the dissipation effect while ignoring the transportation effect in depicting hydraulic loss. Finally, the hydraulic loss analysis in pump mode of a pump-turbine is conducted under SST k-ω model based on local hydraulic loss rate method. With the help of entropy wall function, the local hydraulic loss rate method shows good agreement with the total pressure difference and the error is within 5% apart from the hump region. Transportation effect plays an equal important role as dissipation effect in hydraulic loss and the hydraulic loss proportion in wall region is within 3.1%. Moreover, the local hydraulic loss rate method can better depict hydraulic loss distribution and hydraulic loss evolution process at draft tube outlet, runner inlet and the vaneless region between guide vanes and runner outlet in part load operating points compared to the entropy production method. The local hydraulic loss rate method proposed in this paper might become the most promising method in depicting the hydraulic loss in hydraulic machinery.