Assessment of an implicit mixing plane approach for pump-turbine applications

In the design process of pump-turbines, both in pump and turbine mode, the assessment of the components matching is very important. In order to be fast in this task, the usual procedure is based on steady-state methods, like the frozen-rotor or the mixing-plane method. The frozen-rotor approach is straight forward and relatively easy to implement, but can produce unphysical behavior, mainly depending on the relative position of the components. On the other hand, the mixing-plane has a more physical background, delivering to the downstream component the mixed-out state of the upstream flow. On how the mixed-out state is computed and imposed to the downstream component there are different methodologies. In the present paper a novel, fully-implicit mixing-plane method is presented and applied to pump-turbine applications, both in pump and turbine mode. The major advantage of this approach is its robustness, including the ability to handle back flow at the interface, and accuracy. Compared to currently available methods, both in proprietary and commercial codes, the implicit approach leads to a consistent treatment of the interface, enforcing natively the idea of the mixing-plane, i.e. constant spanwise-distribution of the quantities. This allows to obtain excellent results also at part- and over-load.