How to Improve the Performance Prediction of a Pump as Turbine by Considering the Slip Phenomenon

Abstract

Nowadays Pumps working as Turbines (PaT) are devices widely used to perform energy recovery in hydraulic grids, thus improving their overall efficiency, and to build small hydropower plants. In this work, a centrifugal pump has been numerically investigated in turbine operating mode by means of the open-source CFD code OpenFOAM with emphasis on the flow field at the runner outlet. Due to the reduced number of blades in a PaT, the mean outlet relative velocity angle differs from the blade angle. In order to account for this phenomenon, the slip factor is introduced. The slip factor is investigated and its application to a 1D model is shown in order to highlight the improvement in predicting the characteristic curve of a centrifugal pump used in reverse mode as a turbine (PaT) especially at its part-load.

Slip Factor Correction in 1-D Performance Prediction Model for PaTs

“…al. [21]. For all the analyzed cases, the mass flow rate has been imposed at the inlet considering a uniform inlet velocity distribution and a constant turbulent intensity equal to 3%.…”

“…Moreover, the value of the mass flow leakage, which flows through the annular seal, has been modeled as exiting from the impeller case and incoming axially upstream the impeller eye with a 45 • of swirl with respect to the tangential direction. The leakage was calculated a priori according to a one-dimensional empirical model because the geometry of the seal has not been included in the computational domain [21].…”

“…tangential direction. The leakage was calculated a priori according to a one-dimensional empirical model because the geometry of the seal has not been included in the computational domain [21]. A uniform pressure distribution was imposed at the outlet of the domain.…”

“…Differently from a previous work [21], herein the authors extended their investigations on the slip phenomenon at the outlet of PaTs. Arranging their work in three parts, the first, (Sections 2 and 3) and the last (Sections 5 and 6) parts being new.…”

“…Indeed, numerical investigations have been carried out on a simplified PaT runner geometry, designed according to potential flow solution, in order to study the slip phenomenon and be able to more easily describe the sources of the flow deflection (see Sections 2 and 3). In the second part, the work deals with the prediction of the performance of a real commercial PaT, numerically simulated in both direct and inverse mode, with an emphasis on the flow field through the impeller [21] (see Section 4). The proposed numerical investigation aims to put in evidence the slip phenomenon on a real configuration.…”