Numerical study on the effect of viscosity on a multistage pump running in reverse mode

Utilization of pump as turbine (PAT) systems is known as an economically beneficial method to harvest hydropower energy, especially in a remote area. Despite the extensive investigations on the operation and parametric study of the PAT components, the turbine mode of multistage PATs in case of a viscous working fluid is found as a literature gap. In this paper, a numerical simulation method is employed as a predicting tool to investigate the consequences of changing the fluid viscosity affecting the flow patterns within the multistage PATs. The commercial solver ANSYS CFX 16 was used for simulation. Validation of numerical solution was done for the single-stage pump considering water and 48 cSt viscous fluid. The numerical results in the single-stage PAT showed that increasing the viscosity causes an efficiency drop mainly at the part-load condition and translates the maximum efficiency point to higher flow rates, where the vortices at impeller passage and draft tube are weaker. Increasing the viscosity up to 48 cSt, results in 12.5% reduction of efficiency at the best efficiency point (BEP) in two stage PAT. Based on the two-stage PAT results, the total hydraulic efficiency at the BEP has been increased by 1% and 1.5% for water and the 48 cSt viscous fluid, respectively, in comparison with the single-stage PAT. According to the post-processed streamlines at different stages of the two-stage PAT, the undesired vortical structures were caused by the diffuser and the return channel, which were mainly designed for the pump mode. The hydraulic analysis of two-stage PAT shows that using multistage pumps in reverse mode is reasonable for power generation in high head and flow rate sites. (C) 2019 Elsevier Ltd. All rights reserved.