Comparative study of passage and radial guide vanes in a multistage pump-as-turbine: Experimentally validated numerical simulation and entropy production analysis

Passage guide vanes and radial guide vanes are two guide-vane configurations commonly used in multistage pumps operated as turbines (PATs). To clarify how these two configurations affect PAT efficiency, a two-stage centrifugal PAT was investigated through a combined experimental and numerical study, focusing on the turbine-mode external characteristics, vortex distribution, and energy loss. The results show that the passage guide vane delivers consistently higher efficiency than the radial guide vane: the peak efficiency increases by approximately 3.35%, the average advantage is about 4%, and the high-efficiency range is widened by 47.56%. Within the tested flow-rate range, the turbulent entropy production of the passage guide vane increases from 8.49 W to 80.34 W, whereas that of the radial guide vane increases from 23.85 W to 276.00 W, reaching approximately three times that of the passage guide vane, with an average growth rate about 3.5 times higher. In terms of wall entropy production, the passage guide vane reduces wall-related losses by approximately 20%–23%.For the passage guide vane, turbulent entropy production is mainly concentrated in the forward guide vane, accounting for about 50% of the total; for the radial guide vane, it is mainly concentrated in the transition section, accounting for about 52%. Vorticity associated with the radial guide vane is distributed throughout the entire flow passage, whereas markedly fewer vortical structures are observed for the passage guide vane. The velocity-field analysis further reveals that the passage guide vane mitigates the mismatch between the incoming flow angle and the blade angle. Overall, under turbine operating conditions, the passage guide vane outperforms the radial guide vane in terms of efficiency, energy utilization, and flow stability, providing guidance for guide-vane selection and structural optimization of high-head multistage PATs.