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This paper presents entire flow path simulations in a prototype four-nozzle Pelton turbine under three water heads, from pipe flow to nozzle jet water sheet flow on rotating bucket and drop from bucket brim on the air. Different type flows in the Pelton turbine are analyzed by adopting the three-dimensional transient air-water two-phase flow simulation method. Hydraulic performance of stationary parts and rotating runner are evaluated along the entire flow path. Flow analyses indicate that the pressure pulsation is very low and the water flow could be regarded as steady flow in the stationary parts. The hydraulic loss in these parts drops as the water head increases due to a reduction of the frictional loss. Afterwards, the interactions between the jets and buckets in the rotating runner are discussed. The pressure pulse on the bucket surface pulsates with the spreading of the water sheet flow and takes up 10%–25% water energy. The hydraulic performance of the bucket is highest at optimal water head and decreases as the water head varies. Unsteady flow analyses show that there is a potential for interference between the two adjacent jets if the water head or the angle between the two jets decreases. [ABSTRACT FROM AUTHOR]

The present paper aims to find out the influence of operating head on the rotating bucket free surface sheet flow and hydrodynamics performance for a Pelton turbine. Three-dimensional unsteady air-water two-phase flow simulations in the rotating buckets were performed by adopting the shear stress transport curvature correction turbulence model and homogenous model. The sensitivities of the unsteady simulation results to moving mesh resolution and computational fluid dynamics solver time-step have been evaluated to discuss the effect of Courant–Friedrichs–Lewy conditions on the two-phase flow simulation. The accuracy of the numerical predicted flow pattern and the hydrodynamic performance results are reasonable when compared with the experimental data. The simulation results indicate that the remaining kinetic energy carried by the outflow under the nondesigned water head is the main reason for the efficiency loss in the Pelton turbine. Under low water head conditions, jet distortion caused by the Coanda effect and flow interference will lead to more severe efficiency deterioration. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to numerically analyze the flow characteristic and to explore the hydrodynamic mechanism of the S-shaped region formation of a Francis pump-turbine. Design/methodology/approach – Three-dimensional steady and unsteady simulations were performed for a number of operating conditions at the optimal guide vanes opening. The steady Reynolds averaged Navier-Stokes equations with the SST turbulence model were solved to model the internal flow within the entire flow passage. The predicted discharge-speed curve agrees well with the model test at generating mode. This paper compared the hydrodynamic characteristics of for off-design cases in S-shaped region with the optimal operating case, and more analysis focusses particularly on very low positive and negative discharge cases with the same unit speed. Findings – At runaway case toward smaller discharge, the relative circumferential velocity becomes stronger in the vaneless, which generates the “water ring” and blocks the flow between guide vane and runner. The runner inlet attack angle becomes larger, and the runner blade passages nearly filled with flow separation and vortexes. The deterioration of runner blade flow leads to the dramatic decrease of runner torque, which tends to reduce the runner rotation speed. In this situation, the internal flow cannot maintain the larger rotating speed at very low positive discharge cases, so the unit discharge-speed curves bend to S-shaped near runaway case. Originality/value – The analysis method of four off-design cases on S-shaped region with the comparison of optimal operation case and the calculated attack angles are adopted to explore the mechanism of S characteristic. The flow characteristic and quantitative analysis all explain the bending of the unit discharge-speed curves. [ABSTRACT FROM AUTHOR]