Mitigation of Pressure Pulsations in Francis Turbine Draft Tube with a GuideVane System : A Numerical Investigation

Abstract: The use of renewable energy such as water and wind to produce electricity has been proven extremely effective in Sweden. The ability of these renewable resources to produce clean output energy counters the adversities caused by non-renewable resources. The use of hydraulic turbines is a good example of favoured technique for energy and power production using renewable resources. The hydro-turbines are designed to operate at best efficiency point (BEP). Varying energy demands in recent years implies on the need of flexible operation of hydraulic turbines. The issue of pressure pulsations in the draft tube of hydro-turbines, observed at lower operating conditions has been unresolved for many years. These pressure pulsations are related to the ‘rotating vortex rope’ (RVR) observed at part load operation and, affects the lifespan and performance of the hydro-turbine adversely. Several techniques have been investigated in the past to reduce the pressure pulsations in the draft tube at part load operation and enhance the flexibility of the turbine. During the present research study, a passive flow control technique was investigated numerically by implementing a guide vane system in the draft tube of the Francis-99model turbine. Guide vanes are mechanical devices that can direct the flow in a desired direction. The current study presents the possibility of reducing the pressure pulsations in the draft tube by mitigating the RVR using a guide vane system in the draft tube. At the initial stages of the research study, a reduced numerical model of the Francis model turbine was developed by only considering the draft tube domain. The motive was to develop a reduced model to perform the parametric analysis for the guide vane system in the draft tube with reduced computational time, power, and storage. The results obtained from the numerical study were found to be in good agreement with theFrancis-99 semi-model with passage domains. A parametric study was performed to achieve a guide vane system design that could mitigate RVR with minimum losses. During this study, the number of guide vanes, the chord and the span of the guide vanes were investigated. It was found that a set of three guide vane system with chord of 86% of runner radius and leading-edge span of 30% of runner radius is an ideal design that mitigates RVR above 95%.

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