Abstract
When operating in off-design conditions, Francis Hydraulic Turbines features flow phenomena related with low-frequency pressure pulsations. Those pulsations mechanisms can lead the system to resonance or instabilities. In this situation, high-pressure pulsations can cause structural or mechanical components failure, wear by cavitation, besides reducing energy generation quality. Inserting air into the draft tube is one of the most common measure to mitigate pressure pulsations. In this panorama, the present work aim to apply a one-dimensional distributed parameter mathematical model of the hydraulic circuit of a Francis turbine in order to predict full load instability. The Vortex hydroacoustic parameters are computed by CFD simulations. In order to verify the model, and evaluate aeration effectiveness, a design of a mechanical device that can control the axial atmospheric aeration valve to enable tests in a turbine prototype is developed. Further, field tests shows that a discharge of air of 1.3% of the rated discharge can reduce pressure pulsations by 90% in a specific full load operating point. Also, in this case study applied to ITAIPU turbines, the mathematical modelling result has successfully predict full load instability observed in field test under variation of aeration level. Keywords: Francis turbine, Aeration, Hydraulic Instability, CFD.
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