Investigation of a Francis turbine during speed variation: Inception of cavitation

Abstract

Variable-speed operation of a hydro turbine is considered as an alternative option to meet fluctuating energy demand as it allows high-ramping rate. Cavitation can be a limiting factor to utilize the variable-speed technology at full potential in a hydro power plant. This work investigates the cavitation characteristics and unsteady pressure fluctuations as turbine ramps up, to meet the energy demand. The investigated Francis turbine consists of 15 blades and 15 splitters, and the reference diameter is 0.349 m. Numerical model of complete turbine is prepared and hexahedral mesh is created. Rayleigh Plesset algorithm is activated for cavitation modelling. Available experimental data of model acceptance test are used to prescribe boundary conditions, and to validate the numerical results at distinct points. Transient behaviour of the cavitation is studied, and the results are quite interesting. At certain time instants, the cavitation effect is extremely predominant, and as a result of cavitation bubble bursts, the amplitudes of pressure fluctuations are significantly high.