Abstract
To have a safe structural design, an analysis of the dynamic behavior of a Francis turbine runner with consideration of the added mass effects of surrounding water is necessary during design phase. Both in design and at off-design operations, large-scale forms of attached cavitation may appear on runner blades and can change the added mass effects of the surrounding fluid in relation to a single water domain. Consequently, a numerical investigation of the modal response of a Francis runner has been carried out by reproducing the presence of various sizes of leading edge cavitation (LEC) and trailing edge cavitation (TEC). The fluid–structure interaction problem has been solved by means of an acoustic-structural coupling method. The calculated added mass effects with cavitation have been compared with those corresponding to the pure water condition without cavitation. Firstly, a single blade has been investigated to evaluate the level of significance for the proposed cavity shapes and dimensions. Afterwards, based on the results obtained, the complete runner structure has been considered, factoring in similar cavity shapes and locations. The results prove that significant added mass effects are induced on the entire runner by the attached cavitation that increase the natural frequencies of the first modes. Moreover, the added mass effects increase with cavity size and amplitude of blade deformation below the cavity.
Highlights
Hydropower, a clean and renewable energy resource, has been developed worldwide for the economic growth and improvement of people’s living standards
15, the large size for both types cavitation in the single blade, natural frequencies of the runner submerged in pure water and with cavitation are compared in TECL, have been modeled on all the blades of the investigated Francis runner in separate cases
The acoustic–structural coupling method was applied to calculate the natural frequencies and mode shapes of the structure surrounded by a fluid
Summary
Hydropower, a clean and renewable energy resource, has been developed worldwide for the economic growth and improvement of people’s living standards. At a given water head, the power output of Francis turbines can be controlled by adjusting the flow discharge with the wicket gate opening This changes the flow pattern dramatically within the runner blade channels. Extreme hydrodynamic conditions, provoking strong pressure reductions, are found by the irregular flow inside the runner channels when the turbine operates both at low part loads and over loads, below and above the best efficiency point, respectively. Both at off-design and at design operation conditions, two-phase flows with large scale cavitation forms may occur inside the Francis runners [1].
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