Abstract
Kaplan turbines operating at full-load conditions may undergo excessive vibration, noise and cavitation. In such cases, damage by erosion associated to tip vortex cavitation can be observed at the discharge ring. This phenomenon involves design features such as (1) overhang of guide vanes; (2) blade profile; (3) gap increasing size with blade opening; (4) suction head; (5) operation point; and (6) discharge ring stiffness, among others. Tip vortex cavitation may cause erosion at the discharge ring and draft tube inlet following a wavy pattern, in which the number of vanes can be clearly identified.Injection of pressurized air above the runner blade centerline was tested as a mean to mitigate discharge ring cavitation damage on a scale model. Air entrance was observed by means of a high-speed camera in order to track the air trajectory toward its mergence with the tip vortex cavitation core. Post-processing of acceleration signals shows that the level of vibration and the RSI frequency amplitude decrease proportionally with air flow rate injected. These findings reveal the potential mitigating effect of air injection in preventing cavitation damage and will be useful in further tests to be performed on prototype, aiming at determining the optimum air flow rate, size and distribution of the injectors.
Highlights
Depending on operating conditions and main design features, cavitation in Kaplan turbines may develop at different locations, mostly on both sides and tip of the blades, hub and discharge ring, following different patterns of structure, extension and dynamic behavior [1]
4.1 Efficiency of air injection The results presented in this work show the effectiveness of the air injection in the attenuation of the level of vibration due the tip vortex cavitation (Table 2)
A series of cavitation tests were carried out on a scale model Kaplan turbine and air injection above the runner centerline was implemented with the aim of analyzing its influence on the mitigation of cavitation damage and levels of vibration
Summary
Depending on operating conditions and main design features, cavitation in Kaplan turbines may develop at different locations, mostly on both sides and tip of the blades, hub and discharge ring, following different patterns of structure, extension and dynamic behavior [1]. In connection with the development of tip vortex cavitation, Kaplan turbines may experience, when operating at full-load condition, excessive cavitation along with severe vibration and noise emission Under such conditions, the outflow of the guide vanes and its pressure field are not uniform along the circumference. The outflow of the guide vanes and its pressure field are not uniform along the circumference This may interfere with the non-stationary pressure field derived by the passage of the runner blades, giving rise to the so-called rotor stator interaction (RSI) [3]. This phenomenon, often seen in Francis and pump-turbines, might appear in Kaplan turbines when guide vane overhang is likely to occur, as discussed in previous works [4, 5].
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