Cavitation Effects on the Structural Resonance of Hydraulic Turbines: Failure Analysis in a Real Francis Turbine Runner

Abstract

When discussing potential resonances in hydraulic turbine runners, cavitation effects are usually neglected. Nevertheless, recent studies have experimentally proved, that large cavitation volumes in the proximity of flexible simple structures, such as hydrofoils, greatly modify their natural frequencies.In this paper, we analyze a resonance case in a Francis runner that leads to multiple fractures on the trailing edge of the blades, after just one day of operation at deep part load. If simple acoustic Fluid-Structure-Interaction (FSI) simulations are used, where the runner's surrounding fluid is considered as a homogenous acoustic medium (water), the risk of structural resonances seems to be limited as the predicted natural frequencies are far enough from the excited frequencies by the flow. It is shown that the only hydraulic phenomenon which could have produced such fractures in the present case is the Rotor Stator Interaction (RSI). In order to analyze possible cavitation effects on the natural frequencies of the turbine runner, CFD simulations of the deep part load conditions have been performed, which predict large inter-blade vortex cavities. These cavities have been then introduced in the acoustical FSI model showing that under such conditions, natural frequencies of the runner increase approaching to some of the RSI excited frequencies. In particular, a possible resonance of the four-nodal diameter (4ND) mode has been found which would explain the fast behavior of the crack propagation. Furthermore, the shape and the position of the real fracture found agree with the local maximum stress spots at the junction between the trailing edges and the crown.Energies 2018, 11, 2320 2 of 16 almost one century ago. More recently, the researches of Amabili et al. [4,5] and Kwak [6] revise and improve the analytical work performed by Lamb. Furthermore, many studies performed in the last decade show that acoustical FSI simulations are perfectly capable to predict natural frequencies of submerged structures [7][8][9]. In all the aforementioned studies simple thin plates have been considered.In the particular case of reaction hydraulic Turbines, which are completely submerged, added mass effects play an important role and have to be considered for the calculation of the natural frequencies as well. In this case the added mass phenomenon is even more complicated, as the gaps turbine-casing (usually considered as rigid boundaries) are extremely small (less than 1% of the turbine diameter), the turbine is rotating, and the water couples the turbine with the casing. Some effects such as rotation [10], influence of non-rigid surfaces [11] and acoustic modes [12] have been recently discussed using numerical FSI simulations and contrasting experimental results. Nevertheless, most of these effects have not been yet determined in real turbines. A review of actual research works shows that for turbine runners in still water, natural frequencies and mode shapes obtained with finite element simulations have a good agreement with exp...

Added Mass Effects on a Francis Turbine Runner with Attached Blade Cavitation

“…The study compared the results of a single-phase fluid, water, with those of a two-phase fluid, vapor and water, by reproducing the typical leading edge or trailing edge attached In summary, the linear regressions permitting calculation of the FRRs of the runner when submerged in pure water, in water with the LECL or in water with the TECL, are indicated in Equations (18)- (20), respectively, as a function of the mode order. δ w = 6.36(X ND + 1) + 4.94 (18) δ LECL = 6.78(X ND + 1) + 0.45 (19) δ TECL = 6.65(X ND + 1) − 0.13 (20)…”

“…Nevertheless, the fitted line for the case without cavities has a smaller slope but larger intercept than the line estimated for the case with cavities. In summary, the linear regressions permitting calculation of the FRRs of the runner when submerged in pure water, in water with the LECL or in water with the TECL, are indicated in equations (17)- (19), respectively, as a function of the mode order.…”

“…However, the possible effect of cavitation inside hydraulic turbines has not been sufficiently addressed, and only one recent work has numerically investigated the possibility that inter-blade cavitation vortices are responsible for multiple fractures that have appeared on the trailing edge of full-scale Francis runner blades [19]. For example, the added mass effects of the presence of attached cavitation on the runner blades have not been calculated yet.…”

Characterization of the Effects of Ingested Bodies on the Rotor–Stator Interaction of Hydraulic Turbines

“…It is also possible that they remain stuck betw Damage mechanisms in hydraulic turbines have been reviewed in recent years [2]. In reaction turbines (such as Francis turbines or pump turbines), damage is typically a result of problems such as material fatigue [3][4][5][6], cavitation [7] or sediment erosion [8], which may deteriorate both the runner and distributor, taking its toll on the remaining useful life of the machine.…”

An Indirect Measurement Methodology to Identify Load Fluctuations on Axial Turbine Runner Blades

“…They can experience large pressure fluctuation, fatigue, erosion, and cavitation, particularly close to the trailing edge [ 4 , 5 , 6 , 7 ]. This can change the natural frequency of the runner blade and damage the blade [ 8 ]. In addition, frequent load variations may induce fatigue issues in the Kaplan turbine’s shaft due to runner blade movement [ 9 ].…”