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

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.

Air Injection as a Solution to the Problem for Reducing High-Frequency Acoustic Noise in Some Operating Modes of Francis Hydro Turbines

“…The natural frequencies of the same elements in water will be 60-70 % less than in air [1], [2]. In table 2 natural frequencies closed to main sound frequency (300-370 Hz) are shown.…”

Fatigue Strength Analysis of a Prototype Francis Turbine in a Multilevel Lifetime Assessment Procedure Part II: Method Application and Numerical Investigation

“…It is therefore essential that numerical results are properly validated (see, e.g., [23]). Including two-phase flows into the considerations results in complex investigations, as shown in [24], where added mass effects were combined with blade cavitation.…”

Effect of Boundary Conditions on Fluid–Structure Coupled Modal Analysis of Runners

“…Compared with the twoway fluid-structure coupling method, the acoustic-structure coupling method simplifies the Navier-Stokes equation to a certain extent, and is highly efficient; thus, it is widely used. Huang et al studied the effect of cavitation on the modes of a Francis turbine runner using the acoustic-structure coupling method [11] and calculated the static and dynamic stresses of the runner during operation [12]. Rodriguez et al [13] measured the modes of an underwater circular plate using the acoustic-structure coupling method and compared it with the experimental data, which further proved that this method can accurately predict the natural frequencies of underwater structures considering the influence of rigid walls.…”

“…However, in the research of the dynamic characteristics of the runners, the setting of the constraints is not uniform. Without considering the rotating shaft, line constraint (i.e., fixed constraint on bolt center line) [11,12], surface constraint (i.e., fixed constraint on connecting surface) [15] and rope suspension [2,5,6] are often adopted. Among them, the rope suspension is usually used in the experimental modal analysis to eliminate the influence of constraints or other structures on the dynamic characteristics.…”