Multiscale Simulation of the Hydroabrasive Erosion of a Pelton Bucket: Bridging Scales to Improve the Accuracy

Erosive wear of hydraulic machines is a common issue, which results in efficiency degradation, the enhancement of cavitation, and the need for expensive maintenance. Although numerical simulations of the erosion process could be very useful, both for understanding and predicting the process, its multiscale nature renders it very difficult to simulate. A previously validated multiscale model of erosion is presented. It consists of two coupled sub-models: On the microscale, the sediment impacts are simulated by means of comprehensive physical models; on the macroscale, the turbulent sediment transport and erosion accumulation are calculated. A multiscale simulation of the erosion of a prototype-scale Pelton bucket impacted by a sediment-laden water jet is presented. The simulation results, namely the erodent flux and the distributions of average impact angle and velocity on the bucket surface, bring insight into the erosion process. Furthermore, the results explain the obtained erosion distribution, which is in very good agreement with the experimental erosion measurements available in the literature for the same test case.

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Siamak Alimirzazadeh, François Avellan, Ebrahim Jahanbakhsh, Sebastián Camilo Leguizamón Sarmiento, Audrey Paulette Solange Maertens
2019
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https://infoscience.epfl.ch/record/265793?ln=fr

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Multiscale simulation of erosive wear in a prototype-scale Pelton runner

Siamak Alimirzazadeh, François Avellan, Ebrahim Jahanbakhsh, Sebastián Camilo Leguizamón Sarmiento

The technical capacity to predict the erosion process is instrumental for the optimization of the runner designs and operation strategies of hydroelectric plants. A multiscale model of erosion recently formulated by the authors and validated on a laboratory-scale jet impingement case is used to study the erosion of a prototype-scale Pelton runner. The model is shown to provide physically-sound descriptions of the sediment impact condition distributions on the bucket surface; furthermore, the erosion distribution obtained is explained in terms of these underlying impact condition distributions. The model predictions for the erosion depth distribution on the bucket surface are validated with the corresponding experimental data, resulting in an average error of 35% for eight point-wise comparisons, 14% for line-averaged values along four transversal sections, and 4% for the surface-averaged erosion depth, directly linked to the total eroded mass. A careful error propagation analysis of the comparison between simulation and field data yields an uncertainty of ±22%. Based on these considerations, the modeling error is estimated to be 26±22%. The results obtained, namely quantitative predictions of the erosion of industrial-scale hydraulic machines, have no precedent in the literature; they demonstrate the accuracy and transferability of the multiscale model of erosion and suggest an improvement over the state-of-the-art computational fluid dynamics models based on empirical erosion correlations.

2020

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Dynamics of vortex cavitation involved in the erosion of hydraulic machines

François Avellan

In order to investigate the dynamics of the vortex cavitation, which is found to be responsible of the cavitation erosion in hydraulic machinery, a cavitation vortex generator (CVG) is used. By producing the cyclic growth and collapse of cavitation vortex, this device provides a good simulation of the realistic hydrodynamics situation corresponding to the travelling cavities downstream a fixed leading-edge cavity. The simultaneous use of the shock and the pressure time-history recordings and the high speed Cranz-Schardin visualizations allows us to describe the instantaneous cavity motion in the CVG test section . The most important result is the evidence of a strong spherica1 shock waves emission at the time of the cavitation vortex collapse, leading to overpressure of more than 10 kbar.

1987

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Shock pressure generated by cavitation vortex collapse

François Avellan, Mohamed Farhat

Most of the severe erosion of hydraulic machines are found to be associated with the collapse of transient cavitation vortices downstream of a leading edge cavity. The dynamics of such a type of cavitation is investigated in a Cavitation Vortex Generator (CVG). By producing the cyclic growth and collapse of single cavitation vortex, this device provides a way to investigate the mechanisms involved in the final stage of the cavity collapse. Very high-speed visualization, at a rate of 500 '000 frames per second, reveals a strong spherical shockwave emitted at the time of the cavitation vortex collapse. Thus, the image processing of the high-speed photography allows us to measure the cavity wall velocity and the shock wave celerity in order to estimate the shock pressure. These estimations lead to mean values of 900 MPa and extreme values as high as 2'200 MPa. Moreover, a new optical arrangement can make the shock waves visible both in the fluid and in a Perspex specimen fitted in the test section.

The ASME1989

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