FVPM numerical simulation of the effect of particle shape and elasticity on impact erosion

The erosion of hydraulic machines by solid particle impacts is a widespread problem that leads to outage for expensive repairs, efficiency reduction, and cavitation enhancement. Numerical simulations can be used to study the phenomenon, provided they feature accurate thermomechanical and contact modeling. Numerical investigations often assume that the impacting particles are spherical and rigid, with only some recent studies modeling them as elastic polyhedrons or spheres. However, in the specific case of the erosion of hydraulic machines, particles are far from spherical or polyhedral and are less rigid than the base material. The present investigation focuses on the effect of the particle shape and elasticity on the erosion of oxygen-free copper and martensitic stainless steel 13Cr-4Ni impacted by quartz sediments. First, a novel algorithm to generate realistically-shaped sediment discretizations is presented, bypassing the need to use simplified shapes such as polyhedrons. The algorithm is shown to produce particle discretizations of predefined characteristic size that closely follow the objective sphericity value selected, which can cover the full range of sphericity values found in real sediments. Then, the effect of the particle elasticity on the impact damage is investigated, revealing that an error of up to 38 % is introduced by assuming that the particles are rigid. The effect of the particle shape is then assessed. For the case of copper, sharp sediments generate an increase in damage per unit mass of up to 225 % with respect to spherical particles; a comparable effect is expected on the erosion rate. For the martensitic stainless steel, the shape effect is similar in character but significantly weaker in magnitude. The results are analyzed and explained in terms of the known erosion mechanisms and their dependence on the particle shape, the material ductility and hardness.

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

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

Cavitation erosion power as a scaling factor for cavitation erosion of hydraulic machines

François Avellan, Mohamed Farhat

The Cavitation Erosion Power based on the assumption that severe erosion is often due to the repeated collapse of the traveling vortex generated by a leading edge cavity has been formerly introduced. We have shown that cavitation erosion intensity may be scaled by a simple set of flow parameters depending on the mean upstream velocity U, the Strouhal number St corresponding to the main cavity fluctuations, the cavity length I and the pressure in the cavity closure. Further tests have been done to validate this model. In this work we present the results of synchronous pressure fluctuations measurements downstream a leading edge cavity, cavitation induced vibrations and main cavity dimensions. These results show that the pressure fluctuations may be used to qualify the aggressiveness of leading edge cavitation and that the concept of the Cavitation Erosion Power is a good basis to scale cavitation damage. Furthermore, the envelope of the vibrations signals is found to be coherent with transient pressure signals.

1993