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Prediction of Erosion Damages in Hydraulic MAchines

Final Report Summary - PREDHYMA (Prediction of Erosion Damages in Hydraulic MAchines)

Hydraulic turbines can undergo severe damaging during operation, because of low quality water or detrimental flow conditions. Damaging induces maintenance costs and power production losses, and can also endanger safety of installations. Hydropower plants operators and turbine manufacturers are interested in extending overhaul periods by reducing damaging intensity and protecting turbine components with surface treatments. Accurate and reliable prediction of damaging is however missing. The PREDHYMA project aims at developing predictive tools addressing damaging mechanisms and helping engineers to better manage installations’ lifecycle.
Four main damaging mechanisms are encountered in hydraulic machines: hydro abrasive erosion, impacts of gravels and stones, cavitation and impacts of water droplets. They result from complex interactions between fluids and solids. The PREDHYMA project aims at predicting these four mechanisms by means of numerical simulations. Current approaches of damaging model micro-scale interactions between fluid and solid. They are not generalist and require calibration often valid on specific configurations only. Besides they do not really account for the shape evolution of turbine components as damaging develops. The PREDHYMA project aims at overcoming these limitations by introducing a multi-scale approach in space and time. Micro scale simulations will compute damaging rate directly from very detailed and local simulations and will feed macro-scale simulations encompassing full turbine components along their operation lifetime.
The cornerstone of the approach is an accurate coupling between a solid solver (based on FEM) and a fluid solver (based on SPH-ALE) for fast dynamics. The coupling algorithm preserves energy at the interface. It has been improved to better manage “incompatible time steps”, i.e. situations in which one domain requires much smaller time steps than the other domain. The coupled simulation framework was successfully applied to study the impact of a stone on a Pelton bucket, highlighting the different characteristics between the so-called dry and wet impacts. The final damage results from a complex combination of a water cushioning effect that tends to decrease the impact velocity of the projectile and an entrainment of the projectile by the water flow that reduces the bouncing off and prolongs the contact between the projectile and the body. Accounting for the presence of water around the stone is thus of primary importance in the case of a Pelton bucket, validating the approach followed in the project. As expected, the two main other influencing parameters are the impact velocity and the size of the projectile.
Initial investigations on cavitation erosion aimed at predicting the appearance of vapor and research focused on the development of a phase change model in SPH-ALE, based on the existing multiphase model. In order to preserve the good aspects of separated phases, the phase change model was based on a sub-particle scale. Results could be obtained for one-dimensional cases but difficulties appeared for the extension to 2D and 3D cases. The project then focused on the collapse of a gas bubble surrounded with liquid. The implementation of a Stiffened Gas equation of state in the SPH numerical method for the fluid was achieved, together with the addition of an equation for the conservation of energy. These modifications were necessary to properly manage the strong compression of the gas phase. The numerical solution of the bubble collapse in free field has been compared to the analytical model of Rayleigh-Plesset. The collapse of the bubble close to a solid wall was then simulated with the coupled Fluid-Structure approach, allowing the simultaneous prediction of the mechanical stresses in the material. The fluid simulation was able to capture the main characteristics of the collapse close to a wall, namely the formation of a re-entrant jet crossing the bubble and oriented toward the wall, followed by the collapse of the gas cavity. Both phenomena were found to be responsible for the generation of intense pressure waves, with the distance to the wall acting as a major influencing parameter. Mechanical stresses in the base material were found to be alternatively in traction and compression. Accordingly the damaging mechanism is expected to be related to fatigue due to repeated cavitation loadings.
The study of hydro-abrasive erosion relied on the implementation of an equation of motion for sediment particles in SPH-ALE, enriched with an ad-hoc model for the generation of turbulent fluctuations. These fluctuations were found to have the greatest influence on the impact velocity and position for most of the sediment particles’ sizes. The coupled fluid-structure simulation framework was used to determine the damaging from isolated sediment impacts. These results were then combined with the typical statistical composition of the sediment content in the water passing through a turbine. Despite the biggest particles being the most aggressive considering isolated impacts, the study revealed that taking into account the number of sediments in each class of size and the number of impacts, sediment particles of medium size (dp=50) are the most detrimental for the Pelton buckets. This is an important finding from the project, and a major illustration of the necessity of combining the micro-scale and the macro-scale approaches.
The study of erosion from droplet impacts made an extensive use of the coupled fluid-structure simulation framework. Two damaging criteria were considered and implemented in order to compute the “residual life” of numerical elements because of material fatigue and thus predict the material removal with time. A complete algorithm was finally put in place and tested in order to fictively repeat sequences of similar isolated impacts leading to a deformation of the shape of the body. This work is key for multi-scale approach in time, allowing the link between micro-scale (isolated impact) and macro-scale (complete life time) simulations. The influence of a superficial layer of coating material was also investigated.
The combination of detailed and direct simulations of isolated impacts or events with a macro-scale view, based on a coupled fluid-structure numerical approach, has proven to be the ideal choice throughout the PREDHYMA project. Great progress in the understanding and prediction of the four identified damaging mechanisms could be achieved during the project, also identifying limitations and needs for further improvements.

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