• Energy Research

Abstract Slurry erosion and cavitation erosion tribometers are used to evaluate wear on materials of hydromachinery components, in this work numerical simulations based on computational fluid dynamics (CFD) were implemented with the aim to analyze the influence of parameters such as shape and size of cavitation inducers, rotational speed, particle concentration and cavitation inducer-specimen separation over the mass loss rate. Operating parameters were determined, which are suitable to reproduce typical cavitation erosion and slurry erosion conditions presented in turbomachinery devices like Francis turbine runners under off-design conditions or exposed to a high amount of sediments. CFD analysis provides a numerical insight about the flow field and its behavior due to the influence of shape and location of the cavitation inducers aiming to predict the zone where the cavitation process takes place. Also, it was found that CFD simulation coupled with particle tracking methods provide an understanding about the interaction between the hard particles and the surfaces tested, and its influence over erosion rates.

Abstract Erosive wear has been a serious concern in mainly run-of-the-river medium and small Francis turbines from both economic and technical perspectives. With the aim of finding ways to mitigate erosive wear, this paper proposes a methodology to obtain, via an optimization approach, geometries that maximize the resistance to erosive wear by hard particles and cavitation of the internal components (runner, guide vanes and cover labyrinths) of a Francis turbine. This improvement was implemented to reduce the costs of corrective maintenance and to maximize the machines’ availability and energy generation profits. The methodology used computational fluid dynamics (CFD) and optimization techniques, such as the design of experiments of the factorial type, artificial neural networks and genetic algorithms with a multi-point approach, which includes two operation points, and a multi-objective approach, which simultaneously considers erosive wear by hard particles, cavitation damage and efficiency. It was found that the new geometries of the analysed components of the turbine can allow a decrease of up to 73% in the wear rate, maintaining an efficiency close to the original value throughout the operating range. With the optimized geometry, a mechanical check was performed using finite element simulations to validate that the optimal geometries had the required strength.

Abstract Small-scale hydroelectric plants, primarily run-of-the-river designs, are regularly subjected to hard particle wear and cavitation erosion due to the wide range of operating points. Depending on the severity of the operating conditions and erosion damage experienced by the machine throughout its service life, the operating companies of these facilities will be impacted. The impact will be technical, operational, logistical, and economic. A small-scale generation plant located in Amaime River in Colombia, is one such case, where severe wear occurs in the turbine components, with a consequent reduction of efficiency. In this study, the analysis of the erosion damage has been expanded and supplemented by computational fluid dynamics (CFD). From this approach, correlations between the wear rate and power output were obtained. Likewise and in conjunction with the computer estimates, a methodology to analyse the costs associated with wear based on historical data of operation was developed, creating a strategy of operation based on a stopping criterion that depends primarily on sediment concentration, turbinated flow, and wear level. The methodology optimizes the use of generators, which takes into consideration the revenue generation and the costs associated with operation and maintenance of pieces under conditions of intrinsic erosion wear in the facility.

Abstract This paper presents the analysis of a 16-bucket Pelton impeller from a hydroelectric plant in Colombia, a plant with two turbo generators with a nominal capacity of 2.33 MW each. Multiple cracks were detected one year subsequent to geometry reconstruction welding repair. Metallographic analyses were performed on the impeller zones near the cracks, including an analysis of the fracture surface, a computational simulation of the fluid dynamics using finite volume software that permitted the establishment of the impeller’s load state and a simulation via finite element analysis to determine the state of the impeller’s nominal stress. It was concluded that the material presented multiple defects such as non-metallic inclusions and microcracks, and trapped slag was found in the filler material. The computational fluid dynamics analysis permitted the identification of low-pressure zones caused by an inadequate geometry for the bucket profile where cavitation is present. The finite element analysis permitted the identification of critical points on the bucket zone neck, coinciding with the crack found. This zone is under tensile stress due to the effects of centrifugal force and compression when the bucket comes in contact with the jet, causing fatigue.

Abstract One of the challenges posed by hydraulic energy generation stems from the exploitation of hydrological resources that carry significant amounts of sediment that erodes the surfaces of turbines. This is the case for the Amaime hydroelectric plant, which is located in the western mountainous region of Colombia and was seriously affected by sediment after a brief period of operation. The main symptom indicating failure was a rise in the temperature of the bearings caused by an increase of almost two bars in the pressure between the cover on the side of the generator and the runner, which was caused by the wearing of the seal labyrinths. Inspections that were carried out after six months of operation indicated that there was a 300% increase in the clearance between the covers and the runner, which caused a higher axial thrust on the bearing. The inspections verified that severe wear had occurred on important elements of the turbine, such as the runner, guide vanes and turbine covers, which required major repairs to the two generation groups of the plant in less than 2 years, which is a much shorter time between repairs than is recommended by international standards. Analyses of the material, medium, particles and the worn surfaces demonstrated that the wear on the turbine was mainly due to erosion by hard particles, which was caused by the high sediment concentration and the low hardness of the material used to construct the turbine.

Abstract Computational fluid dynamics (CFD) is a useful tool to predict the erosion behaviour over geometries exposed to conditions of severe erosion wear. This work shows how CFD can be used to virtually characterize the wear behaviour of materials used in several hydraulic components, including turbomachinery systems, in which it is important to consider the effect of particle size. In addition, this work develops a methodology for determination and validation of the constants involved in the well-known Tabakoff-Grant model for erosion prediction using the erosion wear obtained via erosion testing in a jet tribometer reported in the literature as a reference. From the experimental data, an optimization algorithm was performed to determine the optimal values of Tabakoff-Grant model constants for ASTM A743 grade CA6NM martensitic stainless steel. The simulated erosion rate agrees with the experimental data for the material analysed in jet erosion simulations with impact angles ranging from 15° to 90°. The change of the angle of maximum erosion rate for small particles, which has been reported in the literature via experimentation, was explained satisfactorily. The results showed that the erosion rate with smaller particles is affected by fluid flow, since small particles tend to follow the flow streamlines, while larger particles move according to the conditions imposed by the jet at its outlet. The effective impingement angle against the surface for small particles is lower than the impingement angle for large particles; therefore, the angle of maximum erosion rate, measured between the jet and the sample's surface, for small particles increases.