A Review on Sediment Erosion Challenges in Hydraulic Turbines

Sediment constitutes several mineral compositions depending upon the geological formation and geography. In many of the rivers in Himalayas and Andes, Quartz is found as a main constituent (more than 50%), along with feldspar and other hard minerals. These particles have hardness more than 5 Moh’s scale, which is capable to erode turbine components. In hydraulic turbines, flow is highly turbulent and unsteady, which can aggravate the erosion problems. Depending upon the nature of the flow, different components of turbines are eroded with different mechanisms. This chapter will provide a review on how various flow phenomena is responsible for particular types of erosion in turbines and their potential consequences. Some examples of the effect in existing power plants will be shown. This chapter will also discuss about some preventive measures that have been proposed and implemented to reduce the impact of the sediment particles in hydraulic machineries

FSI analysis of Francis turbines exposed to sediment erosion

Sediment erosion is one of the key challenges in hydraulic turbines from a design and maintenanceperspective in Himalayas and Andes. Past research works have shown that the optimization of theFrancis turbine runner blade shapes can decrease erosion by a signicant amount. This study conductedas a Master's Thesis has taken the proposed designs from past works and conducted a CFDanalysis on a single passage of a Francis runner blade to choose an optimized design in terms of erosionand eciency. Structural analyses have been performed on the selected design through one-way andtwo-way FSI to compare the structural integrity of the designs.Two types of cases have been considered in this thesis work to dene the boundary condition of thestructural model. In the rst case, a runner blade is considered to have no in uence of the joint andother stier components. In the second case, a sector of the whole runner has been modeled withnecessary boundary conditions. Both one-way and two-way FSI have been performed on the casesfor the designs. Mesh independent studies have been performed for the designs, but only for the rstcase, whereas in the second case, a ne mesh has been used to make the analysis appropriate.The loads have been imported into the structural domain from the uid on the interfaces for one-wayFSI. In the case of two-way FSI, the Multi-Field Solver (MFX) supported by ANSYS has been usedto solve the coupled eld analysis. A fully coupled FSI in ANSYS works by writing an input le inthe structural solver containing the information about the interfaces in the structural domain, whichis imported in the uid solver. The interaction between the two domains is dened in ANSYS-CFX,including the mesh deformation and solver setups. The results have been post-processed in CFX-Post,where the results from both the elds are included. It has been found that the structural integrity ofthe optimized design is better than the reference design in terms of the maximum stress induced inthe runner. The two-way FSI analysis has been found as an inevitable part of the numerical analysis.However, with the advancement of the computational capability in the future, there could be a greatscope in the research eld to carry out a fully-coupled transient simulation for the whole runner toget a more accurate solution

Numerical investigation of the effet of leakage flow through erosion-induced clearance gaps of guide vanes on the performance of Francis turbines

Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine’s performance and compares different GV profiles to minimize such effects

Numerical investigation of the effect of leakage flow through erosion-induced clearance gaps of guide vanes on the performance of Francis turbines

Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine’s performance and compares different GV profiles to minimize such effects

The numerical and experimental investigation of erosion induced leakage flow through guide vanes of Francis turbine

In Guide Vanes (GV) of Francis turbines, a portion of the pressure head of water converts into velocity head. This causes high acceleration of the flow in GV before reaching the runner. Furthermore, GVs are accompanied with a small clearance gap at both ends to adjust the opening angle based on various operating conditions. In the case of sediment affected power plants, the hard fine particles mixed in water erode the connecting ends due to horse-shoe vortices. This erosion together with the head cover deflection due to water pressure increases the size of the gap. Due to the adjacent pressure and suction sides in GV, the flow passes through the gap from high pressure side to low pressure or suction side. This leakage flow disturbs the main flow in the suction side, which can be observed in the form of a vortex filament. Depending upon the GV profile and opening angle, the vortex can have different characteristics. This study uses numerical and experimental techniques to study the potential effects of the leakage flow in overall performances of the turbine. The experiment is done to measure the velocity field around GV using Particle Image Velocimetry (PIV) technique on a GV cascade rig. The GV in this rig corresponds to 1:1 scale model of 4.1 MW Francis turbine, with the chord length of 142 mm and span height of 97 mm. Similarly, 14 pressure senssors are placed around the GV cover plate to measure the GV loading. The velocity and pressure field are compared with with the results from CFD. In the study, two GV-profiles and 7 GV angels are studied. Results show that at Best Efficiency Point (BEP) and small opening or closing, the pressure difference between the adjacent sides of GV and consequently, the leakage flow and the intensity of the vortex filament in NACA4412 is less than in NACA0012. However, at high opening angle or during full load, the direction of the leakage flow in NACA4412 is in opposite direction due to small or negative GV loading compared to BEP. It is shown how these vortices affect the runner performances and how the particles erode the runner inlet as a consequence of these vortices

Selection of Optimal Number of Francis Runner Blades for a Sediment Laden Micro Hydropower Plant in Nepal

The present study is conducted to identify a better design and optimal number of Francis runner blades for sediment laden high head micro hydropower site, Tara Khola in the Baglung district of Nepal. The runner is designed with in-house code and Computational Fluid Dynamics (CFD) analysis is performed to evaluate the performance with three configurations; 11, 13 and 17 numbers of runner blades. The three sets of runners were also investigated for the sediment erosion tendency. The runner with 13 blades shows better performance at design as well as in variable discharge conditions. 96.2% efficiency is obtained from the runner with 13 blades at the design point, and the runners with 17 and 11 blades have 88.25% and 76.63% efficiencies respectively. Further, the runner with 13 blades has better manufacturability than the runner with 17 blades as it has long and highly curved blade with small gaps between the blades, but it comes with 65% more erosion tendency than in the runner with 17 blades

Leakage Vortex Progression through a Guide Vane’s Clearance Gap and the Resulting Pressure Fluctuation in a Francis Turbine

A clearance gap (CG) between guide vanes (GVs) and facing plates exists at both ends of a Francis turbine and allows the opening angle to be adjusted for varying operating conditions. Leakage flow is induced through this gap due to the pressure difference between the two sides of the guide vanes. While some research works have used qualitative approaches to visualize and predict the strength of a leakage vortex (LV), this paper presents a method for quantifying vortices along a trajectory. In this paper, a prototype high-head Francis runner with specific speed of 85.4 is considered as a reference case. A systematic investigation across both space and time is carried out, i.e., analysis of the spatial temporal progression of LV for three operating conditions. While travelling from the CG to runner leading edge, LV evolution and trajectory data are observed and the values of vorticity and turbulent kinetic energy are calculated for the LV trajectory. Frequency spectrum analyses of pressure oscillations in the vaneless space, runner blade, and draft tube are also performed to observe the peak pressure pulsation and its harmonics. Unsteady fluctuations of the runner output torque are finally studied to identify the patterns and magnitudes of torque oscillations