CFD Analysis on Francis Turbine to Analyse Erosion Wear Due to Sediment Flow

: In present work Computational fluid dynamics analysis based erosion wear prediction is performed for Francis turbine components, especially the runner. For the geometrical parameters, Francis turbine model with steady state condition and viscous flow turbulence SST model using ANSYS Fluent. The erosion effect on all the three component such as spiral casing, runner & draft tube has been studied for different concentration of sand particles from 1% - 6%. For each of those concentration the effect of variation in size has been studied for different sizes 10 μm - 80 μm. Further the effect of total erosion was also analyzed for different particle size. Erosion damage is found close to the upper and lower portions of the leading edge of the stay vane. some erosion spots at guide vane on the blade pressure side where suction side has minimum erosion. Maximum erosion damage observed on runner especially at the middle of the blade. The draft tube situated closer to runner having highest velocity due to high absolute velocity of water coming out from the runner does not produce any serious erosion effect. Results shows that erosion rate is maximum on runner at particle size 80 μm for all sand concentration 1% to 6% and minimum at 30 μm. Thus, 30 μm is the optimum size of sand particles for the erosion.


INTRODUCTION
Sediment erosion is caused by the dynamic action of sediment, which flows with the water hitting a solid surface and creating surface wear on the hydraulic turbine components such as the housing, channel, and suction pipe. This wear not only reduces the turbine's efficiency and life but also causes operational and maintenance problems, which ultimately lead to economic losses. The high concentration of sediment combined with a high percentage of quartz in the water causes severe damage to the components of the hydraulic turbine. The intensity of the erosion depends on the type of sediment and its properties such as shape, size, hardness, concentration, etc. Hydraulic turbine design and operating conditions such as flow, height, velocity, velocity, acceleration, turbulence, angle of attack, etc. Minimizing sediment erosion problems requires a multidisciplinary approach. Further research and development is needed to examine the relationship between particle motion and erosion in a turbine and to determine the operational strategy for turbine operation. The erosion process is highly dependent on the particle size, shape, concentration, and operating conditions of the turbine. The reduction of erosion is associated not only with the reduction of particle velocity but also with the reduction of flow separation, which further depends on the shape, size, and concentration of the particle. A significant reduction in the erosion rate can be achieved by running the turbine at its best efficiency.
II. LITERATURE REVIEW R.D. Aponte et al. [1] Erosive wear is a major problem both economically and technically in the running water turbines of medium and small Francis turbines. The methodology uses computational fluid dynamics (CFD) and optimization techniques such as: which at the same time takes into account the erosive wear of hard particles, cavitation damage and efficiency. It was found that the new geometries of the turbine components analyzed allow a reduction in the wear rate of up to 73% and maintain an efficiency close to the original value over the entire operating range.
Gyanendra Tiwari et al. [2] The main goal of the work is to critically review various calculation methods to achieve various hydraulic design goals and performance evaluation of hydraulic turbines. To this end, various objectives of the computer studies of water turbines are discussed in detail such as the derivation of the performance characteristics, the analysis of various unstable phenomena, the prediction and analysis of cavitation and the determination of the various losses. Gyanendra Tiwari et al. [3] Analyzes the effects of the cavitation phenomenon in different operating states of a Francis turbine prototype with a capacity of 3 MW with the CFD code ANSYS CFX. Detailed flux range analyzes are performed for partial loads of 60% and 80%, full load, and overload of 120% with and without cavitation. A critical VOL.7, ISSUE 6, JUNE 2021 www.ijoscience.com 2 examination of the variation of the different flow parameters for cases with and without cavitation is one of the highlights of the work. S. Gautam et al. [4] this article presents a case study of a specific low-speed Francis turbine power plant in India that is severely affected by sediment erosion problems. A digital flow analysis is performed inside the turbine to investigate the causes of various erosion patterns in the turbine components.
The results of the CFD are compared with the actual erosion of the turbines.

III. OBJECTIVE
There are following objective are to be expected from the present work: 1. To study the theoretical concepts of wear/Erosion in Francis turbine at various components such as casing, runner and draft tube.

2.
To perform the fluid flow analysis on Francis turbine with erode particles at various parameters sand concentration, particles size and shape factor. 3. To identify critical zones of erosion and proposed optimizes operating conditions based upon particle size and sand concentration.

IV. METHODOLOGY
The mechanisms of erosive wear are not constant, but are controlled by the angle of impact of a particle, its speed, size and phase of the material of which the particles are made. The angle of impact is the angle between the eroded surface and the trajectory of the particle just before impact. A small angle of attack promotes wear-like wear processes, as particles tend to migrate to the worn surface after impact. A high angle of incidence causes wear mechanisms typical of erosion. Algorithm used for Computational fluid dynamics analysis

B. CAD Geometry of Spiral Casing:
The spiral casing shown below consists of an inlet section followed by gradually decreasing cross-sectional area which leads the flow to the stay vane inlet. 3D views of casing are shown in figure no. 2.        It has been observed that the sediments concentration is much higher during the monsoon season as compared with other season. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. Thus, 30 μm is the optimum size of sand for the erosion. Figure 5.3 shows that the penetration rate of 3.87 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. Thus, 30 μm is the optimum size of sand for the erosion. fig. 29 shows that the penetration rate of 5.27 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size.  It has been observed from above fig. 30 (a) that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value of 4.31 kg/m 2 s occurring at 30 μm particle size that indicates the optimum size of sand for the erosion. Maximum erosion rate of 9.926 kg/m 2 s have been observed at 80 μm particles size. The average erosion density rate ranging between 1.05E-01 kg/h-m 2 at 10 μm to 2.12E-01 kg/h-m 2 at 80 μm as shown in 30(b). Sand Erosion rate analysis at 30000 PPM sediments concentration:     It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. Thus, 30 μm is the optimum size of sand for the erosion. fig. 38 shows that the penetration rate of 5.27 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 3% sand concentration are 6.67, 5.97, 5.68, 6.28, 8.5, 9.64, 11.05 & 12.46 Kg/m 2 s.      fig. 47 shows that the penetration rate of 9.2 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 3% sand concentration are 9.20, 8.  Sand Erosion rate analysis at 50000 PPM sediments concentration:     fig. 56 shows that the penetration rate of 13.78 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size.    It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. Thus, 30 μm is the optimum size of sand for the erosion. figure 65 shows that the penetration rate of 15.57 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. . Further the effect of total erosion was also analyzed for different particle size. Based on the results of the study following conclusions were drawn.

J. Governing Equations
❖ After performing sand erosion rate analysis for the Francis turbine at sand concentration rate 1%. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 3.87 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 1% sand concentration are 3.87, 3.43, 2.94, 3.7, 4.73, 5.86, 6.63 & 7.39 Kg/m 2 s. The average erosion density rate ranging between 7.20E-02 kg/h-m 2 at 10 μm to 1.48E-01 kg/h-m 2 at 80 μm. ❖ After performing sand erosion rate analysis for the Francis turbine at sand concentration rate 2%. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 5.27 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 5.27 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 3% sand concentration are 6.67 , 5.97, 5.68, 6.28, 8.5, 9.64, 11.05 & 12.46 Kg/m 2 s. The average erosion density rate ranging between 1.25E-01 kg/h-m 2 at 10 μm to 2.50E-01 kg/h-m 2 at 80 μm. ❖ After performing sand erosion rate analysis for the francis turbine at sand concentration rate 4% It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 9.2 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 3% sand concentration are 9.20 , 8.5, 8.33, 8.88, 11.01, 12.09, 13.43 & 14.76 Kg/m 2 s. The average erosion density rate ranging between 1.60E-01 kg/h-m 2 at 10 μm to 3.17E-01 kg/h-m 2 at 80 μm. ❖ After performing sand erosion rate analysis for the francis turbine at sand concentration rate 5%. It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 13.78 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. The erosion rate observed due to 5% sand concentration are 13.78 , Kg/m 2 s. The average erosion density rate ranging between 1.89E-01 kg/h-m 2 at 10 μm to 3.72E-01 kg/h-m 2 at 80 μm. ❖ After performing sand erosion rate analysis for the francis turbine at sand concentration rate 6% It has been observed that as the particle diameter increases, the penetration rate first decreases then increases, with the minimum value occurring at 30 μm particle size. The penetration rate of 15.57 kg/m 2 s at 10 μm particle size is even higher than that of 20 μm particle size. at 80 μm. It has been observed from above conclusion that erosion rate is maximum at particle size 30 μm for all six concentration (1% to 6%). Thus, 30 μm is the optimum size of sand particles for the erosion. Erosion damage is found close to the upper and lower portions of the leading edge of the stay vane. some erosion spots at guide vane on the blade pressure side where suction side has minimum erosion. Maximum erosion damage observed on runner especially at the middle of the blade. This is due to the blade profile-tail vortex flow which leads to higher erosion rate density in the blade outlet. The draft tube situated closer to runner having highest velocity due to high absolute velocity of water coming out from the runner which causes sediment erosion and generally does not produce any serious erosion effect in draft tube.