Sediment and Cavitation Erosion in Francis Turbines—Review of Latest

energies

Review Review SedimentSediment andand CavitationCavitation ErosionErosion inin FrancisFrancis Turbines—ReviewTurbines—Review ofof LatestLatest ExperimentalExperimental andand NumericalNumerical TechniquesTechniques

AdnanAdnan AslamAslam NoonNoon 11 andand Man-HoeMan-Hoe KimKim 2,2,*

11 DepartmentDepartment of of Mechanical Mechanical Engineering, Engineering, FET. FET. International International Islamic Islamic University, University, Islamabad 44000, Pakistan; [email protected]@iiu.edu.pk 22 SchoolSchool of of Mechanical Mechanical Engineering Engineering & & IEDT, IEDT, Ky Kyungpookungpook National National University, University, Daegu 41566, Korea ** Correspondence:Correspondence: [email protected]; [email protected]; Tel.: Tel.: +82-53-950-5576 +82-53-950-5576

Abstract:Abstract: SedimentSediment andand cavitationcavitation erosionerosion ofof thethe hydroelectrichydroelectric powerpower turbineturbine componentscomponents areare thethe fundamentalfundamental problemsproblems inin thethe riversrivers ofof HimalayasHimalayas andand Andes.Andes. InIn the present work, the latest research conducted in bothboth thethe fieldsfields byby variousvarious investigatorsinvestigators andand researchersresearchers areare discusseddiscussed andand criticallycritically analyzed at differentdifferent turbineturbine components.components. Analysis shows that both typestypes ofof erosionerosion dependsdepends onon flowflow characteristics,characteristics, surface,surface, andand erodenterodent materialmaterial properties.properties. DesignDesign optimizationoptimization tools,tools, coalescedcoalesced effect (CE)(CE) ofof sedimentsediment andand cavitationcavitation erosionerosion andand wellwell conductedconducted experimentsexperiments willwill yieldyield resultsresults thatthat areare beneficialbeneficial forfor erosionerosion identificationidentification andand reduction.reduction. AlthoughAlthough somesome researchersresearchers havehave donedone experimental work on thethe coalescedcoalesced effecteffect (CE)(CE) ofof sedimentsediment andand cavitationcavitation erosion,erosion, veryvery limitedlimited Computational Fluid Dynamics (CFD) work isis availableavailable inin literature.literature. The presentpresent researchresearch workwork willwill be beneficial beneficial for for practitioners practitioners and and researchers researchers in inthe the future future to address to address the theerosion erosion problem problem suc- successfully.cessfully.

Citation: Noon, A.A.; Kim, M.-H. Citation: Noon, A.A.; Kim, M.-H. Keywords:Keywords: hydroelectric powerpower turbine;turbine; sedimentsediment erosion;erosion; cavitationcavitation erosion;erosion; CFD;CFD; coalescedcoalesced effecteffect Sediment and Cavitation Erosion in Sediment and Cavitation Erosion in Francis Turbines—Review of Latest Francis Turbines—Review of Latest Experimental and Numerical Experimental and Numerical Techniques. Energies 2021, 14, x. Techniques. Energies 2021, 14, 1516. 1.1. IntroductionIntroduction https://doi.org/10.3390/xxxxx https://doi.org/10.3390/en14061516 HydroelectricHydroelectric power generation generation is is one one of of the the most most important important kinds kinds of renewable of renewable en- Academic Editors: John Anagnos- energyergy which which contributes contributes 70% 70% of the of total the totalenergy energy production, production, but at the but same at the time same it requires time it Academic Editors: topoulos and John M. Cimbala requiressubstantial substantial initial investment, initial investment, rigorous rigorous management, management, and maintenance. and maintenance. Today’s Today’s inven- John Anagnostopoulos and John M. Cimbala inventorytory of non-sustainable of non-sustainable reservoirs reservoirs need need to be to converted be converted to sustainable to sustainable infrastructures infrastructures for Received: 14 January 2021 forfuture future generations. generations. Sedimentation Sedimentation is a global is a global problem problem which which is found is found to exist to in exist the inrivers the Accepted: 4 March 2021 Received: 14 January 2021 riversof Himalayan of Himalayan region, region, China in China Asia, in and Asia, the andAndes the in Andes South inAmerica South America[1]. It is estimated [1]. It is Published: 10 March 2021 9 9 Accepted: 4 March 2021 estimatedthat on average, that on 20 average, × 10 tons 20 ×of10 sedimenttons of reach sediment oceans reach in oceansa year. inEvery a year. year, Every reservoir year, Published: 10 March 2021 reservoirsedimentation sedimentation causes an causesestimated an estimated1% reduction 1% reduction[2] in the total [2] in capacity the total of capacity the reservoirs. of the Publisher’s Note: MDPI stays neu- reservoirs.Figure 1 shows Figure the1 showsaerial view the aerial of a large view dam of a largelocated dam in locatedItaipu, Brazil. in Itaipu, Brazil. tral with regard to jurisdictional Publisher’s Note: MDPI stays neutral claims in published maps and institu- with regard to jurisdictional claims in tional affiliations. published maps and institutional afﬁl- iations.

Copyright: © 2021 by the authors. Submitted for possible open access Copyright:publication under© 2021 the by terms the and authors. con- Licenseeditions of MDPI,the Creative Basel, Commons Switzerland. At- Thistribution article (CC is BY) an openlicense access (http://crea- article distributedtivecommons.org/licenses/by/4.0/). under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Figure 1. Itaipu Dam, Brazil.

Energies 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/energies Energies 2021, 14, 1516. https://doi.org/10.3390/en14061516 https://www.mdpi.com/journal/energies Energies 2021,, 14,, xx FORFOR PEERPEER REVIEWREVIEW 2 of 21

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Figure 1. Itaipu Dam, Brazil.

China,China, byby constructingconstructing 65396539 damsdams overover 3030 mm height,height, hashas thethe highesthighest hydrohydro potentialpotential inin thethe worldworld andand accountsaccounts forfor 43%43% ofof all all dams dams worldwide. worldwide. WithWith installedinstalled hydropowerhydropower capacitycapacity ofof 300300 GWGW [[3],3], accountingaccounting forfor 27%27% ofof suchsuch hydropowerhydropower capacitycapacity worldwide,worldwide, thethe lengthlength ofof waterwater deliverydelivery canalscanals hashas exceededexceeded 13,80013,800 km,km, andand hydraulichydraulic tunnelstunnels areare overover 10,00010,000 kmkm inin length.length. FigureFigure2 2 shows shows the the comparison comparison regarding regarding number number of of dams dams between between ChinaChina andand otherother countriescountries ofof thethe world.world. Meanwhile,Meanwhile, ChinaChina isis facingfacing problemsproblems regardingregarding sedimentation;sedimentation; itsits ThreeThree GorgesGorges DamDam (TGD)(TGD) hashas encounteredencountered seriousserious problemsproblems afterafter itsits 3 2 completion.completion. The The sediment sediment erosion erosion level level has has reached reached 21 21 × 104× 104 m3/km m /km2 aa yearyeara year[3].[3]. TheThe [3]. flush-flush- The ﬂushinging of gravel of gravel and and rocks rocks carried carried through through Sichuan Sichuan in in the the Yangtze Yangtze could could not be removedremoved becausebecause ofof theirtheir weightweight andand size,size, andand itit isis predictedpredicted thatthat thethe reservoirreservoir wouldwould depositdeposit upup andand becomebecome unusableunusable withinwithin thethe nextnext tenten 1010 years.years.

FigureFigure 2.2. NumberNumber ofof damsdams higherhigher thanthan 3030 mm inin majormajor countriescountries [[3].3].

TarbelaTarbela damdam hydroelectrichydroelectric projectproject (TDHP)(TDHP) isis builtbuiltbuilt ononon riverriverriver IndusIndusIndus andandand locatedlocatedlocated north-north-north- westwest of of Islamabad, Islamabad, Pakistan, Pakistan, which which is theis the largest largest rock rock and and earth earth ﬁlled filled dam dam in the in world. the world. The excessiveThe excessive sedimentation sedimentation has resultedhas resulted in about in about 35% 35% reduction reduction of the of reservoirthe reservoir capacity capacity [4] since[4] since its impoundment.its impoundment. The The sediments sediments contain contai sand,n sand, silt, silt, and and clay, clay, which which are are the the source source ofof damagedamage toto thethe unitunit generatinggenerating power,power, tunnelstunnels units,units, andand ultimatelyultimately toto allall thethe plantplant equipmentequipment [[4].4]. FigureFigure3 3 shows shows the the aerial aerial view view of of the the different different features features of of the the dam. dam.

Figure 3. Aerial view of Tarbela Dam Hydroelectric Project [5].

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Abid and Siddiqi [4] had conducted numerical study to analyze the flow behavior for TDHP reservoir for the first time in 2010. Multiphase flow (water and air) analysis has been performed for maximum inflow through upland catchment assuming summer season and discharging water through different spillways. They have tried to address the critical problem of sedimentation. The sediments flowing along water have damaged the structure of tunnels and caused erosion at different turbine components, which were then analyzed by another researcher. Abid and Noon [5] investigated the damage occurred at the TDHP tunnels with and without considering the effect of sediment particles during summer, winter, and mid seasons. The results of velocity, erosion rate, and Pressure rate are discussed in detail. Numerically calculated erosion results were validated through the experimental data. It is concluded in their work that sediment particles are damaging the TDHP equipment extensively and suggested few techniques for the reduction in losses. Thapa et al. [6] showed that severe damage is observed at Jhimruk Hydro Plant JHP (3 × 12.6 MW) at edges of turbine near to band of outlet during the first 4000 h of operation. Drop in runner efficiency due to erosion wear at JHC was quantified by two thermodynamic efficiency measurements for a duration of 11 weeks of monsoon season. It was found that during the working period of a single monsoon season, the average loss of efficiency estimated at BEP (maximum efficiency location on efficiency vs. volume flowrate curve) was 4% and 2.5 mm loss was found in mean thickness of runner. A similar study was carried out by Koirala et al. [7] at a hydropower project in Nepal with a capacity of (3 × 48 MW). They analyzed the sediment samples and turbine materials. During 16,500 operational hours, the clearance gap had enlarged to 1.9 mm and 3.6 mm for leading and trailing edges, respectively. It was observed that the higher sediment load, impact velocity, quartz content, the higher the inefficiency of the flushing system, and operation at small guide vane angles are all main contributors to erosive wear in the system. Therefore, it is concluded that sediment erosion decreases the turbine efficiency as well as it damages the turbine structure. Sediment erosion is a localized issue found in the Himalayan and Andes region but secondary flow phenomenon is a global problem and it causes cavitation erosion in hydraulic machinery. Cavitation is the phenomenon of pitting of metallic surface. It generates instability and highly erratic flow behavior producing excessive noise, vibrations, and drop in efficiency in Francis, Kaplan and other turbines. Both the Kaplan and Francis turbines are the reaction type turbines with Francis inward and Kaplan axial flow turbines. Francis Turbines (FT) are used worldwide due to their relatively compact structure, high efficiency, and working under water head ranging from 100 to 300 m with efficiency from 90% to 95%. Goyal and Gandhi [8] presented the dynamic problems of FT in the present energy production development. The turbine suffered from various dynamic instabilities during transient and off-design operations. Numerous high-frequency and low pressure variations were observed during both transient and steady state conditions. Tomaz et al. [9] had performed various studies to locate the different features of cavitation phenomena such as pressure pulsation, cavitation bubble collapse, vapor volume fraction, cavitation vibration, noise, and rotating cavitation. Furthermore, it was discussed that cavitation visualization is going to be an important feature of scientific research in model testing. Chitrikar et al. [10] focused on developing a numerical model of the test rig for investigating the behavior of leakage flow and validated their results through experiments. It was observed from both Computational Fluid Dynamics (CFD) and experiment that the leakage flow generates a passage vortex, which drifts away from the wall while moving downstream. A non- dimensional term, “Leakage Flow Factor” (Lff), was used for the clearance gap of several hydrofoils to compare the possible leakage flow, starting from the reference case. It is shown that Lff is reduced by 4.45 times for the NACA-4412 profile and the average value of the circumferential velocity, at the runner inlet only by 1.31%, compared to the base case. The drop in intensity and size of leakage flow and passage vortex shows minimization of the coalesced impact of secondary flow and sediment erosion. Comparison of various Energies 2021, 14, 1516 4 of 19

studies show that efficiency of FT varies between 3% and 6% based on changes in sediment concentration and secondary flow phenomena. However, around 2 to 3.5 mm loss in thickness of runner is observed. In all of the discussed studies it is found that cavitation phenomena need the latest equipment for its detection and visualization. Moreover, a lot of work is needed to assess the cavitation numerically. The current study reviews the possible ways of sediment and cavitation erosion detection and reduction for the hydroelectric power turbines in the Himalayan region in particular and the Andes, etc. in general. Both types of erosion depend on flow characteristics, surface, and erodent material properties. The latest CFD techniques have been used by many researchers, which are validated through experiments. The present work presents the comprehensive updates on sediment and cavitation, which causes wear in hydraulic turbines. This aspect has been rarely discussed in much detail in the previous studies. For future work, it is recognized that the design optimization tools, coalesced effect (CE) of sediment, and cavitation erosion and well organized experiments will produce results that will be beneficial for erosion identification and reduction.

2. Experimental and Numerical Investigation for Sediment Erosion 2.1. Introduction The fundamental reason of erosion wear and energy loss is friction, and it is reported that one-third of the world’s energy resources are currently utilized to overcome friction in several ways. The presence of sediment in water is a common issue in hilly regions and most run-of-river plants. The hydro turbines experience severe problems of sediment erosion during their operation. Erosive wear is caused by the solid particle’s impact with a solid surface. The flow medium contains particles that have enough velocity to deface a metallic surface. Various studies have been conducted by different researchers to minimize the effect of sediment erosion at different locations of FT components. Field studies, experimental measurements, empirical modeling, CFD work, etc. are performed for this purpose. Various empirical models have been utilized to describe the erosion wear in terms of material and fluid properties. Rao and Buckley [11] presented a long term impingement erosion problem. They have proposed the modeling methods, which includes the curve fitting approach and a power law relationship. For ductile materials, it is found that the maximum erosion occurs at an impact angle of 30◦, whereas for brittle materials, in the range of 80–90◦.

2.2. Experimental Approaches Different experimental techniques are used in the literature for slurry erosion quantification, such as rotating disc apparatus (RDA), slurry pot test, jet erosion test, rotating pipe test, etc. Rajkarnikar et al. [12] used the RDA setup for analyzing erosion wear in Francis runner blades. The blade is scaled down to 1/4 and the material used is aluminum plus 6% Copper and 4% Zinc, and they were fixed in the rotary disc. Due to the micro erosion from the high rotational motion of the sand particles, it was found that the erosion is dominant at the outer region. The experimental setup for this study is shown in Figure4. Chitrakar et al. [13] reported that the coalesced nature of the sediment and cavitation erosion enhances vibrations, fatigue, mass losses, and ultimately failure of the turbine. They emphasized that studying the relationship between the two phenomena is essential. The methods for the prediction and minimization of the combined effect are presented. Ghenaietet et al. [14] developed a semi empirical erosion model to assess the mass loss and the geometry degradation. Critical areas of erosion wear are identified Koirala et al. [15] discussed the hydraulic and mechanical effects due to sediment erosion in guide vanes and its presence in FT. The Guide Vane (GV) system in FT, flow phenomenon around it, and possible measures to find erosion under normal conditions are some of the important works done. An experimental study was conducted with a rotating disc apparatus for selecting a suitable profile. Unsymmetrical Guide Vane profiles were found to be effective in handling erosion, thus maintaining consistency in turbine overall performance. The Energies 2021, 14, x FOR PEER REVIEW 5 of 21

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GV inlet velocity triangle used in this study is shown in Figure5. Javaheri et al. [ 16] reviewed the literature on some basic parameters causing the slurry erosion of steels, (a) emphasizing on those developed(b) for pipeline applications. They( discussedc) the test rigs Energies 2021, 14, x FOR PEER REVIEW 5 of 21 of erosion, the involved mechanism, and different microstructure behavior under slurry Figure 4. (a) Rotating disc apparatus (RDA) apparatus; (b) apparatus components; (c) erosion behavior observed on the erosion conditions. blade [13].

Chitrakar et al. [13] reported that the coalesced nature of the sediment and cavitation erosion enhances vibrations, fatigue, mass losses, and ultimately failure of the turbine. They emphasized that studying the relationship between the two phenomena is essential. The methods for the prediction and minimization of the combined effect are presented. Ghenaietet et al. [14] developed a semi empirical erosion model to assess the mass loss and the geometry degradation. Critical areas of erosion wear are identified Koirala et al. [15] discussed the hydraulic and mechanical effects due to sediment erosion in guide vanes and its presence in FT. The Guide Vane (GV) system in FT, flow phenomenon around it, and possible measures to find erosion under normal conditions are some of the important works done. An experimental study was conducted with a rotating disc apparatus for selecting a suitable profile. Unsymmetrical Guide Vane profiles were found to be effective in handling erosion, thus maintaining consistency in turbine overall performance. The GV inlet velocity triangle used in this study is shown in Figure 5. Javaheri et al. [16] reviewed the literature on some basic parameters causing the slurry erosion of (a) steels, emphasizing on those developed(b for) pipeline applications. They discussed(c) the test FigureFigure 4. 4.(a )(a Rotating) Rotating disc discrigs apparatus apparatusof erosion, (RDA) (RDA) the apparatus; apparatus;involved ( b (mechanism,)b apparatus) apparatus components; components;and different ( c()c) erosion microstructureerosion behavior behavior observed behaviorobserved on onunder thethe blade [13]. slurry erosion conditions. blade [13].

Chitrakar et al. [13] reported that the coalesced nature of the sediment and cavitation erosion enhances vibrations, fatigue, mass losses, and ultimately failure of the turbine. They emphasized that studying the relationship between the two phenomena is essential. The methods for the prediction and minimization of the combined effect are presented. Ghenaietet et al. [14] developed a semi empirical erosion model to assess the mass loss and the geometry degradation. Critical areas of erosion wear are identified Koirala et al. [15] discussed the hydraulic and mechanical effects due to sediment erosion in guide vanes and its presence in FT. The Guide Vane (GV) system in FT, flow phenomenon around it, and possible measures to find erosion under normal conditions are some of the important works done. An experimental study was conducted with a rotating disc apparatus for selecting a suitable profile. Unsymmetrical Guide Vane profiles were found to be effective in handling erosion, thus maintaining consistency in turbine overall performance. The GV inlet velocity triangle used in this study is shown in Figure 5. Javaheri et al. [16] reviewed the literature on some basic parameters causing the slurry erosion of steels, emphasizing on those developed for pipeline applications. They discussed the test rigs of erosion, the involved mechanism, and different microstructure behavior under slurry erosion conditions.

Figure 5. Inlet velocity triangle at Best Efﬁciency Point (BEP) [15].

2.3. CFD Work

CFD has emerged as a new and useful tool in simulating the sediment flow as the water passes through turbine. Noon and Kim [17] conducted CFD analysis for erosion wear predictions and consequently the efficiency losses for FT components, especially at the runner. Gradual removal of the base material has changed the profile of two components: the guide vanes and the runner blades of the turbine as seen in Figures6 and7. As a result of this degradation, the turbine structure becomes weak. Thermodynamic efficiency measurements are carried out on turbine unit number 11. Figure8 shows the results of the efficiency at TDHP during 12 weeks of monsoon period (rainy season) to quantify the effect of erosion wear.

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Figure 5. Inlet velocity triangle at Best Efficiency Point (BEP) [15]. Figure 5. Inlet velocity triangle at Best Efficiency Point (BEP) [15]. 2.3. CFD Work 2.3. CFD Work CFD has emerged as a new and useful tool in simulating the sediment flow as the CFD has emerged as a new and useful tool in simulating the sediment flow as the water passes through turbine. Noon and Kim [17] conducted CFD analysis for erosion water passes through turbine. Noon and Kim [17] conducted CFD analysis for erosion wear predictions and consequently the efficiency losses for FT components, especially at wear predictions and consequently the efficiency losses for FT components, especially at the runner. Gradual removal of the base material has changed the profile of two compo- the runner. Gradual removal of the base material has changed the profile of two components: the guide vanes and the runner blades of the turbine as seen in Figures 6 and 7. As nents: the guide vanes and the runner blades of the turbine as seen in Figures 6 and 7. As a result of this degradation, the turbine structure becomes weak. Thermodynamic effi- a result of this degradation, the turbine structure becomes weak. Thermodynamic effi- Energies 2021, 14, 1516 ciency measurements are carried out on turbine unit number 11. Figure 8 shows the6 results of 19 ciency measurements are carried out on turbine unit number 11. Figure 8 shows the results of the efficiency at TDHP during 12 weeks of monsoon period (rainy season) to quantify of the efficiency at TDHP during 12 weeks of monsoon period (rainy season) to quantify the effect of erosion wear. the effect of erosion wear.

(a) (b) (a) (b) FigureFigure 6. Guide6. Guide vanes vanes with with covers, covers, (a) ( ata) actualat actual site, site, (b )(b erosion) erosion rate rate density density proﬁle profile [13 [13].]. Figure 6. Guide vanes with covers, (a) at actual site, (b) erosion rate density profile [13].

(a) (b) (a) (b) Figure 7. Runner at Tarbela dam hydroelectric project (TDHP), (a) at actual site, (b) erosion rate density profile [17]. FigureFigure 7. 7.Runner Runner at at Tarbela Tarbela dam dam hydroelectric hydroelectric project project (TDHP), (TDHP), (a) ( ata) actualat actual site, site, (b) ( erosionb) erosion rate rate density density proﬁle profile [17 [17].].

Noon et al. [17] found that during the working period of one monsoon season, the mean loss of efficiency recorded at Best Efficiency Point (BEP) is about 4%. Eltvik [18] preferred Tabakoff erosion model over Finnie’s erosion model as it indicates erosion ten- dency closer to the reality. They performed simulations which can be utilized to predict the location of severe erosion and to identify the maintenance intervals. Alveyro et al. [19] presented the case study of the small hydroelectric plant Rio Cali I, in Columbia, which was built about 100 years ago. The turbine efficiency in this power plant has reduced in the past few years due to the erosion of its components. CFD analysis were performed to gain improved geometry that provides elevated efficiency in a FT of 500-kW. The work was divided into two stages: The first stage of the work was to focus the covers, stay/guide vanes, and other parts of the unit. The second stage of the work was improving the profile of the runner blade. Aponte et al. [20] performed CFD analysis to predict erosion by hard particles. They used the experimental data to determine the coefficients for the Tabakoff- Grant erosion model. The model coefficients were validated to perform a computational study to consider the effect of particle size. Figure9 shows a comparison of the erosion behavior among the particle sizes of 30, 100, and 300 µm. EnergiesEnergies 20212021, 14, 14, x, FOR 1516 PEER REVIEW 7 of7 of21 19

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Figure 8. Turbine efﬁciency measurements at TDHP [17]. Figure 8. Turbine efficiency measurements at TDHP [17].

Noon et al. [17] found that during the working period of one monsoon season, the mean loss of efficiency recorded at Best Efficiency Point (BEP) is about 4%. Eltvik [18] preferred Tabakoff erosion model over Finnie’s erosion model as it indicates erosion ten- dency closer to the reality. They performed simulations which can be utilized to predict the location of severe erosion and to identify the maintenance intervals. Alveyro et al. [19] presented the case study of the small hydroelectric plant Rio Cali I, in Columbia, which was built about 100 years ago. The turbine efficiency in this power plant has reduced in the past few years due to the erosion of its components. CFD analysis were performed to gain improved geometry that provides elevated efficiency in a FT of 500-kW. The work was divided into two stages: The first stage of the work was to focus the covers, stay/guide vanes, and other parts of the unit. The second stage of the work was improving the profile of the runner blade. Aponte et al. [20] performed CFD analysis to predict erosion by hard particles. They used the experimental data to determine the coefficients for the Tabakoff- Grant erosion model. The model coefficients were validated to perform a computational study to consider the effect of particle size. Figure 9 shows a comparison of the erosion behavior among the particle sizes of 30, 100, and 300 µm.

Figure 9. Erosion rate behavior for different particle sizes [20]. Figure 9. Erosion rate behavior for different particle sizes [20]. It was found that the effective impingement angle against the surface for small particles is lowerIt was thanfound the that impingement the effective angleimpingement for large angle particles. against Kocak the surface et al. [for21] small performed par- ticlesanalytical is lower computations than the impingement and numerical angle simulations for large particles. together toKocak design et al. an [21] FTrunner performed blade. analyticalSingle blade computations was designed and numerical by using simulations the Bovet approach. together to Net design head an and FT runner ﬂow rate blade. were 3 Singlechosen blade as 33.53 was mdesigned and 1048 by m using/s, respectively the Bovet approach. for design point.Net head and flow rate were chosen as 33.53 m and 1048 m3/s, respectively for design point. 2.4. Effect of Surface Roughness 2.4. EffectDue of to Surface the manufacturing Roughness techniques used in the machining processes, the bare surface has some absolute roughness value, which is increased during machine operation through Due to the manufacturing techniques used in the machining processes, the bare sur- abrasion or erosion. Therefore, surface texture causes increased energy efﬁciency losses face has some absolute roughness value, which is increased during machine operation during its operation. through abrasion or erosion. Therefore, surface texture causes increased energy efficiency losses during its operation. Liu et al. [22] studied the damage at the turbine runner through increased energy losses due to friction. It was suggested that friction losses should be given special consideration, mainly in the runner where the relative velocity is the maximum. The hydraulic energy efficiency of an FT against the sand grain roughness height and the discharge is plotted in Figure 10. It was found that efficiency losses are increased with increasing surface roughness. Maruzewski et al. [23] studied the specified losses in each component of an FT, which were quantified through CFD simulations. Different water passage surface roughness heights were used to obtain the results. Khanal et al. [24] obtained the optimum blade design in terms of erosion rate and efficiency. They discussed different techniques for designing the runner blade of Francis turbine, which includes the optimal outlet and blade angle distribution to get minimum possible erosion for a given volume flow rate, head, rpm. It is concluded that the optimum blade is a blade with profile curvature position of 0.25. Although the efficiency of an optimized blade is 0.25% less than reference design, but substantial decrease in rate of erosion has been observed; the erosion is 31.5 times less than the reference blade which is a significant reduction. They found that by varying the runner blade profile parameters, significant improvement in minimizing erosion can be made while maintaining the efficiency. The improved profile can also increase the runner blade life.

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Liu et al. [22] studied the damage at the turbine runner through increased energy losses due to friction. It was suggested that friction losses should be given special consideration, mainly in the runner where the relative velocity is the maximum. The hydraulic energy efficiency of an FT against the sand grain roughness height and the discharge is plotted in Figure 10. It was found that efficiency losses are increased with increasing surface roughness. Maruzewski et al. [23] studied the specified losses in each component of an FT, which were quantified through CFD simulations. Different water passage surface roughness heights were used to obtain the results. Khanal et al. [24] obtained the optimum blade design in terms of erosion rate and efficiency. They discussed different techniques for designing the runner blade of Francis turbine, which includes the optimal outlet and blade angle distribution to get minimum possible erosion for a given volume flow rate, head, rpm. It is concluded that the optimum blade is a blade with profile curvature position of 0.25. Although the efficiency of an optimized blade is 0.25% less than reference design, but substantial decrease in rate of erosion has been observed; the erosion is 31.5 times less than the reference blade which is a significant reduction. They found that by varying the Energies 2021, 14, xrunner FOR PEER blade REVIEW profile parameters, significant improvement in minimizing erosion can be 9 of 21 made while maintaining the efficiency. The improved profile can also increase the runner blade life.

Figure 10. StressFigure distribution 10. Stress on the distribution blade from on two-way the blade FSI from [25 two-way]. FSI [25]. 2.5. FSI as a Multiphysics Approach 2.5. FSI as a Multiphysics Approach Fluid structureFluid interaction structure (FSI) interaction is a Multiphysics (FSI) is approacha Multiphysics that is approach essential that for obtain- is essential for ob- ing accurate results.taining However, accurate veryresults. few However, studies are very found few instudies literature are found solving in sedimentsliterature solving sedi- and cavitation erosionments and problems cavitation using erosion FSI methodology. problems using Chitrakar FSI methodology. [25] conducted Chitrakar a study [25] conducted to assess the structurala study to integrity assess the of the structural turbine integrity design through of the turbine both one-way design through and two-way both one-way and FSI. It was observedtwo-way that FSI. the It optimizedwas observed design that yieldsthe optimize betterd results design as yields far as better the runnerresults as far as the structural integrityrunner is concerned. structural integrity The maximum is concerned. stress was The enhanced maximum by stress around was 14% enhanced from by around the base design14% and from approximately the base design by 52% and for approximatel the optimizedy by design, 52% for as the compared optimized to thedesign, as com- one-way FSI aspared shown to the in Figureone-way 10 .FSI Muller as shown et al. in [ 26Figure] worked 10. Muller on the et two-phase al. [26] worked flow on the two- field at the runnerphase exit. flow The field experimental at the runner techniques exit. The presentedexperimental by techniques Muller et al. presented utilized by Muller et Laser Doppleral. Velocimetry utilized Laser (LDV) Doppler and high-speed Velocimetry visualizations (LDV) and high-speed to address thevisualizations unstable to address fluid-structurethe interaction unstable fluid-structure mechanisms between interaction the mechan unsteadyisms draft between tube the flow unsteady and the draft tube flow runner shaft. Moreover,and the runner this approach shaft. Moreover, was compared this approach at the same was compared time with measuredat the same wall time with meas- pressure oscillationsured wall in the pressure draft tube oscillations cone and in thethe torquedraft tube on thecone runner and the shaft. torque on the runner shaft. Table1 shows someTable of 1 the shows erosion some models of the which erosion are models considered which in hydraulicare considered machines. in hydraulic ma- The review madechines. in this The part review of the made paper in [ 11this–26 part] concludes of the paper that [11–26] sediment concludes erosion that severely sediment erosion damages the componentsseverely damages of hydroelectric the components power of plants. hydroele Surfacectric power damage plants. and Surface efficiency damage and ef- losses were examinedficiency throughlosses were experimental examined andthrough CFD experi techniquesmental by and various CFD techniques researchers. by various researchers. Numerical methods are gaining popularity. It is proposed that at the initial stage, the components such as runner stay and guide vanes, draft tube, etc. can be analyzed individually, and then the effect of combined components can be evaluated to determine erosion wear and efficiency losses.

Table 1. Erosion models list for hydroelectric power turbines developed by various researchers.

Model Equation Parameters Km = the material factor and Kf = the flow factor Thapa et al. 𝐸 𝐶.𝐾𝐾𝐾𝐾.𝑎𝑠𝑖𝑧𝑒 C = silt concentration kg/m3 [6] Kshape and Khardness = shape and hardness factors respectively. ei = cumulative erosion after test i in mg/gm, W0 is weight of test Rajkarnikar 𝑊 𝑊 𝑒 100 specimen at the beginning of the experiment in gm, Wi is et al. [12] 𝑊 weight of test specimen after test i in gm. Teran et al. E = Dimensionless mass loss, N is the rate of number of 𝐸 𝐸∗ 𝑁∗ 𝑚 [16] particles, mp is average particle mass.

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Numerical methods are gaining popularity. It is proposed that at the initial stage, the components such as runner stay and guide vanes, draft tube, etc. can be analyzed individually, and then the effect of combined components can be evaluated to determine erosion wear and efﬁciency losses.

Table 1. Erosion models list for hydroelectric power turbines developed by various researchers.

Model Equation Parameters

Km = the material factor and Kf = the ﬂow factor b 3 Thapa et al. [6] Er = C.KhardnessKshapeKmK f .a(size) C = silt concentration kg/m Kshape and Khardness = shape and hardness factors respectively.

ei = cumulative erosion after test i in mg/gm, W0 is weight of test W0−Wi W Rajkarnikar et al. [12] e = W × 100 specimen at the beginning of the experiment in gm, i is weight of test 0 specimen after test i in gm. E = Dimensionless mass loss, N is the rate of number of particles, m is Teran et al. [16] E = E ∗ N ∗ m p r p average particle mass.

NE is the dimensionless normalized erosion, Er is the erosion rate in

Er kg/s, Vj is the average velocity of jet in m/s, A0 is the jet outlet Aponte et al. [20] NE = V A ρ C C j 0 H2O ( 1−C ) cross-sectional area, is concentration of sand, and ρH2O is density of water in kg/m3.

3. Experimental and Numerical Investigation for Cavitation Erosion 3.1. Introduction Formation of vapor bubbles due to decrease in static pressure below its vapor pressure at local temperature and then collapse of these bubbles due to sudden increase in the static pressure in a fluid is called cavitation. The phenomenon of cavitation leads to various problems such as noise and vibration at the trailing edges of turbine blades and in draft tubes, which eventually reduces the service of the components and consequently reduces the plant efficiency. The main types of cavitation that can occur in FT are leading edge, draft tube swirl, traveling bubble, and von Karman vortex cavitation. Different experimental techniques are employed like tribometers configuration, Par- ticle Image Velocimetry (PIV), Laser Doppler Velocimetry (LDV) measurements, etc. by different researchers. It is very critical to develop an appropriate shape and mounting position of the cavitation inducers (CIs) to study the cavitation phenomenon. From the last two decades, the CFD technique has been used to identify cavitation by examining the regions of pressure below vapor pressure with a single-phase model. In most of the studies, the influence of a cavitation bubble on the flow field is ignored. But they cannot provide detailed information such as the effect of cavitation on the efficiency or a more accurate prediction of the magnitude of a cavitation bubble. For this reason, the CFD method in two-phase simulation becomes necessary. The Rayleigh–Plesset model is the most effective tool for analyzing two-phase flow problems. Jain et al. [27] have worked on various turbines appropriate for micro-hydropower plants: also gravity micro hydro turbines, e.g., Archimedes screws and gravity water wheels do not suffer about sediment erosion. The historical development of PAT (Pump as turbine) regarding theoretical investigations are discussed. Several pumps that were suitable to run in turbine mode in case of low capacity power production in micro-hydropower plants were analyzed. The flow path was divided into five parts, namely rear chamber, front chamber, volute, passage of pallor, and outlet pipe, as shown in Figure 11. Jain et al. [27] concluded that in comparison to conventional turbines, PAT can payback in two years in the range of 1–500 kW. Ghiban et al. [28] studied the cavitation erosion and abrasive erosion developed in hydraulic machineries containing high resistant metallic material structure. The synergy of both cavitation erosion and abrasive erosion is also discussed. Energies 2021, 14, x FOR PEER REVIEW 10 of 21

NE is the dimensionless normalized erosion, Er is the erosion 𝐸 Aponte et al. 𝑁 rate in kg/s, Vj is the average velocity of jet in m/s, A0 is the jet 𝐶 [20] 𝑉 𝐴 𝜌 outlet cross-sectional area, C is concentration of sand, and ρH2O 1𝐶 is density of water in kg/m3.

3. Experimental and Numerical Investigation for Cavitation Erosion 3.1. Introduction Formation of vapor bubbles due to decrease in static pressure below its vapor pressure at local temperature and then collapse of these bubbles due to sudden increase in the static pressure in a fluid is called cavitation. The phenomenon of cavitation leads to various problems such as noise and vibration at the trailing edges of turbine blades and in draft tubes, which eventually reduces the service of the components and consequently reduces the plant efficiency. The main types of cavitation that can occur in FT are leading edge, draft tube swirl, traveling bubble, and von Karman vortex cavitation. Different experimental techniques are employed like tribometers configuration, Par- ticle Image Velocimetry (PIV), Laser Doppler Velocimetry (LDV) measurements, etc. by different researchers. It is very critical to develop an appropriate shape and mounting position of the cavitation inducers (CIs) to study the cavitation phenomenon. From the last two decades, the CFD technique has been used to identify cavitation by examining the regions of pressure below vapor pressure with a single-phase model. In most of the studies, the influence of a cavitation bubble on the flow field is ignored. But they cannot provide detailed information such as the effect of cavitation on the efficiency or a more accurate prediction of the magnitude of a cavitation bubble. For this reason, the CFD method in two-phase simulation becomes necessary. The Rayleigh–Plesset model is the most effective tool for analyzing two-phase flow problems. Jain et al. [27] have worked on various turbines appropriate for micro-hydropower plants: also gravity micro hydro turbines, e.g., Archimedes screws and gravity water wheels do not suffer about sediment erosion. The historical development of PAT (Pump as turbine) regarding theoretical investigations are discussed. Several pumps that were Energies 2021, 14, 1516 suitable to run in turbine mode in case of low capacity power production in micro-hydro-10 of 19 power plants were analyzed. The flow path was divided into five parts, namely rear chamber, front chamber, volute, passage of pallor, and outlet pipe, as shown in Figure 11.

Energies 2021, 14, x FOR PEER REVIEW 11 of 21

Figure 11. Different flow domains of PAT.

Jain et al. [27] concluded that in comparison to conventional turbines, PAT can payback in two years in the range of 1–500 kW. Ghiban et al. [28] studied the cavitation erosion and abrasive erosion developed in hydraulic machineries containing high resistant metallic material structure. The synergy of both cavitation erosion and abrasive erosion is also discussed. Figure 11. Different ﬂow domains of PAT.

3.2. 3.2.Experimental Experimental Techniques Techniques AmarendraAmarendra et al. et [29] al. [29have] have experimentally experimentally examined examined and andcompared compared the thealpha-beta alpha-beta brassbrass test material test material for cavitation for cavitation induced induced erosion. erosion. Triangular Triangular bluff bodies bluff are bodies used are as cav- used as itationcavitation inducers inducers which were which utilized were in utilized a slurry in pot a slurryof rotating pot disk of rotating tribometer disk at tribometer different at anglesdifferent and lengths. angles It and was lengths. estimated It was that estimated the cumulative that the mass cumulative loss is 250% mass higher loss is for 250% spec- higher imenfor tested specimen in slurry tested with in slurry 30° CI with than 30 spec◦ CIimen than specimentested without tested CI. without Figure CI. 12 Figure shows 12 theshows variationthe variation of mass ofloss mass with loss different with different types of types cavitation of cavitation inducers inducers and without and without them. them.

Figure 12. Cumulative mass loss as a function of test duration without and with the cavitation inducers [29]. Figure 12. Cumulative mass loss as a function of test duration without and with the cavitation inducers [29].

It is concluded that the brass specimen placed at a distance of 8 mm with CI yields maximum cavitation erosion. Haosheng et al. [30] analyzed the effect of shape of sand particles on cavitation erosion and found that the irregular shape particles have a negligible effect on the number of the cavities, but it causes abrasion on the solid surface. Fur- thermore, it was observed that the suspended particles caused severe cavitation erosion than those without suspension. Franc et al. [31] studied the cavitation pitting by analyzing the incubation period. They used three different metals i.e., Nickel Aluminum Bronze alloy, Aluminum alloy, and a Duplex Stainless Steel to conduct pitting tests in a cavitation tunnel. The impact of flow velocity on a couple of parameters—characteristic diameter and coverage time—was studied, and a standard law of power was established for the effect of the flow velocity on the rate of pitting for all three metals. Pereira et al. [32] presented a test case with a turbine prototype of 444 MW rated power. They used polynomial bi-variate functions based on Hermite polynomials to build a hill chart that can calculate

Energies 2021, 14, 1516 11 of 19

It is concluded that the brass specimen placed at a distance of 8 mm with CI yields maximum cavitation erosion. Haosheng et al. [30] analyzed the effect of shape of sand particles on cavitation erosion and found that the irregular shape particles have a negligible effect on the number of the cavities, but it causes abrasion on the solid surface. Furthermore, it was observed that the suspended particles caused severe cavitation erosion than those without suspension. Franc et al. [31] studied the cavitation pitting by analyzing the incubation period. They used three different metals i.e., Nickel Aluminum Bronze alloy, Aluminum alloy, and a Duplex Stainless Steel to conduct pitting tests in a cavitation tunnel. The impact of ﬂow velocity on a couple of parameters—characteristic diameter and Energies 2021, 14, x FOR PEER REVIEWcoverage time—was studied, and a standard law of power was established for the12 effect of 21

of the flow velocity on the rate of pitting for all three metals. Pereira et al. [32] presented a test case with a turbine prototype of 444 MW rated power. They used polynomial bi- thevariate power functions output, based the discharge, on Hermite the polynomialsefficiency, and to the build cavitation a hill chart parameters. that can calculateThapa et the power output, the discharge, the efficiency, and the cavitation parameters. Thapa al. [33] conducted analysis along the chord plane, from wall of the guide vane to its mid- et al. [33] conducted analysis along the chord plane, from wall of the guide vane to its span. Flow velocity exceeding 35 m/s, at the runner intake of FT, is reported for the first mid-span. Flow velocity exceeding 35 m/s, at the runner intake of FT, is reported for the time from such experimental studies. Venturini et al. [34] acquired the performance data first time from such experimental studies. Venturini et al. [34] acquired the performance experimentally from four different centrifugal pumps, running in both pump and PAT data experimentally from four different centrifugal pumps, running in both pump and mode that are characterized by specific speed values in the range of 1.53–5.82. The model PAT mode that are characterized by specific speed values in the range of 1.53–5.82. The developed by Vinturini et al. showed in this paper shows a powerful and robust tool for model developed by Vinturini et al. showed in this paper shows a powerful and robust predicting PAT performance curves over the whole range of operation based on pump tool for predicting PAT performance curves over the whole range of operation based on parameters. pump parameters.

3.3. Cavitation Research by CFD CFD has become an essential tool for cavitationcavitation research, but it needsneeds validationvalidation against smartly designed and executed experiments as well as good knowledge of ﬂuidfluid mechanics. A lot of resources can be saved by using appropriate scaled down models and using symmetry. Gohil etet al.al. [35[35]] investigated investigated the the unsteady unsteady cavitation cavitation ﬂow flow in FTin atFT part at part load, load, rated rated load load(BEP), (BEP), and overloadand overload conditions. conditions. A small A small variation variation of pressure of pressure distribution distribution was observed was ob- servedfor the for surface the surface of the bladeof the blade under under part load part condition.load condition. It was It was found found that that more more pressure pres- suredifference difference found found at overload at overload condition condition in comparison in comparison to rated to load rated condition load condition as shown as in shownFigure 13in .Figure 13.

(a) (b)

Figure 13. Pressure variation on runner (a) for rated load, ((b)) forfor overloadoverload conditioncondition [[35].35].

Vapor bubblesbubbles formationformation and and turbine turbine efﬁciency efficiency loss loss towards towards suction suction side side of of the the runner run- nerblade blade are foundare found to be to maximum be maximum under under overload over condition.load condition. Sreedhar Sreedh et al.ar [et36 ]al. reviewed [36] re- viewedthe work the done work in done the ﬁeld in the of cavitationfield of cavitation erosion researcherosion research both in waterboth in and water liquid and sodium liquid sodiumas these as are these important are important media in media nuclear in nucl reactors.ear reactors. They formulated They formulated the cavitation the cavitation bubble bubble collapse and estimated the collapse pressure. The methods for measurement of cavitation damage were also discussed. Scliesu et al. [37] studied the vapor cavity rope and its centerline which depends on the turbine setting level. They conducted measurements for the velocity flow field and the volume of a compact unsteady vapor cavity through a PIV two-phase application. Filtering procedures and specific image acquisition techniques are implemented for the examination of cavity volume. It was the first time that they developed a rope in diffuser cone of FT model. Arispe et al. [38] improved the draft tube geometry to recover higher static pressure. The modified draft tubes has better efficiencies and lower coefficient of loss, corresponding to the original one. Mohanta et al. [39] analyzed the vibration condition by monitoring of the electrical and mechanical equipment used in the hydro power stations along with a brief explanation of vibration

Energies 2021, 14, 1516 12 of 19

collapse and estimated the collapse pressure. The methods for measurement of cavitation damage were also discussed. Scliesu et al. [37] studied the vapor cavity rope and its centerline which depends on the turbine setting level. They conducted measurements for the velocity flow field and the volume of a compact unsteady vapor cavity through a PIV two-phase application. Filtering procedures and specific image acquisition techniques are implemented for the examination of cavity volume. It was the first time that they developed a rope in diffuser cone of FT model. Arispe et al. [38] improved the draft tube geometry to recover higher static pressure. The modified draft tubes has better efficiencies and lower Energies 2021, 14, x FOR PEER REVIEW 13 of 21 coefficient of loss, corresponding to the original one. Mohanta et al. [39] analyzed the vibration condition by monitoring of the electrical and mechanical equipment used in the hydro power stations along with a brief explanation of vibration related defects considering previousrelated defects literature considering in the past previous 30 years. literature Kang et in al. the [40 past] analyzed 30 years. the Kang incipient et al. cavitation [40] ana- bylyzed using the theincipient method cavitation of wavelet by using time-frequency the method andof wavelet wavelet time-frequency packet decomposition and wavelet to processpacket decomposition the cavitation noise to process signals. the The cavitation results showed noise signals. that the The characteristics results showed of incipient that the cavitationcharacteristics can beof foundincipient in thecavitation auditory can frequency be found band. in the auditory frequency band.

3.4.3.4. VortexVortex RopeRope FormationFormation ForFor hydraulichydraulic turbines,turbines, thethe operatingoperating pointspoints atat partialpartial ﬂowflow raterate areare aa sourcesource ofof vortexvortex formationformation atat thethe runnerrunner outletoutlet inin thethe draftdraft tubetube conecone [[41].41]. CavitationCavitation developeddeveloped intointo thethe low-pressurelow-pressure zonezone ofof thethe vortexvortex corecore (see(see FigureFigure 1414).). TheThe vortexvortex roperope isis helicalhelical inin shape,shape, thatthat is,is, cavitationcavitation inin itsits corecore andand thethe cavitationcavitation volumevolume variesvaries withwith thethe pressurepressure variation. variation.

FigureFigure 14.14. VaporVapor corecore roperope developmentdevelopment forforσ σ= = 0.3800.380 [[41].41].

ZhangZhang etet al.al. [[41]41] comparedcompared thethe variousvarious techniques techniques for for hydro hydro turbines turbines for for vortex vortex identi- iden- fication.tification. They They discussed discussed and and summarized summarized plenty plenty of of theoretical theoretical and and experimental experimental worksworks performedperformed forfor vortexvortex observation.observation. The geometricalgeometrical parameters, such as thethe innerinner flowflow swirling,swirling, typestypes ofof turbine,turbine, conditioncondition ofof flow,flow, etc.,etc., flowflow conditions,conditions, etc.etc. areare thethe influencinginfluencing parametersparameters forfor vorticesvortices inin thethe hydrohydro turbines.turbines. FT,FT, vortexvortex rope,rope, waswas visiblevisible inin thethe draftdraft tubetube andand inter-bladeinter-blade vortices vortices were were observed observed near near the the runner. runner. Columnar Columnar vortices vortices were were appearing appear- ating the atrunner the runner inlet andinletstreamwise and streamwise vortices vortic werees distributedwere distributed in the inter-bladein the inter-blade channels. chan- Kc etnels. al. [Kc42] et presented al. [42] thepresented results forthe FTresults designed for FT for designed a small scale for hydro-powera small scale planthydro-power through theplant simulation through workthe simulation of the unsteady work flowof the field unsteady resulting flow from field the rotor-statorresulting from interaction the rotor- by takingstator fullinteraction fluid passage. by taking The time-dependentfull fluid passage. analysis The time-dependent shows the contact analysis between shows the static the andcontact rotating between domains the static and and the rotating effect of domains the fluid and flow the in effect different of the components. fluid flow in Fourteen different locationscomponents. were Fourteen chosen, andlocations time changing were chosen, pressure and fluctuations time changing and pressure vibration fluctuations level in the componentsand vibration were level recorded in the components in spiral casing, were runner recorded blade, in spiral and draft casing, tube. runner Three blade, locations and eachdraft weretube. chosenThree locations on spiral each casing were and chosen pressure-suction on spiral casing side ofand the pressure-suction runner blade and side five of werethe runner chosen blade in the and draft five tube were were chosen labeled in the as seendraft intube Figure were 15 labeled. as seen in Figure 15.

Energies 2021, 14, 1516 13 of 19 Energies 2021, 14, x FOR PEER REVIEW 14 of 21

FigureFigure 15.15. Pressure recordingrecording locationslocations inin spiralspiral casing,casing, runnerrunner blade,blade, andand draftdraft tubetube [[42].42].

CelebiogluCelebioglu etet al. al. [ 43[43]] studied studied the the various various cavitation cavitation types, types, such such as suctionas suction side, side, pressure pres- side,sure side, leading leading edge, edge, travelling travelling bubble, bubble, draft draft tube tube swirl, swirl, and and inter-blade inter-blade vortex vortex cavitation, cavita- etc.tion, Inetc. order In order to prevent to prevent cavitation, cavitation, runner runne bladesr blades must must be be designed, designed, taking taking thethe ﬂowflow characteristicscharacteristics into account. Cavitation caused erosion, reduction in efﬁciency,efficiency, vibration,vibration, instabilityinstability ofof operation,operation, andand noise.noise. SimulationsSimulations were performedperformed for 3333 operatingoperating points.points. CavitationCavitation limits and the the operating operating ranges ranges we werere determined determined using using nume numericalrical hill hill charts charts to toobserve observe the the existing existing and and newly newly designed designed turbin turbineses as shown as shown in Figure in Figure 16. 16To. conclude To conclude the thereview review in this in thissection section [27–43], [27– it43 is], found it is found that cavitation that cavitation is a complex is a complex phenomenon. phenomenon. Table Table2 shows2 shows summary summary of some of some cavitation cavitation erosion erosion investigations.Many investigations.Many parameters parameters and and fea- featurestures remain remain to tobe be studied studied and and investigated. investigated. Cavitation Cavitation inducers inducers and and some visualization methodsmethods like PIV, LDV, etc. are are used used by by numerous numerous researchers researchers as as experimental experimental means means to Energies 2021, 14, x FOR PEER REVIEWto study the cavitation erosion phenomenon. In the past decade, CFD has been15 used of 21 study the cavitation erosion phenomenon. In the past decade, CFD has been used exten- extensivelysively and reasonable and reasonable results results have havebeen beenobserved. observed.

Table 2. Summary of some investigations on cavitation erosion.

Investigators Details of Tests/Models Major Conclusions Franc et al. 8 Cavitation rate depends on: pitting rate, coverage 𝑁 𝑒/ [31] 𝜋𝛿𝜏 rate, and depth of deformation rate Correlations developed for cavitation rate and 𝑚 0.0081536 𝑇.𝐻𝑠.𝑉. Gohil et al. normalized efficiency loss, useful for the plant 𝜂 [35] operators. To predict the degradation rate of 1.4550 𝑇.𝐻𝑠.𝑉.𝑒. performance. Head coefficient and discharge coefficients are used 2𝑔𝐻 𝜓 to plot a numerical hill chart. The methodology Celebioglu et 𝜔 𝑅 developed for the minimization of cavitation at off- al. [43] 𝑄 𝜑 𝜋𝜔𝑅 design. The cavitation limit is determined by using the cavitating and non-cavitating operating points.

FigureFigure 16.16.Hill Hill chartchart forfor showingshowing cavitationcavitation andand non-cavitationnon-cavitation operating operating points points [ 43[43].].

4. Experimental and Numerical Investigation for Coalesced Effect of Sediment and Cavitation Erosion 4.1. Introduction

It is very important to identify the regions and components in FT where the coalesced effect of sediment and cavitation erosions take place simultaneously. It means that one problem is both the cause and the effect of another problem. Usually, it is observed at guide vanes and runner blades of a FT where flow velocity is high. As can be seen in Figure 17, the inlet area of FT is observed to be mostly damaged by the coalesced effect, where it touches with the hub and shroud.

Figure 17. Damaged location of turbine runner with sand erosion and combined influence demar- cated.

Zhang et al. [44] worked on microscopic interactions of particles and cavitation bubbles. It is found that the number of pits is strongly dependent on the particle size. As the

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Energies 2021, 14, 1516 14 of 19

Table 2. Summary of some investigations on cavitation erosion.

Investigators Details of Tests/Models Major Conclusions Cavitation rate depends on: pitting rate, coverage Franc et al. [31] N = 8 e−(2D/δ) πδ2τ rate, and depth of deformation rate Correlations developed for cavitation rate and . 0.9726 0.3573 4.9927 normalized efficiency loss, useful for the plant Gohil et al. [35] mcav = 0.0081536 T Hs V 2 operators. To predict the degradation rate of η = 1.4550 T0.2247 Hs0.1724V−5.1779e[3.2497 (lnV) ] losscav performance. Head coefficient and discharge coefficients are used to plot a numerical hill chart. The 2gH methodology developed for the minimization of Celebioglu et al. [43] ψ = 2 2 ω R cavitation at off-design. The cavitation limit is ϕ = Q πωR3 determined by using the cavitating and Figure 16. Hill chart for showing cavitation andnon-cavitating non-cavitation operating operating points. points [43].

4. Experimental and Numerical Investigation for Coalesced Effect of Sediment and 4. Experimental and Numerical Investigation for Coalesced Effect of Sediment and CavitationCavitation ErosionErosion 4.1.4.1. IntroductionIntroduction ItIt isis veryvery importantimportant toto identifyidentify thethe regionsregions andand componentscomponents inin FTFT wherewhere thethe coalescedcoalesced effecteffect ofof sedimentsediment andand cavitationcavitation erosionserosions taketake placeplace simultaneously.simultaneously. ItIt meansmeans thatthat oneone problemproblem is is both both the the cause cause and and the the effect effect of another of another problem. problem. Usually, Usually, it is observed it is observed at guide at vanesguide andvanes runner and bladesrunner ofblades a FT whereof a FT ﬂow where velocity flow isvelocity high. As is canhigh. be As seen can in beFigure seen 17 in, theFigure inlet 17, area the of inlet FT area is observed of FT is toobserved be mostly to be damaged mostly bydamaged the coalesced by the coalesced effect, where effect, it toucheswhere it with touches the hubwith and the shroud.hub and shroud.

FigureFigure 17.17. DamagedDamaged locationlocation of turbine runner with sand erosion and combined combined influence inﬂuence demar- demar- cated.cated.

ZhangZhang etet al.al. [[44]44] workedworked onon microscopicmicroscopic interactionsinteractions ofof particlesparticles andand cavitationcavitation bub-bubbles.bles. ItIt isis foundfound thatthat thethe numbernumber ofof pitspits isis stronglystrongly dependentdependent onon thetheparticle particlesize. size. AsAs thethe particle size increased from 100 nm to 1200 nm, the damage on the sample surface increases with a maximum at 500 nm, and then it decreases. Hu et al. [45] compared the cavitation erosion (CE) in pure water and cavitation-sediment erosion (CSE) in sand suspensions experimentally on 304 stainless steel. The damage was evaluated by mass loss, scanning electron microscopy, roughness, and surface residual stress. The effects of the particles shape and concentrations on the CSE were analyzed. EnergiesEnergies 20212021,, 1414,, xx FORFOR PEERPEER REVIEWREVIEW 1616 ofof 2121

particleparticle sizesize increasedincreased fromfrom 100100 nmnm toto 12001200 nm,nm, thethe damagedamage onon thethe samplesample surfacesurface in-in- creasescreases withwith aa maximummaximum atat 500500 nm,nm, andand thenthen itit decreases.decreases. HuHu etet al.al. [45][45] comparedcompared thethe cavitationcavitation erosionerosion (CE)(CE) inin purepure waterwater andand cavitation-sedimentcavitation-sediment erosionerosion (CSE)(CSE) inin sandsand sus-sus- pensionspensions experimentallyexperimentally onon 304304 stainlessstainless steel.steel. TheThe damagedamage waswas evaluatedevaluated byby massmass loss,loss, Energies 2021, 14, 1516 scanningscanning electronelectron microscopy,microscopy, roughness,roughness, andand surfacesurface residualresidual stress.stress. TheThe effectseffects 15 ofof of thethe 19 particlesparticles shapeshape andand concentrationsconcentrations onon thethe CSECSE werewere analyzed.analyzed.

4.2.4.2. UseUse ofof CavitationCavitation InducersInducers RoaRoa etet al.al. [[46][46]46] employedemployedemployed thethe slurryslurry andand cavitationcacavitationvitation erosionerosion tribometerstribometers to to getget infor-infor- mationmation aboutabout wearwearwear damage.damage.damage. ChangesChanges inin rotationalrotatirotationalonal speed,speed,speed, particleparticleparticle concentration,concentration,concentration, shapeshapeshape andand sizesize ofof cavitationcavitation inducers,inducers, etc.etc.etc. werewere observed.observed. CFDCFD simulationssimulations werewere combinedcombined withwithwith particleparticle tracking trackingtracking methods methodsmethods which whichwhich provided providedprovided an anan understanding understandingunderstanding about aboutabout the thethe contact contactcontact between betweenbetween the hardthethe hardhard particles particlesparticles and andand the surfacesthethe surfacessurfaces tested, tested,tested, and an itsandd impact itsits impactimpact over overover erosion erosionerosion magnitude. magnitude.magnitude.Figure FigureFigure 18a shows18a18a showsshows the tribometerthethe tribometertribometer conﬁguration configurationconfiguration for for thefor thethe conventional conventionalconventional slurry slurryslurry pot potpot tester testertester with withwith triangu- trian-trian- largulargular prismatic prismaticprismatic bluff bluffbluff bodies bodiesbodies as asas Cavitation CavitationCavitation Inducers InInducersducers (CIs). (CIs).(CIs). Figure FigureFigure 18 18b18bb shows showsshows the thethe boundary boundaryboundary conditionsconditions forforfor thethethe cavitationcavitationcavitation andandand erosionerosionerosion simulations.simulations.simulations.

((aa)) ((bb)) FigureFigure 18.18. ((aa)) TribometerTribometer conﬁguration,configuration,configuration, (((bbb))) boundary boundaryboundary conditions: conditions:conditions: slurryslurryslurry potpotpot testertestertester cavitation cavitationcavitation and andand erosion erosionerosion simulation simulationsimulation [ [46].46[46].].

◦ RoaRoa et etet al. al.al. [ 46[46][46]] concluded concludedconcluded that thatthat the thethe sample samplesample with withwith a 15 aa 15°15°CI andCICI andand a rotation aa rotationrotation speed speedspeed of 3500 ofof 3500 rpm3500 yieldedrpmrpm yieldedyielded wear wear verywear similar veryvery similarsimilar to wear toto conditions wearwear conditioconditio presentednsns presentedpresented in the leading inin thethe leadingleading edge of edgeedge the FT ofof runner thethe FTFT asrunnerrunner shown asas inshownshown Figure inin 19 FigureFigure. 19.19.

FigureFigure 19.19. VelocityVelocityVelocity profileprofile proﬁle andand and sandsand sand particlesparticles particles angleangle angle ofof of attackattack attack ononon thethe the samplesample sample withwith with 15°15° 15 CavitationCavitation◦ Cavitation Inducers (CI) and 3500 rpm in the tribometer [46]. InducersInducers (CI)(CI) andand 35003500 rpmrpm inin thethe tribometertribometer [[46].46].

Gohil et al. [47] predicted the weight loss of turbine due to the combined effect of sediment and cavitation erosion in hydro turbines. The erosion wear caused problems in operation and maintenance as well as reduced the plant efﬁciency, which ultimately leads to ﬁnancial losses. The curves showing the weight loss due to sediment, cavitation erosion, and coalesced action of sediment and test of erosion due to cavitation for various sample sizes of steel are shown in Figure 20 respectively. Energies 2021, 14, x FOR PEER REVIEW 17 of 21

Gohil et al. [47] predicted the weight loss of turbine due to the combined effect of sediment and cavitation erosion in hydro turbines. The erosion wear caused problems in operation and maintenance as well as reduced the plant efficiency, which ultimately leads to financial losses. The curves showing the weight loss due to sediment, cavitation erosion, Energies 2021, 14, 1516 and coalesced action of sediment and test of erosion due to cavitation for 16various of 19 sample sizes of steel are shown in Figure 20 respectively.

(a)

1400

1200 No. 2 No. 3 1000 No. 4 800 No. 5

600 No. 6

WEIGHT LOSS, MG 400

200

0 Energies 2021, 14, x FOR PEER REVIEW 18 of 21 2 4 6 8 101214 TIME (HOURS)

(b)

(c)

Figure 20. (a) WeightFigure loss 20.curves(a) Weightfor the losscavitation; curves for(b) theweight cavitation; loss curves (b) weight of the loss sediment curves erosion; of the sediment (c) weight erosion; loss curves (c) for the coalesced effectweight [47]. loss curves for the coalesced effect [47].

It was observed by Gohil et al. [47] that weight loss was minimum in cavitation erosion, intermediate for the sediment erosion, and maximum for the coalesced effect. The analysis of the combined and simultaneous effect of sediment and cavitation erosion in FT was a challenging problem for research [44–47]. There is an opportunity for researchers to work on this demanding area through appropriate CFD methodology supported with validation of experiments. Trivedi1 and Dahlhaug1 [48] critically reviewed the verification and validation methods applied to the study of hydraulic turbines and they suggested future research directions for the CFD simulation work. Future CFD studies should be focused on dynamic mesh and variable speed. They also addressed that challenges will increase when a coupled research of the transient CFD and FSI (fluid-structure interaction) is required. A well- established guide to perform CFD studies for hydraulic turbines, including proper verification and validation for FSI, is an urgent need.

5. Current Status and Future Prospects This paper covers research work done during last decade on the sediment and cavitation erosions by various researchers. For future work, following research areas can be addressed: Coalesced Effect: It is essential to identify the locations where combined or simultaneous effect of sediment and cavitation erosion can occur. Comparison can be made on the basis of material degradation and efficiency losses between combined and individual effects. Optimization studies: Operating parameters like head values, discharge values, best efficiency point, etc. and geometric parameters such as blade angles and blade thickness, hub, and shroud radii, etc. can be optimized through robust algorithms. Experimental work: • Appropriate testing time should be utilized in which the true behavior of erosion can be obtained during the experiments. It is possible to “speed up” the events in the model which may occur over a relatively long time in the prototype by utilizing the time scales appropriately. • Selection of suitable similarity formulae and conditions, such as dynamic, kinematic, and geometric similarity requirements should all be satisfied. • Avoid using distorted models, and if they are utilized, interpretation of the results should be done carefully.

Energies 2021, 14, 1516 17 of 19

It was observed by Gohil et al. [47] that weight loss was minimum in cavitation erosion, intermediate for the sediment erosion, and maximum for the coalesced effect. The analysis of the combined and simultaneous effect of sediment and cavitation erosion in FT was a challenging problem for research [44–47]. There is an opportunity for researchers to work on this demanding area through appropriate CFD methodology supported with validation of experiments. Trivedi1 and Dahlhaug1 [48] critically reviewed the verification and validation methods applied to the study of hydraulic turbines and they suggested future research directions for the CFD simulation work. Future CFD studies should be focused on dynamic mesh and variable speed. They also addressed that challenges will increase when a coupled research of the transient CFD and FSI (fluid-structure interaction) is required. A well-established guide to perform CFD studies for hydraulic turbines, including proper verification and validation for FSI, is an urgent need.

5. Current Status and Future Prospects This paper covers research work done during last decade on the sediment and cavitation erosions by various researchers. For future work, following research areas can be addressed: Coalesced Effect: It is essential to identify the locations where combined or simultaneous effect of sediment and cavitation erosion can occur. Comparison can be made on the basis of material degradation and efficiency losses between combined and individual effects. Optimization studies: Operating parameters like head values, discharge values, best efficiency point, etc. and geometric parameters such as blade angles and blade thickness, hub, and shroud radii, etc. can be optimized through robust algorithms. Experimental work: • Appropriate testing time should be utilized in which the true behavior of erosion can be obtained during the experiments. It is possible to “speed up” the events in the model which may occur over a relatively long time in the prototype by utilizing the time scales appropriately. • Selection of suitable similarity formulae and conditions, such as dynamic, kinematic, and geometric similarity requirements should all be satisfied. • Avoid using distorted models, and if they are utilized, interpretation of the results should be done carefully.

6. Conclusions Both experimental and numerical studies for sediment and cavitation erosion have been critically analyzed. In some studies, the coalesced effect of both erosion phenomena are discussed. The following are the conclusions drawn in this review paper: • Sediment erosion severely damages turbine parts in hydroelectric power plants. It is observed that the size, shape, and concentration of sediment particles are important erosion parameters. Water flowrate and head are significant flow properties. Surface of the erodent is yet another important parameter. Sediment erosion not only deteriorates the surface of the turbine components, but it also causes efficiency loss and high maintenance cost is required periodically. The technology advancements have led to extensive use of computational tools for solving sediment erosion problems. • Cavitation inducers and some latest visualization techniques like PIV, LDV etc. are used by several researchers as experimental means to study the cavitation phenomena. In the last decade, numerical methodology has been used extensively and tangible results have been achieved. • Study of the coalesced effect of sediment and cavitation erosion in hydroelectric power turbines is a challenging issue for future research. Therefore, it is recommended to develop an appropriate CFD methodology validated through experimental techniques for the quantification of combined effect. Energies 2021, 14, 1516 18 of 19

Author Contributions: Conceptualization, methodology, formal analysis, and original draft prepara- tion by A.A.N., and M.-H.K. supervised the research and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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