Abstract: Erosion wear is a major challenge for hydraulic machinery operating in sediment-laden rivers. Erosion wear tests were conducted on a hydrofoil at 0° of attack with five different flow rates (143， 153， 163， 173， and 183 m3/h)， and an optical microscope was used together with an optical 3D profiler to improve the understanding of the wear morphologies. The results show four main wear morphologies on the hydrofoil which includes pits， smooth grooves， sharp grooves， and scratches. Additionally， iridescent rings and ripple patterns are observed. On the leading edge of the hydrofoil， pits and smooth grooves are mainly observed. Four wear morphologies and iridescent rings are discovered as well in the middle of hydrofoil. On the trailing edge， pits， sharp grooves， and ripple patterns are found. The dominant wear mechanisms and sub-mechanisms on the hydrofoil are surface fatigue (indenting)， abrasion (microploughing， micro-cutting， and sliding)， and tribochemical reactions (heating).

Key words: hydraulic machinery;hydrofoil;erosion wear;wear morphology

CLC Number: S277.9" Document Code: A" Article No: 1674-8530(2024)08-0794-08

DOI：10.3969/j.issn.1674-8530.23.0139

Received date: 2023-07-18; Accepted date: 2023-10-20; Publishing time online: 2024-07-15

Online publishing: https://link.cnki.net/urlid/32.1814.th.20240710.0934.006

Supporting funds: National Natural Science Foundation of China (52279090，51779186)

First author information: ZHONG Qinqin(1999—)， female， master (qqzhong@whu.edu.cn)， researching in hydraulic machinery.

Corresponding author information: QIAN Zhongdong(1976—)， male， professor (zdqian@whu.edu.cn)， researching in hydraulic machinery.

鐘琴琴，錢忠東. 含沙水流下水翼磨損形貌試驗研究［J］. 排灌機械工程學(xué)報，2024，42（8）:794-801.

ZHONG Qinqin， QIAN Zhongdong. Experimental study of erosion wear on hydrofoils with emphasis on wear morphology［J］. Journal of drainage and irrigation machinery engineering(JDIME)，2024，42（8）:794-801.(in Chinese)

含沙水流下水翼磨損形貌試驗研究

鐘琴琴 錢忠東

摘要： 泥沙磨損是含沙河流上運行的水力機械面臨的關(guān)鍵難題.文中以水翼為研究對象，在0°攻角和5種不同流量工況（143， 153， 163， 173， 183 m3/h）下開展泥沙磨損試驗研究.采用光學(xué)顯微鏡和三維白光干涉輪廓儀對水翼表面的磨損形貌進(jìn)行觀測與分類，并分析泥沙磨損的機制.結(jié)果表明，水翼表面有4種主要磨損形貌，包括沖擊坑、微犁削引起的光滑溝槽、微切削引起的尖銳溝槽和劃痕.此外，還觀察到空化引起的彩虹環(huán)和邊界層轉(zhuǎn)捩引起的條紋狀形貌.其中，水翼前緣主要分布沖擊坑和光滑溝槽，水翼中部觀察到4種主要磨損形貌和彩虹環(huán)，水翼尾緣主要分布沖擊坑、尖銳溝槽和條紋狀形貌.水翼的泥沙磨損的機制包括擠壓、微犁削、微切削、滑擦和熱效應(yīng)等.

關(guān)鍵詞： 水力機械；水翼；泥沙磨損；磨損形貌

The rivers in the Himalayan region in Asia and the Andes Valley in South America contain a large number of sediment particles［1-2］， including the Yellow River in China. Along the Yellow River， many power plants and pumping stations have been built for electricity generation and agricultural irrigation. Due to the high sediment load in the water， the components of hydraulic machinery suffer from severe erosion wear， which directly influences the operation efficiency of the machinery and eventually causes economic loss［3-4］.

Erosion wear is a crucial type of wear produced by solid particles hitting a solid surface［5］. It is generally accepted that erosion wear is mainly influenced by three factors: the properties of the particles， such as shape， size， concentration， impact angle， and impact velocity; the properties of the target material， such as the hardness and density. And the properties of the flow， such as the flow velocity and viscosity［6-7］.

The erosion wear process and erosion mechanism have been characterized in various ways over the years. In general，erosion wear is caused by four mechanisms: abrasion， surface fatigue， adhesion， and tribochemical reaction［8］. In detail， the erosion mechanism for duc-tile materials can be subdivided into four categories: indenting， microploughing， microcutting， and sliding， depending upon the parameters， such as the impact velocity， impact angle， shape of the particles， and target material properties［9］. Indenting is formed when blunt or spherical-shaped particles strike the surface at an impact angle of approximately 90°［8-9］. Microploughing occurs due to particles with rounded shapes at a large angle， and it usually leads to materials piling up at the sides and the exit［10］. Microcutting occurs when sharp particles strike the surface at a high impact velocity and a small impact angle and lead to volume loss［11］. Sliding is produced by particles with sharp edges at an angle close to 0°， with a shallow erosion scar［10］.

Many researchers have conducted field testing to better comprehend the actual erosion wear phenomena of hydraulic machinery. KOIRALA et al.［12］ summari-zed the erosion wear on guide vanes of Francis turbines in power plants in Nepal. The leading edge， trailing edge， found to be surface of the guide vane， and clea-rance gap were seriously eroded. Indenting and microcutting were the main mechanisms on the eroded surface of the guide vane. GAUTAM et al.［13］ conducted a case study of the Bhilangana Ⅲ hydro-power plant in India and found that the guide vanes， runner blades， and facing plates of Francis turbines experienced serious erosion. Indenting， microploughing， and microcutting were found to be the dominant mechanisms on the guide vane［13］， as shown in Fig.1a. SHARMA et al.［14］ conducted field measurements of Francis turbine components and noticed that the trailing edge of the blade was easily worn， and there was an apparent microcutting mechanism. As shown in Fig.1b， the runner of the Francis turbine suffered severe erosion at the Trishuli Power Plant (2.5 MW) in Nepal， where all four wear sub-mechanisms were found［13］.

Laboratory tests derived from practical engineering cases have been used to investigate the mechanisms of erosion wear on hydraulic machinery. TIAN et al.［15］ conducted an experimental test with a 3D profiler on the erosion wear over guide vanes of Francis turbines and found the erosion on the leading and trailing edges to be indenting and sliding mechanisms. PANG et al.［16］ studied the wear mechanism of Francis turbine blades and revealed that the type of wear on the leading edge was deformation erosion， while the surface of the blade showed microcutting erosion. DONG et al.［17］ conducted a centrifugal pump impeller erosion test with painted blades. The results showed that the pits were easily visible on the eroded surface of the impeller. SHARMA et al.［18］ performed an erosion experiment on a centrifugal slurry pump and observed that the erosion surfaces were worn by deformation， plowing， platelet formation， crack formation， and scratches. However， the flow pattern related to the formation of the wear morphology was too complicated， which hindered further discussion in detail in previous studies.

A hydrofoil， simplified from guide vanes and blades of hydraulic machinery， is adopted in this study to investigate the possible wear mechanisms. Importantly， the wear morphology on the hydrofoil is investigated qualitatively and quantitatively using an optical microscope and a white light interference 3D surface profiler. The connection between the flow pattern and the erosion wear of the hydrofoil is specifically discussed.

1" Experimental method

1.1" Sediment characteristics

The sediment particles used in the experiments were collected from the Yellow River. The impurities were removed by a coarse sieve， and the sediment particles were dried in a drying box before analysis. Fig.2 showed the particle size distribution obtained by the laser particle size analyzer. Where Dp denoted the diameter of sediment particles and" indicated the percentage of particles smaller than a certain size. The maximum particle size was 1 124 μm， and 90% of the particles were less than 344 μm in diameter. The mean particle size D50 was 136 μm.

Quartz accounted for a large proportion of these sediment particles. The designed concentration of the sediment-laden water was 10 kg/m3， which was a typical concentration along the Yellow River. The surfaces of the sediment particle samples were analyzed by scanning electron microscope (SEM)， as shown in Fig.3. The sediment particles were irregularly shaped with sharp or rounded edges.

1.2" Experimental setup

The test rig used in this study was a closed water circulation system consisting of a water tank， centrifugal pump， pressure gauge， flowmeter， valve， and wear test section (Fig.4). The water and sediment particles entered the circulation system from the water tank， passed through the centrifugal pump and wear test section and finally returned to the water tank. The water tank and pipe were made of stainless steel. The lower part of the water tank was designed as an inverted triangular cone to prevent sediment particles from settling.

Cooling coils were also installed in the water tank to cool the sediment-laden water. The centrifugal pump had a rated flow rate of 165 m3/h and was driven by three-phase asynchronous motors rated at 15 kW and 2 900 r/min. The flowmeter used in the experiment was OPTIFLUX2300C-EX with an accuracy of 0.3%. The pressure gauge used was MPM4760 with an accuracy of 0.25%. The flow rate was regulated by the flow control valve. The wear test section consisted of two transition sections and one rectangular pipe and was connected to the rest of the test rig using international standard flanges. The main parameters of the wear test section are as listed in Tab.1. Whereby Ltotal was total length of the test section， D was diameter of the round pipe， H was height of the test section， W was width of the test section， Lchord was chord length of the hydrofoil， and Lspan was span length of the hydrofoil.

For removal and mounting， the rectangular pipe was connected to the upper end of a detachable cover plate， and the lower end of the cover plate was connected with the NACA 634-021 hydrofoil made of ZI101 aluminum alloy.

The experiments were tested at 0-degree attack angle of the hydrofoil. Several flow rates ranging from 143 m3/h to 183 m3/h were selected to study the possible wear morphologies. The total testing time for each case was 48 hours， which was validated to be sufficient to show the wear morphology. After the wear test， the hydrofoil was removed， washed and dried. The wear morphology was observed and measured using an optical microscope and a white light interference 3D surface profiler.

2" Results

To distinguish the erosion mechanism， the wear morphology at different positions of the hydrofoil was first analyzed. The hydrofoil was divided into four areas， A， B， C， and D， along the streamwise direction and three areas， 1， 2， and 3， along the spanwise direction， as shown in Fig.5.

The positions marked by white crosses in Fig.5 indicated the locations for observation. Which was indicated by the index， e.g.， A-1. The eroded surfaces of the hydrofoil were observed by an optical microscope.

The results showed that the wear morphologies at different flow rates were of high repeatability. For simplicity， the maximum flow rate (183 m3/h) was selected for further wear morphology analysis. Fig.6 show the optical micrographs of the eroded surfaces of the hydrofoil (183 m3/h).

There were four main wear morphologies: pit， smooth groove， sharp groove， and scratch. Along the spanwise direction， the wear morphology in area 3 was more abundant than that in areas 1 and 2， as shown in Fig.6a. The pits were observed in area 1 of the hydrofoil. The wear morphology in area 2 was mainly pits and scratches. In area 3， the most diverse wear morphologies appeared， including all four wear morphologies and the group of pits and sharp grooves， as shown in Fig.6b.

Along the streamwise direction， pits and smooth grooves appeared in area A. Wear morphologies in areas B and C were the most abundant， containing all four wear morphologies. In addition， the iridescent ring， which was displayed as a ring area with a color gradient around the erosion pits［19］， was mainly observed in the upper and middle parts of area C. In area D， the surface of the hydrofoil showed a ripple pattern， and its direction was perpendicular to the flow direction， which was also discovered in the actual power plant， as shown in Fig.1b. Pits and sharp grooves were the major wear morphologies in this area.

3" Discussion

The size characteristics of the morphologies from A-2 to D-2 (Fig.7) are especially observed by the white light interference 3D surface profile. Where L and H indicate the length and height of the measuring line on the hydrofoil respectively.

The observed pit in A-2 has a diameter of 900 μm and a depth of 68 μm. The diameter and depth of the pit in B-2 are 42 μm and 10 μm， while they are 108 μm and 15 μm， respectively， in C-2. In D-2， the pit is 326 μm in diameter and 24 μm in depth. The pit size in A-2 is the largest， almost 21 times wider and seven times deeper than the pit in B-2 and approximately three times wider and three times deeper than the pit in D-2. The pit with a large diameter and depth in A-2 is possibly caused by large particles moving in the primary direction of flow and hitting the leading edge of the hydrofoil at a large angle. Because large particles have a large Stokes number， when the sediment-laden water crosses the hydrofoil， large particles can not follow the fluid motion immediately and thus hit the leading edge.

Some typical morphologies， such as smooth grooves， scratches， clusters of sharp grooves， and clusters of pits， are also quantitatively observed in Fig.8. In Fig.8a， the smooth groove appears to have a droplet shape， with a relatively large erosion scar length of 824 μm， a depth of 49 μm， and a small scar width. The scratch in Fig.8b has a small width and depth of 45 μm and 1 μm， respectively.

Fig.8c， 8d show a special phenomenon， where the same wear pattern appears in groups. Fig.8c shows a cluster of sharp grooves containing four similar grooves shaped as obtuse triangles. The length and depth of one typical groove are 371 μm and 18 μm， respectively. Considering the sharp edges of the sand， the shallow depth of the erosion scars shows that the surface materialis possibly remove by sharp particles under a small impact angle. Fig.8d shows a cluster of pits， including three lines of similarly shaped pits. As shown in" Fig.8d， the diameter of the pits gradually increases along the mainstream direction， and the largest diameter and depth of the cluster of pits are 518 μm and 35 μm， respectively. However， the lip is on the side of the incoming flow， as shown in C-3-1 and C-3-3 (Fig.8).

It is inferred that the particle hits the surface of the hydrofoil opposite to the mainstream direction. The unusual phenomenon is probably because the particle trajectory is affected by the vortex， causing particles to move in the direction opposite to that of the main-stream. This kind of vortex induced at the corner between the pipe wall and the surface of the hydrofoil is known as a corner vortex［20］.

The major wear morphologies in different areas of the hydrofoil， including pits， smooth grooves， sharp grooves， and scratches， as well as the possible responsible flow patterns， are summarized in a schematic diagram， as shown in Fig.9. Two special phenomena (iridescent ring and ripple) are also marked based on experimental tests. The appearance of these wear morphologies is closely related to the local flow pattern. The iridescent ring is usually produced by the high temperature， which is probably related to cavitation， is shown around the erosion pit in the upper part of the hydrofoil. In the process of cavitation bubble collapse， high temperatures easily occur［21-22］. The ripple pattern partially due to the boundary layer transition is marked close to the trailing edge， and this kind of wear pattern has also been found in many hydraulic machines［23］.

The main wear mechanisms and the sub-mechanisms for the discussed wear morphology are summarized in Tab.2. The items marked with ″*″ in the table are found on the eroded surface of the hydrofoil. Erosion wear of the hydrofoil is mainly caused by surface fatigue， abrasion， and tribochemical reactions. Their sub-mechanisms are indenting (a sub-mechanism of surface fatigue)， microploughing， microcutting， sliding (sub-mechanisms of abrasion)， and heating (a sub-mechanism of tribochemical reactions)， which are characterized as the wear morphologies of pits， smooth grooves， sharp grooves， scratches， and the iridescent ring， respectively.

4" Conclusions

1) Optical micrographs show that pits， smooth grooves， sharp grooves， and scratches appear on the eroded surface of the hydrofoil. Pits and smooth grooves are found on the leading edge of the hydrofoil. All four wear types are discovered in the middle of the hydrofoil. On the trailing edge， the wear morphology is mainly pits and sharp grooves. In particular， the iridescent ring is found in the middle position， and the ripple pattern appears on the trailing edge. Along the spanwise direction， the wear morphology is more abundant in the lower part of the hydrofoil.

2) According to the 3D surface topographies， the sizes of pits along the streamwise direction are quite different. Deep pits with large diameters appear on the leading edge of the hydrofoil， those with small diameters and shallow depths appear in the middle position， and those with medium-sized diameters and depths appear on the trailing edge.

3) Surface fatigue (indenting)， abrasion (microploughing， microcutting， and sliding)， and triboche-mical reactions (heating) are the dominant wear mechanisms and sub-mechanisms on the hydrofoil， which are responsible for the morphology of pits， smooth grooves， sharp grooves， scratches， and the iridescent ring， respectively.

References

［1］" ZHAO Z， QIAN Z， GUO Z， et al. Erosion wear on the runner shroud of a Francis turbine ［J］. Journal of energy engineering， 2021， 147(6): 04021048.

［2］" QIAN Z， ZHAO Z， GUO Z， et al. Erosion wear on runner of Francis turbine in Jhimruk Hydroelectric Center ［J］. Journal of fluids engineering， 2020， 142(9)：094502.

［3］" PENG G J， WANG Z W， XIAO Y X， et al. Abrasion predictions for Francis turbines based on liquid-solid two-phase fluid simulations ［J］. Engineering failure analysis， 2013， 33: 327-335.

［4］" SHEN Z， CHU W， LI X， et al. Sediment erosion in the impeller of a double-suction centrifugal pump—a case study of the Jingtai Yellow River irrigation project， China［J］. Wear， 2019， 422/423: 269-279.

［5］" CHITRAKAR S， NEOPANE H P， DAHLHAUG O G. Study of the simultaneous effects of secondary flow and sediment erosion in Francis turbines ［J］. Renewable energy， 2016， 97: 881-891.

［6］" MASOODI J H， HARMAIN G A. A methodology for assessment of erosive wear on a Francis turbine runner ［J］. Energy， 2017， 118: 644-657.

［7］" AZIMIAN M， BART H-J. Computational analysis of erosion in a radial inflow steam turbine ［J］. Enginee-ring failure analysis， 2016， 64: 26-43.

［8］" GAHR K H Z. Wear by hard particles ［J］. Tribology international， 1998， 31(10): 587-596.

［9］" TARODIYA R， LEVY A. Surface erosion due to particle-surface interactions-A review ［J］. Powder tech-nology， 2021， 387: 527-559.

［10］" RAI A K， KUMAR A， STAUBLI T. Analytical modelling and mechanism of hydro-abrasive erosion in pelton buckets ［J］. Wear， 2019， 436/437: 203003.

［11］" FINNIE I. Erosion of surfaces by solid particles ［J］. Wear， 1960， 3(2): 87-103.

［12］" KOIRALA R， THAPA B， NEOPANE H P， et al. A review on flow and sediment erosion in guide vanes of Francis turbines ［J］. Renewable and sustainable energy reviews， 2017， 75: 1054-1065.

［13］" GAUTAM S， NEOPANE H P， ACHARYA N， et al. Sediment erosion in low specific speedfrancis turbines: a case study on effects and causes ［J］. Wear， 2020， 442-443: 203152.

［14］" SHARMA S， GANDHI B K， PANDEY L. Measurement and analysis of sediment erosion of a high head Francis turbine:a field study of Bhilangana-Ⅲ hydropower plant， India ［J］. Engineering failure analysis， 2021， 122: 105249.

［15］" TIAN W， LIU X， LI J， et al. Experimental study on sediment abrasion over guide vanes in mixed-flow hydroturbines of high head hydropower stations ［J］. Journal of Chinese Society of Power Engineering， 2020， 40(8): 686-692.

［16］" PANG J， LIU H， LIU X， et al. Study on sediment erosion of high head Francis turbine runner in Minjiang River basin ［J］. Renewable energy， 2022， 192: 849-858.

［17］" DONG J， QIAN Z， THAPA B S， et al. Alternative design of double-suction centrifugal pump to reduce the effects of silt erosion ［J］. Energies， 2019， 12(1): 12010158.

［18］" SHARMA S， GANDHI B K. Experimental investigation on rotating domain wear of hydrodynamic machine due to particulate flow ［J］. Powder technology， 2022， 410: 117884.

［19］" HAOSHENG C， JIANG L. A ring area formed around the erosion pit on 1Cr18Ni9Ti stainless steel surface in incipient cavitation erosion ［J］. Wear， 2009， 266(7): 884-887.

［20］" CHIANG T P， HWANG R R， SHEU W H. On end-wall corner vortices in a lid-driven cavity ［J］. Journal of fluids engineering， 1997， 119(1): 201-204.

［21］" PHILIPP A， LAUTERBORN W. Cavitation erosion by single laser-produced bubbles ［J］. Journal of fluid mechanics， 1998， 361: 75-116.

［22］" FLINT E B， SUSLICK K S. The temperature of cavitation ［J］. Science， 1991， 253(5026): 1397-1399.

［23］" KARIMI A， SCHMID R K. Ripple formation in solid-liquid erosion ［J］. Wear， 1992， 156(1): 33-47.

［24］" FISCHER A， DUDZINSKI W， GLEISING B， et al. Analyzing mild- and ultra-mild sliding wear of metallic materials by transmission electron microscopy［M］// ARAMBERRI J.Advanced Analytical Methods in Tribo-logy. Cham： Springer Berlin Heidelberg， 2018: 29-59.

［25］" WIMMER M A， LOOS J， NASSUTT R， et al. The acting wear mechanisms on metal-on-metal hip joint bearings: in vitro results ［J］. Wear， 2001， 250(1): 129-139.

［26］" JAHANMIR S， SUH N P， ABRAHAMSON E P. The delamination theory of wear and the wear of a composite surface ［J］. Wear， 1975， 32(1): 33-49.

［27］" GAHR K H Z. Modelling of two-body abrasive wear ［J］. Wear， 1988， 124(1): 87-103.

［28］nbsp; NGUYEN V B， NGUYEN Q B， LIU Z G， et al. A combined numerical-experimental study on the effect of surface evolution on the water-sand multiphase flow characteristics and the material erosion behavior ［J］. Wear， 2014， 319(1): 96-109.

［29］" SIKORSKI M E. The adhesion of metals and factors that influence it ［J］. Wear， 1964， 7(2): 144-162.

［30］" QUINN T F J. Review of oxidational wear: part I: the origins of oxidational wear ［J］. Tribology international， 1983， 16(5): 257-271.

［31］" LANDOLT D， MISCHLER S， STEMP M. Electrochemi-cal methods in tribocorrosion: a critical appraisal ［J］. Electrochimica acta， 2001， 46(24): 3913-3929.

（責(zé)任編輯" 朱漪云）