Technical Solution to Decrease Cavitation Effects in the Kaplan Turbine Blade

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Kaplan turbines are used at sites with a typical head range of 2 to 40 meters, with 15-100% efficiency at full discharge of water [1]. The main problems in Kaplan hydro turbines are cavitation, erosion, fatigue, and material defects [2-4]. Hydraulic turbine deterioration is now largely due to cavitation, which decreases turbine efficiency, increases turbine vibrations, and blade wear, leading to reduce the turbine operating life [5; 6]. Xavier et al. have described cavitation as the state at which vapor cavities are created and expanded because of dynamic pressure reductions to the liquid’s vapor pressure at constant tempe- rature [7]. It is a fact that in order to increase energy production, turbines operate in ways that worsen the problem of cavity erosion [8]. Also, cavitation and other complex flow phenomena in the flow field leads to structural fatigue failures [9]. To promote research on cavitation in Kaplan hydro turbines, many scholars have summarized the related studies. Kjolle has discovered that the main causes of damage to water turbines is due to cavitation problems [10]. In this regard, runners and draft tube cones in Kaplan turbines are the turbine components most susceptible to cavitation. It has been stated that the impact of cavitation erosion may be lessened by enhancing hydraulic component production and design, using materials resistant to erosion, and positioning the turbines to operate within the permissible range of cavitation conditions [11]. The advantages of cavitation monitoring in hydraulic turbines through vibration methods have been demonstrated [12]. This method was applied to verify a small alteration to its distributor that aimed to decrease the severity of the cavitation and, consequently, the associated erosion. Karimi and Avellan have presented a new cavitation erosion device that generates vortex cavitation [13]. To confirm their vortex cavitation generator, a comparative research study between various cavitation erosion conditions was conducted. They concluded that the hardened surface layers in specimens exposed to flow cavitation were thicker than those in specimens exposed to vibratory cavitation, which results in faster rates of erosion. A hydro turbine blade online monitoring system has been implemented by Shi et al. [14]. In this research, Continuous Sound Monitoring (CSM) was done for both audible sound (20 Hz-20 kHz) and ultrasound (50-300 kHz). The signal properties were assessed, including the standard deviation, noise level, and frequency components. In addition, the evaluation results were stored in a database in association with the operating condition determined by the water head and wicket gate opening or power output. To this end, sound produced by cavitation was separated from other sounds like water flow and mechanical sound based on its frequency characteristics. Therefore, it was possible to determine the cavitation intensity at various water heads and powers. Alligne et al. have investigated the ability of hydroelectric power plants to adapt to changes in the use of electrical power networks [15]. Also, under specific circumstances, the swirling flow leaving the runner of a turbine may operate as an excitation source for the entire hydraulic system. The purpose of their study was to determine how the location of the full load excitation source affect the eigen values, shape modes, and the stability of the system. In summary, there is much research on different failure processes on the Kaplan turbine, which are the main reason for the authors of this paper to produce a state-of-the-art survey focusing on the cavitation problem alone to evaluate the current cavitation phenomenon for Kaplan turbine more effectively. Accordingly, the main aim of the present paper is to overview the Kaplan turbines’ different failures based on the cavitation and introduce some practical solutions to reduce damages. Cavitation phenomenon is defined as the development of cavities (vapor bubbles) as a result of a pressure drop below saturated vapor pressure [16]. Kaplan turbine is prone to cavitation, which has the potential to reduce performance and harm the blade surfaces. Water vaporization and water vapor condensation are both involved in the twophase (water and water-vapor) interaction known as cavitation. When the local pressure in the flow field is lower than the vapor pressure of water (e.g., 3.17 kPa at 25 °C), vapor bubbles are created [17]. Noise and vibrations are produced as a result of the high pressures that are briefly formed when the bubbles are compressed and collapsed. Industrial experience shows that vibrations and noise produced by cavitation cause cracks, particularly in Kaplan turbines [18]. Figure 1 presents examples of localized damage to a pump blade due to cavitation. Up to 90% of hydro turbines suffer cavitation damage, and cavitation-erosive damage is most likely to occur on the low-pressure side of the turbine runner blades [18]. The main types of cavitation in axial reaction turbines are tip vortex cavitation pheno- mena, lead edge cavitation, surface cavitation, hub cavitation, draft, and inter-blade vortex cavitation. In the following, a brief description of them will be given. As shown in Figure 2, tip vortex cavitation can occur in the low-pressure sites generated over the turbine blades and in the wake of propellers and control surfaces [19]. When there are small bubbles or other cavitation nuclei in the core of a concentrated vortex and the core pressure shifts into tension, tip vortex cavitation begins to form. In the region of a vortex cavitation collapse, tip vortex cavitation may cause surface damage, noise, and a decrease in mechanical efficiency [20]. to cause significant erosive damage. From Figure 3, this kind of cavitation is distinguished by a partial vapor cavity that separates from a lifting body's leading edge and spreads downriver [21]. Also, due to operating at a higher head than the machine’s design head, it manifests as a connected cavity on the suction side of the runner blades [11]. This is a very frequent and complex type of cavitation, and depending on the hydrodynamic circumstances, it can exhibit several regimes. The high swirl of the flow in the blade slipstream close to the rotational axis is where the wasted energy is first used up. This swirl will cause a pressure drop in relation to ambient pressure, which will produce an unfavorable drag force on the blade [22]. Also, when the swirl is high, axial thrust will not be produced by the energy transfer to the fluid at the hub, and turbulence will be dispersed by mixing. In addition, hub vortex cavitation, as shown in Figure 4, has a sizable cavity. As a result, the lift force that is meant to be created may be lost if the rudder or any other control surfaces are positioned parallel to the propeller-shaft system axis. Eventually, depending on the axial load distribution on the propeller and hub geometry, the vortices around the hub cause an increase in energy loss, which lowers the propeller’s efficiency. On the other hand, hub vortex cavitation may cause vibration, noise, and, in certain instances, surface erosion [23]. In this type of cavitation, the cavitation nuclei, also known as microbubbles, travel through the flow field until they reach the lower pressure zones, where they transform into large macroscopic cavitation bubbles before collapsing at pressure recovery zones. According to Figure 5, the inter- actions between the produced bubbles and nearby walls or other bubbles often result in complicated shapes for the bubbles [24]. In general, cavitation-erosion damage occurs in two stages: 1 - incubation stage, during which the surface deforms plastically but no weight loss is visible and is typically characterized by a duration or incubation period; 2 - erosion stage, during which weight loss and cracking occur at varying rates depending on the time [25]. The incubation period was first described by Leith as the time when significant plastic deformation of the test surface occurs without any visible weight loss [26]. Based on the maximum mean penetration rate, some researchers have developed relationships to estimate the incubation duration. In this regard, the volume loss in the sample divided by the exposed area yields the maximum depth of penetration. This criterion is very helpful for evaluating materials with varying densities and incubation times in various cavitation devices [26]. For materials that have been exposed to cavitation, residual stress measurements and information about early microcrack formation by employing X-ray analysis have been utilized to predict early weight loss [27]. X-ray photoelectron spectroscopy or hyper spectral photography can be used to identify the composition of surface materials and surface degradation [28]. An increase in the first subharmonic of the ultrasonic driving frequency indicates the start of cavitation. The beginning of instability in enormous bubbles just before they begin to collapse is the cause of this pheno- menon [29]. The acoustic signs might be different since the measuring approach is based on detecting the bubble collapse inside the bulk liquid near a surface [30]. The CaviSensor and CaviMeterTM systems are two of the cavitation sensors developed by The National Physics Laboratory (NPL) in the United Kingdom [31]. A hydrophone device called the Hygea ultrasonic activity meter by ultrawave uses a 15 mm diameter probe to measure the frequency and acoustic pressure inside an ultrasonic bath [32]. With a straightforward display for ultrasonic frequency (5-50 kHz) and power (10-100%), this device is geared toward end users and is suitable for comparing measurements over time. The following measures to detect cavitation have been undertaken: vibration, pressure, acoustic emission, sound measurements [33]. Cavitation severely harms turbines by destroying the runners’ and flow channels’ surfaces. Moreover, noise generation is significantly increased by cavitation-induced vibration. In hydropower plants, equipment failure brought on by vibration results in shutdowns or a catastrophe [2; 34]. Excessive vibrations wear down components like guide vanes, runner blades, rims, bearings, shaft seals, and runner labyrinths through fatigue, as shown in Figure 6. Sometimes hydropower units are operated in the draft tube surge region to accommodate the various power systems. Draft tube vibrations also arise during remote operations of a unit with an operator in a surging region. The power plant operator can feel or hear some noise during this procedure, and as a result, it can take the necessary precautions to leave this dangerous zone. Several techniques, including plasma nitriding, shot peening, deep rolling, and coating depo- sition, have been discussed to enhance cavitation erosion resistance by hardening the material’s surface. The improvement depends on the sub- strate, coating material, and testing conditions, but even when these factors are the same, there are occasionally noticeable variances in the improvement [37]. Furthermore, a protective layer or coating component’s production conditions may occasionally be slightly altered, which may even result in a reduction in resistance against untreated material. Plasma nitriding can increase resistance up to 20, but this is not uncommon. Also, up to five times more erosion resistance can be achieved with friction stir processing or shot peening [38]. When increased cavitation-erosion resistance is required, cobalt alloys are employed. Due to the development of an extremely tough, supersaturated fcc-phase known as expanded austenite, or S-phase, low-temperature plasma nitriding is known to signifycantly improve the CE resistance of austenitic and duplex stainless steels [38]. PVD coatings were one of the other efficient treatments (an improvement of up to 40 times) created from a very soft reflective coating to one that is extremely hard and resistant to fatigue even at high temperatures [39]. However, the majority of PVD coating applications are linked to their high hardness, elastic modulus, fracture strength, strong tribological properties-particularly low friction coefficient-good oxidation resistance, good fatigue endurance, and high wear resistance [40, 41]. Due to their greater hardness, fracture toughness, and subsequently increased wear resistance compared to their traditional counter- parts, nanostructured WC-Co based thermal spray coatings have garnered considerable attention in recent years. The strong tendency of these coatings to decarburize and dissolve during the thermal spraying process, which can impair their mechanical qualities, has long been a challenge for nanostructured WC-Co-based coatings [42]. In addition, the results of studies showed that HVOF sprayed nanostructured WC-Co based coatings had higher wear, corrosion, and cavitation erosion resistance than conventional ones and were better bonded to the substrate with a high fraction of retained near-nano WC grain, low porosity, and a low quantity of harmful reaction products [43]. To increase the wear resistance of component surfaces, various techniques such as thermal spraying, plasma nitriding, chemical vapor depo- sition, physical vapor deposition, laser cladding, and hardening have been developed. Also, for different machinery components, thermal spraying techniques like Air Plasma Spraying (APS), Detonation Gun (D-Gun), and HVOF have been used. In hydro power plant facilities, where mechanical components are subjected to severe abrasive wear, HVOF spraying has been extensively utilized to apply WC-based coatings. Due to their exceptional resistance to abrasive wear, WC-based nanostructured and multifaceted coatings have received a lot of interest. By pro- viding an efficient means of controlling processing variables during thermal spraying application, the WC-Co coatings produced by HVOF that contain nanoparticle sizes have better mechanical and tribological properties.

About the authors

Mohammed Ridha W. Khalid

RUDN University

Email: 1042218144@rudn.ru
ORCID iD: 0009-0009-0798-4317

Ph.D. student, Department of Mechanical Engineering Technologies, Academy of Engineering

Moscow, Russia

Kazem Reza Kashyzadeh

RUDN University

Email: reza-kashi-zade-ka@rudn.ru
ORCID iD: 0000-0003-0552-9950

Candidate of Technical Sciences, Professor, Department of Transport, Academy of Engineering

Moscow, Russia

Siamak Ghorbani

RUDN University

Author for correspondence.
Email: gorbani-s@rudn.ru
ORCID iD: 0000-0003-0251-3144
SPIN-code: 8272-2337

Candidate of Technical Sciences, Associate Professor, Department of Mechanical Engineering Technologies, Academy of Engineering

Moscow, Russia

References