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During their operational life span of around 20 years, the individual components of a wind turbine, especially the rotor blades, are exposed to extreme environmental influences. This is the result of the continuous exposure of wind turbines to the elements and of particularly high rotor blade tip speeds, which exceed a velocity of 90 m/s. These eff...
Citations
... Due to the strong adhesive used to secure the samples on the rotor blades, the samples could not be removed without causing damage. Since the erosion of the blade has its maximum at the leading edge, the 3D topography characterization was therefore focused on these areas [49]. The surface topology was measured with 20 nm vertical and 1 µm lateral resolution. ...
Ice accumulation on lift-generating surfaces, such as rotor blades or wings, degrades aerodynamic performance and increases various risks. Active measures to counteract surface icing are energy-consuming and should be replaced by passive anti-icing surfaces. Two major categories of surface treatments—coating and structuring—already show promising results in the laboratory, but none fulfill the current industry requirements for performance and durability. In this paper, we show how femtosecond laser structuring of stainless steel (1.4301) combined with a hydrocarbon surface treatment or a vacuum treatment leads to superhydrophobic properties. The anti-ice performance was investigated in an icing wind tunnel under glaze ice conditions. Therefore, flexible steel foils were laser-structured, wettability treated and attached to NACA 0012 air foil sections. In the icing wind tunnel, hydrocarbon treated surfaces showed a 50 s ice build-up delay on the leading edge as well as a smoother ice surface compared to the reference. To demonstrate the erosion resistance of these surfaces, long-term field tests on a small-scale wind turbine were performed under alpine operating conditions. The results showed only minor erosion wear of micro- and nano-structures after a period of six winter months.
... Additionally, a decrease in lift coefficient for higher angles of attack was ascertained. The results also implied a significant loss in annual energy yield by 7 % for an increased drag coefficient of 80 %, which underlines the necessity of an independent, comparable and repeatable erosion test (Liersch and Michael, 2014). ...
Leading edge erosion (LEE) is one of the most critical degradation mechanisms that occur with wind turbine blades (WTBs), generally starting from the tip section of the blade. A detailed understanding of the LEE process and the impact on aerodynamic performance due to the damaged leading edge (LE) is required to select the most appropriate Leading Edge Protection (LEP) system and optimize blade maintenance. Providing accurate modeling tools is therefore essential. This paper presents a two-part study investigating Computational Fluid Dynamics (CFD) modeling approaches for different orders of magnitudes in erosion damage. The first part details the flow transition modeling for eroded surfaces with roughness in the order of 0.1–0.2 mm, while the second part focuses on a novel study modeling high-resolution scanned LE surfaces from an actual blade with LEE damage in the order of 10–20 mm (approx. 1 % chord). 2D and 3D surface resolved Reynolds Average Navier Stokes (RANS) CFD models have been applied to investigate wind turbine blade section in the Reynolds number range of 3–6 million. From the first part, the calibrated CFD model for modeling flow transition accounting roughness shows good agreement of the aerodynamic forces for airfoils with leading-edge roughness heights in the order of 140–200 μm, while showing poor agreement for smaller roughness heights in the order of 100 μm. Results from the second part of the study indicate that up to 3.3 % reduction in AEP can be expected when the LE shape is degraded by 0.8 % of the chord, based on the NREL 5MW turbine. The results also suggest that under fully turbulent condition the eroded LE shapes show the least amount of influence on the aerodynamic performances and results in negligible difference to AEP.
... Rainfall KE helps to quantify the impact of droplets hitting perpendicular to the surface and hence rainfall erosion on blades which is the accumulated aggregate of multiple impacts stochastically distributed over the surface of the coated laminate (Bech et al., 2018). In Whirling arm rain erosion test (WA-RET) (ASTM- G73-10, 2017;Liersch et al., 2014;DNVGL-RP-0171, 2014), the industrial standard for measuring durability of leading-edge structures, specimens are subjected in controlled velocity and rain conditions to assess the damage caused by droplet impacts (Bech et al., 2018). Considering the rapid growth of offshore wind industry as sustainable clean energy solution in the Americas and Asia with monsoon seasons, proper representation of KE and R is important in quantifying LEE in wind turbines. ...
To calculate the effect of rainfall in detaching particles and initiating soil erosion, or in eroding wind turbine leading edge, it is important to measure recorded drop size distributions (DSD) and fall velocity over long period. Commonly used relationships between kinetic energy (KE) and rainfall rate (R) exhibit strong dependence on the temporal resolution at which the analysis is carried out. Here we aim at developing a new scale invariant relationship relying on the framework of Universal Multifractals (UM), which is widely used to analyze and characterize geophysical fields that exhibit extreme variability across wide range of scales.
Rainfall data is collected using three optical disdrometers working on different underlying technologies (one Campbell Scientific PWS100 and two OTT Parsivel² instruments) and operated by the Hydrology Meteorology and Complexity laboratory of École des Ponts ParisTech in Paris area (France). They provide access to the size and velocity of drops falling through sampling areas of few tens of cm². Such data enables estimation of rainfall DSD, R and KE at various resolutions. The temporal variations of this geophysical data over wide range of scales are then characterized in the UM framework, which was never done for KE. A new power law relation is developed and tested against the theoretical framework assuming gamma DSD for describing the dependence between KE and R. The developed equation using scale invariant features of UM does not rely on gamma DSD assumption, performs as well as the existing tools, and is valid not only at a single scale, but also across scales.
... Rain erosion, or water droplet erosion (WDE) is defined as the progressive loss of original material from a solid surface due to continuous impingements of water droplets [1]. Recently, attention towards WDE has increased due to the issues noted in different components, such as: efficiency drop of gas turbine compressor due to WDE induced by over-sprayed droplets during inlet fog cooling process [2,3]; degradation of steam turbine blades during operation [4][5][6]; performance drop of wind turbine blades due to leading edge erosion [7]; and rain erosion of aeroengine fan blades. The latter is the core of the current study, which has not been covered by any past research studies. ...
Rain erosion of turbofan blades creates problems in the aeronautics community, since the changes of profiles on leading edges affect aerodynamic performance that subsequently leads to significant efficiency drop for the aircraft engine. Water-hammer pressure induced by droplet impingements is primarily responsible for the damaged induced during the initial compression stage. As the surface roughened, the damage mechanism is then governed by lateral flow jetting assisted with hydraulic penetration. Studies on WDE are restricted to laboratory testing with simplified conditions and samples. Hence, this will be the first study to demonstrate real-life WDE damage on real turbofan blades during their ‘complicated’ in-service conditions.
A set of Ti-6Al-4V ex-service turbofan blades are sectioned to examine their in-service degradation. Different parts of the blade, with impact velocities ranging from 144 m s−1 to 396 m s−1, were selected to study the effect of impact velocity on rain erosion damage. Erosion morphology and the damage mechanisms are characterised with Alicona profilometer and SEM. The results reveal that rain erosion occurs exclusively at the leading edges. Increase of rain erosion severity with increasing impact velocity is identified. The normalised volume loss is proportional to Vn, where V is the impact velocity, ranging from 286 m s−1 to 348 m s−1, and the exponent (n) is estimated to be around 8 for Ti-6Al-4V ex-service turbofan blade, which agrees with the literature from laboratory tests. The damage features during early incubation period cannot be detected due to the roughening of the leading edges. However, intergranular fracture is detected at the tip of the leading edge with less severe damage. In the steady state, material removal appears to be due to coalescence of microcracks to form erosion craters. The cracks continue to propagate from the side wall and bottom of the erosion craters, attributable to the joint effect of lateral outflow jetting and hydraulic penetration induced by repetitive droplet impingements. Thus, the present research is an invaluable opportunity to validate the results obtained from laboratory tests by comparing with the real-life WDE, which then helps to further elucidate the damage mechanisms behind WDE.
... The industrial standard for measuring the durability of leading edge strucures is the whirling arm rain erosion test (WA-RET) (ASTM G73-10, 2010, Liersch 2014). In the WA-RET the test specimens are mounted on a rotor spinning at high velocity in an artificially generated rain field. ...
Impact fatigue caused by collision with rain droplets, hail stones and other airborne particles, also known as rain erosion, is a severe problem for wind turbine blades. Each impact on the leading edge adds an increment to the accumulated damage in the material. After a number of impacts the leading edge material will crack. This paper presents and supports the hypothesis that the vast majority of the damage accumulated in the leading edge is imposed at extreme precipitation condition events, which occur during a very small fraction of the turbines operation life. By reducing the tip speed of the blades during these events, the service life of the leading edges significantly increases from a few years to the full expected lifetime of the wind turbine. In the worst case at the cost of a negligible reduction of annual energy production (AEP) and in the best case with a significant increase in AEP.
Severe erosion or delamination at the leading edge of blades has an adverse influence on the power loss of wind turbines. Obtaining relevant quantitative data and its mechanisms would help in the efficient management and maintenance of these turbines. In the present work, detailed analysis on the effects of various levels of leading edge delamination on the aerodynamics and flow characteristics of an S809 airfoil and, in turn, on the performance of the NREL phase VI rotor is conducted. The results indicated that an elongated leading edge separation bubble and two enclosed vortex systems appeared in the delaminated region of the airfoil when the delamination levels aggravate to a certain degree, while the trailing edge separation vortex can be observed for all delaminated airfoils. The aerodynamic performance of the wind turbine decreases with increasing delamination depth and increases with growing delamination depth. The most influential delamination length is 1% c, and the maximum power loss is 23.09% for the blade with the severe delamination case. A small area of deep leading edge delamination or defects may cause great power loss, which should not be ignored in practice engineering.
The hot corrosion behaviors of directed energy deposited Inconel 718/Haynes 25 functionally graded materials (FGMs) coated with 75% Na2SO4+25% NaCl molten salt at 700 °C and 900 °C for 100 h were investigated. The hot corrosion products at different temperatures were characterized by scanning electron microscope (SEM) in gradient direction. The microstructural evolution as well as elemental distribution in layers containing different powder contents were observed by optical microscopy and SEM. The results showed that the columnar grains became equiaxed and smaller with the increase of Haynes 25 content. The hot corrosion resistance in gradient direction was improved due to the increasing of Cr content, narrower grain boundaries and finer grains. In particular, the microstructual evolution and its effect mechanism on hot corrosion resistance were revealed.
This chapter describes the wind resource, siting, wind distribution, and the design basis for the onshore and offshore Wind DFIG plants, with protective equipment to operate safely with the grid for reactive power exchange, voltage control, and ride through conditions during system disturbances.
Engine tests under flight conditions are extremely important at the stage of its complex debugging. These conditions include the simulation of icing, check for the penetration of birds in the inlet vanes, modeling hail and rain. Simulation of rain due to its significant unsteadiness need providing requirements for the concentration of water in the air, the distribution spectrum of droplets by size, the value of the average diameter of the droplets, the mass flow rate curves for the injectors and the conditions of interaction of droplets with air. This paper contains information about the developed method of calculating the characteristics of the injector for the necessary modes of water injection into the engine as well as experimental study of the injector and conducted studies of the parameters of its operation process, including the choice of optimal modes. As a result of the research, an injector was developed that allows to obtain the average median diameter of 2.66 mm and droplets of the certification spectrum. Also there was worked out the schematic diagram of the collector module of the installation for rain simulation that meets the certification requirements: simulation of rain in the “idle power” and “take-off” modes with the condition of simultaneous provision of water concentration in the air of 20 g/m ³ and the average median diameter of 2.66 mm. This installation provides operation in transient modes with the condition of maintaining the water concentration in the air of 20 g/m ³ .
Impact fatigue caused by collision with rain droplets, hail stones
and other airborne particles, also known as leading-edge erosion, is a severe
problem for wind turbine blades. Each impact on the leading edge adds an
increment to the accumulated damage in the material. After a number of
impacts the leading-edge material will crack. This paper presents and
supports the hypothesis that the vast majority of the damage accumulated in
the leading edge is imposed at extreme precipitation condition events, which
occur during a very small fraction of the turbine's operation life. By
reducing the tip speed of the blades during these events, the service life of
the leading edges significantly increases from a few years to the full
expected lifetime of the wind turbine. This life extension may cost a
negligible reduction in annual energy production (AEP) in the worst case, and
in the best case a significant increase in AEP will be achieved.
