Showing papers on "Turbine blade published in 2015"

A review of damage detection methods for wind turbine blades

Abstract: Wind energy is one of the most important renewable energy sources and many countries are predicted to increase wind energy portion of their whole national energy supply to about twenty percent in the next decade. One potential obstacle in the use of wind turbines to harvest wind energy is the maintenance of the wind turbine blades. The blades are a crucial and costly part of a wind turbine and over their service life can suffer from factors such as material degradation and fatigue, which can limit their effectiveness and safety. Thus, the ability to detect damage in wind turbine blades is of great significance for planning maintenance and continued operation of the wind turbine. This paper presents a review of recent research and development in the field of damage detection for wind turbine blades. Specifically, this paper reviews frequently employed sensors including fiber optic and piezoelectric sensors, and four promising damage detection methods, namely, transmittance function, wave propagation, impedance and vibration based methods. As a note towards the future development trend for wind turbine sensing systems, the necessity for wireless sensing and energy harvesting is briefly presented. Finally, existing problems and promising research efforts for online damage detection of turbine blades are discussed.

Novel structural modeling and mesh moving techniques for advanced fluid-structure interaction simulation of wind turbines

Abstract: Summary In this paper, we target more advanced fluid–structure interaction (FSI) simulations of wind turbines than reported previously. For this, we illustrate how the recent advances in isogeometric analysis of thin structures may be used for efficient structural mechanics modeling of full wind turbine structures, including tower, nacelle, and blades. We consider both horizontal axis and vertical axis wind turbine designs. We enhance the sliding–interface formulation of aerodynamics, previously developed to handle flows about mechanical components in relative motion such as rotor–tower interaction to allow nonstationary sliding interfaces. To accommodate the nonstationary sliding interfaces, we propose a new mesh moving technique and present its mathematical formulation. The numerical examples include structural mechanics verification for the new offshore wind turbine blade design, FSI simulation of a horizontal axis wind turbine undergoing yawing motion as it turns into the wind and FSI simulation of a vertical axis wind turbine. The FSI simulations are performed at full scale and using realistic wind conditions and rotor speeds. Copyright © 2014 John Wiley & Sons, Ltd.

Repair and manufacturing of single crystal Ni-based superalloys components by laser powder deposition—A review

Abstract: Laser powder deposition is one of the most promising methods for the repairing of the single crystal Ni-based superalloys components used in the hot-section of gas turbine engines in order to extend their lifetime and reduce their overall cost The microstructure of Ni-based superalloys deposited on single crystal substrates of similar materials depends mainly on the materials involved, on the orientation of the deposited tracks in relation to the substrate and on the deposition parameters In the present paper these relations are discussed and illustrated for the case of single and multiple layer depositions of NiCrAlY and Rene N4 on (100) single crystal substrates of SRR99 and CMSX-4 Ni-based superalloys On the other hand, when the aging treatment is applied directly to the solidification microstructure resulting from laser deposition, abnormal γ/γ′ microstructures may result, due to the inhomogeneity created by alloying elements partition during solidification Performing a homogenization annealing before aging circumvents this difficulty The homogenizing annealing also eliminates undesirable brittle phases present in the solidification microstructure, such as carbides and topologically close-packed compounds

Review of aerodynamic developments on small horizontal axis wind turbine blade

Abstract: Wind energy is innately renewable, abundant in the earth and can possibly reduce the dependency on fossil fuels. Wind is an incarnation of sun and is always nourished by the latter. Approximately 10 million MW of energy can be continuously generated from the wind sources. In contrast to the large horizontal axis wind turbines (HAWT), which are established in the area with optimum wind conditions, small wind turbines are being installed to produce power irrespective of favourable wind conditions. Parameters associated with blade geometry optimization are important, because once optimized, shorter rotor blades could produce power comparable to larger and less optimized blades. A detailed review of various blade profiles and aerofoil geometry optimization processes to achieve high power coefficient in small wind turbines that falls below Reynolds number 500,000 have been presented in this paper.

Control of Thick Airfoil, Deep Dynamic Stall Using Steady Blowing

Abstract: The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...

Composite Repair in Wind Turbine Blades: An Overview

Abstract: Renewable energy sources such as wind energy—together with energy-efficient technologies—are essential to meet global energy demands and address climate change. Fiber-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the design and manufacture of modern wind turbines, especially turbine blades, for demanding service conditions. However, with the current global growth in onshore/offshore wind farm installations (with total global capacity of ∼282 GW by the end of 2012) and trend in wind turbine design (∼7–8 MW turbine capacity with ∼70–80 m blade length for offshore installations), one of the challenges that the wind energy industry faces with composite turbine blades is the aspect of structural maintenance and repair. Although wind turbines are typically designed for a service life of about 20 years, robust structural maintenance and repair procedures are essential to ensure the st...

Condition Monitoring and Damage Location of Wind Turbine Blades by Frequency Response Transmissibility Analysis

Abstract: Incipient defects occurring in long wind turbine (WT) blades are difficult to detect using the existing condition monitoring (CM) techniques. To tackle this issue, a new WT blade CM method is studied in this paper with the aid of the concept of transmissibility of frequency response functions. Different from the existing CM techniques that judge the health condition of a blade by interpreting individual CM signals, the proposed method jointly utilizes the CM signals measured by a number of neighboring sensors. This offers the proposed technique a unique capability of both damage detection and location. The proposed technique has been experimentally verified by using the real CM data collected during the fatigue and static tests of a full-scale WT blade. The experiment has shown that the new technique is effective not only in damage detection but also in damage location when either fiber Bragg grating strain gauges or accelerometers are used for data acquisition.

Forces and Moments on Flat Plates of Small Aspect Ratio with Application to PV Wind Loads and Small Wind Turbine Blades

Abstract: To improve knowledge of the wind loads on photovoltaic structures mounted on flat roofs at the high angles required in high latitudes, and to study starting flow on low aspect ratio wind turbine blades, a series of wind tunnel tests were undertaken. Thin flat plates of aspect ratios between 0.4 and 9.0 were mounted on a sensitive three-component instantaneous force and moment sensor. The Reynolds numbers varied from 6 × 104 to 2 × 105. Measurements were made for angles of attack between 0° and 90° both in the free stream and in wall proximity with increased turbulence and mean shear. The ratio of drag to lift closely follows the inverse tangent of the angle of incidence for virtually all measurements. This implies that the forces of interest are due largely to the instantaneous pressure distribution around the plate and are not significantly influenced by shear stresses. The instantaneous forces appear most complex for the smaller aspect ratios but the intensity of the normal force fluctuations is between 10% and 20% in the free-steam but can exceed 30% near the wall. As the wind tunnel floor is approached, the lift and drag reduce with increasing aspect ratio, and there is a reduction in the high frequency components of the forces. It is shown that the centre of pressure is closer to the centre of the plates than the quarter-chord position for nearly all cases.

Optimal smoothing length scale for actuator line models of wind turbine blades

Abstract: The actuator line model (ALM) is a commonly used method to represent lifting surfaces such as wind turbine blades within Large-Eddy Simulations (LES) In the ALM the lift and drag forces are replaced by an imposed body force which is typically smoothed over several grid points using a Gaussian kernel with some prescribed smoothing width $\epsilon$ To date, the choice of $\epsilon$ has most often been based on numerical considerations related to the grid spacing used in LES However, especially for finely resolved LES with grid spacings on the order of or smaller than the chord-length of the blade, the best choice of $\epsilon$ is not known In this work, a theoretical approach is followed to determine the most suitable value of $\epsilon$ Firstly, we develop an analytical solution to the linearized flow response to a Gaussian lift and drag force and use the results to establish a relationship between the local and far-field velocity required to specify lift and drag forces Then, focusing first on the lift force, we find $\epsilon$ and the force center location that minimize the square difference between the velocity fields induced by the Gaussian force and 2D potential flow over Joukowski airfoils We find that the optimal smoothing width $\epsilon^{\rm opt}$ is on the order of 14-25\% of the chord length of the blade, and the center of force is located at about 13-26\% downstream of the leading edge of the blade, for the cases considered These optimal values do not depend on angle of attack and depend only weakly on the type of lifting surface To represent the drag force, the optimal width of the circular Gaussian drag force field is shown to be equal to the momentum thickness of the wake

A comparison of wake measurements in motor-driven and flow-driven turbine experiments

Abstract: We present experimental data to compare and contrast the wake characteristics of a turbine whose rotation is either driven by the oncoming flow or prescribed by a motor. Velocity measurements are collected using two-dimensional particle image velocimetry in the near-wake region of a lift-based, vertical-axis turbine. The wake of this turbine is characterized by a spanwise asymmetric velocity profile which is found to be strongly dependent on the turbine tip speed ratio (TSR), while only weakly dependent on Reynolds number (Re). For a given Re, the TSR is controlled either passively by a mechanical brake or actively by a DC motor. We find that there exists a finite region in TSR versus Re space where the wakes of the motor-driven turbine and flow-driven turbine are indistinguishable to within experimental precision. Outside of this region, the sign of the net circulation in the wake changes as TSR is increased by the motor. Shaft torque measurements show a corresponding sign change above this TSR threshold set by circulation, indicating a transition from net torque due to lift to net torque due to drag produced by the turbine blades, the latter of which can give wake measurements that are inconsistent with a flow-driven turbine. The results support the claim that the turbine kinematics and aerodynamic properties are the sole factors that govern the dynamics of its wake, irrespective of the means to move the turbine blades. This has significance for both experimental and computational studies where it may be necessary, or perhaps more economical, to prescribe the turbine kinematics in order to analyze its aerodynamic characteristics.

Damage tolerance and structural monitoring for wind turbine blades.

Abstract: The paper proposes a methodology for reliable design and maintenance of wind turbine rotor blades using a condition monitoring approach and a damage tolerance index coupling the material and structure. By improving the understanding of material properties that control damage propagation it will be possible to combine damage tolerant structural design, monitoring systems, inspection techniques and modelling to manage the life cycle of the structures. This will allow an efficient operation of the wind turbine in terms of load alleviation, limited maintenance and repair leading to a more effective exploitation of offshore wind.

Trailing edge noise reduction of wind turbine blades by active flow control

Abstract: In the current study, we investigate a route to reduction of the turbulent boundary layer–trailing edge interaction noise. The trailing edge noise is generated by surface pressure fluctuations beneath a turbulent boundary and scattered at the trailing edge of wind turbine blades. Trailing edge noise is considered to be the dominant noise source of modern wind turbines. Therefore, efforts are constantly made to attenuate the noise. Today, noise emission can be reduced by proper airfoil design or passive devices, such as trailing edge serrations. A further improved candidate technology for trailing edge noise attenuation is active flow control in the form of wall-normal suction. With active flow control, the boundary layer features responsible for trailing edge noise generation can be manipulated, and correspondingly the trailing edge noise can be reduced. Detailed experimental investigations were performed at the Universities of Tel-Aviv and Stuttgart. The tests showed that steady wall-normal suction has a positive effect on the trailing edge noise by reducing the boundary layer thickness, and with it the integral length scales of the eddies within the boundary layer.

A Novel Method and its Field Tests for Monitoring and Diagnosing Blade Health for Wind Turbines

Abstract: A new diagnostic method is proposed to efficiently monitor the structural health and detect damages in wind turbine blades. A high-resolution real-time blade condition monitoring system that considers the harsh turbine operating environment and uses optical sensors and a wireless network is presented. A hybrid algorithm, which merges probabilistic analysis, design loads, and real-time load estimates, is introduced to enhance operational safety and reliability. Moreover, the alarm limits are updated every 10 min through a learning algorithm to further improve reliability. The proposed algorithm was implemented in a blade monitoring system. The effectiveness of the proposed algorithm was demonstrated for a 3-MW wind turbine in the Yeongheung wind farm.

An Innovative Way to Produce γ-TiAl Blades: Spark Plasma Sintering

Abstract: A study is conducted to show that the production of turbine blades in γ-TiAl alloys through spark plasma sintering (SPS) is a viable manufacturing route, by demonstrating that near-net shaping of γ-TiAl blades is successful, and by developing an alloy with balanced and outstanding mechanical properties at room and high temperatures in a single run and without subsequent thermal treatments. It is demonstrated that replacing nickel-based superalloys by such Intermetallic γ-TiAl alloys with high specific modulus and mechanical strength, combined with an exceptional resistance to oxidation, is an effective contribution to the development of lighter aircraft engines with high performance. Casting processes such as centrifugal cast have demonstrated that sound TiAl parts with a complex shape such as low-pressure turbine blades can be achieved.

Model of wind shear conditional on turbulence and its impact on wind turbine loads

Abstract: We analyse high-frequency wind velocity measurements from two test stations over a period of several years and at heights ranging from 60 to 200 m, with the objective to validate wind shear predictions as used in load simulations for wind turbine design A validated wind shear model is thereby proposed for flat terrain and that can significantly decrease the uncertainty associated with fatigue load predictions for wind turbines with large rotors An essential contribution is the conditioning of wind shear on the 90% quantile of wind turbulence, such that the appropriate magnitude of the design fatigue load is achieved The proposed wind shear model based on the wind measurements is thereby probabilistic in definition, with shear jointly distributed with wind turbulence A simplified model for the wind shear exponent is further derived from the full stochastic model The fatigue loads over different turbine components are evaluated under the full wind measurements, using the developed wind shear model and with standard wind conditions prescribed in the IEC 61400-1 ed 3 The results display the effect of the Wohler exponent and reveal that under moderate turbulence, the effect of wind shear is most pronounced on the blade flap loads It is further shown that under moderate wind turbulence, the wind shear exponents may be over-specified in the design standards, and a reduction of wind shear exponent based on the present measurements can contribute to reduced fatigue damage equivalent loads on turbine blades Although the influence of wind shear on extreme loads was found to be negligible, the IEC 61400-1 wind shear definition was found to result in non-conservative estimates of the 50 year extreme blade deflection toward the tower, especially under extreme turbulence conditions Copyright © 2014 John Wiley & Sons, Ltd

Modeling and Development of Cooling Technology of Turbine Engine Blades

Abstract: The paper aims to develop adequate means to measure the effectiveness of cooling technology of turbine blades. The used methodologies are the mathematical modeling of movement and heat transfer in the boundary layer of the working medium near the curved surface of the turbine blade and also using empirical data on the critical flow regimes. The mathematical model of film cooling of turbine engine blades under film formation on the punched surface with blind damping cavities is offered. On the basis of numerical research with the use of the offered model the option for essential (providing in analyzed conditions decrease in adiabatic temperature of a wall on 200 K) increase in efficiency of film cooling at the expense of a partial laminarization of a turbulent interface on the punched surface is revealed.

A hybrid multi-objective evolutionary algorithm for wind-turbine blade optimization

Abstract: A concurrent-hybrid non-dominated sorting genetic algorithm (hybrid NSGA-II) has been developed and applied to the simultaneous optimization of the annual energy production, flapwise root-bending moment and mass of the NREL 5 MW wind-turbine blade. By hybridizing a multi-objective evolutionary algorithm (MOEA) with gradient-based local search, it is believed that the optimal set of blade designs could be achieved in lower computational cost than for a conventional MOEA. To measure the convergence between the hybrid and non-hybrid NSGA-II on a wind-turbine blade optimization problem, a computationally intensive case was performed using the non-hybrid NSGA-II. From this particular case, a three-dimensional surface representing the optimal trade-off between the annual energy production, flapwise root-bending moment and blade mass was achieved. The inclusion of local gradients in the blade optimization, however, shows no improvement in the convergence for this three-objective problem.

Thermographic non-destructive inspection of wind turbine blades using unmanned aerial systems

Abstract: Wind turbines are gaining importance in the last years because of their high efficiency during energy production without greenhouse gas emission. Furthermore, they can be installed both on- and off-shore. However, wind blade maintenance operations have become difficult and expensive due basically to size aspects and operative costs related with their location and out-off service period. A non-destructive technique capable of detecting most significant in service defects in composite wind blade is infrared thermography (IRT). However, this method can only be applied by developing cost efficient in situ inspection strategies. This work presents a feasibility study for defect detection during maintenance operations in wind blades using unmanned aerial systems (UASs). IRT inspections are performed by means of passive IRT methodology first (i) for specimen located at the ground level with artificial defects (like delaminations, cracks, impact damage and debondings) and later (ii) during flight operatio...

The aerodynamic interference effects of a floating offshore wind turbine experiencing platform pitching and yawing motions

Abstract: Generally, a floating offshore wind turbine experiences larger motions than onshore or ground fixed wind turbines because of its free motions of the platform on the water. The accurate prediction of unsteady aerodynamic performances and loads for the rotating blades installed on a floating offshore wind turbine is a numerically complex problem and it is still challenging works. In this study, both the conventional numerical methods based on an unsteady blade element momentum theory and the advanced computational fluid dynamic method have been applied for the rotating blades with oscillating motions. Unsteady aerodynamic characteristics of the rotating blades with the platform pitching and yawing motions are numerically investigated in detail. It is shown that there are flow interaction phenomena between the rotating wind turbine blades with oscillating motions and generated blade-tip vortices. Finally, the unsteady aerodynamic blade loads for the pitching motion are also calculated and the results are compared for each rotating blade in order to show the effect of aerodynamic load variations during its oscillating motion.