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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

The effects of atmospheric and wake turbulence on wind turbines and wind turbine wakes

Farr, Thomas D. January 2015 (has links)
Wind tunnel studies using model wind turbines have been used to investigate the effects and characteristics of neutral and unstable atmospheric boundary layers on their operation and wake behaviour. Wind turbine arrays have also been arranged to observe the effect of wake interaction. Single-point two-component and two-point single-component velocity measurements have been made using laser Doppler anemometry in conjunction with cold-wire anemometry to interrogate the modelled boundary layer. The manufacture and installation of a second traverse mechanism in the wind tunnel was necessary to perform the two-point measurements, along with the development of laboratory software for control and data analysis. In order to allow for measurements of turbine performance, a current sensor was developed so that correlations could be made between velocity and torque fluctuations. Investigation of larger arrays, up to 12 turbines, required the production of additional turbines and installation and subsequent integration of the associated control systems. Measurements made in the neutral flow conditions show that there is an increasing correlation between the upstream turbulence and torque fluctuations with proximity to the turbine, especially in the wake of another turbine where the flow is rapidly evolving. Two-point velocity measurements, with a lateral separation, have shown that there is little effect of the turbine on the correlation of the flow over the rotor disc. Analysis of data from this type of measurement also shows that in an array of four aligned turbines, the spatial structures reach an equilibrium state and are of larger size after the second turbine. Furthermore, the velocity-torque correlation magnitude decreases after the first turbine, but then increases with distance through the array owing to the increased correlation over the rotor disc, although not to the level observed for the first turbine. The turbulence approaching the first turbine behaves in a frozen-flow manner, but this is not true for the second and subsequent turbines, although the idea of convection time still applies. Measurements made in the modelled unstable atmospheric boundary layer show that the length and time scales are changed in the flow, in addition to the alteration of the profiles of mean velocity and Reynolds stresses. The increased turbulence caused by the convective boundary layer increases the rate of wake deficit recovery and does not result in the same spatial structures as the neutral conditions. Temperature effects are of secondary importance with regard to wake and turbine behaviour, with the main driving force behind the performance being the increased turbulence levels.
12

Development of offshore wind operational expenditure model and investigation of optimum operation and maintenance fleet

Dalgic, Yalcin January 2015 (has links)
Offshore wind turbine technology is moving forward as an alternative to the fossil fuelled power production. However, there are a number of challenges in further offshore areas; wind turbines are subject to loads that are not often experienced onshore and more importantly challenging wind and wave conditions limit the operability and accessibility of the vessels needed to access offshore wind farms. Therefore, operating further from shore increases the logistic challenges of offshore wind operation and maintenance (O&M) activities. In contrast with the prospects, operational expenditure (OPEX) of the offshore wind farms has been increasing, reflecting greater risk for potential investors and current operators. As the power generation capacity improves constantly, advanced logistics planning of O&M activities, which supports the developers in achieving reduced downtime, optimised availability and maximised revenue, has gained vital importance. In order to sustain the competitiveness of the offshore wind industry against other renewable energy sources, the cost of offshore wind needs to come down to today's onshore cost. This cost reduction target can be achieved through improving the offshore related operations, which contribute the most to the OPEX of the offshore wind farms. Available vessels in the market and the variety of benefits & drawbacks of different vessel chartering strategies have to be considered in the O&M planning. In this research, an offshore wind operation and maintenance expenditure model has been developed. A time domain Monte-Carlo simulation approach is implemented, which includes analyses of environmental conditions (wind speed, wave height, and wave period), operational analyses of transportation systems, investigation of failures (type and frequency), and simulation of repairs. The model enables the quantification of the influence of cost drivers and provide an improved understanding about the key aspects associated operational decisions. The results of this research can assist offshore wind farm operators in developing midterm/long-term O&M plans. Through this extensive study, it is concluded that O&M related costs can be reduced significantly while availability and productivity of the turbines can be increased by selecting correct O&M fleet in terms of size and vessel capabilities.
13

Experimental investigation on a rotary compressor-expander heat pump performance

Kouhestani, Mohammad Salehi January 2016 (has links)
This research investigates the performance and impacts of integration of an internal expander within a typical heat pump system for the UK climate. In this novel system, the expansion process takes place within an expander rather than a traditional expansion valve, and it is expected that recovered work by the expander could provide a portion of the required power to drive the compressor. This project comprises a literature review, test rig design and fabrication, control system design instrument selection and calibration, data-logging system setup and installation, implementation of the BS standard for heat pumps performance testing, implementation of tests, and data collection according to relevant standards and data analysis. A test rig is developed and improved throughout this research. More than 20 sets of tests have been carried out between 2011 and 2014, and the most appropriate of them are presented and discussed. To determine the impact of expander integration, the rotary compressor-expander unit was tested with and without an expander, and the system COP, capacity, and isentropic efficiency are discussed. This thesis commences by introducing key drivers of this research, the energy crisis, global warming issues, and the contribution of the domestic sector to energy consumption in the UK. Then it continues by classifying heat pumps according to their energy sources. A detailed review and evaluation of major advanced heat pumps and refrigeration technologies then follows. Test rig design and development (according to the BS EN14511-2:2011) are discussed and outlined comprehensively. The test scenarios are presented, obtained data are discussed, test outcomes are evaluated, and the issues and their causes are addressed in detail. To address the issues and interpret the achieved results, theoretical analysis is carried out as well, then actual outcomes are compared with theoretical anticipated results and the differences are addressed. Results of compressor tests and system integration are presented separately and discussed comprehensively. The last chapter outlines conclusions and recommends further possible work on this research topic and offers practical suggestions to improve the performance of this developed system. However, this research reveals that such integration of an expander and compressor is not suitable in practice for typical UK winter conditions, though it may offer some benefits for extreme winter conditions where the ambient temperature drops to -10°C or below. In addition, it reveals that the designed compressor is not able to overcome a significant temperature increase (which is the case for medium-temperature heating applications and retrofit applications) due to internal refrigerant leakage.
14

Fatigue analysis and testing of wind turbine blades

Greaves, Peter Robert January 2013 (has links)
This thesis focuses on fatigue analysis and testing of large, multi MW wind turbine blades. The blades are one of the most expensive components of a wind turbine, and their mass has cost implications for the hub, nacelle, tower and foundations of the turbine so it is important that they are not unnecessarily strong. Fatigue is often an important design driver, but fatigue of composites is poorly understood and so large safety factors are often applied to the loads. This has implications for the weight of the blade. Full scale fatigue testing of blades is required by the design standards, and provides manufacturers with confidence that the blade will be able to survive its service life. This testing is usually performed by resonating the blade in the flapwise and edgewise directions separately, but in service these two loads occur at the same time. A fatigue testing method developed at Narec (the National Renewable Energy Centre) in the UK in which the flapwise and edgewise directions are excited simultaneously has been evaluated by comparing the Palmgren-Miner damage sum around the blade cross section after testing with the damage distribution caused by the service life. A method to obtain the resonant test configuration that will result in the optimum mode shapes for the flapwise and edgewise directions was then developed, and simulation software was designed to allow the blade test to be simulated so that realistic comparisons between the damage distributions after different test types could be obtained. During the course of this work the shortcomings with conventional fatigue analysis methods became apparent, and a novel method of fatigue analysis based on multi-continuum theory and the kinetic theory of fracture was developed. This method was benchmarked using physical test data from the OPTIDAT database and was applied to the analysis of a complete blade. A full scale fatigue test method based on this new analysis approach is also discussed.
15

Optimization of a CFD based design of a straight blade vertical axis wind turbine (SB-VAWT)

Almohammadi, Khaled Mohammad January 2014 (has links)
Enhancing the extraction of the wind energy in urban regions using micro and small wind turbines becomes a necessity with the increasing power consumption. The focus of this thesis is to optimize micro and small SB-VAWT performance by analysing CFD techniques, several modelling characteristics and design parameters where the performance is measured by the power coefficient. In this thesis, the SB-VAWT is optimized by employing sophisticated optimization techniques, namely such as the GA and the NLPQL, which are employed on response surfaces created from several design sampling methods. The optimization is based on three parameters, namely, camber, thickness and chord. A novel airfoil geometry has been introduced. The new airfoil geometry increases the power coefficient by about 42% at the optimized tip speed ratio and increases the peak of the turbine power coefficient by 4% at a low tip speed ratio. However, it was necessary to assess the computational process by examining the mesh and the computational method in order to ensure that the optimized design of the SB-VAWT is only resulting from the optimization process. Therefore, several physical phenomena have been investigate including the dynamic stall, laminar-turbulent transition and laminar bubbles. Also, several computational techniques and schemes have been critically analysed. Further, several mesh independency techniques have been implemented and it was found that the fitting method may be suitable for SB-VAWTs due to the presence of oscillations in the convergence of the power coefficient which may be caused by the presence of dynamic stall, laminar-turbulent transition and laminar bubbles. The physics of these flow conditions are only captured when the transitional model is employed. The optimization of the SB-VAWT in this thesis is based on a 2D model. It was found that the 2D model produce a results similar to the 3D model at mid span of the turbine blade. Therefore, the 2D model of the turbine sufficiently represents the flow physics around the blades qualitatively, and thus the 2D model is employed for the optimization of the SB-VAWT.
16

Novel methods for fatigue data editing for horizontal axis wind turbine blades

Pratumnopharat, Panu January 2012 (has links)
Wind turbine blades are the most critical components of wind turbines. Full-scale blade fatigue testing is required to verify that the blades possess the strength and service life specified in the design. Unfortunately, the test must be run for a long time period. This problem led the blade testing laboratories to accelerate fatigue testing time. To achieve the objective, this thesis proposes two novel methods called STFT- and WT-based fatigue damage part extracting methods which are based on short-time Fourier transform (STFT) and wavelet transform (WT), respectively. For WT, different wavelet functions which are Morl, Meyr, Dmey, Mexh and DB30 are studied. An aerodynamic computer code, HAWTsimulator, based on blade element momentum theory has been developed. This code is used to generate the sets of aerodynamic loads acting along the span of a ‘SERI-8 wind turbine blade’ in the range of wind speed from cut-in to cut-out. SERI-8 blades are installed on 65 kW wind turbines. Each set of aerodynamic loads is applied on the finite element model of the SERI-8 blade in structural software (ANSYS) to generate a plot of von Mises stress at the critical point on the blade versus wind speed. By relating this relationship to the wind speed data, the stress-time history at the critical point on the SERI-8 blade can be generated. It has the same sampling rate and length as the wind speed data. A concept of applying accumulative power spectral density (AccPSD) distribution with time to identify fatigue damage events contained in the stress-time history has been introduced in this thesis. For STFT, AccPSD is the sum of power spectral density (PSD) of each frequency band at each time interval in the spectrogram. For WT, AccPSD is the sum of PSD of wavelet coefficients of each scale at each time interval in the scalogram. It has been found that the locations of AccPSD spikes imply where the fatigue damage events are. Based on an appropriate AccPSD level called a cutoff level, the fatigue damage events can be identified at time location of the stress-time history. A fatigue computer code, HAWTfatigue, based on stress-life approach and Miner’s linear cumulative damage rule has been developed. Basically, the code is used for evaluating the fatigue damage and service lifetime of horizontal axis wind turbine blade. In addition, the author has implemented STFT- and WT-based fatigue damage part extracting methods into the code. Fatigue damage parts are extracted from the stress time history and they are concatenated to form the edited stress-time history. The effectiveness of STFT- and WTbased algorithms is performed by comparing the reduction in length and the difference in fatigue damage per repetition of the edited stress-time histories generated by STFT and WT to those of the edited stress-time history generated by an existing method, Time Correlated Fatigue Damage (TCFD) used by commercial software. The findings of this research project are as follows: 1. The comparison of the reduction in length of the edited stress-time histories generated by TCFD, STFT and WT indicates that WT with the Mexh wavelet has the maximum reduction of 20.77% in length with respect to the original length, followed by Meyr (20.24%), Dmey (19.70%), Morl (19.66%), DB30 (19.19%), STFT (15.38%), and TCFD (10.18%), respectively. 2. The comparison of the retained fatigue damage per repetition in the edited stress-time histories generated by TCFD, STFT and WT indicates that TCFD has the retained fatigue damage per repetition less than the original fatigue damage per repetition by 0.076%, followed by Mexh (0.068%), DB30 (0.063%), STFT (0.045%), Meyr (0.032%), Dmey (0.014%), and Morl (0.013%), respectively. 3. Both comparison of reduction in length and comparison in the retained fatigue damage per repetition of the edited stress-time histories suggest that WT is the best method for extracting fatigue damage parts from the given stress-time history. It has also been indicated that not only do STFT and WT improve accuracy of fatigue damage per repetition retained in the edited stress-time histories, but also they provide the length of the edited stress-time histories shorter than TCFD does. Thus, STFT and WT are useful methods for performing accelerated fatigue tests. 4. It has been found that STFT is controlled by two main factors which are window size and cutoff level. Also, WT is controlled by three main factors which are wavelet decomposition level, cutoff level and wavelet type. To conclude, the edited stress-time history can be used by blade testing laboratories to accelerate fatigue testing time. STFT- and WT-based fatigue damage part extracting methods proposed in this thesis are suggested as alternative methods in accelerating fatigue testing time, especially for the field of wind turbine engineering.
17

On the prediction of the effect of interstage liners in turbofan engines

Maldonado, Ana Luisa Pereira January 2016 (has links)
The current trends for next generation turbofan engines are towards shorter nacelles and increased distances between the fan and the outlet guide vanes. This leads to an overall reduction in lined surface areas as well as an increase in the relative importance of the interstage liner, which is the liner placed between the rotor blades and the stator vanes. The interstage is different in that the liner is subject to a mean flow with a strong swirl component and shear. This project will contribute to understanding and predicting the effect of the swirl on liner attenuation and consists of 4 steps: To model an eigenvalue problem that includes sheared and swirling mean flows and acoustic absorption, to develop a code based on this eigenvalue problem and to validate it, to compare results from this code with experimental results and to carry out a parametric study to evaluate how the swirling flow afects liner attenuation and optimum impedance. Two models were developed. The first one considers a ducted sheared mean flow and is based on the Pridmore-Brown equation and the second one takes into consideration a mean flow with swirl and shear and is based on the Linearized Euler Equations. For both cases an eigenvalue problem was obtained by applying the normal mode analysis to the governing equations together with the impedance boundary condition. Both models were discretized using a Finite Difference Method. The codes were exhaustively validated against predicted values obtained by other methods for uniform, sheared and swirling mean flows and hard-walled and lined ducts. The swirling mean flow, when present, is a combination of rigid body and vortex swirl. A cross-validation between the Finite difference code based on the Linearized Euler Equation and the JM66 code from Rolls-Royce was carried out for a more realistic case. Axial wavenumbers and pressure and velocity eigenvectors obtained with the JM66 code were compared with the current predictions. A comparison has been conducted of predictions from the current Finite Difference code with measured data at a single frequency for a range of spinning mode numbers. Qualitative agreement is obtained for the measured Power Transmission loss (TL) but the low Mach numbers and modest TL levels meant that the effect of swirl was small nad it was difficult to validate the accuracy at the Finite Difference code specifically for the swirl case. Finally, a parametric study was undertaken for hard-walled and lined ducts for realistic interstage conditions to evaluate the effect sound propagation in swirling flows. This confirmed that the effect of swirl is higher for radial modes near cut-off and tends to vanish for higher radial mode orders. The swirl strongly changes the modal content. When swirl is included, the modal distribution for positive and negative azimuthal mode orders is no longer symmetrical. The higher the swirling flow magnitude, the more the modal content is shifted to negative circumferential mode orders. Co-rotating modes become more cut-on and contra-rotating modes become more cut-off. When acoustic absorptive liners are considered, the swirl changes the liner optimum resistance and reactance and affects the optimum insertion loss. The optimum resistance becomes considerably higher and the change in optimal liner reactance is not as pronounced. The swirling flow also reduces attenuation; the insertion loss is lower when swirl is considered. As a conclusion, swirling flow should be considered when designing liners.
18

Improving wind turbine aerodynamic performance using iterative learning control applied to smart rotors

Blackwell, Mark W. January 2015 (has links)
Currently there is significant research into the inclusion of localised active flow control on wind turbine rotor blades, with the aim, in conjunction with collective and individual pitch control, of improving the aerodynamic performance of the rotor. These blades are termed smart rotors. The unique contribution of this research is the application of Iterative Learning Control to wind turbine smart rotors to reduce blade loading from lift disturbances. The smart devices act locally, at different spanwise positions, and include actuation to manipulate local lift (e.g. trailing edge flaps, blowing/suction, circulation control); sensing to determine the current turbine loading (e.g. pressure sensors, strain gauges, LIDAR); and a suitable control scheme to achieve predefined objectives. The principal objective is to reduce fatigue loads, although mitigating the effects of extreme loads is also of interest. The reduction of these loads leads to lighter, larger and more reliable turbines. Traditionally blade loads have been managed using stall regulation, pitch control, torque control or a combination of all three. Smart rotors are an evolutionary step in the control of turbines and have the advantage of deploying variable control along the blade with quicker response times to variations in flow conditions, leading to a potential increase in energy production, an increase in turbine reliability and a reduced energy requirements. The aerodynamic loads on a wind turbine blade have periodic and non-periodic components, and the nature of these strongly suggests the application of iterative learning control. The research within this PhD thesis employs a 2D computational fluid dynamics model (vortex panel method), with nonlinear wake effects, to represent flow past an aerofoil. The CFD model uses a potential flow approximation which is valid for inviscid and attached flow only. This is acceptable because smart devices typically operate under such conditions. Circulation control (actuation) and pressure sensors (load sensing) are modelled to represent a 2D section of a smart rotor. The model is used in conjunction with a first-order lag actuator model to undertake a detailed investigation into the level of control possible by, as in other areas, combining iterative learning control with classical control action with emphasis on how performance can be eeffectively measured. Typical turbine flow regimes are simulated by generating multiple upstream vortices, drifting turbine time periods, stochastic in flow conditions and a combination of all three regimes. Results indicate that cyclical and stochastic loadings on turbine blades can be effectively managed using Iterative Learning Control, with significant reductions in both fatigue and extreme loads for a range of flow conditions.
19

Performance analysis of doubly-fed induction generator (DFIG)-based wind turbine with sensored and sensorless vector control

Kareem, Amer Obaid January 2016 (has links)
Conventional energy sources are limited and pollute the environment. Therefore more attention has been paid to utilizing renewable energy resources. Wind energy is the fastest growing and most promising renewable energy source due to its economically viability. Wind turbine generator systems (WTGSs) are being widely manufactured and their number is rising dramatically day by day. There are different generator technologies adopted in wind turbine generator systems, but the most promising type of wind turbine for the future market is investigated in the present study, namely the doubly-fed induction generator wind turbine (DFIG). This has distinct advantages, such as cost effectiveness, efficiency, less acoustic noise, and reliability and in addition this machine can operate either in grid-connected or standalone mode. This investigation considers the analysis, modeling, control, rotor position estimation and impact of grid disturbances in DFIG systems in order to optimally extract power from wind and to accurately predict performance. In this study, the dynamic performance evaluation of the DFIG system is depicted the power quantities (active and reactive power) are succeed to track its command signals. This means that the decouple controllers able to regulating the impact of coupling effect in the tracking of command signals that verify the robust of the PI rotor active power even in disturbance condition. One of the main objectives of this study is to investigate the comparative estimation analysis of DFIG-based wind turbines with two types of PI vector control using PWM. The first is indirect sensor vector control and the other type includes two schemes using model reference adaptive system (MRAS) estimators to validate the ability to detect rotor position when the generator is connected to the grid. The results for the DFIG-based on reactive power MRAS (QRMRAS) are compared with those of the rotor current-based MRAS (RCMRAS) and the former scheme proved to be better and less sensitive to parameter deviations, its required few mathematical computations and was more accurate. During the set of tests using MATLAB®/SMULINK® in adjusting the error between the reference and adaptive models, the estimated rotor position can be obtained with the objective of achieving accurate rotor position information, which is usually measured by rotary encoders or resolvers. The use of these encoders will conventionally lead to increased cost, size, weight, and wiring ii complexity and reduced the mechanical robustness and reliability of the overall DFIG drive systems. However the use of rotor position estimation represents a backup function in sensor vector control systems when sensor failure occurs. The behavioral response of the DFIG-based wind turbine system to grid disturbances is analyzed and simulated with the proposed control strategies and protection scheme in order to maintain the connection to the network during grid faults. Moreover, the use of the null active and reactive reference set scheme control strategy, which modifies the vector control in the rotor side converter (RSC) contributes to limiting the over-current in the rotor windings and over-voltage in the DC bus during voltage dips, which can improve the Low Voltage Ride-through (LVRT) ability of the DFIG-based wind turbine system.
20

The performance of inward radial flow turbines under unsteady flow conditions

Newport, John D. January 1974 (has links)
No description available.

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