582 research outputs found
Experimental Study Of Flow Field Of An Aerofoil Shaped Diffuser With A Porous Screen Simulating The Rotor
No full textThis study presents an experimental investigation on a diffuser augmented wind turbine (DAWT). A screen mesh is used to simulate the energy extraction mechanisms of a wind turbine in experiment. Different screen porosities corresponding to different turbine loading coefficients are tested. Measurements of the axial force and of the velocity distribution in radial direction are reported. The general purpose is to highlight the dependency between the diffuser and the screen, and to compare the radial velocity distributions in the diffuser between unloaded and loaded conditions. It is shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition. Moreover, the thrust on the diffuser largely depends on the screen loading. For the present configuration, the thrust on the screen with high loading coefficient contributes for more than 70% of the total thrust on the DAWT. Smoke visualizations and radial velocity profiles reveal that the high loading screen induces flow separation on the outer surface of the diffuser, justifying the results of the thrust measurements. It is also inferred that the flow separation leads to loss of thrust and has a great effect on the total pressure drag. It should be emphasized that the experimental results indicate that the flow field around the diffuser is strongly affected by the choice of screen porosity, that is, turbine loading. And that, the thrust coefficient of the diffuser does not show a linear dependence on the thrust coefficient of the screen. The axial momentum theory, therefore, is not a solid predictor for DAWT performance with high loaded screens
Computational Study Of Diffuser Augmented Wind Turbine Using Actuator Disc Force Method
No full textIn this paper, a computational approach, based on the solution of Reynolds-averaged-Navier–Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported. In order to reduce the computational cost, the turbine is modeled as an actuator disc (AD) that imposes a resistance to the passage of the flow. The effect of the AD is modeled applying two body forces, upstream and downstream of the AD, such that they impose a desired pressure jump. Comparison with experiments carried out in similar conditions shows a good agreement suggesting that the adopted methodology is able to carefully reproduce real flow features
Experimental investigation on the effect of the duct geometrical parameters on the performance of a ducted wind turbine
No full textThis paper reports an experimental investigation on the effect of the duct geometry on the aerodynamic performance of an aerofoil shaped ducted wind turbine (DWT). The tested two-dimensional model is composed of an aerofoil equipped with pressure taps and a uniform porous screen. The experimental setup is based on the assumption that the duct flow is axisymmetric and the rotor can be simulated as an actuator disc. Firstly, different tip clearances between the screen and the aerofoil are tested to point out the influence of this parameter on the DWT performance in terms of aerofoil pressure distribution, aerofoil lift and flow field features at the duct exit area. Then, the combined effect of tip clearance, of the angle of attack and of the screen position along the aerofoil chord is evaluated through a Design of Experiments (DoE) based approach. The analysis shows that, among the analysed range of design factor variation, increasing angle of attack and the tip clearance leads to a beneficial effect on the lift and back-pressure coefficients, while they show a poor dependence upon the screen axial position. Finally, the configuration characterized by the maximum value of all three main factors (15 degree of angle of attack, 5% of tip clearance and 30% backward to the nozzle plane), has the best values of lift coefficient and back-pressure coefficient.Wind Energ
Towards improving the aerodynamic performance of a ducted wind turbine: A numerical study
No full textThis paper aims to study the aerodynamic performance of ducted wind turbines (DWT) using inviscid and viscous flow calculations by accounting for the mutual interaction between the duct and the rotor. Two generalized duct cross section geometries are considered while the rotor is modelled as an actuator disc with constant thrust coefficient. The analysis shows the opportunity to significantly increase the overall aerodynamic performance of the DWT by a correct choice of the optimal rotor loading for a given duct geometry. Present results clearly indicate that the increased duct cross section camber leads to an improved performance for a DWT. Finally, some insights on the changes occurring to the performance coefficients are obtained through a detailed flow analysis
Effects of yawed inflow on the aerodynamic and aeroacoustic performance of ducted wind turbines
No full textDucted Wind Turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform inflows might be the cause of aerodynamic and acoustic performance degradation. For this reason, an aerodynamic and aero-acoustic analysis of DWTs in yawed inflow condition is performed for two duct geometries: a baseline commercial DWT model, DonQi®, and one with a duct having a higher cross-section camber with respect to the baseline, named DonQi D5. The latter has been obtained from a previous optimization study. A numerical investigation using Lattice-Boltzmann Very-Large-Eddy Simulations is presented. Data confirm that the aerodynamic performance improvement, i.e. increase of the power coefficient, is proportional to the increase of the duct thrust force coefficient. It is found that, placing the DWT at a yaw angle of 7.5 , the aerodynamic performances of the DonQi D5 DWT model are less affected by the yaw angle. On the other hand, this configuration shows an increase of broadband noise with respect to the baseline DonQi® one, both in non-yawed and yawed inflow conditions. This is associated to turbulent boundary layer trailing edge noise due to the turbulent flow structures developing along the surface of the duct
Ducted wind turbines in yawed flow: A numerical study
No full textDucted wind turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform
flows are caused by the presence of buildings or other surface discontinuities. For this reason, the aerodynamic
performance of DWTs in yawed-flow conditions must be characterized depending upon their geometric parameters and operating conditions. A numerical study to investigate the characteristics of flow around two DWT
configurations using a simplified duct-actuator disc (AD) model is carried out. The analysis shows that the aerodynamic performance of a DWT in yawed flow is dependent on the mutual interactions between the duct and
the AD, an interaction that changes with duct geometry. For the two configurations studied, the highly cambered
variant of duct configuration returns a gain in performance by approximately 11 % up to a specific yaw angle
(α = 17.5◦
) when compared to the non-yawed case; thereafter any further increase in yaw angle results in a performance drop. In contrast, performance of less cambered variant duct configuration drops for α > 0
◦
. The gain
in the aerodynamic performance is attributed to the additional camber of the duct that acts as a flow-conditioning
device and delays duct wall flow separation inside of the duct for a broad range of yaw angle
Multi-element ducts for ducted wind turbines: A numerical study
No full textMulti-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.</p
Simulation of the phenological development of wheat and maize at the global scale
No full textTo derive location-specific parameters that reflect the geographic differences among cultivars in vernalization requirements, sensitivity to day length (photoperiod) and temperature, which can be used to simulate the phenological development of wheat and maize at the global scale. Location: Global. Methods: Based on crop calendar observations and literature describing the large-scale patterns of phenological characteristics of cultivars, we developed algorithms to compute location-specific parameters to represent this large-scale pattern. Vernalization requirements were related to the duration and coldness of winter, sensitivity to day length was assumed to be represented by the minimum and maximum day lengths occurring at a location, and sensitivity to temperature was related to temperature conditions during the vegetative development phase of the crop. Results: Application of the derived location-specific parameters resulted in high agreement between simulated and observed lengths of the cropping period. Agreement was especially high for wheat, with mean absolute errors of less than 3 weeks. In the main maize cropping regions, cropping periods were over- and underestimated by 0.5-1.5 months. We also found that interannual variability in simulated wheat harvest dates was more realistic when accounting for photoperiod effects. Main conclusions: The methodology presented here provides a good basis for modelling the phenological characteristics of cultivars at the global scale. We show that current global patterns of growing season length as described in cropping calendars can be largely reproduced by phenology models if location-specific parameters are derived from temperature and day length indicators. Growing seasons can be modelled more accurately for wheat than for maize, especially in warm regions. Our method for computing parameters for phenology models from temperature and day length offers opportunities to improve the simulation of crop productivity by crop simulation models developed for large spatial areas and for long-term climate impact projections that account for adaptation in the selection of varieties
Cost-sensitivity Analyses for Gearbox Condition Monitoring Systems Offshore
No full textTo compete more successfully with other sources of energy, a decrease in the costs of offshore wind energy needs to be achieved. Operation and maintenance costs represent a large share of these costs. In order to reduce these costs new developments and strategies are considered for operation and maintenance of wind turbine components. Condition monitoring systems (CMSs) could be a vital tool to decrease these costs, especially for expensive components such as a gearbox. This thesis focusses on the gearbox as it is the component, with one of the highest downtime per failure and failure costs. A literature study reveals that the replacement of a gearbox offshore might lead to months of downtime and costs might sum up to one million euro for a 6 MW wind turbine. In order to prevent such high costs, CMSs are assessed in this thesis. CMSs comprise of sensors providing data, which reflects the health status of the component. This data is subsequently analysed by a data-mining technique, capable of detecting trends and anomalies to predict upcoming failures. If the system is sufficiently accurate, a large failure can be prevented, which leads to significant savings in the lifetime gearbox maintenance costs A cost-benefit study is performed to determine the required performance of a CMS in order to be profitably implemented in an offshore wind turbine. The CMS performance is described by two parameters: one reflecting the ability of the system to prevent large gearbox failures, and a second parameter describing its ability to prevent waiting downtime, caused by weather window waiting time, spare part logistics and vessel mobilisation. Based on Monte Carlo simulations, the gearbox maintenance costs are quantified over the wind turbine lifetime. Subsequently a sensitivity analysis is performed. Results show that differences in gearbox failure rate and wind farm distance significantly affect the maintenance costs. Hence to break-even different performance requirements of the CMS are necessary. The model reveals that a system, capable of preventing large failures and/or preventing waiting downtime, can reduce the lifetime maintenance costs to a great extent. The actual performance of a CMS, needed to break-even, can be considered low, as the condition monitoring costs are in no proportion to the total gearbox maintenance costs and thus the potential revenue of the CMS.Aerospace EngineeringSustainable Energy Technolog
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