Alessandro Bianchini

Start-Up Behavior of a Three-Bladed H-Darrieus VAWT: Experimental and Numerical Analysis

Despite increasing attention paid by both the industrial and the academic worlds, an effective diffusion of Darrieus wind turbines is still hindered by productivity lower than that of classical HAWTs, mainly connected to the critical behavior of these machines during the transient phases and in particular, during the start-up transitory, which has not been investigated in depth in the past. In this paper, a numerical code for the evaluation of the transient behavior of H-Darrieus turbines is presented. The time-dependent code was based on a theoretical approach derived from the Momentum Models and completed by several sub-models for the evaluation of the main secondary and parasitic effects. The new software was validated with an extended experimental campaign in a wind tunnel on a three-bladed H-Darrieus turbine, obtaining constant agreement with experimental data. A sensitivity analysis was then performed in order to investigate the start-up behavior of a generic small size three-bladed H-Darrieus rotor. In particular, for a fixed turbine layout, the influence of the airfoil type and the blade shape on the startup capabilities of the rotor was investigated as a function of the initial position of the rotor and the oncoming wind velocity.Copyright

An Improved Model for the Performance Estimation of an H-Darrieus Wind Turbine in Skewed Flow

Small turbines are considered one of the most promising technologies for an effective diffusion of renewable energy sources in new installation contexts with a high degree of integration with human activity (e.g. the urban environment). In these new installations, however, the real working conditions can be far from the nominal ones. In particular, the turbine functioning can be noticeably affected by misalignments between the oncoming flow and the axis of the rotor; differently from horizontal-axis wind turbines, whose performance is decreased by a skew angle, H-Darrieus turbines are thought to take advantage from this condition in some cases. In this study, an improved model for the performance prediction of H-Darrieus rotors under skewed flow was developed. In detail, a theoretical approach based on Momentum Models was properly modified to account for the variations induced by the new direction of the flow which invests the rotor. In particular, the modifications in the aerodynamic characteristics of the airfoils, the swept area and the streamtubes distribution were modeled. The performance predictions of the new model were compared both with experimental data available in the technical literature and with the results of wind tunnel tests purposefully carried out on a full scale model of an H-Darrieus turbine. Notable agreement has been constantly obtained between simulations and experiments.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part I: Flow Curvature Effects

A better comprehension of the aerodynamic behavior of rotating airfoils in Darrieus Vertical-axis wind turbines (VAWTs) is crucial both for the further development of these machines and for improvement of conventional design tools based on zero or one-dimensional models (e.g. BEM models).When smaller rotors are designed with high chord-to-radius (c/R) ratios so as not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be heavily impacted by a virtual camber effect imparted on the blades by the curvilinear flow they experience.To assess the impact of this virtual camber effect on blade and turbine performance, a standard NACA0018 airfoil and a NACA0018 conformally transformed such that the airfoil’s chord line follows the arc of a circle, where the ratio of the airfoil’s chord to the circle’s radius is 0.25 were considered. For both airfoils, wind tunnel tests were carried out to assess their aerodynamic lift and drag coefficients for Reynolds numbers of interest for Darrieus VAWTs.Unsteady CFD calculations have been then carried out to obtain curvilinear flow performance data for the same airfoils mounted on a Darrieus rotor with a c/R of 0.25. The blade incidence and lift and drag forces were extracted from the CFD output using a novel incidence angle deduction technique.According to virtual camber theory, the transformed airfoil in this curvilinear flow should be equivalent to the NACA0018 in rectilinear flow, while the NACA0018 should be equivalent to the inverted transformed airfoil in rectilinear flow.Comparisons were made between these airfoil pairings using the CFD output and the rectilinear performance data obtained from the wind tunnel tests and XFoil output in the form of pressure distributions and lift and drag polars.Blade torque coefficients and turbine power coefficient are also presented for the CFD VAWT using both blade profiles.Copyright

A Model to Account for the Virtual Camber Effect in the Performance Prediction of an H-Darrieus VAWT Using the Momentum Models

This study reports an investigation on the influence of the virtual camber effect on the performance prediction of an H-Darrieus VAWT using a theoretical approach based on the Momentum Models. In detail, a simplified model is proposed which highlights a limit point below which the virtual camber effect must be neglected to correctly evaluate the performance of the airfoils. This correction provides a more accurate description of the unstable region of the power curve, for which very few in-depth studies are available. An accurate description of the left side of the power curve is, however, of particular importance for a correct prediction of the self-starting capabilities of the rotor, which are presently considered as a decisive requirement for small-power machines. On this basis, the effects of the new virtual camber estimation on the transient study of an H-Darrieus rotor are also presented here. This has been carried out by means of a specifically developed numerical code. All the theoretical conjectures and the numerical models have been validated with the experimental data of two wind-tunnel campaigns on two different models of H-Darrieus turbine.

Parametric and Comparative Assessment of Navier-Stokes CFD Methodologies for Darrieus Wind Turbine Performance Analysis

Navier-Stokes computational fluid dynamics simulations are expected to provide the basis for a deeper understanding of the real behavior of vertical-axis wind turbines. The prediction of the flow field past Darrieus rotors, a popular turbine of this type, requires a good resolution of both the unsteadiness caused by the periodic variation of the modulus and direction of the relative velocity perceived by the blades, and the interaction between the wakes shed by the blades in the upstream region of the rotor and the downstream blades traveling through such wakes. This paper presents a comparative assessment of the predictive capabilities of two substantially different timedependent Navier-Stokes Reynolds-averaged-based approaches to the analysis of Darrieus turbines. One is based on a commercial code, and the other on an academic research code. A Darrieus rotor configuration previously analyzed by other researchers was selected for this study, which focused on the turbine flow at a tip-speed ratio of 3.3. Aggregate power coefficient comparisons at other regimes are also provided. Solution sensitivity analyses to spatial and temporal grid refinement, domain size and boundary condition modeling aspects for each of the two approaches are provided in the study. A very good agreement is obtained between the two simulation sets, and a fairly good agreement is found between both simulations and available measured data.

A Systematic Approach to Estimate the Impact of the Aerodynamic Force Induced by Rotating Stall in a Vaneless Diffuser on the Rotordynamic Behavior of Centrifugal Compressors

One of the main challenges of the present industrial research on centrifugal compressors is the need of extending the left margin of the operating range of the machines. As a result, interest is being paid in accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine to further operate, in case their amplitude is too high. These vibrations are often contained thanks to the stiffness of the oil journals.Centrifugal compressors design is, however, going towards alternative journal solutions having lower stiffness levels (e.g. Active Magnetic Bearings or Squeeze Film Dampers), which hence will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in presence of dynamic external forces like those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions.Within this scenario, experimental tests were carried out on an industrial impeller operating at high peripheral Mach number. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings.The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement.Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.© 2013 ASME

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part II: Post-stall Data Extrapolation Methods

Accurate post-stall airfoil data extending to a full range of incidences between −180° to +180° is important to the analysis of Darrieus vertical-axis wind turbines (VAWTs) since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios encountered during startup.Due to the scarcity of existing data extending much past stall, and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, BEM codes). Most of these models assume post-stall performance to be virtually independent of profile shape.In this study, wind tunnel tests were carried out on a standard NACA0018 airfoil and a NACA 0018 conformally transformed to mimic the “virtual camber” effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25.Unsteady CFD results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades.Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna-Corrigan, Montgomerie and Kirke, was analyzed and discussed.Copyright

Energy-Yield-Based Optimization of an H-Darrieus Wind Turbine

Small-size H-Darrieus wind turbines are gaining more popularity in the wind energy market, thanks to some particular benefits (simplicity, reliability and low noise emissions) and to the efforts of industrial manufacturers to propose new exterior solutions coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution.With this goal in mind, 5400 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. To this purpose, the wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The limits related to the wind turbine start-up behavior and to the structural loads (i.e. maximum rotational speed and maximum wind velocity) were also taken into account.The analysis, developed in terms of dimensionless parameters, highlighted the configurations which ensure the largest annual energy yield for each wind distribution. In addition, the differences between the results of a design process oriented to the maximum power output or to the maximum annual-energy-yield were analyzed; in particular, the comparison showed that a design oriented to the maximum energy-yield is assumed to provide a notable increase in the extracted energy (up to around 20%, with the selected design assumptions) whenever wind distributions with low average wind speeds are considered.Copyright

Effects of Airfoil's Polar Data in the Stall Region on the Estimation of Darrieus Wind Turbine Performance

Interest in vertical-axis wind turbines (VAWTs) is experiencing a renaissance after most major research projects came to a standstill in the mid 1990s, in favor of conventional horizontal-axis turbines (HAWTs). Nowadays, the inherent advantages of the VAWT concept, especially in the Darrieus configuration, may outweigh their disadvantages in specific applications, like the urban context or floating platforms. To enable these concepts further, efficient, accurate, and robust aerodynamic prediction tools and design guidelines are needed for VAWTs, for which low-order simulation methods have not reached yet a maturity comparable to that of the blade element momentum theory for HAWTs’ applications. The two computationally efficient methods that are presently capable of capturing the unsteady aerodynamics of Darrieus turbines are the double multiple streamtubes (DMS) theory, based on momentum balances, and the lifting line theory (LLT) coupled to a free vortex wake model. Both methods make use of tabulated lift and drag coefficients to compute the blade forces. Since the incidence angles range experienced by a VAWT blade is much wider than that of a HAWT blade, the accuracy of polars in describing the stall region and the transition toward the “thin plate like” behavior has a large effect on simulation results. This paper will demonstrate the importance of stall and poststall data handling in the performance estimation of Darrieus VAWTs. Using validated CFD simulations as a baseline, comparisons are provided for a blade in VAWT-like motion based on a DMS and a LLT code employing three sets of poststall data obtained from a wind tunnel campaign, XFoil predictions extrapolated with the Viterna–Corrigan model and a combination of them. The polar extrapolation influence on quasi-steady operating conditions is shown and azimuthal variations of thrust and torque are compared for exemplary tip-speed ratios (TSRs). In addition, the major relevance of a proper dynamic stall model into both the simulation methods is highlighted and discussed. [DOI: 10.1115/1.4034326]

Some Guidelines For The Experimental Characterization Of Vaneless Diffuser Rotating Stall In Stages Of Industrial Centrifugal Compressors

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors, as it is often connected with the onset of detrimental subsynchronous vibrations which can prevent the machine from operating beyond this limit.With particular reference to the vaneless diffuser stall, much research has been directed at investigating the physics of the phenomenon, the influence of the main design parameters and the prediction of the stall inception. Few of them, however, focused attention on the evaluation of the aerodynamic unbalance due to the induced pressure field in the diffuser, which, however, could provide a valuable contribution to both the identification of the actual operating conditions and the enhancement of the compressor operating range by a suitable choice of the control strategy.Although advanced experimental techniques have been successfully applied to the recognition of the stall pattern in a vaneless diffuser, the most suitable solution for a wider application in industrial test-models is based on dynamic pressure measurements by means of a reduced number of probes. Within this context, a procedure to transpose pressure measurements into the spatial pressure distribution was developed and validated on a wide set of industrial test-models.In this work, the main guidelines of the procedure are presented and discussed, with particular reference to signals analysis and manipulation as well as sensors positioning.Moreover, the prospects of using a higher number of sensors is analyzed and compared to standard solutions using a limited probes number.Copyright