Showing papers on "Blade element momentum theory published in 2002"

Blade Row Interaction in a High-Pressure Steam Turbine

Abstract: This paper presents a study of the three-dimensional flow field within the blade rows of a high-pressure axial flow steam turbine stage. Compound lean angles have been employed to achieve relatively low blade loading for hub and tip sections and so reduce the secondary losses. The flow field is investigated in a Low-Speed Research Turbine using pneumatic and hot-wire probes downstream of the blade row. Steady and unsteady numerical simulations were performed using structured 3D Navier-Stokes solver to further understand the flow field. Agreement between the simulations and the measurements has been found. The unsteady measurements indicate that there is a significant effect of the stator flow interaction in the downstream rotor blade. The transport of the stator viscous flow through the rotor blade row is described. Unsteady numerical simulations were found to be successful in predicting accurately the flow near the secondary flow interaction regions compared to steady simulations. A method to calculate the unsteady loss generated inside the blade row was developed from the unsteady numerical simulations. The contribution of various regions in the blade to the unsteady loss generation was evaluated. This method can assist the designer in identifying and optimizing the features of the flow that are responsible for the majority of the unsteady loss production. An analytical model was developed to quantify this effect for the vortex transport inside the downstream blade.Copyright © 2002 by ASME

The nebulous art of using wind tunnel aerofoil data for predicting rotor performance

Abstract: The objective of this study was threefold: to evaluate different two-dimensional S809 aerofoil data sets in the prediction of rotor performance; to compare blade element momentum rotor predicted results with lifting surface, prescribed wake results; and to compare the NASA Ames combined experiment rotor measured data with the two different performance prediction methods. The S809 aerofoil data sets evaluated included those from Delft University of Technology, Ohio State University and Colorado State University. Substantial differences in prediction performance resulted from the different two-dimensional aerofoil data sets. Predicted performance comparison with NASA Ames data documents shortcomings of these methods and recommends the use of the lifting surface, prescribed wake method over blade element momentum theory for future analytical improvements. Copyright © 2002 John Wiley & Sons, Ltd.

An Integrated Time-Domain Aeroelasticity Model for the Prediction of Fan Forced Response due to Inlet Distortion

Abstract: The forced response of a low aspect-ratio transonic fan due to different inlet distortions was predicted using an integrated time-domain aeroelasticity model A time-accurate, nonlinear viscous, unsteady flow representation was coupled to a linear modal model obtained from a standard finite element formulation The predictions were checked against the results obtained from a previous experimental program known as Augmented Damping of Low-aspect-ratio Fans (ADLARF) Unsteady blade surface pressures, due to inlet distortions created by screens mounted in the intake inlet duct, were measured along a streamline at 85 percent blade span Three resonant conditions, namely 1F/3EO, IT & 2F/8EO and 2S/8EO, were considered Both the amplitude and the phase of the unsteady pressure fluctuations were predicted with and without the blade flexibility The actual blade displacements and the amount of aerodynamic damping were also computed for the former case A whole-assembly mesh with about 2,000,000 points was used in some of the computations Although there were some uncertainties about the aerodynamic boundary conditions, the overall agreement between the experimental and predicted results was found to be reasonably good The inclusion of the blade motion was shown to have an effect on the unsteady pressure distribution, especially for the 2F/1T case It was concluded that a full representation of the blade forced response phenomenon should include this feature

Fatigue Estimation for a Rotating Blade of a Wind Turbine

Abstract: Le calcul de la fatigue d'une pale rotative peut nous aider a prevoir les problemes de structure generalement rencontres dans l'utilisation des aerogenerateurs (eoliennes rapides), ou la rupture des pales est tres frequente. Ce calcul doit passer par plusieurs etapes parmi elles le calcul des frequences et des modes propres ainsi que le calcul des deplacements et des contraintes qui agissent sur ces pales. Dans ce travail deux methodes differentes sont utilisees pour le calcul des modes propres : la premiere (...)

The prediction of unsteady three-dimensional aerodynamics on wind turbine blades

Abstract: After introducing the main features of the aerodynamics of wind turbines, a review of key theoretical studies and aerodynamic modelling methods provides the opportunity to focus on predictive methods and the main technical challenges associated with the aerodynamics of HAWTs. The basic aerodynamic method adopted in this study, a classic Blade Element Momentum theory model, BEM, is described next and its extension to yawed flow is detailed for completeness. Analysis then focuses on how stall delay due to three-dimensional effects can be predicted on a HAWT. Implementation of a semi-empirical stall delay model shows sensitivity to blade geometry but no dependency on wind velocity or rotational speed. This seems to be physically incorrect and suggests that a deeper understanding of 3-D effects is still needed if better algorithms are to be developed. The work then examines the onset of dynamic stall. A 2-D semi-empirical correlation of vortex stall onset, developed previously at Glasgow University, is implemented and validated through available field data from the NREL turbine Phases II and IV. The comparison of measured and predicted locations of dynamic stall onset highlights some interesting features of the three-dimensionality of the process; after the local inception, earlier dynamic stall appears to be triggered in adjacent stations. An attempt to study how 3-D stall delay interacts with the onset of dynamic stall, shows that stall delay appears not to influence the inception of dynamic stall in the way it does static stall. Moreover, the firsts signs of dynamic stall onset are generally best characterised by the correlation when it assumes locally 2-D flow. This is a significant result, as it demonstrates that the earliest signs of dynamic stall onset on wind turbines can be correctly predicted using 2-D tools. A closer examination of the discrepancies between the predictions and measurements has highlighted the particular aerodynamic characteristics of the S809 aerofoil, utilised as the blade section of the NREL turbines. The unusual stalling characteristics of this aerofoil bring into question the significance of the static stall angle in relation to dynamic stall. It is show that other features of the static behaviour may provide a more appropriate link to dynamic stall for some aerofoils. Finally, the phenomenon of tower shadow on a downwind turbine is studied. Unaveraged pressure measurements and integrated normal force coefficients from tests conducted at Glasgow University are analysed. The analysis highlights many interesting features of the tower shadow response. In particular, as the blade enters the tower shadow region, there is a rapid reduction in normal force due to the tower wake velocity deficit. As the blade leaves the tower shadow, the recovery is consistently slower and more progressive and apparently extends further than the edge of the velocity deficit region. These observations are then used in a examination of tower shadow modelling. A steady model, based on a cosine shaped velocity deficit is evaluated by comparison with the wind tunnel measurements. Unfortunately neither the phase nor the intensity of the response is adequately captured. This leads to the implementation of a new model, based on classic unsteady thin aerofoil theory that accounts for the aerofoil wake induced velocities. The unsteady model captures, in a satisfactory manner, the global response of the blade through the tower shadow region, with a negligible computational cost.

Computation for Dynamic Stress of Steam Turbine Blade by Using the Method of Harmonic Response Analysis

Abstract: The method of harmonic response analysis is applied to the analysis of dynamic stress of steam turbine blade in this paper. First the steam turbine blade is modeled and its dynamic frequencies are calculated by using the finite element method. Then, the dynamic response of displacements of blade is computed by using the harmonic response analysis method of modal superposition, after that, the dynamic stress of blade is obtained by the expansion analysis of dynamic responses.

Validation of a Two-Dimensional Numerical Model for Vortex/Blade Interaction

Abstract: The numerical method developed previously to analyze two-dimensional vortex-blade interaction problems is validated using recently measured vortex-induced blade vibration data. It assumed a vortex lattice method to calculate the flow field assuming a distribution of sources and discrete vortices on the blade surfaces and a free wake model for the wake flow. A discrete vortex tracking technique in Lagrangian frame is used to track the path of the vortices. The blade is modeled as elastic structures with two-degree-of-freedom in plunging and pitching direction. The fully coupled fluid-structure interaction problem is resolved by means of a time-marching technique. The flow-field is assumed to be inviscid, incompressible and two-dimensional, with no flow separation occurring on the surfaces of the blade. Two cases were examined and they included a blade-vortex interaction and a blade vortex street interaction problem. In the blade-vortex interaction case, the blade is modeled as rigid; therefore, the response of the structure is purely aerodynamics. The calculated variation of the lift coefficient of the blade with the horizontal missed distance of the convected vortex compares well with known experimental results. In the blade vortex street interaction case, the blade is modeled as elastic and is under the unsteady excitation from a Karman vortex street. The calculated blade responses due to vortex-induced vibration are compared with some recently measured vibration characteristics of a flat plate placed behind a cylinder at different separation distance. Good agreement between calculations and measured vibration amplitudes of the plate at its mid-span is obtained, thus indicating that the numerical method gives a viable model for the analysis of the aerodynamics and structural response in vortex/blade interaction problems.Copyright © 2002 by ASME

Application of circulation control aerofoils to wind turbines

Abstract: Circulation control aerofoils potentially offer an additional means of load and power control for horizontal axis wind turbines by virtue of their rapid response time. Their suitability for these tasks has been assessed with respect to the power which they absorb, their interaction with aerofoils used on modern wind turbines, the infrastructure or hardware which they require and the degree to which they can affect the loads experienced by the turbine blades and other major components. It has been determined that the type of circulation control aerofoil most suited to use on wind turbine blades are those of the jet flap type and it has been realised that an ability to shed, as well as increase loads is advantageous in this application. To this end the behaviour of both negatively and positively deflected jets have been investigated with a two-dimensional computational fluid dynamics code, validated in the course of this work for such modelling. Particular emphasis has been placed on minimising the input power requirements of the circulation control aerofoils and in proposing an overall system that has the required level of robustness and reliability. A 2MW turbine has been modelled with a blade element momentum theory code in order to compare performance with and without circulation control aerofoils. These initial results show that there may be some positive benefits to be gained, but that the energy demands of the system place a hard limit on the degree to which circulation control aerofoils can determine the forces experienced by the turbine.

Numerical Simulation of the 3D Laminar Viscous Flow on a Horizontal-axis Wind Turbine Blade

Abstract: The aim of this paper is to describe the methodology followed in order to determine the viscous effects of a uniform wind on the blades of small horizontal-axis wind turbines that rotate at a constant angular speed. The numerical calculation of the development of the three-dimensional boundary layer on the surface of the blades is carried out under laminar conditions and considering flow rotation, airfoil curvature and blade twist effects. The adopted geometry for the twisted blades is given by cambered thin blade sections conformed by circular are airfoils with constant chords. The blade is working under stationary conditions at a given tip speed ratio, so that an extensive laminar boundary layer without flow separation is expected. The boundary layer growth is determined on a non-orthogonal curvilinear coordinate system related to the geometry of the blade surface. Since the thickness of the boundary layer grows from the leading edge of the blade and also from the tip to the blade root, a domain transfo...