Computational Study of Stalled Wind Turbine Rotor Performance

摘要

Simplification of the aerodynamic control of large horizontalaxis wind turbines (HAWTs) has been identified as an important steptowards improved reliability and reduced cost. At present themajority of large HMrrs use active control to regulate power andloads. A simpler strategy is to use the inherent stalling of therotor blades in high winds to limit power and loads.Unfortunately the performance of stall regulated HAWTs 1S poorlyunderstood; current performance models often fail to correctlypredict peak power levels. The benefits of passive control of powerand loads cannot be utilised because of this uncertainty.This study examines the possible reasons for the poor performanceof current prediction techniques 1n high winds with the objective offonmulating a new model.The available experimental evidence suggests that rotor stall iscaused by turbulent separation at the rear of the blade aerofoil,growing in extent from the root in increasing wind. This 'picture'of the stalling HAW! rotor forms the basis of the approach. The newmodel consists of a prescribed vortex wake, first order panel method(extended to represent the viscous region of trailing edgeseparation) and three dimensional integral boundary layer directlycoupled in an iterative scheme.A sensitivity study of rotorindicates that the most importantperformance to wake geometryfactor is the rate at which thewake is convected downstream. However, it is found that stalledpower levels are insensitive to wake geometry; the study concludesthat the problem of poor prediction of high wind performance lies onthe rotor blades.Before using the complete code to calculate the performance of arotor it 1S first tuned for the aerofoils used on the blade.Aerofoil perfonmance characteristics measured in a wind tunnel aresynthesised by the model. Ideally these characteristics shouldinclude measured pressure profiles below and above stall.Validation of the complete code against detailed measurementstaken under controlled conditions on a three metre diameter machineindicates significant differences in the perfonmance of aerofoilsections on a wind turbine blade when compared to the same sectionwhen tested in a wind tunnel. Derived lift coefficients show areduced lift curve slope and more gentle delayed stall.Similar results are found when the code is applied to two Danishstall regulated machines. These two machines although having verysimilar geometries and using the same family of aerofoils do howevershow differences in derived post stall drag. This is thought to bedue to the different thickness distributions of the two rotors.The validation and applications of the new model show that it canaccurately predict the peak power level of stall regulated machines.