Flow Curvature Effects on Darrieus Turbine Blade Aerodynamics

Abstract

The effects of curvilinear flow on Darrieus turbine blade aerodynamics are described. Analysis shows that these effects can have a sizeable impact on performance for blades of large chord. Experimental data are presented which verify this forecast. Unusually large boundary-layer radial pressure gradients and virtually altered camber and incidence are identified as causal phenomena. Conformaf mapping techniques are used to transform geometric airfoils in curved flow to their virtual equivalents in rectilinear flow. It is argued that flow curvature is an important determinant of Darrieus turbine blade aerodynamic efficiency and that its proper consideration will yield performance improvements, even for blades of small chord. uncovered blade aerodynamic complexities which were beyond initial expectations. Most noteworthy are the peculiar aerodynamic phenomena associated with the orbital motion of the blades. In essence, these blades are subjected to a curvilinear flow and behave very differently than if they were immersed in a rectilinear flow. Furthermore, centrifugal forces alter the boundary layer of the airfoils rotating in this fashion. This situation presents problems in the design and analysis of all cross-flow wind turbines, because virtually all published airfoil data are derived from tests in rectilinear flow. Recent studiesl show that modest improvements in Cp yield desirable reductions in the cost of energy. Since these Cp improvements can be achieved by increasing blade aerodynamic efficiency, there exists ample incentive for considering the aerodynamic idiosyncrasies of rotating blades. In the material which follows, boundary-layer centrifugal effects will be discussed first. Treatment of this subject is brief; its significance has only recently been appreciated and extensive studies of the phenomenon have not been con- ducted. Flow curvature effects are treated next. A simple kinematic analysis demonstrates that the turbine blade relative inflow velocity and angle of attack are unique everywhere on the chord. It is then shown how conformal mapping techniques, which transform airfoils in the curved flow field to their virtual equivalents in rectilinear flow, may be used in the aerodynamic analysis of the turbine blades. The method indicates that flow curvature effects are strongly dependent upon the blade chord to turbine radius C/R. Experimental data are introduced for two sets of blades, both of NACA 0015 airfoil section. The first set of blades had C/R = 0.114 and the second set had C/R = 0.260.