Blade Element Momentum theory and CFD modeling as a tool for optimizing wind turbine blade design

Abstract

The present chapter focuses on the comparison of the capabilities of Blade Element Momentum Theory (BEMT) in relation to computational fluid dynamics (CFD) modeling as a tool for the design and performance optimization of a horizontal axial wind turbine (HAWT). A generated blade is examined in different scales by BEMT with the use of the QBlade software. The same wind turbine blade design was then incorporated in a detailed 3D CFD model (in ANSYS CFX). Computations were performed and the results were compared to the ones produced with the BEMT method. For the CFD modeling, a National Advisory Committee for Aeronautics (NACA) profile was initially validated through a two-dimensional analysis and flow field investigation regarding lift and drag coefficients for a variety of angles of attack (AoA). For computation reasons, a rotating domain was applied. The domain is discretized into 4,320,733 elements, most of which are tetrahedral, creating 781,582 nodes. An extra inflation layer is used on the turbine boundary and mesh density is higher in that vicinity. After the validation of the two-dimensional analysis, the wind turbine blade design was incorporated in a detailed 3D CFD model and computations were performed and compared to the ones of the BEMT method. Detailed transition formulas were applied and compared and a mesh independent solution was achieved. Furthermore, pressure and velocity distribution on the blade are analyzed and Cp graphs were produced. The shear stress transport (SST) turbulence model capability to simulate the flow around an airfoil in the pro stall region was verified, while, the angle of attack at which stall begins could also be predicted using CFD modeling. The study revealed the superior performance and advantages of CFD modeling in relation to BEMT since CFD can take into account the 3D effects of actual flow around a turbine blade which cannot be obtained by BEMT methodology.