Numerical Analysis of Wells Turbine Aerodynamics

Abstract

Ocean wave power can be harnessed by means of a Wells turbine driven by a bi-directional airflow above an oscillating water column. While the Wells turbine research budget remains small, full-scale experiments are prohibitively expensive. Computational fluid dynamics (CFD) is a possible way of generating full-scale data. This paper summarizes a numerical study of Wells turbine performance and aerodynamics using a CFD method, and makes recommendations about its use for Wells turbine studies. Calculations have been performed for a monoplane turbine comprised of straight NACA 0015 blades at a stagger angle of 90 deg, Reynolds number 8 x 105, tip Mach number 0.4, and hub-to-tip ratio 0.6. Flow coefficient, tip clearance, and blade number were varied. The predictions agree favorably with experimental data. Differences can be explained partly by geometric differences between the experimental and numerical turbines. Because of the 90-deg stagger angle of the turbine, accurate predictions require fine resolution of the blade leading-edge region. This makes performance calculations expensive, and it is probably more prudent to use simpler predictive methods. CFD, however, is most useful for the study of turbine aerodynamics. Nomenclature CT = torque coefficient c - blade chord h = ratio of hub diameter to tip diameters N = number of blades in annulus p^ p 2 = static pressure up- and downstream of turbine Q = volume flow rate QR - volume flow rate ratio Rt - tip radius of turbine T = torque Ut = blade tip velocity Vx = axial velocity Ap* = nondimensional ized pressure drop j] = efficiency 6 = tangential coordinate cr = turbine solidity, Ncl[7rRt( + h)] TC = blade clearance at tip as percentage of chord 4> = flow coefficient fl = angular velocity of turbine