Influence of Reynolds number consideration for aerodynamic characteristics of airfoil on the blade design of small horizontal axis wind turbine

ABSTRACT

Airfoil’s aerodynamic characteristics are vital for the design and performance evaluation of wind turbine rotor blades through blade element momentum theory. Generally, the airfoil’s aerodynamic characteristics are evaluated at an approximated fixed Reynolds number. However, during a typical operation of a wind turbine, the Reynolds number varies along the length of the blade as well as with wind speed. Since airfoil’s aerodynamic characteristics are dependent on Reynolds number, inadequate consideration of Reynolds number in blade element momentum theory may result in discrepancy in an optimum design and performance evaluation. In the present work, the influence of inadequate consideration of Reynolds number on design and performance evaluation of a small horizontal axis wind turbine blade is studied. For the study, a wind turbine blade design with various design considerations, including a range of design tip speed ratio (5, 6, 7, and 8) and a range of inadequately approximated fixed (i.e., 0.5, 0.75, 1.00, and 1.25 million) and operational Reynolds number for aerodynamic characteristics of the airfoil, is considered. The study suggests that inadequate fixed Reynolds number consideration for blade design and performance evaluation can result in blade solidity as high as 17.1% with respect to blade design considering operational Reynolds number consideration and discrepancy in power coefficient prediction as high as 3.6% with fixed Reynolds number consideration for aerodynamic characteristics of the airfoil.

KEYWORDS:

• Blade element momentum theory
• artificial neural network
• aerodynamic force coefficients
• Reynolds number
• small horizontal axis wind turbine
• blade design

Acknowledgments

The Authors would like to acknowledge the facilities support provided by the Sardar Vallabhbhai National Institute of Technology – Surat, India.

Nomenclature

$a$ axial induction factor (-)

$U_{rel}$ relative wind speed (m/s)

$a^{\prime }$ tangential induction factor (-)

$B$ number of blades in the rotor (-)Greek symbols

$c$ chord length of airfoil (m)

$\alpha$ angle of attack (degree)

$C_l$lift coefficient of the airfoil (-)

$\theta$ twist angle (degree)

$C_d$ drag coefficient of the airfoil (-)

$\lambda$-tip speed ratio (-)

$C_p$ power coefficient (-)

${\lambda }_r$ local speed ratio (-)

$C_n$ normal force coefficient (-)

$\mu$ dynamic viscosity of air (kg/ms)

$C_t$ tangential force coefficient (-)

$\rho$ density of air (kg/m³)

$C_T$ thrust coefficient (-)

$\sigma$ solidity of blade (-)

$F$ hub-tip loss factor (-)

$\varphi$ flow angle (degree)$F_{tip}$ tip loss factor (-)

$\mathrm{\Omega }$angular velocity of rotor (rad/s) F_hub

hub loss factor (-)$F_1$ Shen’s correction factor (-)

Abbreviations$-$blade element (-)

ANN Artificial Neural Network $N$ total number of blade elements (-)

CFDComputational Fluid Dynamics $R$

Radius of the rotor (m) UAEUnsteady Aerodynamics$Re$Reynolds number (-) Experiment

$r$local blade radius (m) NRELNational Renewable Energy U${ }_{\mathrm{\infty }}$free stream wind speed (m/s) Laboratory

Declaration of Competing Interest

There is no conflict of Interest.