Effects of a blade profile, the Reynolds number, and the solidity on the performance of a straight bladed vertical axis wind turbine

Abstract

This study investigates the effects of parameters such as a blade profile (changing the digit of the 4-digit NACA00xx airfoil), the Reynolds number, and the solidity on the performance characteristics of a straight bladed vertical axis wind turbine (VAWT). A numerical analysis, adopting the multiple stream tube (MST) method, is carried out to evaluate the performance depending on the parameters. The numerical result shows that the variation of a blade profile directly influences the power production, i.e., the high-digit NACA00xx airfoil provides higher power in a low speed zone (BSR < 3; BSR: blade speed ratio (ΩR/U f ), Ω: angular velocity of blade, R: radius of a straight Darrieus wind turbine, U f : free stream velocity) than the low-digit NACA00xx profile. On the contrary, the low-digit NACA00xx airfoil produces higher power in a high speed range (BSR > 5) than the high-digit NACA00xx profile. An enhancement of the power production is observed with increasing the Reynolds number on the whole tested blade speed ratio range (1 < BSR < 12). In particular, the rate of the enhancement of the power is rapidly decreased with the increases of the Reynolds number ( $$Re = {{\rho \bar U_r c} \mathord{\left/ {\vphantom {{\rho \bar U_r c} \mu }} \right. \kern-\nulldelimiterspace} \mu }$$ , ρ: air density, $$\bar U_r$$ : mean resultant velocity acting on a blade with variable rotating speeds in a uniform free stream velocity (U f ), c: blade chord length, µ: air viscosity). For the effect of the solidity on the power production, a marked reduction of the range of the blade speed ratio that can provide the power is observed with increasing the solidity. A pattern of very steep variation of the power around the peak in the low speed zone (BSR 0.3; σ: solidity (N c /R), N: number of blade, c: chord length of an airfoil), and this tendency is conspicuously different from that of the eggbeater-type Darrieus VAWT, which is interpreted as a gradual variation of the power around the peak.