International Journal of Rotating Machinery

Volume 2019, Article ID 6580345, 10 pages

https://doi.org/10.1155/2019/6580345

## Effect of Rotation Friction Ratio on the Power Extraction Performance of a Passive Rotation VAWT

^{1}School of Machinery and Automation, Wuhan University of Science and Technology, Wuhan 430081, China^{2}Hubei Key Laboratory of Mechanical Transmission and Manufacturing Engineering (Wuhan University of Science and Technology), China

Correspondence should be addressed to Jianyang Zhu; moc.361@20gnaynaijuhz

Received 27 April 2019; Accepted 24 June 2019; Published 1 August 2019

Academic Editor: Sourabh V. Apte

Copyright © 2019 Jianyang Zhu and Changbin Tian. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper performs a systematic numerical study to investigate the effect of rotation friction ratio on the power extraction performance of a passive rotation H-type vertical axis wind turbine (H-VAWT). In contrast to the previous literature, the present work does not impose rotation velocity on the turbine, and the rotation friction ratio which reflects the effect of external load characteristics on the turbine is introduced to the governing equation of the turbine. During each iteration, the rotation velocity of the turbine is computed after having determined the aerodynamic torque exerted on the blade of the turbine. This is more consistent with the actual working environment of the H-VAWT. A novel numerical coupling model was developed to simulate the interaction between the fluid and the passive rotation of the H-VAWT; then, the power extraction performance of the turbine with different rotation friction ratio was systematically analyzed. The results demonstrate that the power extraction performance of H-VAWT will be enhanced when the H-VAWT has appropriate rotation friction ratio. It is also found that the flow separation induced by large angle of attack is alleviated essentially if the H-VAWT has appropriate rotation friction ratio, which makes the H-VAWT have better energy extraction performance.

#### 1. Introduction

With increasing of energy crisis and environment pollution, the performance of vertical axis wind turbine (VAWT) which is used for electricity generating has attracted a lot of interest [1]. There are three types of VAWT, named Savonius type, Darrieus eggbeater type, and Darrieus straight type [2]. Among them, the Darrieus straight type represents the most promising design for its special advantages, such as simple design, low cost, and good maintenance [3, 4]. Large numbers of computational and experimental studies have been carried out to investigate the aerodynamic performance of straight type VAWT (H-VAWT) with different parameters, and a considerable number of conclusions have been drawn [5–8].

Peng et al. [9] conducted a series of wind tunnel tests to measure aerodynamic forces on the blades of a high-solidity H-VAWT with different chord widths under different wind speeds and at various tip speed ratios, and the results showed that the high-solidity H-VAWT not only has better self-starting performance, but also possesses better power extraction performance at lower tip speed ratios. To investigate the effect of blade number on the performance of a H-VAWT, Castelli et al. [10] simulated the flow around a H-VAWT with NACA0025 airfoil, and the results indicated that with the increasing of blade number, the peak power coefficient of the turbine will decrease. Li et al. [11] conducted an experimental study to investigate the effect of Reynolds number on the power extraction performance of the H-VAWT. It was found that the higher the Reynolds number is, the better the turbine’s performance possesses. Roh and Kang [12] investigated the effect of solidity on the performance of a H-VAWT by using multiple stream tube method. The results revealed that the larger the solidity is, the narrower the range of the turbine to generate positive power is. Zhu et al. [13] numerically studied the power extraction performance of a H-VAWT with utilizing synthetic jet active flow control technique and concluded that the power extraction performance of the turbine can be enhanced by the jet effect, and the maximum increase of power coefficient is 15.2%.

Zhu et al. [14] investigated the effect of solidity and fixed pitch angle on the aerodynamic characteristics of the H-VAWT by using coupled solving incompressible Navier-Stokes (N-S) equations and passive rotation of the H-VAWT method. It was found that the solidity and fixed pitch angle influence the aerodynamic characteristics of the H-VAWT greatly, and delay stall is observed around the H-VAWT with appropriate solidity and fixed pitch angle, therefore leading the turbine to have better performance characteristics. Later, using the same method, they also studied the effect of fluctuating wind on the self-starting aerodynamic characteristics of H-VAWT, and it was concluded that the self-starting aerodynamic characteristics of VAWT will be enhanced when the fluctuating wind has appropriate fluctuation amplitude and frequency [15].

From the above aforementioned studies, it is concluded that many studies have been carried out to study the power extraction performance of the H-VAWT; however, most of the aforementioned studies consider the energy extraction characteristics of the H-VAWT with prescribed rotational speed; actually, the rotation of the H-VAWT is passive. Therefore, in this paper, we consider a H-VAWT whose rotational speed is passive and determined by the dynamic interaction between the fluid and turbine and investigate the effect of rotation friction ratio on the power extraction performance of the turbine. To this end, the rotation friction ratio which reflects the effect of external load characteristics on the turbine is introduced to the governing equation of the turbine, and a novel numerical coupling model is developed to simulate the interaction between the fluid and the passive rotation turbine. The format of this paper is organized as follows. The physical model and parameter definition are presented in section “Physical Model and Parameter Definition”, followed by a description of the numerical method in section “Numerical Method”. Results and discussions are presented in the next section, and some conclusions are drawn in the final section.

#### 2. Physical Model and Parameter Definition

The object of this work is to analyze the effect of rotation friction ratio on power extraction characteristics of the 2D three blades H-VAWT. The schematic structure of the passive rotation turbine is similar to our previous published paper [14], as shown in Figure 1, where the section blade of the turbine is simplified as NACA0018 airfoil and is the free stream velocity, the turbine passive rotation velocity, the resultant velocity approaching to the blade section, *α* the angle of attack of the airfoil,* θ* the azimuth angle, , the tangential and normal force, and , the lift and drag force. Based on the relationship described in this figure, the angle of attack

*α*can be defined aswhere

*is the tip speed ratio which is defined asand the tangential and normal force and can be calculated from the equation as*

*λ*