Investigation on aerodynamic performance of horizontal axis wind turbine by setting micro-cylinder in front of the blade leading edge

Accepted Manuscript Investigation on aerodynamic performance of horizontal axis wind turbine by setting micro-cylinder in front of the blade leading edge Ying Wang, Gaohui Li, Sheng Shen, Diangui Huang, Zhongquan Zheng PII:

S0360-5442(17)31799-1

DOI:

10.1016/j.energy.2017.10.094

Reference:

EGY 11738

To appear in:

Energy

Received Date: 19 March 2017 Revised Date:

18 October 2017

Accepted Date: 20 October 2017

Please cite this article as: Wang Y, Li G, Shen S, Huang D, Zheng Z, Investigation on aerodynamic performance of horizontal axis wind turbine by setting micro-cylinder in front of the blade leading edge, Energy (2017), doi: 10.1016/j.energy.2017.10.094. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Investigation on Aerodynamic Performance of

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Horizontal Axis Wind Turbine by Setting

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Micro-Cylinder in Front of the Blade Leading Edge

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Ying Wang1,2 Gaohui Li1,2 Sheng Shen1,2 Diangui Huang1,2 * Zhongquan

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Zheng3

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(1.School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093;

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Abstract: For NREL Phase VI horizontal axis wind turbine, a flow control method to suppress the flow separation

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Keywords: Horizontal Axis Wind Turbine; Flow Separation; Aerodynamic Performance; Micro-cylinder

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1. Introduction

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With large global consumption of fossil fuels and increase of environmental problems, new energies such as nuclear energy, solar energy, wind energy and geothermal energy have attracted universal attention from all over the world. As a type of low-cost, renewable and clean energy, wind energy gains sustained attention. The utilization of wind energy refers to the process by which wind turbines convert the movement of wind into electricity. According to the relationship between rotating shaft and blades, wind turbines can be classified into horizontal axis wind turbine (HAWT)and vertical axis wind turbine (VAWT). HAWT is proven to bea more effective energy conversion deviceaccounting for more than 97% of wind turbine generators. However, traditional HAWT still shows many problems.For instance,withrelatively high inlet velocities and large anglesof attack, there follows large scale flow separations which can reduce the torque, resulting in deterioration of aerodynamic performance and reduction of wind power coefficient [1].

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2. Shanghai key laboratory of multiphase flow and heat transfer of power engineering, Shanghai, 200093; 3. Aerospace Engineering Department, University of Kansas, Lawrence, Kansas 66045-7621,USA)

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(Tel: 021-55897317, Email: [email protected])

by setting micro-cylinder in front of theblade leading edge is proposed, and the corresponding numerical simulation analysis for the aerodynamic performance of wind turbine is conducted. Firstly, the results predicted by simulation are confirmed experimentally. Under the same operating condition, the simulation and experimental results of low-speed shaft torque are compared, along with results from other studies.It can be found that the simulation results can accurately reflect the basic physical characteristics of flow field for NREL Phase VI wind turbine. Secondly, the influence of different diameters and positions of micro-cylinders on aerodynamic performance of

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wind turbine is discussed.Numerical results suggest that under different stall conditions, setting appropriate micro-cylinders in front of the blade leading edge can effectively suppress flow separation on wind turbine blades without increasing the load of wind turbine. Moreover, under different wind speeds, micro-cylinders with different diameters and positionshave various impacts on aerodynamic performance of wind turbine. Through numerical calculation, the blade torquecan maximum increase to 27.3% by setting a micro-cylinder with proper diameter and

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position in front of theblade leading edge.

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ACCEPTED MANUSCRIPT Many researchers have conducted studies for HAWT experimentally [2-10]. A series of PHASE blades were developed and tested in the NASA Ames wind tunnel by the National Renewable Energy Laboratory (NREL). Besides, as a simulation database or as reference to the experimental measurement method of the NREL, these data has been widely used by researchers [2-6] . One well-known experimentalHAWT database was conducted in the NASA Ames wind tunnel with a 120m×80m closed test section and using NREL Phase VI wind turbine with diameter of 10.1 m.By removing uncertainty factorsinevitably caused by a varied atmospheric environmental effect in field measurement of wind turbine, this experimentis so far the most comprehensive measurement data of full scale wind turbine. Since the corresponding experiment data is comprehensive and reliable, it has provideda basis for the comparison of numerical simulation [7, 12] . The experiment data was utilized in Zhang[13],Munir[14] and Ghasemian[15]’s research for comparison of CFD simulation, and will also be used and compared in this study.

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A proper turbulence model is required for numerical simulation of flow field around an HAWT. Tacho[16]and Zhang[17]appliedSpalart-Allmaras (S-A) turbulence model on the simulation of NREL Phase VI turbine.FAN [18] used the S-A turbulence model for simulations of aerodynamic performance of Phase VI wind turbine. Results showed that under most of the wind speeds, S-A turbulence model could well predict the aerodynamic performance of wind turbine.However,under stall condition, the flow around the blade root shows serious unsteadiness, and the simulation of large separation at the blade root has large deviation. The k-ɛ series models were rarely used in the simulation of wind turbines [19-23]. Kasmi[22] proposed and amendedthe k-ɛ model and then applied it on the simulation of wind turbine wakes.Thecoincidence between simulation and experimentwere much improved by Kasmi’s research. Wang [24] concluded that the standard k- ɛ turbulence model was inappropriate for the simulation of HAWT. Rocha[21] and Bai[25] summarized that k-ω series turbulence models were appropriate for simulation of aerodynamic forces in HAWT blades. Moshfeghi[19] used k-ω SST turbulence model for the numerical study of aerodynamic performance of NREL PHASE VI, and the simulation results showed strong agreement with experimental data. Tachos[20] compared four turbulence models including k-ɛ, k-ɛ renormalization group, Spalart—Allmaras and k-ω shear stress transport (SST)) for simulating the flow fields of an NREL rotor and suggested the SST k-ω model was better for the NREL Phase VI rotor. Many other researchers [26-34] applied k-ω SST turbulence model on the simulation of NREL PHASE VI turbine. By comparing with the experimental results from NASA, it was found that the k-ω SST turbulence model can provide better description for flow field. Therefore, thek-ωSST is adopted for simulation of NREL PHASE VI wind turbine in this study. More advance review of previous work can be found in reference [1].

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By utilizinginteraction between fluids and through changing flow locally, the control of flow state can be achieved by flow control techniques.There are mainly two types of flow control methods: active control method and passive control method [35]. Most of the active control methods can significantly control the flow separation on blades surface.However, since these active control methods require additional air system, controller, actuator and power supply, and even sometimes large energy input, the applicability of this method is restricted. The passive control methods can be classified intovortex generator[36-40], stall strip[41-43],flap[44-46],micro tab[47, 48],roughness strip [49-51] , bionic concave-convex leading edge[61-54],riblet[55-57], etc.These passive flow control methods with mounting additional structures on blade surfaceare quite simple and practical.However, under

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ACCEPTED MANUSCRIPT the operation condition without flow separation, extra drag force will be generated due to these micro structures distributing on the blade surface. In addition, disturbances caused by these micro structures normally distribute inside of the boundary layer.With the variation of operating conditions,the disturbance caused by micro structures cannot work effectively in a wide range of operating conditions to suppress flow separation. Therefore, these passive control methods can only work in a narrow range of operating conditions.

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Chen [58] proposed a flow control method by setting a micro-cylinder in front of the blade leading edge for VAWT. By adopting this method, the corresponding numerical research for the twodimensional VAWTwas conducted. Numerical results suggested that by setting a micro-cylinder, the flow separation was effectively suppressedat large angles of attack, and there showed lift coefficient enhancement and drag coefficient reduction.Inspired by Chen’s Research, this study will apply this controlling structure which is passive and distanced on the HAWT to suppress the flow separation. By adopting similar structure of stringed instruments, microcylinders can be installed on blade surface by using loop cords set in every certain distance, and the tension of the string can be adjusted by rotating pegs at the end of wind turbine blade, as shown in Fig. 1.The greatest feature of this method is that the small part controlling flow separation leaves certain distance from the blade surface and near the upstream of the flow separation point rather than being set inside or on the blade surface [58]. Namely, this paper mainly utilizes the concept of setting micro-cylinder in front of blade leading edge to suppress flow separation and then apply it on the NREL Phase VI wind turbine. By adopting this method, the aerodynamic performance of HAWTat stall conditionis expected to be improved.Since the wind velocity and blade rotating speed are quite low, the air in flow field can be considered as incompressible fluid. Based on numerical simulations,the effect of this method to suppress flow separation is analyzed, and the influence of different diameters and positions of micro-cylinders on aerodynamic performance of HAWT is discussed.Simulation is conducted based on three-dimensional steady Reynolds averaged Navier-Stokes equations (RANS) for incompressible flow combined with SST k-ω turbulence model.

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2. Mathematical model

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2.1 Governing equation

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The aerodynamic performance of wind turbine mainly depends on low speed shaft torque and bending moment.Low speed shaft toque is the moment from tangential force in the plane of wind turbine rotor acting on wind turbine axis.The work produced by wind on wind turbine rotor through torque show direct relationship with wind energy coefficient. Based on the momentum theorem, the axial thrust of flow field acting on wind turbine rotor can be obtained from the rate of change of momentum.

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T = ρ AU d (U ∞ − U w )

(1)

whereρis the air density, A is the rotor swept area,U∞is the wind speed, Uwisthe velocity at the infinity wind turbine downstream,Udis the axial velocity of fluid passing through the wind rotor. The definition of wind energy coefficient is 3

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P ρ AU ∞3

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(2)

where P = TU d , and Ud is the axial velocity when wind pass through the wind rotor. It can be

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found that the wind energy coefficient is directly related to the torque.Thus, the low speed shaft torque is adopted to evaluatethe aerodynamic performance.

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The bending momentat the blade root is from the axial force acting on blades, and it represents the thrust load on the blades. r

θ * EI

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r

M =∫

(3)

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whereθ is axial bending angle for each blade cross section, EI is rotational stiffness, r is the rotation radius for each blade cross section.

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Besides, the definition of pressure coefficient of blade cross-sections along the spanwise direction is

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C 'p =

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p − p∞ ρ (U ∞2 + w2 r 2 )

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where pis the surface pressure, p∞is the pressure of incoming flow, ω is the angular velocity of rotary wind rotor. With larger difference of pressure coefficient on blade surface, there showssmaller stall area and stronger blade work capacity.

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2.2 Turbulence model

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The flow field around the blade is numerically simulated by employing 3D Reynolds-Averaged Navier-Stokes equations (URANS). The blades usually operate in the turbulent flow regime. The choice of turbulence model depends on many factors including the accuracy required, the physics of problem and the computational power available.As summarized in introduction, the k-ω shear stress transport (SST) model is chosen for simulation of NREL Phase VI in this paper. There are two equations [59, 60] in Menter's k-ω SST model [59-64]:

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One for k, the specific turbulent kinetic energy (m2·s−2), equation (5)

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∂ ∂ ∂ (ρk) + (U iρk) = ∂t ∂x i ∂x j

  ∂ μk  + P~k − β*ρωk k  ∂x j  

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and one for ω, the specific turbulent dissipation rate (s-1) (or specific turbulent frequency)

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∂ ∂ ∂ (ρω) + (U iρω) = ∂t ∂x i ∂x j

  ∂ μω ω  + P~ω − βρω2  ∂x j   4

(5)

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∂

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ω σ ω ,2 ∂x j

∂

k

∂x j

ω (6)

where the effective viscosities (kg·m−1·s−1) are:

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µk = µ + µt

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µω = µ + µt

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(8)

σω

where µt is the modified eddy (or turbulent) viscosity (kg·m−1·s−1), and σ k and σ ω are diffusion

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constants of the model. The Reynolds stresses

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two-equation models with the Boussinesq expression [60-62]:

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τ ij (kg·m−1·s−2) are computed as usual in

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τ ij = − ρ U i'U 'j = 2 µt Sij − ρ kδ ij

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(9)

σ ij is the Kronecker

In Eq. (9), Sij represents the mean rate of deformation component (s−1) and

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delta function. Pω isthe rate of production of ω (kg·m−3·s−2), is given by the Eq. (10):

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2 ∂ Pω = γ [2 ρ Sij ⋅ Sij − ρω ( U i )δ ij ] ∂x j 3

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where γ is a model constant.

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2.3 Boundary condition

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Fig. 2shows the experimental model of NREL Phase VI wind turbine in the NASA Ames wind tunnel.The experiment adopts two kinds of blade shapesincluding standard shape and tip extension shape, where the wind rotor diameter for standard shape is 10.058 m, whilethe wind rotor diameter fortip extension shape is 11.064 m.This paper only considers the blade with standard shape.Table 1 showsthe data for airfoil shapes and the corresponding chord lengths and twisted angles at different positions along the blade spanwise direction.

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Based on the geometrical data of the wind turbine blade airfoils, the geometrical model built up by a 3D modeling software is showed in Fig.3.

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Boundary conditions for NREL Phase VI wind turbine are showed in Fig. 4. The rotationalperiodic boundary condition is used on the symmetric plane of the computation domain. The velocity boundary condition is used at inlet where the turbulence intensity equals to 5%, the pressure boundary condition is set at the outlet, and the no slip boundary condition is adopted on

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ACCEPTED MANUSCRIPT the blade surface. The detailed numerical set up for simulation of HAWT with micro-cylinder is showed in Table 2.

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2.4 GeometricalModel for NREL Phase VI HAWT by Setting Micro-cylinder in front of the Blade Leading Edge

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The diameters and positions of micro-cylinder should have influence on suppressing flow separation on blade. In order to clearly express the positions of micro-cylinder, parameters including ∆, R and d which represent vertical distance between micro-cylinder and blade leading edge, horizontal distance between micro-cylinder and blade leading edge, distance between center of micro-cylinder and blade surface, respectively, are showed in Fig. 5. By adopting the maximum value of the blade chord which is 25% of the blade height as the characteristic length, the nondimensionalization for distance and diameter are conducted, and three significant digits are adopted.

D=

D% ∆% R% d% ,∆ = , R = ,d = c c c c

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the horizontal distance, For example, R(2.71×10-2)Δ(8.14×10-2)D(1.36×10-2) means: which is between the center of micro-cylinder and blade leading edge, is 2.71×10-2 times of the maximum blade chord; the vertical distance is 8.14×10-2 times of the maximum blade chord; the diameter is 1.36×10-2 times of the maximum blade chord.As shown in Fig. 6, the location of the micro-cylinder is located from r=1.257m which is 25% of the rotor radius to r=5.029m at the blade tip, where the micro-cylinder is bending and the corresponding bending radian shows the same value as the twisted radian of the blade leading edge.

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Table 3 is about the details of different geometrical combinations of the micro-cylinder such as the positions, diameters and vertical distances of the micro-cylinders have been added.

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2.5 Mesh generation

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By considering the rotational symmetry of the flow field, only one of the wind rotor blades is needed to conduct simulation, while hub, tower and other parts can be neglected. The computational domain is semi-cylinderwith five times of rotor radius as its radius. By using Ansys ICEM, the mesh is generated where C-type mesh is used for the area around the blade and O-type and Y-type mesh are adopted for the rest of the areas.Besides, the C-type meshes near the blade are connected by interface. The grid profiles along the blade spanwise direction(r=5.029m)with and without micro-cylinder areshowed in Fig.7, and the whole 3D structural mesh is showed in Fig.9.Fig. 10 shows the surface grid on a rotor blade. However, since the grid number is quite large, it is hard to observe the mesh distribution. Thus, the grid section at r/R=0.31 on a rotor blade is showed in Fig. 11. Additionally, a finer mesh is required to predict an accurate separation point on turbineblade surface. In this way, the boundary layer flow can be resolved through setting around the turbineblade. Moshfeghi[63]investigatedthe effects of near-wall grid spacing with SST-K-ωmodelfor the aerodynamic model of NRELPhase VI, and recommended the non-dimensional wall distance (y+) value is less than or equal to 1. Thus, y+ on the blade surface is controlled within 1.5.

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ACCEPTED MANUSCRIPT 3. Results

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3.1 Mesh independence study

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This paper conducts numerical simulation for 3D flow under different wind speeds including 7m/s, 13m/s, 15m/s, 20m/s and 25m/s. The rotating speed of the wind rotoris fixed at about 72rpm.In order to ensure the reliability of numerical simulation, the mesh independence study is conducted. Seven sets of mesh numbers are adopted for mesh independence study, as shown in Table 4. The height of the first layer of grid for these seven sets of meshes is 2.2×10-6m. Test results show that y+ 1 in most of the areas. In the rest of the areas, y+ is slightly higher and y+<2. This means that the turbulence model meets the requirement.

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Fig. 12 and Fig. 13 show thetorque and bending moment of numerical simulation for the NREL Phase VI wind turbine by usingdifferent settings of grid numbers. It can be found that the simulation results of torque and bending moment for M01 and M02 are quite different from M03 to M07.Withthe increase ofgrid number, the numerical simulation results for torque and bending moment tend to be stable, especially for cases under high wind speeds. The relative error between M06 and M07 is less than 5%. Since this paper will conduct numerical simulation by adding micro-cylinder in front of blade leading edge, M07 with 17.49 million grid numbers is confirmed and selected for the following research.

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3.2 Model validation

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The simulation results of the low speed shaft torque based onM07mesh grid are compared with the experimental results[5]as well as other simulation results[65-69], as shown in Fig.14. Compared with experimental values, there is deviation for numerical simulation underwind speed of 10m/s,and the relative error is about 26%. This is associated with the influence of stall delayed by 3D rotational effect.Overall, thesimulation results agreed well with the experimental results and are closer to the experimental results than Zhong[65] and Song[68], especially for wind speeds which are greater than 10m/s. Moreover, compared with MO[67], Sorensen [26] and Huang[69]’s research, simulation results under wind speed of 7m/s~11m/s show higher relative errors.Thus, it can be seen that the mesh setting of M07is quite reliable, andthe correspondingsimulation results are accurate enoughto reflect the basic physical characteristics of flow field for NREL Phase VI wind turbine.

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Moreover, pressure coefficient distributionson NREL Phase VI wind turbine blade at different spanwise directions as well as wind speeds are compared with experimental data, as shown in Fig. 15. When wind speeds are 7m/s and 10m/s, the pressure coefficient distributions on blade surface at different spanwise directions show good agreement with the experiment results. When wind speed increases to 13m/s, the pressure coefficient distributions on the blade suction side show large deviation with the experimental values, especially at 30% and 63% of the spanwise direction. While the pressure coefficient distributions on the blade pressure side fit well with the experimental data. When wind speeds are 20m/s and 25m/s, there is complete separation on the blade suction side and the pressure coefficient distributions from simulation generally agree well with experiment results.

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3.3 Flow Field of Regular NREL Phase VI Wind Turbine

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ACCEPTED MANUSCRIPT Fig. 16to Fig.18show the velocity streamlines at 60% to 90% of cross sections along the blade spanwise direction for the NREL Phase VI HAWT blade. Flow separation area develops from the blade root to the blade tip and lateral. By further increasing the wind speed, the separation zone continuously expands to the lateral of blade.When the wind speed is up to 15 m/s, about 85% of the suction surfaces of the cross section areas are completely in the state of flow separation, accompanied by stall.

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3.4 Aerodynamic performance of micro-cylinder in front of the blade leading edge

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3.4.1 Micro-cylinders with different diameters

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By fixing the center of micro-cylinder, the influence of micro-cylinder diameters on blade aerodynamic performance under different wind speeds is studied, and the corresponding numerical results are showed in Fig. 19 to Fig. 21.

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Fig. 19 and Fig. 20show the influence of center points of micro-cylinders on torque, where R=2.71 × 10-2 and∆=6.78 × 10-2, or R=2.71 × 10-2 and ∆=8.14 × 10-2, while changing micro-cylinder diametersDfrom 1.36×10-3 to 1.35×10-2.

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It can be observed that different micro-cylinder diameters have various influences on blade torque which is related with the effect of disturbance degree of micro-cylinder on flow field. There is no flow separation under low wind speed (U =7m/s) and the torque will instead decrease by adding micro-cylinder. With the increase of wind speed, there occurs flow separation in the boundary layerof suction surface. By adding micro-cylinder at this moment, the blade torque can be improved in a certain extent. According to the simulation results, for cases in light stall (U =7m/s~10m/s), there is larger improvement of blade torque by adding smaller diameter of micro-cylinders; for cases in deep stall (U ≥13m/s), the blade torque is improved more than those cases without adding micro-cylinder in generally. This is much clearer when micro-cylinder is close to the blade surface. Moreover, when micro-cylinder diameter D varies from 1.36×10-3 to 1.35× 10-2, the vertical distance ∆=6.78× 10-2 shows better improvement on blade torque compared with ∆=8.14×10-2. This might because the close distance between micro-cylinder and blade leading edge has larger influence on flow field of blade surface.

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Fig. 21 shows the influence of micro-cylinder diameters on blade root bending moment with vertical distance R=2.71×10-2 and horizontal distance ∆=6.78×10-2. With the increase of micro-cylinder diameter, the blade root bending moment shows small increase.But overall, thebending moment from thrust load for turbine blade varies little with changing the micro-cylinder diameter, and thus the stability of wind turbine will not be influenced by adding micro-cylinder.

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3.4.2 Same micro-cylinder diameter but different positions

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Fig. 22 and Fig. 23 show the influence of horizontal distances R=0 to R=5.43×10-2 on blade torque while keeping the distance between center of micro-cylinder and blade leading edge d=4.34 ×10-2 or d=5.70×10-2, and also keeping themicro-cylinder diameter D=6.78×10-3. As shown in Fig. 23 and 18,for cases in light stall (U =7 m/s ~10m/s), the micro-cylinder has little effect on torque; While for cases at deep stall (U =13m/s and above) and d=4.34×10-2 and D=6.78×10-3, the micro-cylinder always has improvement on torque. While for cases withd=5.70×10-2 and

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Fig. 24 to Fig. 25 compare the influence of vertical distance ∆ (5.43×10-2~8.14×10-2) on the blade torque for cases with horizontal distance R=2.71×10-2 and micro-cylinder diameter D=1.36 ×10-3 and also for cases with horizontal distanceR=0 and micro-cylinder diameterD=6.78×10-3. For cases in light stall (U =7 m/s~10m/s), micro-cylinders with different horizontal distances R and vertical distances D show almost no influence on the blade torque; while for cases with U =13 m/s~18m/s, the micro-cylinder with R=2.71×10-2 and D=6.78×10-3 shows larger improvement compared with micro-cylinder with R=2.71 × 10-2 and D=1.36 × 10-3. For cases with U =18m/s~25m/s, micro-cylinders with R=2.71×10-2 and D=1.36×10-3 have larger influence on blade torque rather than micro-cylinder with R=2.71×10-2 and D=6.78×10-3.

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Fig. 25 and Fig. 26 compare the influence of vertical distance U(5.43×10-2~8.14×10-2) on the blade torque for cases with horizontal distance R=2.71×10-2 and micro-cylinder diameter D=6.78×10-3 and cases with horizontal distanceR=0 and micro-cylinder diameterD=6.78×10-3. It can be found that for cases with U =7m/s~10m/s, different horizontal distances R and diameters D show almost no influence on blade torque; for cases with U =13m/s~18m/s and cases with U =18m/s~25m/s, micro-cylinders with different horizontal distances R and D=6.78×10-3 always show improvement on blade torque; while ∆ varies from 2.30×10-2 to 5.43×10-2 with R=0 and D=6.78×10-3, the micro-cylinder generally shows greater improvement on blade torque compared with ∆ varies from 5.43×10-2 to 8.14×10-2 with R=2.71×10-2 and D=6.78×10-3.

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Fig. 27presents the influence of vertical distance ∆ on the bending moment at the blade root when the horizontal distance R=0 and the micro-cylinder diameter D=6.78×10-3. It can be found that with a smaller vertical distance ∆ for micro-cylinder, the bending moment at the blade root increases slightly when wind speeds are 13m/s and 15m/s. Based on the above analysis and combined with Fig. 22, the variation of vertical distances ∆ between micro-cylinder and blade leading edge almost have no influence on bending moment at the blade root.

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As shown in Fig. 28, for 60%, 75% and 80% of blade spanwise directions, the following operating conditions are chosen for analysis: deep stall with wind speed of 13m/s, the horizontal distance of micro-cylinder isR=0, the diameter of micro-cylinder is D=6.78×10-3, and the vertical distance between micro-cylinder and blade leading edge is ∆=2.30×10-2~5.43×10-2. When the micro-cylinder is close to the blade leading edge point (R=0), the aerodynamic performance has been improved obviously.This is because the pressure difference between pressure surface and suction surface increases by adding micro-cylinder, and the flow separation is improved greatly. Besides,with smaller vertical distance of ∆, there is greater improvement.According to thecalculation, when the vertical distance∆ is 2.30×10-2 for micro-cylinder (D=6.78×10-3), the blade torque shows increase from 7.1% to 27.3%. Certainly, if the value of∆ is too small, the microcylinder is too close to the blade and the flow on the blade surface would be blocked.

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3.4.3 Detailed analysis for flow field

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Based on the above analysis, the micro-cylinder with R(2.71×10-2)∆(8.14×10-2)D(1.36× 10-3) at light stall (U =10/s) and micro-cylinder with R(0)∆(2.30×10-2)D(6.78×10-3) at deep stall have obvious improvement on aerodynamic performanceof NREL Phase VI turbine blade.When U

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Fig. 29 shows that adding micro-cylinder almost has no influence on flow separation for three different positions on wind turbine blade under wind speed of 10m/s. Fig. 30 shows that for case with wind speed equaling to 13m/s and for 60% of cross sectionalong the blade spanwise direction, the flow separation bubble on blade surface obviously moves to the trailing edge and the flow separation on blade surface is suppressed to a certain extent by adding micro-cylinder. For case with 70% and 80% of cross sections along the blade spanwise direction, the flow separation is suppressed partially by micro-cylinder. Fig. 31 shows that for case with wind speed equaling to 15m/s and 70% of cross sectionalong the blade spanwise direction, the large flow separation bubble changes to a small one by adding micro-cylinder and the flow separation is suppressed obviously. In this way, the aerodynamic performance of wind turbine blade can be improved generally and the wind power coefficient can also be expected to increase accordingly.

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As shown in Fig. 32 to Fig. 35, the velocity streamlines at 60% to 90% of cross sectionalong the blade spanwise direction for normal wind turbine blade and for blade adding micro-cylinder of R(0.00)∆(2.98×10-2)D(6.78×10-3) are compared. It can be found that the micro-cylinder at blade leading edge has lower influence on flow field which is lower than 75% of the blade height. However, the blade which is smaller than 60% of the blade height only takes 40% of the work done by wind turbine. Thus, it is not significant to suppress the flow separation at this part. While the wind turbine blade which is higher than 75% of the blade height plays the dominant role in work capability. For this part, the micro-cylinder has obvious influence on flow separationsuppression and is meaningful to increase effect of work.

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Focusing on the characteristics of flow separation for HAWT operating at high angles of attack, this paper proposes a controlling method of adding a micro-cylinder in front of the blade leading edge for NREL Phase VI wind turbine. Based on the Reynolds time-averaged Navier-Stokes equations (RANS) and combined with k-ωSST turbulence model, simulation is conducted for this new type of HAWT and the aerodynamic characteristics of flow field are analyzed. The following conclusions are obtained:

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1. Under different stall conditions, setting an appropriate micro-cylinder in front ofthe blade leading edge can effectively suppress the flow separation on the blade surface without influencing the stability of the wind turbine. Based on the calculation, the blade torque of the wind turbine can be improved up to 27.3% by setting a micro cylinder with appropriate size and position.

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2.By comparing the influence of different positions of micro-cylinders on the torque of NREL Phase VI wind turbine, it is found that micro-cylinder shows no influence on improving torque under low wind speed (7m/s); At light stall, (U =10m/s),micro-cylinder with small diametersuch as 1mm shows greater improvement on torque compared with other micro-cylinders with larger diameters.Besides, with larger vertical distance ∆(∆≤6cm) between micro-cylinder and blade leading edge, there shows higher torque; At deep stall (U 13m/s), micro-cylinders with larger diameters show greater improvement on torque. Moreover, when the micro-cylinder is close to the blade leading edge (R=0cm) with smaller vertical distance ∆(∆≥1.7cm), there shows greater

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3.Based on the analysis of velocity streamlines, adding a proper micro-cylinder in front of the blade leading edgecan suppress flow separation on the blade surface effectively, especially for improving the aerodynamic performance of the key section which is higher than 60% of the cross section along the blade spanwise direction.In this way, the aerodynamic performance of wind turbine can be improved effectively and the wind power coefficient is then increased accordingly. However, other setting programs of micro-cylinders on different types of HAWT need to be further studied.

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This work is supported by National Natural Science Foundation of China Grant Nos. 51536006, Nos. 51406117, and the Program of the Shanghai Science and Technology Commission (No.17060502300).

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x/Chord

(i1) 20m/s

(i2) 25m/s 24

0.8

1.0

ACCEPTED MANUSCRIPT Fig. 15 Pressure distributions for NREL Phase VI wind turbine blade at different spanwise directions and wind speeds

RI PT

584 585

60%

65% 70% 80% 85%

M AN U

90%

SC

75%

Fig. 16 Velocity streamlines at different cross-sections along the blade spanwise direction without setting

AC C

EP

TE D

micro-cylinder,U∞= 10m/s

60% 65% 70% 75% 80%

85% 90%

Fig. 17 Velocity streamlines at different cross-sections along the blade spanwise direction without setting micro-cylinder, U∞=13m/s

586

25

ACCEPTED MANUSCRIPT

RI PT

60% 65% 70% 75% 80% 85%

SC

90%

Fig. 18 Velocity streamlines at different cross-sections along the blade spanwise direction without setting

M AN U

micro-cylinder,U∞=15m/s 2000

1400 1200 1000 800

1600

Nm

Nm

1600

Prototype D(1.35E-2) D(6.78E-3) D(1.36E-3)

1800

1400

Torque

1800

Torque

2000

Prototype D(1.35E-2) D(6.78E-3) D(1.36E-3)

1200 1000

600 400 5

10 U∞

TE D

800

15 m/s

20

600 400

25

5

10 U∞

15 m/s

20

25

Fig. 20 Effect of different cylinder diameters on torque

R=2.71×10-2&∆=6.78×10-2

R=2.71×10-2&∆=8.14×10-2

EP

Fig. 19 Effect of different cylinder diameters on torque

7000

Nm Bending torque

AC C

6000 5000 4000 3000

Prototype D(1.35E-2) D(6.78E-3) D(1.36E-3)

2000 1000 0 5

10 U∞

15 m/s

20

Fig. 21 Effect of different cylinder diameters on bending moment

26

25

R=2.71×10-2&∆=6.78×10-2

ACCEPTED MANUSCRIPT Prototype R(0.00) R(2.71E-2) R(4.07E-2) R(5.43E-2)

Torque (Nm)

1600

1800 1600

1400 1200 1000

1400 1200 1000

800

800

600

600

400

400 5

10

15 U∞ (m/s)

20

25

5

10

RI PT

1800

Prototype R(0.00) R(2.71E-2) R(4.07E-2) R(5.43E-2)

2000

Torque (Nm)

2000

15 U∞ (m/s)

20

25

Fig. 22 Variation of torque by changing the horizontal

Fig. 23 Variation of torque by changing the horizontal

distance R from the blade leading edge

distance R from the blade leading edge

-2

-3

d=5.70×10-2&D=6.78×10-3

2000

1800

1800

1600

1600

Nm

1400

1000

Torque

1200 Prototype ∆(5.43E-2) ∆(6.78E-2) ∆(8.14E-2)

800 600

SC

2000

M AN U

Torque

Nm

d=4.34×10 &D=6.78×10

1400 1200 1000

800 600

Prototype ∆(5.43E-2) ∆(6.78E-2) ∆(8.14E-2)

400

400 5

10

20

5

25

TE D

U∞

15 m/s

10

15 U∞

20

25

m/s

Fig. 24 Effect of different vertical distances ∆ from the

Fig. 25 Effect of different vertical distances ∆ from the

blade leading edge point on torque

blade leading edge point on torquet5

R=2.71×10-2 &D=1.36×10-3 2000

1400 1200

AC C

Torque

Nm

1600

Prototype ∆(2.30E-2) ∆(2.98E-2) ∆(4.34E-2) ∆(5.43E-2)

1000

800 600

10

U∞

15 m/s

5000 4000 3000

Prototype ∆(2.30E-2) ∆(2.98E-2) ∆(4.34E-2) ∆(5.43E-2)

2000 1000 0

400

5

6000 Bending torque (Nm)

EP

1800

R=2.71×10-2 &D=6.78×10-3 7000

20

5

25

10 U∞

15 m/s

20

Fig. 26 Effect of different vertical distances ∆ on torque

Fig. 27 Effect of different vertical distances ∆ on

R=0&D=6.78×10-3

bending moment(R=0&D=6.78×10-3)

27

25

ACCEPTED MANUSCRIPT 75%

原型 Prototype UU(2.30E-2) (2.30×10-2) UU(2.98E-2) (2.98×10-2) UU(4.34E-2) (4.34×10-2) UU(5.43E-2) (5.43×10-2) 0.0

0.2

0.4

0.6

0.8

1.0

SC

587

RI PT

Prototype U (2.30×10-2) U (2.98×10-2) U (4.34×10-2) U (5.43×10-2)

2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0 -7.0 -8.0

588 589

M AN U

Prototype U (2.30×10-2) U (2.98×10-2) U (4.34×10-2) U (5.43×10-2)

Fig. 28 Effect of different cylinder diameters on bending moment

590 Without micro-cylinder

U∞=13m/s, R=0.00&D=6.78×10-3

R(2.71× ×10-2)∆(8.14× ×10-2)D(1.36× ×10-3)

the blade spanwise direction

TE D

60% of cross sections along

70% of cross sections along the blade spanwise

AC C

direction

R(2.71× ×10-2)∆(8.14× ×10-2)D(1.36× ×10-3)

EP

Without micro-cylinder

R(2.71× ×10-2)∆(8.14× ×10-2)D(1.36× ×10-3)

Without micro-cylinder

80% of cross

sections along the blade

spanwise direction

591

Fig. 29 Comparison of flow field details when U = 10m/s

28

ACCEPTED MANUSCRIPT R(0)∆(2.30× ×10-2)D(6.78× ×10-3)

Without micro-cylinder 60% of cross sections along the blade spanwise direction

Without micro-cylinder

×10-3) R(0)∆(2.30× ×10-2)D(6.78×

Without micro-cylinder

R(0)∆(2.30× ×10-2)D(6.78× ×10-3)

70% of cross

RI PT

sections along the blade spanwise direction

80% of cross

SC

sections along the blade spanwise

592

M AN U

direction

Fig. 30 Comparison of flow field details when U = 13m/s Without micro-cylinder 60% of cross sections along the blade spanwise direction

R(0)∆(2.30× ×10-2)D(6.78× ×10-3)

the blade spanwise direction

EP

70% of cross sections along

TE D

Without micro-cylinder

R(0)∆(2.30× ×10-2)D(6.78× ×10-3)

Without micro-cylinder

80% of cross sections along

AC C

the blade

R(0)∆(2.30× ×10-2)D(6.78× ×10-3)

spanwise direction

593

Fig. 31 Comparison of flow field details when U =15m/s

29

ACCEPTED MANUSCRIPT

60%

60%

65%

65%

RI PT

70%

70%

75%

75%

80%

80%

85%

85%

90%

90%

Fig. 33 Detailed velocity streamlineswith setting

micro-cylinder,U∞=13m/s

micro-cylinder,U∞=13m/s

M AN U

SC

Fig. 32 Detailed velocity streamlineswithout setting

60% 65% 70% 75% 80% 85% 90%

TE D

80% 85%

Fig. 35 Detailed velocity streamlineswith setting

micro-cylinder,U∞=15m/s

micro-cylinder,U∞=15m/s

EP

595

70% 75%

Fig. 34 Detailed velocity streamlines without setting

AC C

594

90%

60% 65%

30

ACCEPTED MANUSCRIPT Table 1 Chord lengths and twisted angles for NREL Phase VI blade [5]

c (m) 0.218 0.218 0.183 0.349 0.441 0.544 0.737 0.728 0.711 0.697 0.666 0.636 0.627

r/R 0.101 0.131 0.176 0.200 0.212 0.225 0.250 0.267 0.300 0.328 0.388 0.449 0.466

597

θ (deg) 0.000 0.000 0.000 6.700 9.900 13.400 20.040 18.074 14.292 11.909 7.979 5.308 4.715

T (%) 50.0 50.0 50.0 35.9 33.5 31.9 30.0 30.0 30.0 30.0 30.0 30.0 30.0

r (m) 2.562 2.867 3.172 3.185 3.476 3.781 4.023 4.086 4.391 4.696 4.780 5.000 5.029

r/R 0.509 0.570 0.631 0.633 0.691 0.752 0.800 0.812 0.873 0.934 0.950 0.994 1.000

(deg) 3.425 2.083 1.150 1.115 0.494 -0.015 -0.381 -0.475 -0.920 -1.352 -1.469 -1.775 -1.812

T (%) 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0

Table 2 Numerical set up for simulation of HAWT with micro-cylinder Numerical solution methods

Ansys CFX SSTk-ω turbulence model Global dynamic model control. High resolution First order Coupled solver

M AN U

Simulation software Turbulence model Dynamic model control Discrete scheme of convective term Turbulence numeric option Scheme of pressure-velocity coupling Boundary condition Inlet

TE D

Outlet

Flow field outside of computationdomain Blade Symmetric plane of the computationdomain

598

The velocity boundary condition Intensity=5% Pressure boundary condition Average static pressure Pres. profile blend=0.05 Opening boundary condition No slip boundary condition Rotational periodic boundary condition

EP

Table 3 Details of different geometrical combinations of the micro-cylinder Group 1

Group 2

Different diameters -3

-2

AC C

D=1.36×10 ~1.35×10 -2

-2

-3

Different positions

R=0 ~5.43×10 -2

-2

∆=5.43×10-2~8.14×10-2

-3

-2

-2

R(0.00)d(4.34×10 )D(6.78×10 )

R(2.71×10 )∆(5.43×10 )D(1.36×10 )

R(0.00)∆(2.30×10-2)D(6.78×10-3)

R(2.71×10-2)∆(6.78×10-2)D(6.78×10-3)

R(0.00)d(4.34×10-2)D(6.78×10-3)

R(2.71×10-2)∆(6.78×10-2)D(1.36×10-3)

R(0.00)∆(2.98×10-2)D(6.78×10-3)

R(2.71×10-2)∆(6.78×10-2)D(1.35×10-2)

R(2.71×10-2)d(4.34×10-2)D(6.78×10-3)

R(2.71×10-2)∆(8.14×10-2)D(1.36×10-3)

R(0.00)∆(4.34×10-2)D(6.78×10-3)

R(2.71×10-2)∆(8.14×10-2)D(1.36×10-3)

R(4.07×10-2)d(4.34×10-2)D(6.78×10-3)

R(2.71×10-2)∆(5.43×10-2)D(6.78×10-3)

R(0.00)∆(5.43×10-2)D(6.78×10-3)

R(2.71×10-2)∆(8.14×10-2)D(6.78×10-3)

R(5.43×10-2)d(4.34×10-2)D(6.78×10-3)

R(2.71×10-2)∆(6.78×10-2)D(6.78×10-3)

R(0.00)d(5.70×10-2)D(6.78×10-3)

R(2.71×10-2)∆(8.14×10-2)D(6.78×10-3)

R(2.71×10-2)d(5.70×10-2)D(6.78×10-3) R(4.07×10-2)d(5.70×10-2)D(6.78×10-3) R(5.43×10-2)d(5.70×10-2)D(6.78×10-3)

Tips: 1. ∆: Vertical distance between micro-cylinder and blade leadingedge

2. R: Horizontal distance between micro-cylinder and blade leading edge 3. d: Distance between centerof micro-cylinder and blade surface 4. D: Diameter of the diameter

31

-3

∆=2.30×10-2~5.43×10-2

R(2.71×10 )∆(6.78×10 )D(1.36×10 )

R(2.71×10-2)∆(8.14×10-2)D(1.35×10-2)

599 600 601 602 603

θ

c (m) 0.605 0.574 0.543 0.542 0.512 0.482 0.457 0.451 0.420 0.389 0.381 0.358 0.355

RI PT

r (m) 0.508 0.660 0.883 1.008 1.067 1.133 1.257 1.343 1.510 1.648 1.952 2.257 2.343

SC

596

ACCEPTED MANUSCRIPT 604

Table 4 Parameters for mesh independence study M01

M02

M03

M04

2.8×10-6

2.8×10-6

2.2×10-6

160

180

70

80

Grid growth rate

1.20

Amount of the grid

5.7×10

1.15 5

M05

M06

M07

2.2×10-6

2.2×10-6

2.2×10-6

2.2×10-6

220

270

300

410

520

98

125

155

165

174

1.15

1.12×10

6

2.38×10

5.01×10

1.15 6

1.15

1.05×10

7

1.55×10

AC C

EP

TE D

M AN U

SC

605

1.15 6

32

1.15

RI PT

No. Height of grid for first layer (m) Grid number for blade chord Grid number along blade spanwise direction

7

1.75× ×107

ACCEPTED MANUSCRIPT Highlights (1) Micro-cylinder is added in front of the blade leading edge to increase blade torque. (2) Simulation results are compared with experimental results for validation. (3) Influence of diameters and positions of micro-cylinders are discussed numerically.

RI PT

(4) Blade torque can be improved obviously with proper setting of micro-cylinders.

AC C

EP

TE D

M AN U

SC

(5) Flow separation can be suppressed effectively by setting appropriate micro-cylinders.