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The present work is based on the comparative study between “Blade-Element- Momentum” (BEM) analysis and “Computational-Fluid-Dynamics” (CFD) analysis of small-scale horizontal axis wind turbine blade. In this study, the pitch is considered as fixed and rotor speed is variable. Firstly, the aerodynamic characteristics of three different specialized airfoils were analyzed to get optimum design parameters of wind turbine blade. Then BEM was performed with the application of the open source wind turbine design and performance computation software Q-Blade v0.6. After that, CFD simulation was done by Ansys CFX software. Here, k-ω “Shear-Stress-Transport” (SST) model was conducted for three-dimensional visualization of turbine performance. However, the best coefficient of performance was observed at 6
^{o} angle of attack. At this angle of attack, in the case of BEM, the highest coefficient of performance was 0.47 whereby CFD analysis, it was 0.43. Both studies showed good performance prediction which was a positive step to accelerate the continuous revolution in wind energy sector.

Strenuous exploration is continuously offered for the advancement of wind tur- bine technology. As the part of this revolution, investigation of wind turbine performance related to the aerodynamic characteristics has a significant impact on wind energy sector. Among all the wind technologies, horizontal axis wind turbine is the most efficient and popular one. However, for horizontal axis wind turbine system, performance mainly depends on blade shape and working principle. For lift-type wind turbine blade, the cross-section is made by the airfoil. The airfoil is responsible for creating aerodynamic lift force effect by making pressure difference of air flow between upper and lower surfaces. Due to the lift force, the blade intended to go upward direction resulted into rotation of the blade along the horizontal axis [

Usually, the small scale lift-type wind turbine blade cross-section has single airfoil throughout the whole length. With the progress of interest in wind energy sector, some dedicated airfoils have been introduced for the wind turbine. However, for the small scale wind turbine, the airfoil should be used at a low angle of attack, where the coefficient of drag is much lower as compared to the lift coeffi- cient [

where U is relative wind velocity, c airfoil chord length, and

The radius of the wind turbine rotor was selected in such a way that it fulfills the small scale criteria and as well as the expected or rated power requirement. The rotor radius was estimated from the following power equation of wind turbine.

Here,

The aerodynamic behavior mainly depends on the blade design. For lower wind velocity, the blade is always designed to operate at a small angle of attack with- out flow separation. In this study, the potential flow technique was employed by using X foil software application to predict aerodynamic performance [

Parameters | Value |
---|---|

Expected Power | 10 kW |

Number of Blades | 3 |

Rotor Radius | 5.5 m |

Design Wind Speed | 7 m/s |

Design Tip Speed Ratio | 6 |

Air Density | 1.225 kg/m^{3} |

Design Reynold Number | 230,000 |

From

Horizontal axis wind turbine blade cross-sectionsare made by airfoils which are not uniform throughout the length. The chord lengths and twist angles are varied with blade length. Excluding the hub, the total blade length was divided into nine segments. Usually, small scale wind turbine system has no pitch control unit. As the pitch angles were fixed, the twist angles of the blade in all blade segments were determined in such way that every segment has the optimum angle of attack. The following equations are used to get optimum chord length and twist angle.

where R is rotor radius, r is local rotor radius, B is blade number,

The above values of chord lengths and twist angles were fixedat the midpoints of the blade segments. Then, a continuous linear loft operation was done between two midpoints of blade segments, next to each other. However, this the-

Relative Radius | Chord Length (m) | Twist Angle (degree) | Airfoil Name |
---|---|---|---|

0 | HUB | 0 | HUB |

0.15 | 1.1 | 28.52 | S823 |

0.25 | 0.912 | 15.9625 | S823 |

0.35 | 0.82 | 9.6125 | S823 |

0.45 | 0.704 | 7.3697 | S833 |

0.55 | 0.5812 | 4.9212 | S833 |

0.65 | 0.4946 | 3.2004 | S833 |

0.75 | 0.3846 | 1.427 | S822 |

0.85 | 0.34 | 0.45 | S822 |

0.95 | 0.3048 | −0.329 | S822 |

oretical optimum blade chord and twist distribution sometimes are not viable for manufacturing. Because of the theoretically designed structure, there might be some complexity to manufacture the blade. For this reason, linearization of chord and twist distribution is done by several methods. However, linearization should be done in such a way that the total performance of the wind turbine will remain closer to optimum designed blade performance. Due to simplify the study, this work avoided the linearization of the blade segments parameters. However, a full wind turbine blade was modeled according to the dimensions obtained from

Integration of “Momentum-theory” and “Blade-Element-Theory” results in (BEM) analysis. Momentum-theory deals with responsible forces for producing the motion of the fluid by the rotor. On the other hand, blade element theory is related to the forces on turbine blade due to the flow of the fluid. In this method, the wind turbine blade was divided into small blade sections. After that, the conservation of one-dimensional linear momentum was applied to all segments of the blade which lead to forces and power calculation. In BEM there are two main key factors, the induction factor, and airfoil aerodynamic characteristics. The open source software Q-Blade was used to analyze the BEM [

where,

Here, c, B, r represent chord length, blade number and local radius of the turbine blade respectively [

The CFD analysis is done based on continuity and Navier-Stokes governing equations. In this work k-ω, Shear Stress Transport (SST) turbulence model was executed in Ansys CFX software. The equations are given below

Conservation of mass is defined by

Moreover, Conservation of momentum can be represented as

where

The blade geometry was imported into a computational fluid domain, which is one-third of a complete circular wind section around the blade. The front side and top side of the domain were defined as the air velocity inlet while rear side was defined as pressure outlet. The other two sides were assignedto the periodic boundary condition. The inlet radius of fluid domain is ten times more than the blade radius. For the outlet radius, the ratio is 20 times. The downstream length of a fluid domain is higher than upstream length which allowsobserving the generated wake in fluid domain. After that, the mesh was generated for the entire domain and as well as for the blade geometry (

The accuracy of the analysis depends on how the meshing is done. It is good practice to do trial and error for fixing the mesh element size, inflation layer thickness, the sphere of influence radius and so on. For better and uniform mashing the match control was applied between two periodic boundaries conditioned face. This is called local mesh control. One of the key factors of meshing is the variation of element sizing throughout the geometry. Here, the maximum and minimum face element sizes of the fluid domain are 7.47 m and 0.00747 m respectively. Besidesthis, the element size of the blade surface was maintained at 0.02 m. An inflation layer was created on the blade surface to give the better resolution of boundary layer flow. The transition ratio was maintained at 0.272 with the growth rate of 1.2. A sphere of influence was also added to fine the mesh around the blade. The sphere radius was 10 m, and element size was 0.4 m [

Finally, the fluent pressure based solver was used to get aerodynamics loading, velocity streamlines, and torque generated by the blade. Here, the fluid flow was considered as turbulent. Among all the turbulence model, k-ω “Shear-Stress- Transport” (SST) model is suitable for this analysis, because it can predict the boundary layer separation under the adverse pressure gradient. Menter intro- duced this model. It is a combination of k-ε and k-ω turbulence model [

The governing equations of k-ω SST model are described below

where,

The turbulence stress tensor can be defined as

The turbulence viscosity represented by

where

The function

Here, y denotes the distance to nearest surface

From the k-ε and k-ω turbulence model, the coefficients

where,

Moreover, the coefficient

However, to observe the rotational of the blade the standard moving reference frame was used. This model allows generating a steady state problem on the moving reference. Along with all standard air parameters and operating conditions, the air velocity was applied at the inlet and the top surface of the fluid domain because air flows not only from the horizontal direction but also from all direction.

CFD post was used to observe the results of the simulation.

From the

The pressure distribution on blade span wise is showed in

outer portion compare to the inner part of the wind turbine blade. In other words, there is a low-pressure area at the tip leading edge of the blade. The reason behind this is the three-dimensional rotation effect of the wind turbine blade tip.

In current study performance of wind turbine was determined against different

Tip Speed Ratio (TSR). From coefficient of performance (COP) vs. Tip Speed Ratio (TSR) curve analysis, it is clear that the performance of wind turbine blade by CFD computation is a little bit less than by BEM. The reason behind that, the CFD method can calculate more accurately with 3D calculation than BEM.

Mixed airfoils are not usually used for a small-scale wind turbine. However, this work has shown that mixed airfoil wind turbine could also have satisfactory performance in wind energy sector. The BEM and CFD analyses were done only for the wind turbine blade. The integration of tower, rotor hub, generator housing, and yaw control has much effect on overall efficiency of the wind turbine. Instead of improving design criteria, this work focused on the performance analysis of the turbine. The further improvement of the currently designed wind turbine could be made by FSI analysis and several optimizing techniques.

Authors would express the heartiest deep sense of gratitude to reference papers, books, and websites as mentioned below which are necessary for the research.

Hasan, M., El- Shahat, A. and Rahman, M. (2017) Performance Investigation of Three Combined Airfoils Bladed Small Scale Horizontal Axis wind Turbine by BEM and CFD Analysis. Journal of Power and Energy Engineering, 5, 14-27. https://doi.org/10.4236/jpee.2017.55002