On the role of surface roughness in the aerodynamic performance and energy conversion of horizontal wind turbine blades: a review

Renewable energy is one of the main pillars of sustainable development, especially in developing economies. Increasing energy demand and the limitation of fossil fuel reserves make the use of renewable energy essential for sustainable development. Wind energy is considered to be one of the most important resources of renewable energy. In North African countries, such as Egypt, wind energy has an enormous potential; however, it faces quite a number of technical challenges related to the performance of wind turbines in the Saharan environment. Seasonal sand storms affect the performance of wind turbines in many ways, one of which is increasing the wind turbine aerodynamic resistance through the increase of blade surface roughness. The power loss because of blade surface deterioration is significant in wind turbines. The surface roughness of wind turbine blades deteriorates because of several environmental conditions such as ice or sand. This paper is the first review on the topic of surface roughness effects on the performance of horizontal‐axis wind turbines. The review covers the numerical simulation and experimental studies as well as discussing the present research trends to develop a roadmap for better understanding and improvement of wind turbine performance in deleterious environments. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of a leading edge erosion shield on the aerodynamic performance of a wind turbine blade

Armour EDGE is a novel shield developed to protect the leading edge of wind turbine blades from erosion. The aerodynamic impact on aerofoils of National Renewable Energy Laboratory (NREL) 5MW wind turbine has been investigated using 2D fully turbulent computational fluid dynamics (CFD), with three profiles at critical locations along the blade simulated both with and without the shield to compare aerodynamic performance. Two wind speeds were investigated that reflect regular operating conditions: at rated speed of 11.4 m/s and a below rated speed of 7 m/s. The results showed that the presence of the shield during rated wind speed reduced the drag by as much as 4.5%, where the lift‐to‐drag ratio increased by a maximum of 4%. At the below rated wind speeds, the shield had negligible impact on the performance of all but one National Advisory Committee for Aeronautics (NACA) 64‐618 profile, which resulted in an increase in the drag coefficient of 7%. It was also found that the suction side of the aerofoil is much more sensitive to leading edge protection placement than the pressure side. It was concluded that the erosion shield as a method of leading edge protection, with a gradual transition from shield to blade, will not have a major impact on the aerodynamic performance of a multi‐megawatt wind turbine blade and could slightly increase aerofoil efficiency at high wind speeds.     

The application of passive air jet vortex‐generators to stall suppression on wind turbine blades

An experimental study was performed to assess the feasibility of passive air jet vortex‐generators to the performance enhancement of a domestic scale wind turbine. It has been demonstrated that these simple devices, properly designed and implemented, can provide worthwhile performance benefits for domestic wind turbines of the type investigated in this study. In particular, this study shows that they can increase the maximum output power coefficient, reduce the cut‐in wind speed and improve power output at lower wind speeds while reducing the sensitivity to wind speed unsteadiness. A theoretical performance analysis of a 500 kW stall‐regulated wind turbine, based on blade element momentum theory, indicates that passive air jet vortex‐generators would be capable of recovering some of the power loss because of blade stall, thereby allowing attainment of rated power output at slightly lower average wind speeds. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical Analysis and PIV Measurements of Tip Vortex in an Axial Rotor with Skewed-Swept Blade at Its Leading Edge

Based on the characteristics of axial fans of outdoor units of centralized air-conditioners, using the finite-volume method, applying three-dimensional steady Reynolds-averaged Navier-Stokes equations coupled with Spalart-Allmaras turbulence model equation, and adopting SIMPLE algorithm, numerical analysis is made and applied to analyze the internal flow field of axial rotors with skewed-swept blade at its leading edge. This numerical simulation mainly investigates the formation and development of the tip vortex. Based on the tip vortex characteristics that have been captured, the internal flow numerical results are compared with those obtained by the PIV experiments. This comparison indicates a good agreement between numerical results and PIV results, thus proving the validity of the numerical simulation. In addition, based on the internal flow analyses of the axial rotor with skewed-swept blade at its leading edge, different flow phenomena features are presented. These flow features can be used for further improvements of the present rotor performance characteristics.     

Prospective challenges in the experimentation of the rain erosion on the leading edge of wind turbine blades

Developments in the wind industry reveal intricate engineering challenges, one of them being the erosion on the leading edge of the wind turbine blades. In this review work, the main issues for the wind industry in the experimentation with respect to erosion are examined. After a historical and general overview of erosion, this review focuses on the rain erosion on the leading edge of the wind turbine blades giving prominence to (1) the rain simulations, (2) experimental erosion facilities, and (3) variables to characterise erosion. These three factors have to be improved to establish a research field enabling the prediction of erosion behaviour and providing useful information about how the rainfall affects the leading edge of the wind turbine blades. Moreover, these improvements in the experimentation of the erosion would be a first step to understand and predict the erosion damage of the wind turbine blades. Finally, this review work also will help to cope with experimental investigations and results in the rain erosion on the leading edge with a deeper critical thinking for future researchers.     

Investigation of flow behind vortex generators by stereo particle image velocimetry on a thick airfoil near stall

Stereoscopic Particle Image Velocimetry measurements investigating the effect of vortex generators (VGs) on the flow near stall were carried out in a purpose‐built wind tunnel for airfoil investigations on a DU 91‐W2‐250 profile. Measurements were conducted at Re = 0.9?10⁶, corresponding to free stream velocity U_∞ = 15 m s^?1. The objective was to investigate the flow structures induced by the vortex generators and study their separation controlling behavior on the airfoil. The uncontrolled flow (no VGs) displayed unsteady behavior with separation as observed from large streamwise velocity variations. The corresponding controlled flow (with VGs) showed the same unsteadiness, where the appearance of the vortex structures alternated with a much less separated or even attached boundary layer as also seen in the measured airfoil data: C_L = 1.56, C_D = 0.116 with VGs and C_L = 1.16, C_D = 0.135 without. On average, the controlled flow left an attached flow as opposed to the uncontrolled one. Mixing close to the wall, transferring high momentum fluid into the near wall region, was observed, and the hypothesis of variations in the streamwise velocity component in the boundary layer was supported by a Snapshot Proper Orthogonal Decomposition analysis. This analysis also revealed some of the dynamics of the induced vortices. Copyright © 2012 John Wiley & Sons, Ltd.     

Transversal tube pitch effects on local heat transfer characteristics of the flat tube bank fin mounted vortex generators and their sensitivity to nonuniform temperature of the fin

The tube bank fin is commonly used to increase the area of the heat transfer surface with a small heat transfer coefficient of a heat exchanger. If vortex generators (VGs) are punched on the fin surface, the heat transfer performance of the fin can be improved. This paper focused on the effect of transversal tube pitch on the local heat transfer performance of the three-row flat tube bank fin mounted with VGs. On the fin surface, constructing the flow channel but without mounted VGs, the transversal tube pitch was greater, and the span averaged Nusselt number downstream was larger because fewer interactions of vortices would be generated from different VGs located upstream. When the area goodness factor was used as the criteria on the condition of one tube unit of heat exchanger for commonly used fin materials and fin thickness, the transversal tube pitch has considerable effect on the heat transfer enhancement of VGs. Large transversal tube pitch is more sensitive to fin material than to fin thickness.     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Two improvements to the dynamic wake meandering model: including the effects of atmospheric shear on wake turbulence and incorporating turbulence build‐up in a row of wind turbines

The dynamic wake meandering (DWM) model is an engineering wake model designed to physically model the wake deficit evolution and the unsteady meandering that occurs in wind turbine wakes. The present study aims at improving two features of the model:

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Effect of the vacancy close to heat exchange wall on the heat transfer performance of the solid particles steam generator using waste heat in a methanol steam reforming hydrogen production system

With the massive consumption of fossil fuels and it resulted in significant carbon emissions, it is urgent to find an alternative clean energy source. Hydrogen has been regarded as one of the most promising energy candidates for the next generation. It is a great approach that methane steam reforming for hydrogen production by rational utilization of industrial waste heat, which significantly minimizes carbon emissions and develops methanol steam reforming technology. A solid particle steam generator based on the primary heat exchange method has been proposed, which can provide the heat and steam in the methanol steam reforming hydrogen production system. The quasi-two-dimensional packing heat transfer model of solid particles steam generator was set up.The effect of distance change between the vacancy and the cold wall and distance change between vacancies on heat transfer performance of the steam generator and hydrogen production capacity were studied. As the distance between the vacancy and the wall increases, the heat transfer performance of the steam generator gradually deteriorates, so the steam production of the steam generator decreases, and the system's hydrogen production capacity is reduced, the maximum of the heat flux and the minimum of the apparent thermal resistance are 34.67 kW/m² and 12.02 K/W, respectively. As the distance between vacancies increases, the heat transfer performance of the steam generator is gradually optimized slightly. To maintain the hydrogen production capacity, vacancies should be avoided to appear 2 layers of particles away from the heat exchange wall in the particles steam generator. From the results of the study, the farther the distance between vacancies, the better the steam production and hydrogen production capacity.