Wind turbine blade aerodynamics
Atmospheric ice accretion on wind turbine blade mainly occurs due to collision of super cooled water droplets. These super cooled water droplets may immediately or after some short delay freeze into ice on surfaces. [3] [4]
The main effect of icing on wind turbine blades is generally an altered profile shape which results in a change of its aerodynamic characteristics. This affects the wind turbine torque and power production. The results of such aerodynamic changes can be seen as a decrease in the in-plane (rotating) force because of a decrease in the lift coefficient (CL) and an increase in the drag coefficient (CD). [5] Figure 1 illustrates the general nomenclature of wind turbine, which contains several relevant terms, and the airfoil nomenclature.
Different conditions (operating and thermodynamic) on the wind turbine blades will result in accreted ice of different densities and shapes. [6] The change in accreted ice shapes affects the air flow behaviour on the blade profile and its aerodynamic performance. [7] Bragg et al. [8] categorized four types of ice accretion that affect wind turbine blade aerodynamics differently: dispersed roughness, horn ice, streamwise ice and spanwise-ridge ice, as shown in the figure 2. The change in aerodynamic properties is bigger than the change in the mechanical properties such as change in mass of the rotor, which means that load imbalance of the rotor under icing is mainly due to change in aerodynamic properties. [9]
Better knowledge of atmospheric ice accretion on wind turbine blades is critical in determining which geometric features of the blade profile (airfoil) contribute to the performance degradation during the ice accretion events and how these may differ for different blade profiles. [3] [4] Different researchers have studied the effects of the atmospheric ice accretion on the airfoil performance. Most of these investigations have been performed using either ordinary wind tunnel with artificial ice templates attached to the blade profile or icing wind tunnel. However, in the recent few decades, the CFD based numerical techniques have begun to play a significant role in simulating and determining the performance of wind turbines under icing conditions. [5]
| Angle of attack (AOA, α) | It is the angle between the chord line and the direction of the oncoming flow. |
| Suction surface (upper surface) | Generally associated with the higher velocity and lower static pressure. |
| Pressure surface (lower surface) | It has a comparatively higher static pressure than the suction surface. The pressure gradient between these two surfaces contributes to the lift force generated for a given airfoil. |
| Leading edge | It is the point at the front of the airfoil that has maximum curvature (minimum radius). |
| Trailing edge | It is defined similarly as the point of maximum curvature at the rear of the airfoil. |
| Chord line | It is the straight line connecting leading and trailing edges. The chord length, or simply chord, c is the length of the chord line. That is the reference dimension of the airfoil section. |
| Mean camber line (mean line) | It is the locus of point’s midway between the upper and lower surfaces. Its shape depends on the thickness distribution along the chord. |
| Thickness | The thickness of an airfoil varies along the chord.
Thickness measured perpendicular to the camber line. This is sometimes described as the "American convention"; Thickness measured perpendicular to the chord line. This is sometimes described as the "British convention". |
| Nose circle | At some point on the LE the radius will be a minimum, at which point one could calculate a radius of curvature at that point and draw the radius inward normal to that point and make a circle. |
| Aerodynamic center | Which is the chord-wise length about which the pitching moment is independent of the lift coefficient and the angle of attack. |
| Center of pressure | Which is the chord-wise location about which the pitching moment is zero. |
| Lift force (L) | The component of this force perpendicular to the direction of motion is called lift. |
| Drag force (D) | The component parallel to the direction of motion is called drag. |
| Stall | It is a reduction in the lift coefficient generated by a foil as angle of attack
increases. This occurs when the critical angle of attack of the foil is exceeded. |
| Reynolds number | It is a measure of the ratio of the inertial force (ρv2)/L
to the viscous force μv/L2 of the fluid, it is a dimensionless quantity. |
| Tip Speed Ratio (TSR) | It is the ratio between the tangential speed of the tip of a blade and the actual
speed of the wind, ν. The tip-speed ratio is related to efficiency, with the optimum varying with blade design. |
| MVD | It refers to the midpoint droplet size, where half of the volume of spray is in
droplets smaller, and half of the volume is in droplets larger than the mean, it is not the average mean values of the droplet sizes. |
| LWC | It is the measure of the mass of the water in a cloud in a specified amount
of dry air, and it is typically measured per volume of air (g/m3) or mass of air (g/kg). |
| Supercooled large droplets (SLD) | It defined as those water droplets which exist in liquid form at temperatures
below 0°C with a diameter greater than 50 microns. SLD conditions include freezing drizzle drops and freezing raindrops. |
| The power coefficient (Cp) | It is a quantity that expresses what fraction of the power in the wind is
being extracted by the wind turbine. It is generally assumed to be a function of both TSR and pitch angle. |
More on this topic: Icing on wind turbine blades
References
- ↑ P. Dvork, Lessons learned from ice-resistant turbine tech, Windpower Engineering & Development, October 19, 2016.
- ↑ Sam Lee, Michael B. Bragg, Investigation of Factors Affecting Iced-Airfoil Aerodynamics, Journal of Aircraft, vol. 40, no. 3, pp. 499 – 508, 2003.
- ↑ 3.0 3.1 Kathie Zipp. Understanding costs for large wind-turbine drivetrains, https://www.windpowerengineering.com/understanding-costs-for-large-wind-turbine-drivetrains/.
- ↑ 4.0 4.1 Sohrab Gholahosein Pouryoussefi, Masoud Mirzaei, Mohamed Mahdi Nazemi, Mojtaba Fouladi, Alreza Doostmahmoudi, Experimental study of ice accretion effects on aerodynamic performance of NACA-23012 airfoil, Chinese Journal of Aeronautics, vol. 29, no. 3, pp. 585 – 595, 2016.
- ↑ 5.0 5.1 Matthew C Homola, Muhammad S Virk, and P. J. Nicklasson., Performance losses due to ice accretion for a 5 MW wind turbine, Wind Energy, Vol. 15, no. 3, pp. 379 – 389, 2012.
- ↑ Muhammad S. Virk, Matthew C. Homola, and Per J. Nicklasson, Effect of Rime Ice Accretion on Aerodynamic Characteristics of Wind Turbine Blade Profiles, Wind Engineering, vol. 34, pp. 207 – 218, No. 2, 2010.
- ↑ Jaiwon Shin, Berkowitz Brian, Chen Hsun, and Cebeci Tuncer, Prediction of ice shapes and their effect on airfoil performance, NASA Techical Memorandum 103701, 1991.
- ↑ Sam Lee, Michael B. Bragg, Investigation of Factors Affecting Iced-Airfoil Aerodynamics, Journal of Aircraft, vol. 40, no. 3, pp. 499 – 508, 2003.
- ↑ M. Etemaddar, M. O. L. Hansen, and T. Moan, Wind turbine aerodynamic response under atmospheric icing conditions, Wind Energy, vol. 17, no 2, pp. 241 – 265, 2012.
- ↑ 10.0 10.1 Jia Yi Jin. Study of Atmospheric Ice Accretion on Wind Turbine Blades. Thesis for the degree of Philosophiae Doctor. Narvik, March 2021. UiT ̶ The Arctic University of Norway. Faculty of Engineering Science and Technology.