Multi-Dimensional Extraction of Ice Shape and an Investigation of Its Aerodynamic Characteristics on Iced Wind Turbine Blades

Abstract

The icing of wind turbine blades can cause changes in airfoil shape, which in turn significantly reduces the aerodynamic performance and affects the power generation efficiency of a wind turbine. In this paper, the iced airfoil shape of wind turbine blades with different positions, masses, and angles of attack icing was measured and modeled using 3D scanning technology, and changes in airfoil shape parameters under different icing conditions were obtained. The numerical simulations of icing blades were carried out to investigate the effect of blade icing on aerodynamic characteristics. The results show that ice accumulation thickness tends to increase nonlinearly along the spanwise direction and chord length for both windward and leeward icing. The airfoil angle of attack affects the trend of ice accumulation changes. As shown by the numerical simulation of the aerodynamic characteristic, blade icing changes the airfoil shape, which changes the pressure difference between the leading edge and trailing edge, affects the size and number of the wake vortex structures, and further changes the aerodynamic characteristics of the blade.

1. Introduction

Wind energy is one of the most widely used renewable energy sources and is favored by various countries because of its green and clean characteristics [1]. Due to the high air density and abundance of wind energy resources in cold regions, the installed capacity of wind turbines is 10% higher than that in other regions [2,3]. However, due to the lower temperatures, the blades are prone to ice accumulation [4]. Once the blades of a wind turbine freeze, it will lead to a series of safety problems, such as increasing the noise and load, and even breaking the blades in severe cases [5,6].

In recent years, many research results have been achieved in regard to the icing characteristics of wind turbine blades and changes in aerodynamic characteristics. In terms of blade icing characteristics, Bose et al. conducted icing tests on a horizontal-axis wind turbine with a diameter of 1.05 m and found that most of the open ice accumulated at both the leading edge and the tip of the blades [7]. Shin et al. investigated the impacts of different liquid water contents, incoming wind speeds, etc., on the icing of NACA0012 airfoils and obtained different icing shapes under different icing parameters [8]. Hu and Zhu et al. selected NREL Phase VI as the test condition to study the influence of different parameters on icing distribution on a blade. The results showed that the greater the wind speed and the lower the temperature, the greater the influence on blade icing [9,10]. Drapalik et al. monitored ice formation on small wind turbines and evaluated the ice formation strength. It was shown that the ice geometries were similar to those of large wind turbines but with a significant increase in ice-forming intensity [11]. Shu and Liang et al. quantified ice distribution on horizontal-axis wind turbine blades and found that the ice thickness on the blades increased rapidly from the blade root to the blade middle, and then increased slowly from the blade middle to the blade tip [12,13]. Hui et al. investigated the icing characteristics of offshore wind turbine blades by simulation. It was shown that an increase in liquid water content and medium volume diameter led to an increase in the amount of ice formed on the blade surface, while an increase in medium volume diameter expanded the surface adhesion of ice on the blade [14]. Homola et al. conducted a numerical study of ice accumulation in a 5MW wind turbine blade profile and found a streamlined ice shape at low temperatures and an angular shape at higher temperatures [15]. Virk et al. found that blade airfoil symmetry affects ice accumulation, with more streamlined ice shapes observed along the leading edge for symmetrical airfoils compared with asymmetrical airfoils, by numerically simulating two symmetrical (NACA 0006 and 0012) and two asymmetrical airfoils (NACA 23012 and N-22) [16]. Guo et al. selected NACA 0018 airfoils as their research subject and carried out icing tests with different TSRs under rime ice conditions. It was shown that when the TSR was less than 1, ice covered the entire blade surface and grew by layers. When the TSR was greater than 1, the ice layer was concentrated near the leading edge [17]. Li et al. investigated the blade of NACA7715 airfoils in a wind tunnel and showed that the icing area and the icing rate depend on the angle of attack and the wind speed and determined the relationship between them [18]. In terms of aerodynamic characteristics, Ren et al. numerically simulated a static icing blade and calculated the power coefficients of a wind turbine before and after icing for different tip speed ratios. The results showed that the power coefficient decreased with an increase in the tip speed ratio [19]. Gao et al. investigated changes in wind turbine aerodynamic performance during dynamic icing. The results showed that irregularly shaped ice structures greatly disturbed airflow around the blades, leading to the shedding of unstable vortex structures from the surface of the accumulated ice and causing significant degradation in aerodynamic performance [20]. Oumnia and Kangash et al. simulated the aerodynamic and structural performance of a wind turbine on an iced-up NACA 4412 airfoil and a NACA 64-618 airfoil. The results showed that when ice accumulates on the surface of a blade, the shape of its profile is changed, the lift and drag coefficients change significantly, and the aerodynamic and structural performance of the wind turbine is degraded [21,22]. Jin et al. investigated the aerodynamic characteristics of S832 and S826 airfoils with icing profiles based on CFD numerical analysis, and the CFD analysis showed that the airflow behavior of iced blades changed compared with that of un-iced blades, which led to the degradation of aerodynamic performance, and that the change in the aerodynamic characteristics of S832 was more obvious than in S826 [23]. Aakhash and Li et al. simulated different icing shapes and aerodynamic characteristics of icing airfoils. The results showed that the aerodynamic characteristics of the icing airfoils were degraded by a decrease in the lift coefficient and an increase in drag compared to un-iced shapes [24,25]. Wang et al. suggested an improved icing calculation model based on different time scales to reveal the effects of yaw icing on aerodynamic characteristics. The results showed that aerodynamic properties, such as the lift and drag coefficients of localized airfoils, are further reduced under yaw icing conditions, leading to a reduction in wind turbine power, thrust, and blade root moment; the results are summarized in the following table [26]. Sean et al. investigated the effects of glaze ice and freezing ice conditions on wind turbine aerodynamics through numerical simulations. It was found that more ice is formed at lower temperatures in freezing ice conditions, and the wind turbine performance was reduced by a maximum of 40% [27]. Mustafa et al. investigated the static performance of a wind turbine at different uniform wind speeds between cut-in and cut-out wind speeds by simulating and analyzing a 5 MW wind turbine blade with light icing. The results showed that mild icing changed the aerodynamic and dynamic characteristics of the wind turbine by increasing the cut-in and rated wind speeds and decreasing the thrust and maximum power coefficient by 5.5% and 13.35%, respectively [28]. Aleksei et al. investigated the effect of icing on the performance of a pitch-regulated large wind turbine by performing numerical simulations of six blade sections of an NREL 5 MW wind turbine at different free-stream speeds. The results showed that the blade tips were most affected by icing and the maximum degradation of wind turbine performance was about 24% [29].

Till now, most studies have focused on the blade icing requirement or ice shape change and ice accumulation distribution under different icing conditions. There are few quantitative studies on the change of airfoil shape under different icing conditions along the chord length direction and blade spanwise direction. By quantitatively analyzing the changes in airfoil data from quantitative studies, the changes in aerodynamic characteristics of wind turbine icing blades were refined. The 600 W wind turbine blade under nonicing and different icing conditions was scanned by using a 3D scanner. The thickness and shape of icing on the blade surface were measured and calculated. A database of the ice shape of the wind turbine blade and its icing airfoil was established, and the icing state parameters of the wind turbine blade and its airfoil as a whole under different icing conditions were investigated. Through the icing model obtained from the quantitative study, the aerodynamic characteristics of wind turbine blades were simulated and analyzed using numerical simulation, and the influence law of icing on the pressure field and flow field of the blades was derived. The research results will provide theoretical and data support for a comprehensive understanding of the shape of ice accumulation and its induced changes in aerodynamic characteristics of wind turbines after icing. Through the change of aerodynamic characteristics of the blade after icing, the operational status of the wind turbine will be monitored, laying the foundation for the realization of noncontact monitoring.

2. Icing Theory

Wind Turbine Icing Types

From a thermodynamic point of view, when the temperature of water is decreased below 0 °C, it will become super-cooled water. In cold environments, the super-cooled water droplets hitting the blades will freeze and gradually accumulate into ice [30]. The wind turbine icing is a typical atmospheric structure icing. Wind turbine icing can be categorized as shown Figure 1.

Usually, different types of icing are categorized into rime ice, glaze ice, and mixed ice. The icing state on wind turbine blades in north China is investigated, aiming at glaze ice, in this paper. The main physical characteristics of each ice type [31] are shown in

Table 1. Main physical characteristics of various icing types.

Type	Density/(kg·m−3)	Color	Adhesion
Glaze ice	900	Transparent	Strong
Heavy rime ice	600~900	Nontransparent	Strong
Mild rime ice	200~600	White	Medium
Mixed ice	600~800	Strong	Semi-transparent

.

3. Icing Experiment Device and Three-Dimensional Scanning of Blade

3.1. Blade Icing Test

The blade used in this test was produced by a company in Jiangsu Province, China. It is made of glass fiber composite. Its length is 1170 mm. Its maximum chord length is 170 mm and its weight is 2.14 kg. Two different types of icing were used for the icing test, namely the horizontal and vertical icing of the blade. The horizontal icing of the blade is divided into windward and leeward icing, which are divided into four different icing mass cases. The vertical icing is divided into seven different angle of attack icing cases, as shown in Figure 2.

The icing tests were carried out at Inner Mongolia Agricultural University in the Inner Mongolia Autonomous Region, where the average outdoor temperature was in the range of −12 °C to −19 °C. The test instrument parameters are shown in

Table 2. Blade icing instrument parameters.

Instrument	Parameters	Value
Temperature and relative humidity detector	Temperature range	−40–80 °C
	Temperature accuracy	±0.2 °C
	Humidity range	0–100% RH
	Humidity accuracy	±2% RH
Electric nebulizer	Flow rate	3 L/min
Precision electronic platform	Range	0–10 kg
scale	Accuracy	0.1 g

.

To quantitatively study the icing characteristics of the blade under different icing conditions, the blade is placed horizontally in the blade windward and leeward icing test. The blade windward side and leeward side are facing upwards to be covered in ice. In the vertical icing test, the blade is placed vertically to be covered in ice under different angles of attack. An eight-hole high-pressure electric sprayer is placed in front of the blade to spray water mist. The diameter of the atomized particles is up to 0.3 μm. When the water mist from above is falling freely, the blade windward side, leeward side, and different angle of attack positions will be gradually covered by ice as is shown in Figure 3.

As shown in Figure 4, the density of ice in an icing environment needs to be measured to determine the type of ice in a trial. Each set of tests was repeated 3 times each, with 5 in each group.

According to the measurement, the average density of ice is 920.16 kg/m³, and the ice type is glaze ice.

3.2. The 3D Scanning of Icing Blades

The 3D Scanning Test

As shown in the figure below, the wind turbine blade before and after icing was fixed in a vise to ensure that the blade did not move during the scanning process, and then the blade was placed diagonally in front of the 3D scanner at a distance of 30 cm to ensure that all sides of the blade could be scanned. When scanning the iced blade, the iced side needs to be evenly sprayed with developer to ensure that the scanner probe can fully receive the diffuse reflections from the iced blade and obtain the complete point cloud data. As shown in Figure 5, a 3D scanner is used to scan the blade to obtain the complete point cloud data and import them into Geomagic Studio 11.0 software to encapsulate them in a shell. The shell model is optimized to fill the various defects in the model and obtain a high-quality shell model. Since the shell model cannot be recognized by the Solidworks 2018 software, it is necessary to fit curved surfaces to the model. The final model is imported into the Solidworks 2018 software for modeling and the solid blade model and wind turbine impeller model are obtained.

4. Results and Discussion

4.1. Icing Blade Airfoil Thickness Change

In order to obtain the iced blade airfoil shape under different icing conditions, the airfoil shapes in a 0.2R section of nine icing blades are taken for comparison. Figure 6 shows the location of the taken airfoil sections; Figure 7 shows the comparison of each section of the airfoil.

As can be seen in Figure 7, there is an obvious difference in airfoil shape before and after blade icing. During icing on the windward side and the leeward side, the water droplets will flow lower when they hit the blade at the moment of impact. Thus, windward icing has a greater effect on the trailing edge, while leeward icing has a greater effect on the leading edge. When the leading edge is iced, the thickness at the trailing edge is basically unchanged, while the thickness at the edge of the airfoil changes very significantly. With the increase in the angle of attack, the position of the ice accumulation from the windward side gradually moved to the leading edge, and the thickness of the ice accumulation at the trailing edge gradually decreased. The ice thickness change on the trailing edge is extremely obvious and the influence on the leading edge is small.

In order to accurately describe the effect of icing on the geometrical parameters of the airfoil, the chord length was divided into four equal parts at 0.2R, 0.4R, 0.6R, 0.8R, and 0.95R of the un-iced blade, respectively, with five points distributed at chord length ratios 0, 1/4, 2/4, ¾, and 1, as shown in Figure 8. For the windward side icing, the thickness at five points of the perpendicular intersection is measured with the windward icing contour line. For the leeward side icing, the thickness at five points of the intersection is measured perpendicular to the leeward icing contour line. The trend of thickness changes is shown in Figure 9.

From Figure 9a–e, it can be seen that in the case of icing on the windward side, the initial period of icing leads to the most significant increase in the thickness of ice accumulation near the trailing edge. This is due to the more gentle airfoil shape at the trailing edge, and the droplets at monitoring points 3 and 4 at the initial stage slipped off to form ice accumulation, which increased the thickness. As the icing mass increases, the curvature of the trailing edge airfoil increases and the thickness change stabilizes. With the increase in icing mass, the curvature of the leading edge increases, and the thickness change is more uniform. Monitoring points 2, 3, and 4 are located in the middle of the airfoil, the curvature of the airfoil decreases with the increase in icing mass, and it is easier to form ice accumulation. When the ice accumulation reaches the critical value, there is a sudden increase in thickness. From Figure 9e,f, it can be seen that the thickness of ice accumulation near the leading and trailing edges increases most significantly when icing occurs on the leeward side. This is due to the fact that the droplets at monitoring points 2, 3, and 4 slipped toward the ends and the droplets accumulated at the ends, forming ice accumulation. The amount of change in thickness at the leading and trailing edges decreases as the mass of icing increases. When monitoring points 2, 3, and 4 were in the middle of the blade and at the blade root, the thickness change was obvious with the increase in icing mass, and then the thickness change stabilized. At the tip of the blade, the airfoil curvature varied with increasing icing mass, and the thickness underwent several abrupt changes. Ice thickness on both the windward and leeward sides showed a nonlinear trend.

4.2. Ice Thickness Analysis along the Blade Spanwise Direction

Using four measurement points at point 1 and point 2 and the maximum thickness in Figure 10, the thickness when not iced is subtracted from the measured thickness of the windward side and the leeward side under different icing conditions, and the change of icing thickness at 0.2R, 0.4R, 0.6R, 0.8R, and 0.95R along the blade spanwise direction can be obtained. The trend of icing thickness along the blade spanwise direction is shown in Figure 11.

From Figure 11, it can be seen that the icing thickness on the windward and leeward sides of the blade shows a nonlinear trend along the spanwise direction. From Figure 11a–c, it can be seen that when the windward side is iced, the trailing edge thickness increment changes the most along the spanwise direction, and the thickness of the trailing edge measurement point is always greater than that of the leading edge measurement point. With the increase in icing mass, the change rule of measuring points 1 and 2 along the spanwise direction is changed, and the thickness change of the measuring points at the maximum thickness shows a trend of decreasing first and then increasing. This is due to the change of the wing angle of attack along the spanwise direction, and the windward wing curvature changes more and the location of ice accumulation changes significantly. From Figure 11d–f, it can be seen that when icing occurs on the leeward side, the thickness of the leading edge changes most significantly along the spanwise direction. With the increase in icing mass, the overall change trend of the three measurement points along the spanwise direction remains stable. The change in the angle of attack of the airfoil affects the location of the thickness change of the measurement points along the spanwise direction. The thickness changes on the windward side near 0.4R and 0.8R are obvious. On the leeward side, the thickness at 0.4R increases significantly with the increase in icing mass, and then decreases.

5. Numerical Simulation Study of the Iced Wind Turbine Flow Field

5.1. Calculation Settings

5.1.1. Numerical Model and Grid

In order to study the effects of different icing conditions of blades on the aerodynamic characteristics of wind turbines, the wind turbine blade model obtained from icing experiments is used for simulation. The iced wind turbine model is imported into ANSYS 19.2 software for the creation of the inner and outer basins. Since the wind turbine model has rotational motion, the inner and outer basins were divided into the rotational domain and the stationary domain. The diameter of the wind turbine is 1.95 m. The inner basin is created as a cylinder of 3 m in diameter and 7 m in length, which is 2 m from the front surface of the wind turbine. The outer basin is a cylinder of 5 m in diameter and 35 m in length, which is 5 m from the front surface of the wind turbine. Through continuous debugging and irrelevance verification, it was determined that a total mesh number of about 2.6 million was appropriate. The grid division is shown in Figure 12.

5.1.2. Simulation Solution

The turbulence model was solved using large eddy simulation (LES). In the calculation, the Poisson’s ratio coefficient is set to 0.45 and the size factor is set to 0.6 to ensure the convergence of the fluid calculation. The iteration step size is set to 1 × 10⁻⁵ and the residuals are set to 1 × 10⁻⁶.

5.2. Analysis of Results

5.2.1. Flow Field Pressure Analysis on the Icing Blade Surface

The aerodynamic noise of the flow field is closely related to its pressure variation and distribution, and the analysis of the dynamic situation in the flow field can help to carry out further noise research. In this study, the pressure field was obtained when the rotational speed is 200 r/min and the incoming wind speed is 5 m/s by using LES as an example for analysis.

From Figure 13, it can be seen that with the increase in icing mass, the surface pressure of the windward and leeward side blades increases with it, and the pressure distribution moves towards the blade tip. This is due to the fact that wind turbine blade icing changes the airfoil shape to make the surrounding flow field unstable, which in turn causes the pressure distribution to change. In addition, the turbulent fluctuating pressure generates noise on the airfoil surface, and the interaction between the blade surface and the boundary layer fluid also generates turbulent boundary layer noise.

As the angle of attack increases, the flow on the blade surface becomes more unstable and the separation point moves toward the leading edge and increases the turbulence intensity, leading to an increase in pressure surface pressure. In particular, at an angle of attack of 60°, the pressure surface pressure maximum occurs in the separation zone and at the trailing edge. This indicates that the strongest aerodynamic noise is generated in these regions and is the main source of noise emission generation.

5.2.2. Static Pressure Analysis of Two-Dimensional Airfoil Sections

The pressure at the 0.2R section of the blade for different icing conditions is shown in Figure 14.

As shown in the figure above, the static pressure increases significantly when icing on the windward and leeward sides occurs. When the windward side ices, the pressure separation point moves toward the leeward side. The pressure difference between the leading and trailing edges of the blade decreases as the icing mass increases. When the leeward side ices, the pressure separation point moves towards the windward side. The pressure difference between the leading and trailing edges of the blade increases with increasing icing mass. When icing at different angles of attack, the pressure difference between the leading and trailing edges increases with increasing angle of attack. The change in pressure difference causes a change in the operating speed of the turbine, which will have a further effect on the aerodynamic noise of the wind turbine.

5.2.3. Flow Field Analysis

As shown in Figure 15, the variation of wake vortices on the windward side of the blade for different icing masses is given. The results show that the wake vortices in the flow field are mainly composed of the center vortex and the tip vortex. With the increase in icing mass, the effect of icing on the vortex structure is mainly manifested in the size and number of vortices. The size and number of center vortices and tip vortices increase significantly after icing. This is due to the changes in the airfoil shape and surface roughness values of each blade after icing, resulting in the flow field around the blade becoming unstable. These changes can exacerbate the formation and growth of vortices that spread out to the surroundings, thus affecting the noise performance of the wind turbine.

6. Conclusions

In this paper, through theoretical analysis, icing tests, and ice shape extraction, we quantitatively analyze the ice parameter changes of a 600 W small wind turbine blade in the blade chord length direction and spanwise direction under different icing conditions and numerically simulate the changes of aerodynamic characteristics of the blade by using the obtained icing model. The following conclusions are summarized.

The change rule of blade airfoil shape under different icing conditions was quantitatively investigated. By analyzing the change of icing blade airfoil shape, the ice accumulation shows a nonlinear trend along the direction of blade spread and chord length. With the increase in icing mass, the change of the angle of attack of the airfoil makes the thickness of ice accumulation at the different cross-sections different.

Through numerical simulation studies of icing blades, blade icing causes unsteady changes in the surrounding flow field, leading to vortex aggregation at the tip of the blade. As the icing mass increases, the pressure difference between the leading and trailing edges and between the windward and leeward surfaces changes, and the size and number of center and tip vortices increase significantly. These in turn lead to blade stall and affect blade aerodynamic noise, negatively impacting the aerodynamic characteristics of the turbine.