Research on infrared nondestructive testing and thermal effect analysis of small wind turbine blades under natural excitation

doi:10.21203/rs.3.rs-1992085/v1

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Research on infrared nondestructive testing and thermal effect analysis of small wind turbine blades under natural excitation

https://doi.org/10.21203/rs.3.rs-1992085/v1

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In recent years, with the increase of wind power generation and the rapid growth of the number of wind turbines installed, the safety monitoring of wind turbine blades has also attracted wide attention. To further study the thermal effect of wind turbine damaged blades under natural excitation in summer in northern China, based on the outdoor infrared non-destructive testing of wind turbine blades, a numerical simulation of wind turbine blades with wear damage is carried out by using the fluid solid heat transfer theory in COMSOL multi-physical field coupling software. The best detection light condition is obtained, in which the detection effect of wear damage is the best when the light intensity reaches 1000W.m^− 2 or above at noon. The simulation results are basically in agreement with the experimental results. And compared with the previous physical modeling method using natural convection heat transfer coefficient to replace wind speed, the physical modeling method considering wind speed and air humidity is more consistent with the experimental results, the maximum error value was decreased by 7.46%, which verifies the rationality of the model and the feasibility of the method. Then, the numerical model is used to carry out fluid-heat-solid coupling on the wear-damaged blades, with the increase of wind speed, the temperature on the blade surface gradually decreases and the wind pressure gradually increases. Because the influence of temperature on the stress is greater than that of wind pressure, the stress also shows a gradual downward trend.

wind turbine blade

nondestructive detection

infrared thermal imaging

numerical simulation

fluid-heat-solid coupling

thermal effect

Blade is a key component of the wind turbine, bearing an important role in energy conversion. Its safety and reliability plays a very important role in the stable operation of the whole wind turbine. Wind turbine blades have been served in the harsh natural environment for a long time, not only by the comprehensive effect of their own stress, but also affected by the impact of environmental load [1, 2]. Take the special geographical environment of central and western Inner Mongolia as an example, which is a high incidence of dust weather in China. According to the local meteorological data, the annual dust and dust days account for 30%~50%. In the operation process, the wind turbine blade often causes wear damage and crack due to the impact erosion of sand and ultraviolet radiation. [3]. At present, the conventional methods used in wind turbine blade damage detection mainly include sound emission technology, vibration technology, optical fiber grating sensing technology, X-ray detection technology, ultrasonic technology, and infrared nondestructive testing technology[4, 5, 6, 7]. Infrared thermal imaging detection technology has the advantages of long-distance and large-area testing,[8, 9]and is more suitable for the field detection and analysis of wind turbine blades [10, 11, 12].

However, in the field detection, the air humidity, wind speed, wind direction and light intensity will affect the change of the blade surface temperature, which has certain limitations in the application process [13]. Schwahlen et al. [14, 15], using a thermal imager instrument to detect the wind turbine blade segment indoors, realized the effective observation of the internal structure, and analyzed the influence of relative humidity and airflow on the detection. Doroshtnasir and other [16] monitor the operating wind turbine blades at a long distance based on infrared thermal image technology, realizing the detection and positioning of surface wear.Therefore, using infrared nondestructive testing technology to detect the damage of wind turbine blades needs further development and research.

Therefore, this work based on the infrared non-destructive testing of small wind turbine blades, the whole blade model is taken as the research object for simulation. However, in the simulation research for practical engineering, there are few pure single-field problems, and there are certain associations between different fields, so the coupling behavior between the fields needs to be considered[17]. The problem fluid-heat-solid coupling mainly studies heat transfer of solids and fluids as well as thermal stress and thermal deformation of solids after being heated. It is derived from multi-field coupling. The working environment of fan blades is complicated[18]. The surface of wind turbine blade is affected by air flow and sun radiation, which will form different gradients of pressure and temperature on the blade surface. Therefore, the wind turbine blade are coupled by flow field and temperature difference, and flow field also affects boundary conditions of temperature field, and the fields interact with each other. In addition, the contents of this paper are all carried out under the stationary state of the wind turbine blade, so the centrifugal force generated by the blades during rotation does not have to be considered to influence the flow field and temperature field.

2.1 Experimental specimens

In this work, the small wind turbine blade as the research object, the blade length is 1.5 m, the mass is 4 kg, the blade body structure material is reinforced FRP, and the surface coating material is epoxy resin glue[19]. A wear damage with a diameter of 3 cm is artificially imposed on the windward surface of the wind turbine blade.

2.2 Experimental equipment

The test instrument used is shown in Fig. 1. The FLIR T10xx series thermal imager used in this experiment has excellent image quality, with an infrared resolution of 1024×768 pixels and high accuracy, high sensitivity, and data recording options, ideal for this study. The temperature and humidity recorder using the jingchuang GSP-6 high-precision temperature and humidity recorder, the temperature measurement range is from − 40 to 85℃, The temperature measurement accuracy within the range of -20 ~ 40 ℃ is ± 0.5, the temperature measurement accuracy in other temperature measurement ranges is the ± 1, and the resolution of temperature and humidity is 0.1. Solar power meter using Taiwan ‘Taishi’ recording solar power meter, the measurement range is 0 to 2,000 W/m², accuracy is the ± 10W/m²[20]. Anemometer using the Xima AS859S handheld anemometer, the wind speed measurement range of 0.3 to 45 m/s, the speed measurement accuracy of 0.001 m/s, and the measurement error of ± 0.1.

2.3 Experimental process

In this experiment, under an outdoor environment with good light for five consecutive days from July 26 to July 30, 2021, using the sun as the radiation source to detect wind turbine blade damage by using the infrared thermal imager. The experimental scenario is shown in Fig. 2. At 8:30 every day, the blade was placed in a spacious experimental place for thermal excitation. The data were gathered from 9:00, and the acquisition time was from 9:00 to 17:00. The collection interval was 30min. At the same time, the temperature, air humidity, light intensity, wind speed, and other environmental parameters at the collection time were recorded. The curve of environmental parameters is shown in Fig. 3. The original infrared thermal image at different times is shown in Fig. 4.

Figure 3 show that the light intensity increases gradually from the beginning of the experiment, and the maximum value is 1010 W/m² at 12:30 noon, and then gradually decreases until the end of the trial; The temperature of the day first increased and then decreased, and rose to the highest value of 33.6 ℃ at 14:30. The air humidity basically showed a downward trend in the measured time of the day, while the wind speed showed a wave type in the measured time period of the day, without a fixed change trend.

3.1 Scene construction of the physical models

3.1.1 Establishment of the blade and the flow field model

The wind turbine blade is modeled with the help of profile airfoil data software in ANSYS[21]. The blade length is set as 1.5m, the width on both sides is 0.270m at the widest part and 0.070m at the narrowest part respectively, and the thickest part of the blade is 0.055m. At 30cm away from the blade tip, the wear damage is assigned to the blade by the method of Fixed damage depth. In addition, in the real environment, the working fluid field of the fan blade should be infinite, but this paper belongs to the simulation environment, so we can not set up infinite, so we need to set up a boundary flow field model.In this paper, the blade is taken as the center to set up 2m×2m×5 m, as shown in Fig. 5, the fluid type is wet air, and the air velocity and humidity are the experimental data.The blades are placed horizontally in the fluid domain to simulate the wind field environment of a single blade.

3.1.2 Material properties

The physical parameters of the wind turbine blade test blade body and the damage parts are shown in Table 1[22]. Meanwhile, wet air material is added in the flow field.

Table 1

Physical parameters of specimen of wind turbine blade
region	density (kg·m^− 3)	thermal conductivity (W·m^− 1·K^− 1)	specific heat (J·kg^− 1·K^− 1)
Wind turbine blade body	1800	0.52	535
polyurethane	2500	0.1	1300
damage	2000	0.5	1260

3.1.3 Physical field conditions

1) Physical field of solid fluid heat transfer

Solid selection blade model, fluid selection of 2 * 2 * 5 size fluid domain, fluid attribute is wet air, air humidity value is the experimental measurement data, the initial temperature value is the temperature of each time of the experiment. Define the boundary heat source, in this experiment, the blade was placed outdoors, so only the upper surface of the blade was exposed to sunlight,The measured radiation rate on the blade surface is taken as the measured solar radiation intensity on the blade surface.

2) Physical field of fluid flow

This paper adopts K-${\epsilon }$ The turbulence model is used to simulate the surface flow field of wind turbine blade. During the simulation process, the inlet and outlet should be defined and their boundary conditions should be given according to the environmental parameters measured in the experiment. The positions of the two are shown in Fig. 5. The wind speed, turbulence intensity and turbulence length are defined at the air inlet, and the wind speed is the data measured in the experimental environment,The turbulence intensity selection software defines the medium intensity turbulence intensity, and its value is 0.1. The turbulence length selection is based on the model. As the name implies, the length value of the fluid model is the defined turbulence length value.The pressure condition is defined at the air outlet, and the pressure value at the outlet is 0.

3) Physical field of solid mechanics

In order to study the influence of wind speed on the stress and strain at the site of blade damage, it is necessary to add a solid mechanical physical field, and then the results of blade surface temperature and wind pressure obtained by solid and fluid heat transfer are added to the solid mechanical physical field for fluid-heat-solid coupling analysis.In the physical field of solid mechanics, it is also necessary to define the fixed constraint and add gravity. According to the idea of cantilever beam, the fixed constraint is added at the root of the blade. At the same time, all domains are selected and gravity acceleration is applied in - Z direction.

3.1.4 multi physical field coupling

1)Non isothermal flow

Non isothermal flow is a multi physical field formed by the coupling of fluid flow physical field and heat transfer physical field-${\epsilon }$Solid and fluid heat transfer is selected at the heat transfer interface.

2)Thermal expansion

Thermal expansion is a multi physical field formed by coupling the physical field of heat transfer and the physical field of statics when solving the thermal stress.

3.1.5 Mesh generation

The fluid domain in the model and the mesh generation of the fan blade are all tetrahedral division method. The fluid domain is not the key research object of this paper, so the conventional of the software customization is selected for dividing the size, and the surface temperature and stress changes of the fan blade are the research focus of this paper. Therefore, the mesh size is refined by the software customization, and the mesh division is shown in Fig. 6.

3.2 result analysis

3.2.1 damage detection and analysis

The experimental results at 10:30, 12:30, 14:30 and 16:30 are selected to verify the simulation results, and the results are compared with those of previous physical simulation results using convective heat transfer coefficient instead of wind speed. The results are shown in Fig. 7. From left to right, each group of figures are the experimental results, the physical simulation results using convective heat transfer coefficient instead of wind speed, and the simulation results of fluid thermal solid coupling considering wind speed and air humidity.

Figure 8 show that the simulation results considering wind speed and air humidity are closer to the experimental results than the simulation results using natural convection heat transfer coefficient instead of wind speed.The error values of simulation results and experimental results considering wind speed and air humidity are 1.73%, 0.46%, 0.86% and 1.18%, respectively, while the errors of simulation results and experimental results using natural convection heat transfer coefficient instead of wind speed are 3.46%, 5.24%, 7.76% and 8.64%, respectively, and the accuracy is improved by 1.73%, 4.78%, 6.81% and 7.46%, respectively.Because the natural convection heat transfer coefficient obtained by predecessors' experience is not representative in fact, when the wind speed increases or decreases sharply, and the environmental climate changes, if the natural convection heat transfer coefficient obtained by experience is used to replace the wind speed, although the calculation amount can be reduced to a certain extent, but it will inevitably cause large error.

3.2.2 Influence of wind speed on thermal effect

In order to study the influence of wind speed on blade surface temperature, wind pressure and stress, on the premise that air humidity, ambient temperature and light intensity are the same, ten different wind speeds of 1m/s-10m/s are given to the turbulent inlet respectively, and the relationship between wind speed and blade surface temperature, wind pressure and stress is obtained by numerical model, as shown in Fig. 9.

Figure 9 show that with the increase of wind speed, the temperature of blade surface gradually decreases, the wind pressure increases gradually, and the stress decreases gradually.Because the numerical simulation is based on the infrared nondestructive testing experiment, the blade is flat on the ground, and the wind only blows horizontally on the surface of the blade, so the pressure on the blade is small, that is, the wind speed has little impact on the wind pressure of the blade lying on the ground;With the increase of wind speed, the decrease trend of blade surface temperature is more obvious.The change of stress is mainly affected by temperature and wind pressure. In the experimental situation, the effect of temperature on stress is greater than that of wind pressure, so the stress decreases with the increase of wind speed.

Figure 10 show that the relationship between blade surface temperature and wind speed is power function, and the correlation coefficient between them is 0.99862 after fitting.In heat transfer, wind speed often affects the value of convective heat transfer coefficient, and a large number of studies on the relationship between wind speed and convective heat transfer coefficient show that the relationship between wind speed and heat transfer coefficient is power function. The convective heat transfer coefficient increases gradually with the increase of wind speed. The greater the convective heat transfer coefficient is, the faster the heat transfer between blade surface and air is, and the greater the heat dissipation is, so the temperature gradually decreases,In addition, the relationship between the blade surface temperature and wind speed is also a power function, which is in line with the heat transfer theory.The relationship between temperature and wind speed is expressed as follows

$$T=48.9234{v}^{-0.09945}$$

Figure 11 show that the pressure on the blade surface has a power function relationship with the wind speed, and the correlation coefficient is 0.99994 after fitting.The pressure increases with the increase of wind speed.The relationship between pressure and wind speed is expressed as follows

$$\text{P}=0.01118{\text{v}}^{1.98448}$$

Figure 12 show that the stress corresponding to the highest temperature point on the blade surface is exponentially related to the wind speed, and the correlation coefficient between the two is 0.98896 after fitting. since the blade is flat on the ground, the wind pressure produced by the wind on the blade surface is very small, and the wind speed has a more obvious effect on the change of temperature, so the influence of temperature on stress is greater than that of wind pressure, so that the stress tends to decrease gradually with the increase of wind speed. The relationship between stress and wind speed is expressed as follows:

$${\sigma }=10.79833-0.57243v+0.0211{v}^{2}$$

3.2.3 Influence of wind direction on thermal effect

The simulation results are compared with the experimental results, and they are in good agreement, which can verify the rationality of the fluid-heat-solid coupling physical model. At the same time, in practical engineering application, the wind turbine blade is often connected with the tower wind frame vertically, so the influence of wind direction on it should not be underestimated. Next, the influence of risk on blade surface temperature, wind pressure and stress is studied through two cases of side wind and head wind.

For the sake of the safety of the inspector, the wind turbine must be in the shutdown state during the field infrared nondestructive testing of wind turbine blades. Therefore, in the simulation study, it is not necessary to apply the rotation domain to make the blade rotate, but only to change the position of the air inlet and outlet. At the same time, in order to simplify the calculation, the single blade is still used for the research. The simulation model is shown in Fig. 13.

Figure 14 show that in the case of downwind and headwind wind pressure, the effect of downwind direction on blade surface temperature is greater than that of headwind direction on blade surface temperature, so the temperature of blade under downwind pressure is generally lower than that under adverse wind pressure.In both cases, the temperature of wind turbine blade surface decreases with the increase of wind speed, and the temperature decreases faster when the wind speed is small. When the wind speed increases gradually, the decreasing speed gradually tends to be flat.

Figure 15 show that the force on the surface of the wind turbine blade increases with the increase of the wind speed, regardless of whether the wind turbine blade is subjected to the downwind or the headwind. Moreover, under the same wind speed, the force exerted by the downwind direction on the damaged surface of the wind turbine blade is pressure, and the force exerted by the headwind direction is tensile force, and the value of the force exerted by the downwind direction on the blade surface is obviously greater than that exerted by the headwind direction on the blade surface.

Figure 16 show that under the same wind speed, the stress at the blade damage under the downwind pressure is lower than that at the blade damage under the adverse wind pressure. In both cases, the stress at the damaged part of the fan blade shows a downward trend with the increase of the wind speed, and the downward trend is consistent with the change trend of the temperature with the wind speed. When the wind speed is small, the stress decreases rapidly,The rate of descent gradually flattened.The results show that the influence of temperature on stress is greater than that of wind pressure in the range of measured wind speed.

In this paper, based on the infrared nondestructive testing test of small wind turbine blade, the finite element software is used to simulate it. The factors that may affect the test results, such as light intensity, ambient temperature, humidity and wind speed, are taken into account. The blade model is placed in the flow field for fluid-heat-solid coupling analysis, and the following conclusions were obtained.

Compared with the simulation results using natural convection heat transfer coefficient instead of wind speed, the simulation results considering wind speed and air humidity are closer to the experimental results. Among them, the maximum error between the simulation results considering wind speed and air humidity and the experimental results is 1.73%, while the maximum error between the simulation results using natural convection heat transfer coefficient instead of wind speed and the experimental results is 8.64%, and the accuracy is improved by 6.91%.

With the increase of wind speed, the temperature of blade surface decreases gradually, the wind pressure increases gradually, and the stress decreases gradually. The change of stress is mainly affected by temperature and wind pressure. In the experimental situation, since the blade is flat on the ground, the wind pressure produced by the wind on the blade surface is very small, and the wind speed has a more obvious effect on the change of temperature, so the influence of temperature on stress is greater than that of wind pressure, so that the stress tends to decrease gradually with the increase of wind speed.

In the case of downwind and headwind, the effect of downwind on temperature is greater than that of upwind, so the temperature of blade under downwind pressure is lower than that of blade surface under headwind pressure.In both cases, the temperature of fan blade surface decreases with the increase of wind speed, and the temperature decreases faster when the wind speed is small. When the wind speed increases gradually, the decreasing speed gradually tends to be flat.The change trend of stress with wind speed is consistent with that of temperature with wind speed.Under the condition of the same wind speed, the force exerted on the blade surface by the downwind pressure is the pressure, and the force exerted by the upwind pressure on the blade surface is the tension, and the value of the force exerted by the downwind pressure on the blade surface is obviously greater than that of the upwind pressure on the blade surface.

Ethics approval and consent to participate Not applicable

Consent for publication Not applicable

Availability of data and material Not applicable

Competing interests The authors declare that they have no competing interests.

Funding Not applicable

Authors contributions WANG Chen is the first author and corresponding author of the paper, is the operator of the specific experiment and obtained the full data of the paper, and is also the main author of the paper. GU Yongqiang as the second author of this paper, providing valuable advice on the guidance and proofreading of the paper, and also applied for fund support for the research of this paper.

Acknowledgements Not applicable

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No competing interests reported.

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Research on infrared nondestructive testing and thermal effect analysis of small wind turbine blades under natural excitation

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Abstract

Figures

1. Introduction

2. Infrared Nondestructive Detection Experiment

2.1 Experimental specimens

2.2 Experimental equipment

2.3 Experimental process

3. Numerical Simulation

3.1 Scene construction of the physical models

3.1.1 Establishment of the blade and the flow field model

3.1.2 Material properties

3.1.3 Physical field conditions

3.1.4 multi physical field coupling

3.1.5 Mesh generation

3.2 result analysis

3.2.1 damage detection and analysis

3.2.2 Influence of wind speed on thermal effect

3.2.3 Influence of wind direction on thermal effect

4 Conclusion

Declarations

References

Additional Declarations

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