Icing Wind Tunnel Research Articles

Supercooled Water Droplets Impacting at High Speed on Superhydrophobic Surfaces: Open Questions Related to Icephobicity

It is posited that impact ice adhesion strength on superhydrophobic surfaces is related to droplet impalement. Several gaps still exist in the understanding of how different types of textured surfaces resist the impalement of high‐speed impacting droplets (<102 μm diameter at 102–103 m s−1). A previous study reveals that a pressure balance calculation can predict the wetting state of an impacting drop (≈103 μm diameter at ≈101 m s−1). However, the ability of this model to predict the wetting state of small droplets impacting at high speed or of droplets in a supercooled state has not been reported. To better understand supercooled microdroplet impalement, and to verify the hypothesis that droplet impalement is directly related to ice adhesion strength, herein, the impact behavior of microdroplets in a cold airflow onto surfaces of varying topography and wettability in an icing wind tunnel with support of high‐speed video images are investigated. Although the results do not allow for validating the above hypothesis about impalements and ice adhesion, they show that the difference between supercooled droplet spreading and retraction can be a predictor of ice adhesion strength. However, this relation must be validated with a greater body of data from samples with very diverse surface properties.

Metal Replacement in Aeronautics Applications Using Two‐Dimensional Multifunctional Composites

Carbon‐based composites have replaced metals in many structural components of modern airplanes; however, functional applications still require metal conductors and devices. Here, the production and integration in small‐scale airplane parts of electric devices based on metal‐free layered composites of graphene, boron nitride, and polymers are described. Thanks to their two‐dimensional structure, these materials feature excellent tunable thermoelectrical properties and high flexibility. We describe the production and performance of heaters and ice sensors installed on scaled NACA‐0012 airfoils to avoid ice formation, to remove ice or to detect the ice on the outer skin of the airfoil. We demonstrate that we can tune the electrical conductivity of the composite heating element from 105 to 103 S m−1 that is preserved even after 1000 bending cycles. The heaters successfully achieve anti‐icing and deicing in representative icing conditions and can sustain high‐power densities as required for electrothermal ice‐protection systems in aviation. The composite material can be readapted into a sensor able to detect ice formation in real time in an icing wind tunnel. Moreover, the possibility to use the heaters developed as heating molds to cure composite materials by out‐of‐oven curing, with a significant energy saving potential, is demonstrated.

MVD and LWC prediction in icing environment based on genetic algorithm optimized neural network

Mean volumetric diameter (MVD) and liquid water content (LWC) are two important meteorological parameters that affect aircraft icing, but they are difficult to measure accurately in practice. If these two parameters can be obtained in real time and accurately, it can provide some guidance for ice accumulation prediction and the establishment of aircraft airworthiness certification standards. This paper proposes a prediction model for icing meteorological parameters based on genetic algorithm optimized neural network. With ice thickness and icing rate of different measurement point combinations, ambient temperature, flight speed and wing angle of attack as input parameters, and icing meteorological parameters MVD and LWC as output parameters, the genetic algorithm optimized icing meteorological parameter prediction model is constructed, and the prediction model is used to predict the icing meteorological parameters of the numerical calculation test group data and the icing wind tunnel experimental data. The results show that the prediction model based on genetic algorithm optimized Elman neural network predicts the icing meteorological parameters of the test group within the relative error of the test group 10%, and the relative error of the experimental data is within the relative error of the experimental data 20%. This method has certain feasibility.

Numerical Investigation of Wind Turbine Airfoil Icing and Its Influencing Factors under Mixed-Phase Conditions

Icing is a popular research area in wind energy, and the icing problem of the supercooled droplet–ice crystal mixed-phase condition is one of the new challenges. A numerical method for analyzing the icing characteristics of wind turbine airfoil under mixed-phase conditions is presented. The control equations for the dynamics of supercooled droplets and ice crystals are formulated using the Lagrangian method. Equations for the conservation of mass and energy during the icing process involving supercooled droplets and ice crystals are constructed. The impact of erosion phenomena on the mixed-phase icing process is examined, and methodologies for solving the control equations are introduced. The numerical method is utilized for modeling mixed-phase icing under a range of conditions. The results of these simulations are then compared with data obtained from icing wind tunnel tests to assess the validity of the method. The influence of various mixed-phase conditions on ice shapes is studied. It is found that higher icing temperatures correspond to a larger icing range and amount. The increase in supercooled droplet content, ice crystal content, and ice crystal diameter all contribute to enhanced ice accretion. However, the effects of ice crystal content and diameter are relatively minor.

Investigation of airflow thermal ice melting process under various flight conditions

The challenge of preventing and removing ice from exposed regions of aircraft is a significant engineering concern. Understanding the melting process of ice due to aerodynamic heating during flight is essential for developing UAV flight strategies in icy conditions. This paper analyzes the heat transfer mechanisms involved in airflow over ice and introduces a theoretical model to estimate the ice melting rate, using principles of turbulent heat transfer at the stagnation point. The study also discusses the impact of environmental conditions on ice melting rates. Findings indicate that the melting speed of the ice surface exhibits a negative linear correlation with the initial temperature of the ice, whereas it shows a nonlinear correlation with airflow velocity and total temperature of the incoming flow. Lower airflow velocity or total temperature of the incoming flow enhances the sensitivity of ice melting speed to changes. Additionally, lower ice density results in a higher melting speed, showing an exponential relationship with factors like average droplet diameter, airflow velocity, and airfoil leading edge diameter in the cloud field during icing. Experiments conducted using a small jet test bench and an icing wind tunnel confirmed the impact of varying conditions on ice melting rates. The deviation between experimental results and theoretical predictions was under 5 %. These conclusions offer valuable insights for flight safety planning of unprotected iced aircraft in challenging environments.

Durable one-component polyurea icephobic coatings with energy-saving performance in electrical heating de-icing

Ice formation and accumulation on the surfaces of aircraft, wind turbines, and boat hulls might result in serious economic losses and catastrophic accidents. The development of a new generation of icephobic surfaces has emerged in recent years for anti-icing. However, insufficient durability of the existing icephobic surfaces remains a significant challenge to their applications. In this study, durable one-component polyurea icephobic coatings (OPICs) with a low ice adhesion strength (∼25.3 kPa) were prepared via a solvent-free method by combining the NCO-terminated prepolymer, latent curing agent, and silicone oil. The chemical structure, surface morphology, wettability, mechanical properties, icephobicity, and durability of the OPICs were studied. The optimized design of OPICs with synergistic icephobic mechanisms endowed them with excellent icephobicity and durability. The superior mechanical durability and chemical stability were revealed by the maintaining icephobicity of OPICs even after 3000 continuous Taber abrasion cycles, as well as 50 icing/de-icing cycles, 72 h of NaOH immersion, and 240 h of neutral salt spray test. The effect of OPICs on minimizing the electric power consumption was further investigated by icing wind tunnel. In comparison to the epoxy primer coated and Al alloy coated airfoil surface, airfoil surface coated by OPIC-30 was found to achieve about 43.7 % and 43.9 % energy savings in the keeping entire airfoil surface ice-free, respectively. The one-component polyurea icephobic coatings have great prospects for practical applications that require long-term anti-icing properties.

Research on icing model and calculation methods

Aircraft icing seriously threatens flight safety. This paper describes a modified shallow-water icing thermodynamic model that is applicable to unstructured grids and considers the effects of changes in water physical parameters and sublimation on icing. An icing calculation method enables the automatic determination of whether freezing occurs and the type of icing. An autonomous icing calculation program is developed to extract the geometric coordinates and mesh information of the icing surface and combine this information with the multiphase flow field of air-supercooled water droplets. The icing equations in the Godnov format are discretized to produce a large-scale sparse matrix that is solved iteratively using the biconjugate gradient stabilized method. The output includes parameter distributions for the ice thickness, water film thickness, and equilibrium temperature. Taking the National Advisory Committee for Aeronautic 0012 airfoil as the study object, the results of icing simulations are compared with experimental data from an ice wind tunnel and the ice shapes calculated by the FENSAP-ICE software. The results are found to be in good agreement with the experimental data, and the ice height errors at stagnation points are less than 15%. In the case of rime ice, single-horned ice shapes are simulated for both conventional and large supercooled droplets. For mixed ice and glaze ice, double-horned ice shapes are simulated for both droplet conditions. FENSAP-ICE fails to simulate the ice horns and produces large errors in ice thickness and ice range.

Enhanced Analysis of Ice Accretion on Rotating Blades of Horizontal-Axis Wind Turbines Using Advanced 3D Scanning Technology

This study investigated the meteorological conditions leading to ice formation on wind turbines in a coastal mountainous area. An enhanced ice formation similarity criterion was developed for the experimental design, utilizing a scaled-down model of a 1.5 MW horizontal-axis wind turbine in icing wind tunnel tests. Three-dimensional ice shapes on the rotating blades were obtained and scanned using advanced 3D laser measurement technology. Post-processing of the scanned data facilitated the construction of solid models of the ice-covered blades. This study analyzed the maximum ice thickness, ice-covered area, and dimensionless parameters such as the maximum dimensionless ice thickness and dimensionless ice-covered area along the blade. Under the experimental conditions, the maximum ice thickness reached 0.5102 m, and the ice-covered area extended up to 0.5549 m2. The dimensionless maximum ice thickness and dimensionless ice-covered area consistently increased along the blade direction. Our analysis of 3D ice shape characteristics and the ice volume under different test conditions demonstrated that wind speed and the liquid water content (LWC) are critical factors affecting ice formation on blade surfaces. For a constant tip speed ratio, higher wind speeds and a greater LWC resulted in increased ice volumes on the blade surfaces. Specifically, increasing the wind speed can augment the ice volume by up to 57.2%, while increasing the LWC can enhance the ice volume by up to 149.2% under the experimental conditions selected in this study.

Experimental analysis of ice crystal size–velocity correlation in icing wind tunnel with digital holographic particle tracking velocimetry

The ice crystal icing in aero-engines poses a significant threat to aircraft flight safety, and investigating the correlation between ice crystal size and velocity in icing wind tunnels is essential for correlating ground tests with flight conditions. In this paper, a digital holographic particle tracking velocimetry system is developed and integrated with an icing wind tunnel, and the ice crystal size and velocity are simultaneously measured in experiments at four different wind speeds. The velocity difference between the ice crystal and wind has been experimentally observed, and it increases with both the ice crystal size and wind speed. The normalized velocities of the ice crystals by the wind speeds decrease with size, and the maximum velocity difference in the experiments reaches 28% of the wind speed. Additionally, quantitative correlations between ice crystal size and normalized velocity based on the power function and polynomial model are established. The relationship between the particle Reynolds number of ice crystal and size is also analyzed, resulting in the development of a model based on the power function, and the power index ranges from 1.51 to 1.64. These findings will provide valuable assistance in the calibration and commissioning of the ice crystal simulation system in icing wind tunnels, as well as in exploring the correlation between ground icing tests and flight and in developing corresponding models.

Numerical and experimental investigation of the supercooled large droplets icing of rotating spinner

The icing of aero-engine rotating components is more complex than traditional aircraft icing. A three-dimensional numerical method was developed based on the Eulerian method to study the supercooled large droplets icing of a rotating spinner. Typical dynamic behaviours of supercooled large droplets, such as deformation, breakup, rebound, and splashing, were considered. An icing thermodynamic model applicable to a rotating spinner was established by considering the effects of pressure gradient, air shear force, and centrifugal force on the flow of the water film. Meanwhile, an ice wind tunnel experiment on supercooled large droplets icing of a rotating spinner was carried out. The results showed that the maximum relative deviation of the simulated icing thickness at the cone tip is no more than 15% compared with the experimental result, and the icing thickness on the surface of the rotating spinner is basically consistent with the experimental result. Based on the three-dimensional numerical method developed in this study, the effects of the rotational speed, inflow velocity, and inflow temperature on the supercooled large droplets icing of a rotating spinner were investigated. As the rotational speed increases, the rebound and splashing of supercooled large droplets becomes more obvious, leading to an increase in the mass loss coefficient and a decrease in the collected mass of water droplets on the surface of the rotating spinner. Consequently, the icing thickness at the tail of the rotating spinner decreases with an increase in the rotational speed. Since the liquid water content in the environment is relatively low, water droplets freeze immediately after impacting the surface of the rotating spinner and rime ice is usually formed. When the inflow temperature is relatively high, overflow water exists and glaze ice is formed near the tip of the rotating spinner. The findings of this study can provide valuable support for the design and verification of anti-icing systems for aero-engine rotating spinner.

De-icing behavior and efficiency of electrothermal film surfaces with different non-wettability in low-temperature flow field

The electrothermal anti-/de-icing technology with excellent de-icing efficiency has been widely applied to eliminate the icing threat on airfoil, whose essence is heat transfer process between heating surface and ice. However, the heat transfer behavior and interfacial transformation form of ice accretion are indistinct on traditional non-superhydrophobic surfaces and rapidly-developed superhydrophobic surfaces in the low-temperature flow field. Therefore, the heat transfer and solid-ice interface transform behavior on film heater surface were explored in this work by analyzing the temperature-rising performance and de-icing characteristics of hydrophobic films (HPoF) and superhydrophobic films (SHPoF) in the icing wind tunnel. The results indicated that the faster wind speed enhanced the convection and radiant heat loss of the film heater, and the peak temperature of film heater was directly affected by the environment temperature. Moreover, the SHPoF had lower de-icing consumption than HPoF in an environment without wind field, which was less related to the ice accretion. However, the de-icing form transformed from shearing to shedding mode with the growth of ice accretion under wind field conditions. In the shearing de-icing mode, the lower environment temperature and faster wind speed caused longer de-icing time, whereas the deicing time shortened with the decrease of environment temperature in the shedding mode. When the icing time was 1 min, the de-icing time of SHPoF increased by 12.25%–25.73 % due to the slower heat transfer at the solid-gas-ice interface compared to the solid-liquid-ice interface of HPoF. With the further increase of icing time, the ice accretion was removed after the melt of interfacial ice. The less solid-ice contact points were present at the solid-gas-ice interface on SHPoF surfaces, resulting in energy saving of 20.16%–40.46 % compared to HPoF surfaces. Therefore, the electrothermal superhydrophobic de-icing technology exhibited energy-efficient characteristics when the self-weight of ice is the primary driving force for de-icing process.

Transient Three-Dimensional Measurement of Ice Crystal Accretion Using a Plenoptic Camera

Monitoring the three-dimensional formation of ice layers on airfoils during icing wind tunnel experiments is extremely challenging. For the first time, this paper demonstrates the use of a single plenoptic camera to perform transient, nonintrusive in situ measurements of ice crystal accretion. Experiments have been conducted in the Altitude Icing Wind Tunnel at the National Research Council of Canada under different icing conditions to assess the potential of the new technique. Using the camera in a close-up configuration, the results show the evolution of the three-dimensional shape of the accreted ice in high spatial and temporal resolution and in absolute metric units. The computed surface meshes allow for a detailed analysis in terms of ice shape, surface area, and ice volume. Posttest shapes are compared to measurements taken using a commercial laser scanner. Although not rated for ice surfaces, this device is used as a reference to compare the detailed surface structure after registering the data sets. The results of the two methods are in good agreement and show a mean relative deviation of the plenoptic camera of about 0.15 mm.

Modeling two-dimensional ice shape based on fractal interpolation

The ice accretion data obtained from ice wind tunnel tests reveal a multiscale structure and rough surface. In the follow-up aerodynamic evaluation of icing airfoils, simplified two-dimensional ice shapes are generally used as substitutes, but this simplification changes the aerodynamic effects of the original ice shape. Therefore, finding a simple and effective two-dimensional ice shape simulation method is urgent. Due to the self-similarity characteristics of ice shape surfaces, fractal interpolation is proposed for ice shape simulation. First, the geometric characteristics of the ice shapes are analyzed to determine interpolation points, and an iterative function system is constructed for interpolation simulation. Considering the influence of various characteristics of ice shapes on aerodynamics, interpolation parameters are limited to simulating more realistic ice shapes. High-order numerical simulation methods were utilized to numerically simulate and analyze the aerodynamic characteristics of icing airfoil while also verifying the feasibility of fractal interpolation for simulating ice shapes. The analysis revealed that this method could effectively simulate ice profiles of various feature scales with minimal ice shape data. These simulated shapes closely resemble real ice formations and maintain the original aerodynamic characteristics of the icing airfoil. This method can be used to improve the computational accuracy of ice accretion codes and provide improvement strategies for complex ice shape prediction; thus, this method has great application prospects in engineering.

Characterization of the Spray System of the TerraXcube Icing Wind Tunnel

Characterization of the Spray System of the TerraXcube Icing Wind Tunnel

Icing Wind Tunnel and Erosion Field Tests of Superhydrophobic Surfaces Caused by Femtosecond Laser Processing

Ice accumulation on lift-generating surfaces, such as rotor blades or wings, degrades aerodynamic performance and increases various risks. Active measures to counteract surface icing are energy-consuming and should be replaced by passive anti-icing surfaces. Two major categories of surface treatments—coating and structuring—already show promising results in the laboratory, but none fulfill the current industry requirements for performance and durability. In this paper, we show how femtosecond laser structuring of stainless steel (1.4301) combined with a hydrocarbon surface treatment or a vacuum treatment leads to superhydrophobic properties. The anti-ice performance was investigated in an icing wind tunnel under glaze ice conditions. Therefore, flexible steel foils were laser-structured, wettability treated and attached to NACA 0012 air foil sections. In the icing wind tunnel, hydrocarbon treated surfaces showed a 50 s ice build-up delay on the leading edge as well as a smoother ice surface compared to the reference. To demonstrate the erosion resistance of these surfaces, long-term field tests on a small-scale wind turbine were performed under alpine operating conditions. The results showed only minor erosion wear of micro- and nano-structures after a period of six winter months.

Influence of the contraction section configuration and deflector layout density on the wind-snow flow characteristics inside the ice snow wind tunnel for railway vehicles

The significant accumulation of snow and ice in the bogie region severely compromises operational quality and poses a safety hazard to high-speed trains. To enhance testing capabilities for assessing ice and snow accumulation within bogie regions, this study conducts numerical investigations into the influence of contraction section configuration and deflector layout density on wind-snow flow quality within an ice and snow wind tunnel. The predictive accuracy of the numerical method is comprehensively validated against wind-snow wind tunnel tests, involving a comparison of airflow velocity in the test section and snow accumulation thickness on the channel's floor. The results demonstrate that the coupled numerical method of Improved Delayed Detached Eddy Simulation (IDDES) and Discrete Phase Model (DPM) is a reliable tool for analyzing wind-snow flow characteristics within an ice and snow wind tunnel. Furthermore, compared to the vickers curve and bicubic curve, quintic curves applied to the contraction section consistently compress and accelerate airflow. This contributes to a more uniform flow characterized by lower stream-wise fluctuation levels, pitch angles, and yaw angles in the test section, thereby significantly enhancing the overall flow quality of wind and snow. Thus, the quintic curves or smoother curves are recommended for the design of the contraction section in ice and snow wind tunnels. Moreover, increasing the deflector number leads to a noticeable improvement in flow quality inside the test section, while resulting in increased snow accretion on the windward surfaces of deflectors. Hence, a medium-level deflector density is suggested for designing ice and snow wind tunnels to achieve a better balance between flow quality and the additional snow accretion inside the channel.

Preparation of n-Tetradecane Phase Change Microencapsulated Polyurethane Coating and Experiment on Anti-Icing Performance for Wind Turbine Blades

Icing is a common physical phenomenon, and the icing of wind turbine blades can significantly affect the performance of wind turbines. Therefore, researching methods to prevent icing is of great significance, and the coating method of anti-icing is an effective way to delay icing, with advantages such as low energy consumption and easy implementation. In this study, using the coating method as the background, tetradecane phase change microcapsules were prepared, with a melting enthalpy of 90.8 J/g and a crystallization enthalpy of 96.3 J/g, exhibiting good coverage and energy storage efficiency. After mixing tetradecane phase change microcapsules (PCMS) with polyurethane coating (PUR) and coating them on wind turbine blades, after a 5 min icing wind tunnel test, the coating could significantly delay the icing on the blade surface, with the highest anti-icing rate reaching 60.41%. This indicates that the coating has a good anti-icing effect and provides basic research data for exploring new anti-icing methods.

An Experimental Study of Surface Icing Characteristics on Blade Airfoil for Offshore Wind Turbines: Effects of Chord Length and Angle of Attack

Offshore wind turbines operating in frigid and humid climates may encounter icing on the blade surface. This phenomenon adversely impacts the aerodynamic efficiency of the turbine, consequently diminishing power generation efficacy. Investigating the distribution characteristics of icing on the blade surface is imperative. Hence, this study undertook icing wind tunnel tests on segments of DU25 airfoil, a prevalent type for offshore wind turbines, to examine such characteristics as different chord lengths and angles of attack. The results show a simultaneous increase in the blade icing area and growth rate of the net icing area with augmenting the chord length and angles of attack. The total icing area rate decreases by a factor of two when the chord length is doubled. The relative positioning of icing and the average icing thickness remain consistent across the airfoil blades with varying chord lengths. Comparing the icing shapes on blades of varying scales shows a similarity ranging from 84.06% to 88.72%. The results of this study provide insight into the icing characteristics of offshore wind turbines.

Ice shedding tests for the assessment of hybrid ice protection systems

In technical areas, the prevention of atmospheric icing on structures is imperative due to safety risks and potential technical failures it poses. Recent development projects have focused on hybrid ice protection systems, combining active elements (heaters/mechanical actuators) with passive icephobic coatings. However, there is a lack of test strategies for the early development stages of these coating materials. An efficient test design for an ice wind tunnel is described here, developed to quantitatively assess the ice shedding effects facilitated by icephobic surfaces in electro-thermal ice protection systems. Dependencies on test and surface parameters are explored, laying the groundwork for defining test procedures for subsequent surface evaluations. During testing, utilizing a hydrophobic model coating, a reduction in energy of over 30% was observed compared to uncoated aluminium. This finding highlights the potential effectiveness of icephobic surfaces in mitigating icing effects. The presented test serves as a valuable tool for pre-selecting the most promising surfaces for further advanced tests, particularly those involving aerodynamic profiles within the ice wind tunnel test facility. It constitutes a vital component of a comprehensive test pyramid, encompassing ice-related tests with increasing complexity. Furthermore, the results obtained from these tests are utilised to establish correlations with surface properties, thereby enhancing our understanding of the significance of these findings. This approach provides a well-founded testing strategy for evaluating and advancing icephobic surface technologies.

Research on icing scaling technology for large civil aircraft icing wind tunnel test

This article explores the icing scaling technology for large civil aircraft icing wind tunnel tests. Based on the typical critical icing conditions of large civil aircraft and the calibration of icing wind tunnel, and through the similarity analysis of water droplet trajectories, water collection, and energy balance, four similar parameters, namely modified inertia parameter, drop energy transfer parameter, stagnation freezing coefficient, and accumulation parameter, were selected as similarity criteria for icing scaling. Based on this criteria, icing scaling was conducted on the critical icing conditions of large civil aircraft, and the scaling results were evaluated by icing code. The results show that the water droplet collection and the ice shape before and after the scaling are in good agreement, and the similarity criteria used are appropriate. Finally, icing wind tunnel test was conducted in 3m × 2m test section using scaled icing conditions. The test results show that the main characteristics of the experimental ice shape are in good agreement with the calculated ice shape, and the calculated ice shape is more critical. The calculated ice shape can be used as a critical ice for aerodynamic wind tunnel tests and flight tests with simulated ice.