Preventing Ice Accretion: Design Strategies for Anti-Icing Surfaces.

A comparison study on the thermal effects in DBD plasma actuation and electrical heating for aircraft icing mitigation

In today’s world, green energy has become a key initiative as an alternative energy resource. Wind turbines are widely used to harvest wind energy in seasonal and cold environments. Although efficient, cold weather conditions negatively affects wind turbine operations due to ice formation. Damage from icing is seen on blade-tips when super-cooled water droplets that form in colder environments rapidly freeze and accumulate. Different forms of ice structures are formed along the leading edge to the trailing edge of the turbine blade and are classified into horn, rime and glaze ice. These various ice structures can cause power losses, mechanical and electrical failures and pose serious safety hazards (e.g., ice throwing). Ongoing efforts have been in place to develop anti-icing and de-icing strategies, but only a few are available on the market. In this computational study using ANSYS 14, a variable pitched National Renewable Energy Laboratory (NREL) and National Advisory Committee for Aeronautics (NACA) airfoils are used to determine the effects of various ice formations along the cord of turbine blade. Ice accretions on turbine blade can cause significant performance issues such as decreased lift and increased drag leading to performance and energy losses. Understanding the flow behavior of iced airfoil is critical in determining what geometric features of ice contributes to the performance degradation and aerodynamic failures in wind turbines. This study may help optimize future designs and implementation of ice mitigations systems to maximize turbine power output.

Chapter 6 - Hydro-/ice-phobic coatings and materials for wind turbine icing mitigation

A critical review is provided to summarize our recent efforts to utilize the state-of-the-art bio-inspired icephobic coatings/surfaces, i.e., 1). Lotus-leaf-inspired superhydrophobic surfaces (SHS) and 2). Pitcher-plant-inspired slippery liquid-infused porous surfaces (SLIPS) for aircraft icing mitigation. By leveraging the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT), an experimental campaign was performed to evaluate the effectiveness of using SHS and SLIPS coatings to suppress impact ice accretion over the surfaces of typical airfoil/wing models. While both SHS and SLIPS were found to be able to suppress ice accretion over the airframe surfaces where strong aerodynamic forces are exerted, ice was still found to accrete in the vicinity of the airfoil stagnation line where the aerodynamic forces are at their minimum. A novel hybrid anti-/de-icing strategy concept to combine icephobic coatings with minimized surface heating near airfoil leading edge was demonstrated to effectively remove impact ice accretion over entire airfoil/wing surfaces. An experimental investigation was also conducted to examine the durability of the icephobic coatings/surfaces to resist “rain erosion” effects (i.e., the damage to the surface coatings due to continuous impingement of water droplets at high speeds) in considering the practical usage for aircraft icing mitigation. The rain erosion effects were characterized based on the variations of the ice adhesion strengths and surface morphology of the eroded test surfaces coated with SHS and SLIPS. The research findings are very helpful to elucidate the underlying physics for the development of novel and robust anti-/de-icing strategies for aircraft icing mitigation.

I CE accretion on cold surfaces is a topic of great concern for a number of engineering applications. Ice formation and accretion on power cable and radio masts have been found to cause significant damage or completely destroyed the electric equipment on numerous occasions [1]. Aircraft icing is widely recognized as one of the most serious weather hazards to aircraft operations [2]. The importance of proper ice control for aircraft operation in cold climates was highlighted by many aircraft crashes in recent years, like the Continental Connection Flight 3407, which crashed in Buffalo, New York due to ice buildup on its wing, killing all 49 people aboard and one person on the ground, as the plane hit a residential home on 14 February 2009.Wind-turbine icing represents themost significant threat to the integrity of wind turbines in cold weather. It has been found that ice accretion on turbine blades would decrease power production of the wind turbines significantly [3]. Ice accretion and irregular shedding during wind-turbine operation would lead to load imbalances, as well as excessive turbine vibration, often causing the wind turbine to shut off [4]. Icing was also found to affect the reliability of anemometers, thereby leading to inaccuratewind-speed measurements and resulting in resource estimation errors [5]. Advancing the technology for safe and efficient operation of numerous functional devices in atmospheric icing conditions requires a better understanding of the icing physics. While a number of theoretic and numerical studies have been conducted in recent years to develop ice prediction tools for improved ice protection system designs [6–9], many details of important microphysical processes that are responsible for the ice formation and accretion on frozen cold surfaces are still unclear. Fundamental icing physics studies capable of providing accurate measurements to quantify important microphysical processes associated with icing phenomena are highly desirable in order to elucidate the underlying physics. In this study, we report an experimental icing physics study to quantify the transient behavior of the phase-changing and heattransfer processes within small water droplets impinging onto a frozen cold plate. It should be noted that this is a fundamental icing physics study. Instead of reproducing every detail of the icing phenomena for a specific engineering application, the present study was aimed to elucidate underlying fundamental physics to improve our understanding about the important microphysical processes pertinent to various icing phenomena found in nature, which include power cable icing, wind-turbine icing and aircraft icing. To the best knowledge of the authors, this is the first effort of its nature. The new findings derived from the icing physics studies, as the one reported here, will lead to a better understanding of the important microphysical processes, which could be used to improve current icing accretion models for more accurate prediction of ice formation and accretion on frozen cold surfaces, as well as the development of effective icing mitigation and protection systems for various engineering applications.

Anti-icing strategies are on the way

All-day superhydrophobic photo-thermal and electro-thermal icephobic surfaces based on ZrN/MoSe2 composite

We report the research progress made in our research efforts to utilize the thermal effects induced by DBD plasma actuation to suppress dynamic ice accretion over the surface of an airfoil/wing model for aircraft icing mitigation. While the fundamental mechanism of thermal energy generation in DBD plasma discharges were introduced briefly, the significant differences in the working mechanisms of the plasma-based surface heating approach from those of conventional resistive electric heating methods were highlighted for aircraft anti−/de-icing applications. By leveraging the unique Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT), a comprehensive experimental campaign was conducted to quantify the thermodynamic characteristics of a DBD plasma actuator exposed to frozen cold incoming airflow coupled with significant convective heat transfer. By embedding a DBD plasma actuator and a conventional electrical film heater on the surface of the same airfoil/wing model, a comprehensive experimental campaign was conducted to provide a side-by-side comparison between the DBD plasma-based approach and conventional resistive electrical heating method in preventing ice accretion over the airfoil surface. The experimental results clearly reveal that, with the same power consumption level, the DBD plasma actuator was found to have a noticeably better performance to suppress ice accretion over the airfoil surface, in comparison to the conventional electrical film heater. A duty-cycle modulation concept was adopted to further enhance the plasma-induced thermal effects for improved anti−/de-icing performance. The findings derived from the present study could be used to explore/optimize design paradigm for the development of novel plasma-based anti−/de-icing strategies tailored specifically for aircraft icing mitigation.

Tuning Ice Nucleation by Mussel-Adhesive Inspired Polyelectrolytes: The Role of Hydrogen Bonding

Materials against ice formation and accretion are highlydesirablefor different industrial applications and daily activities affectedby icing. Although several concepts have been proposed, no materialhas so far shown wide-ranging icephobic features, enabling durabilityand manufacturing on large scales. Herein, we present gradient polymersmade of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane(V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate(PFDA) deposited in one step via initiated chemical vapor deposition(iCVD) as an effective coating to mitigate ice accretion and reduceice adhesion. The gradient structures easily overcome adhesion, stability,and durability issues of traditional fluorinated coatings. The coatingsshow promising icephobic performance by reducing ice adhesion, depressingthe freezing point, delaying drop freezing, and inhibiting ice nucleationand frost propagation. Icephobicity correlates with surface energydiscontinuities at the surface plane resulting from the random orientationof the fluorinated groups of PFDA, as confirmed by grazing-incidenceX-ray diffraction measurements. The icephobicity could be furtherimproved by tuning the surface crystallinity rather than surface wetting,as samples with random crystal orientation show the lowest ice adhesiondespite high contact angle hysteresis. The iCVD-manufactured coatingsshow promising results, indicating the potential for ice control onlarger scales and various applications.

Icing on blade surface of the wind turbine set in cold regions is a serious problem. To invest the mechanism of icing and ice accretion on blade surface, wind tunnel tests were carried out on a static straight blade used for the straight-bladed vertical axis wind turbine by using a simple icing wind tunnel. The icing and ice accretions on blade surface at some typical attack angles were observed and recorded under different wind speeds and flow discharges of a water spray nozzle set in the wind tunnel outlet. The maximum icing thickness on the leading edge and trailing edge of blade surface were also measured and compared. Based on the test results, the factors affecting the icing and ice accretion on the static straight blade surface for wind turbine were discussed.

Directional motion of water droplets enhances anti-icing failure of structural superhydrophobic surfaces

Ultrasonic inspection for ice accretion assessment: effects on direct wave propagation in composite media

Significant progress in the field of icephobic coatings has raised a demand for evaluation criteria to assess and monitor the related icephobic effects and their durability. The initial coating performance in preventing ice formation and reducing ice adhesion needs to be proven over a given period by withstanding technically relevant stressors. In this study, silanized polyurethane (PUR) coatings are assessed in conjunction with a standardized accelerated ultraviolet (UV)-ageing procedure in order to identify potential monitoring tools that are also applicable during in-service inspections. Wettability and roughness parameters are recorded after pre-defined ageing intervals, compared with the ice adhesion strength, and tested using a modified centrifuge. Correlation assessments indicate that the chosen parameters cannot generally be used for the monitoring of icephobic effects for the selected material class. It is more likely that specific coating parameter sets need to be defined for in-service monitoring, as an important step towards the integration of icephobic coatings into technical applications.

IntroductionNorthern China is considered a global hotspot of biodiversity loss due to dramatic climate and land use change characterized by rapid urban expansion. However, little is known that the impacts of these two drivers in shaping the future availability of habitat for plants in urban areas of Northern China, especially at a high spatial resolution.MethodsHere, we modelled the habitat suitability of 2,587 plant species from the flora of Northern China and estimated how future climate and urban expansion may affect species-level plant biodiversity across three shared socioeconomic pathway (SSP) scenarios for the year 2050 in main city clusters.ResultsThe results suggested that climate and urban expansion combined could cause a decline of up to 6.5% in plant biodiversity of Northern China, while urban expansion alone may cause 4.7–6.2% and climate change cause 0.0–0.3% by 2050. The contribution of urban expansion was higher in urban areas, while the contribution of climate change was higher in natural areas. Species may lose an average of 8.2–10.0% of their original environmentally suitable area. Our results verified that the process of urban expansion would necessarily result in large-scale biodiversity loss.DiscussionThe plant biodiversity loss in city clusters of Northern China was mainly determined by urban expansion rather than climatic change. The impact of climate change should not be ignored, since climate change will likely cause a higher reduction of area for some species. Based on these findings, we proposed that plant biodiversity loss in Northern China will accelerate in the future unless both urban expansion change and climate change are minimized.