Tobias Strobl

Feasibility Study of a Hybrid Ice Protection System

A key design factor impacting the use of electrical power to drive aircraft systems and subsystems is energy efficiency. With the design of an all-electric, hybrid ice protection system, energy consumption can be reduced to a large extent. The hybridization is achieved through an intentional partitioning of the ice at the stagnation line by melting via surface heating and ice shedding in the unheated regions of the airfoil surface via an electromechanical deicing system based on piezoelectric multilayer actuators. To further reduce energy consumption, the adhesion forces between the ice and the airfoil surface can be reduced using an ultrasmooth, nanostructured surface with water- and ice-repellent properties that encourages ice shedding. Experimental investigations, performed in a laboratory-scale icing wind tunnel for a small-scale configuration, reveal that the hybrid approach for ice protection reliably sheds the ice accreted on the airfoil surface. Compared with an all-thermoelectric system for ice p...

Simulating the Freezing of Supercooled Water Droplets Impacting a Cooled Substrate

To study ice adhesion at the droplet scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled volume-of-fluid and level-set method to capture the air–water interface, and an enthalpy-porosity method is used to capture the liquid–solid interface. Calibration of a simulation parameter Amush is performed in order to match experimental data for different ideal surface types and surface temperatures. The simulation strategy successfully predicts the overall droplet response for several droplet impact conditions.

Evaluation of Roughness Effects on Ice Adhesion

In this study, the adhesion strength of ice to bare aluminum substrates with different values of surface roughness is investigated. A thin glaze ice accumulation is formed on the substrate surface within an icing wind tunnel facility. The adhesion strength between the ice and the metal samples is measured by means of a permanent magnet shaker. Ice de-bonding occurs in the interface between the ice and the aluminum. The experimental results reveal that the ice adhesion strength is significantly dependent on the degree of surface roughness of the respective aluminum substrate.

Feasibility Study of a Hybrid Ice Protection System Based on Passive Removal of Residual Ice

Aircraft icing is considered a serious weather hazard during flight. Against the background of a more-electric aircraft, electro-thermal systems for ice protection are wellsuited to remove in-flight ice accretions from aircraft components. This effort intends to investigate the performance of a wet-running electro-thermal ice protection system and the extent to which surface coatings with low ice adhesion properties enable passive, aerodynamically-induced ice shedding. A heater systematically partitions the ice accreted around the leading edge into an upper and a lower part by melting the ice in the region near the stagnation line. The region of the airfoil aft of the heated region is treated to reduce the adhesion of the ice to the surface and facilitate aerodynamically-induced ice shedding. To demonstrate the feasibility of this approach, a prototype system is tested in a laboratoryscale icing wind tunnel. A numerical model of a NACA 0012 airfoil with a slender thermoelectric heater at the stagnation line is also studied. The aerodynamic forces acting on the residual ice shapes are predicted using a computational fluid dynamics simulation. The resulting loads acting on the ice shape are incorporated into a finite element analysis to determine if the stresses in the ice shape produce delamination from the airfoil surface. The aerodynamic forces required to shed the ice in the unheated area are numerically analyzed and compared to experimental data obtained in a laboratory-scale icing wind tunnel. It can be concluded from both the numerical simulations and the experimental investigations that, except for the warm and mushy glaze ice cases, passive ice shedding due to the aerodynamic forces of the airstream is obtained once the ice layer is thick enough.

Effects of Surface Characteristics and Droplet Diameter on the Freezing of Supercooled Water Droplets Impacting a Cooled Substrate

Reducing the accretion of ice on aerodynamic surfaces remains a challenge. Certain surface types have been hypothesized to reduce ice buildup. In this paper, a previously developed numerical method is employed to investigate the effects of droplet size and surface characteristics on the solidification of a supercooled water droplet as it impacts a cooled surface at high speed. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquidsolid interface. We consider high-speed Supercooled Large Droplet impacts on hydrophilic, hydrophobic, and superhydrophobic surfaces. Results show that superhydrophobic surfaces may not be icephobic for larger diameter droplets.

Chapter 26 – Synergic Effects of Passive and Active Ice Protection Systems

Abstract Ice accretion on aerodynamic surfaces can catastrophically impact the safety of an aircraft; it leads to a sudden lift drop and a relevant drag rise, compromising the aircrafts flight capability. Typical ice protection systems (IPS) are either concurrently or alternately hampering the ice accretion (anti-icing) or removing the ice itself before it achieves a dangerous consistency (deicing). Thermoelectric-resistance, pneumatic, and mechanic-hydraulic IPSs are among the most common devices currently implemented on aircraft. Those IPSs require a consistent amount of power and need sufficient room inside the leading edge, the critical wing zone for ice protection. For this reason, more and more research departments, aerospace industries and airline companies are devoting efforts worldwide to the study of ice generation and growth phenomena, with the goal of developing safer, simpler, and cheaper IPSs. The work at hand focuses on three different IPS solutions and how they can be combined for energy-efficient ice protection. The first was a passive technique, implementing a nanostructured surface that reduces the actual wetted surface, thus limiting droplet adhesion. The other two were based on a forced heat transfer to hinder ice formation on the leading edge at the stagnation line, and a mechanical device using piezoelectric actuators to induce ice fragmentation aft of the heated zone. Combined, these devices may need only limited electrical power. The design process used computational fluid dynamics (CFD) numerical tools to describe the ice development and its interaction with the aerodynamic flow. In particular, the ice melting due to thermoelectrical system action was modeled with its downstream motion, illustrating how the melting particles give rise to ice reformation once they move far from the heated zone. The aerodynamic action necessary to ultimately produce ice detachment in this region was then estimated; this information is used to size the piezoelectric actuator system, in terms of actuator thickness, extension, location, and excitation conditions. The icephobic surface characteristics were defined to minimize the wet surface and facilitate the removal action of the aerodynamic and mechanical forces. Laboratory tests were carried out on a scaled airfoil model to estimate the adhesion strength of both rime and glazed ice on both a conventional surface and on a nanostructured icephobic surface. This parameter was used to set the numerical tools. Experiments were then conducted in a wind tunnel environment. There, the ices behavior was observed from formation and throughout its evolution: from its melting at the stagnation point, the downstream movement of the resulting particles, and its reassembly, until its final removal via flow action. The results were compared with the theoretical predictions, appreciating and quantifying the combined action of the three elements of the energy-efficient IPS.