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We report a comparative study to evaluate the effects of surface coatings with different hydrophobicities and icephobicities on the performance of a hybrid anti-/de-icing system that integrates surface heating with hydro-/ice-phobic coating for aircraft icing mitigation. While a flexible electric film heater wrapped around the leading edge of an airfoil/wing model was used to heat the airfoil frontal surface to prevent ice accretion near the airfoil leading edge, three different kinds of coatings were applied to coat the airfoil model at three distinct spanwise locations, which included an icephobic coating with an outstanding icephobicity but a weak hydrophobicity; a superhydrophobic surface (SHS) coating with outstanding water repellency but a moderate icephobicity; and a commonly used hydrophilic coating with poor hydrophobicity and poor icephobicity. Surface wettability was found to play a more important role than icephobicity in affecting the performance of the hybrid anti-/de-icing systems. In comparison to the approach of forceful heating the hydrophilic airfoil surface, the hybrid approach with the SHS coating was found to be able to achieve about 90% energy savings in keeping the entire airfoil surface ice-free; the corresponding energy savings for the hybrid system with the icephobic coating was only about 10%.

 

An experimental study was conducted to examine the dynamic ice accretion process upon the impingement of microsized, airborne ice particles/crystals onto a heated test surface pertinent to aeroengine icing phenomena. The experimental study was conducted in a specially designed ice crystal icing test facility to generate and inject microsized ice particles into a frozen-cold airflow. The microsized ice particles were forced to impinge onto a heated test plate with controllable surface temperatures. Upon impingement of the ice particles onto the heated test surface, the dynamic ice accretion process was found to take place over the heated surface in three distinct stages: 1) an ice-melting stage at the beginning, followed by 2) an ice/water mixture formation stage, and then 3) a water refreezing stage, causing the formation of a solid ice layer accreted on the heated test surface eventually. After impinging onto the test plate, while small ice particles with spheric shapes were found to be more ready to bounce off from the test surface, large, nonspheric-shaped ice particles experienced a catastrophic fragmentation process and break up into smaller pieces with noticeable impingement residues remaining on the test surface. The formation of a liquid water film layer on the test surface due to the melting of the impinged ice particles was found to be very beneficial to make more impinged ice particles stay sticking on the test surface, resulting in a rapid growth of the water/ice layer accreted on the heated test surface. A comprehensive theoretical analysis was also performed to examine the unsteady heat transfer characteristics during the dynamic ice accretion process. The theoretic predictions of the collection efficiency of the impinged ice particles on the heated test surface and the temperature variations of the water layer at the initial ice-melting stage were found to agree well with the experimental measurement results. 

An experimental study was conducted to characterize the dynamic ice accretion process over the surface of a typical aeronautic Pitot probe model under different icing conditions. The experimental study was conducted in the Icing Research Tunnel available at Iowa State University. While a high-speed imaging system was used to record the dynamic ice accretion process, a three-dimensional (3D) scanning system was also used to measure the 3D shapes of the ice layers accreted on the test model. While opaque and grainy ice structures were found to accrete mainly along the wedge-shaped lip of the front port and over the front surface of the probe holder under a dry rime icing condition, much more complicated ice structures with transparent and glazy appearance were observed to cover almost entire surface of the Pitot probe under a wet glaze icing condition. While a flower-like ice structure was found to grow rapidly along the front port lip, multiple irregular-shaped ice structures accreted over the probe holder under a mixed icing condition. The characteristics of the icing process under different icing conditions were compared in terms of 3D shapes of the ice structures, the profiles of the accreted ice layers, the ice blockage to the front port, and the total ice mass on the Pitot probe model. The acquired ice accretion images were correlated with the 3D ice shape measurements to elucidate the underlying icing physics. 

The accretion of ice on the surface of a wind turbine blade causes a drastic reduction in the aerodynamic performance and as a result, the power output, in addition to posing a safety hazard. To quantify this phenomenon, an experimental study was conducted in the Iowa State University Icing Research Tunnel (ISU-IRT) to understand the dynamic ice accretion process and resultant aerodynamic performance degradation specifically experienced by offshore wind turbines at higher Liquid Water Content (LWC) levels. Four different LWC values were tested for both glaze and rime ice conditions each, to cover the possible spectrum of typical icing conditions. A high-speed imaging camera was used to capture the dynamic ice accretion process, while a Digital Image Projection (DIP) technique was used to perform the 3D qualification of the ice accretion characteristics. Two highly sensitive multi-axis force and moment transducers were used to measure the lift and drag forces acting upon the airfoil. The lift force was found to decrease, and the drag force was found to increase with the formation of ice. The amount of change in the unsteady aerodynamic forces was found to depend on the ambient temperature, the LWC, as well as the accreted ice structure. 

An experimental campaign was conducted to study dynamic ice accretion on rotating aeroengine fan blades and evaluate the icing-induced performance degradation to the fan rotor. The experiments were performed in an icing research tunnel with a scaled spinner-fan model exposed to typical dry rime and wet glaze icing conditions. Although the accreted ice layers were found to conform well with the shapes of the fan blades under the rime icing condition, the performance of the fan rotor was found to degrade substantially due to the much rougher blade surfaces, causing up to 60% reduction in the pressure increment after 360 s of the rime icing experiment. More complicated, needlelike icicles were found to grow rapidly over the rotating spinner and fan blades under the glaze icing condition due to the combined effects of the aerodynamic forces and the centrifugal forces associated with the rotation motion. The irregular-shaped glaze ice structures were found to induce tremendous detrimental effects on the fan rotor, making the airflow depressurized, instead of pressurized, after passing the iced fan rotor. The iced spinner-fan model was always found to consume more power, regardless of rime or glaze ice structures accreted on the fan blades.