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1450 Y. TIAN et al. In-flight icing is caused by supercooled water droplets in clouds which are usually in a metastable condition. When an aircraft encounters supercooled water droplets in flight, the droplets may either change their phase to form ice crystals the moment they impact the surface, or collect into a thin water film and flow downstream along the aircraft surface by the resultant force of aerodynamic, gravity, and surface adhe- sion forces. The typical atmospheric temperature associated with aircraft icing ranges from 40 C to 0 C.1 Ice accretion on aircraft surface is usually classified as clear (or glaze) ice, rime ice, mixed ice, and hoar frost.2 Clear ice always occurs at temperatures ranging from 18 C to 0 C, and the size of supercooled water droplets leading to clear ice is relatively large. In clear-ice situations, when supercooled droplets impact aircraft surface, the freezing process is rela- tively gradual due to the latent heat released in the freezing process, allowing some of the water droplets to flow rearwards before they solidify. Thus a sheet of solid, clear, glazed ice is formed with very little air enclosed, and has a density of 0.917 kg/m3 close to that of water.2,3 Rime ice occurs at lower temperatures (40 C to 10 C), and the size of supercooled water droplets to be frozen to rime ice is relatively tiny. Because of the smaller volume of supercooled water droplets, the freezing process is so fast that they solidify upon impact- ing. In the process of rime ice formation, air is entrapped between frozen droplets, that’s why rime ice is with a white appearance. Therefore, rime ice is a mixture of tiny ice parti- cles and trapped air with a density of 0.88 kg/m3.2,3 Actually, different moistures and droplet sizes commonly exist in clouds, and this variation can produce a mixture of clear ice (from lar- ger drops) and rime ice (from smaller droplets), known as mixed ice, which is the most common ice encountered in flight. Hoar frost occurs when moist air comes in contact with air- craft surface at sub-zero temperatures, where water vapor changes directly to ice and deposits in the form of frost without condensing to liquid water, and thus it is a white crystalline coating which can be brushed off.2,3 The locations where icing occurs most frequently on air- craft are always the leading edges of wings and empennages, the propellers, hubs, engine nacelles, rotor blades, windshields and some sensors.4 Ice accretion on wings and rotor blades can increase drag and decrease lift, and moreover, change the moment characteristics.5–7 Propellers’ thrust efficiency will decrease once ice forms on them. Ice accretion on engine nacelles and hubs will reduce the efficiency of the engine, once the dislodged ice crystals are ingested, the engine is very possi- bly to be destroyed.8–10 Windshields icing will influence the view of pilots and sensors icing can mislead pilots.11,12 Gener- ally speaking, Aircraft icing is a dangerous phenomenon which threatens flight safety. To ensure flight safety, anti-icing and deicing methods are indispensable for aircraft. Anti-icing refers to prevention of any ice formation during flight. Deicing refers to the case when ice has already formed on aircraft surface, and then is removed with some methods.1 Methods for anti-icing and deicing under investigation or in use are mainly classified into four kinds: (A) freezing point depressant method; (B) thermal method; (C) mechanical method; (D) surface modification method. The freezing point depressant method usually uses anti-icing/ deicing fluids such as ethylene glycol and propylene glycol to lower the freezing point of supercooled water droplets and melt existing ice accretion. The shortcomings of this method lie in its low efficiency at extreme conditions and its pollution to the environment.13–16 The thermal method is to deliver heat flux to the aircraft surface, forming a hot boundary layer on the surface to prevent the phase transition of supercooled water droplets into ice or to melt solidified ice crystals. This kind of method involves using hot compressor gases, hot exhaust gases, hot oil, or electrical energy. Even though this method has features of simplicity and fast response, the energy consumption is very high.1,16,17 The mechanical method is to deform the aircraft surface to break the structure of ice accre- tion, smash the ice, and remove the ice dregs from the surface by air flow and gravity force. This kind of method includes pneumatic boots, Electromagnetic Impulse DeIcing (EIDI), and electromagnetic expulsive boots. It can only deice, and additional drag is unavoidable1,18,19The surface modification method is to modify the physical and chemical properties of the surface to change the contact state of water and the sur- face, such as contact angle and roll-off angle, to reduce ice adhesion and accumulation on the surface. The most popular method of this kind is hydrophobic and superhydrophobic coatings.20 However, the serviceability of coatings in humid environments has been doubted, and much more research is needed before real applications.16,21–23 Although lots of anti-icing and de-icing methods have been in use or studied for quite a long time, new methods are still in great demand in both civil and military aircraft communities. Dielectric Barrier Discharge (DBD) plasma actuation has been widely studied and applied in flow control such as flow separa- tion suppression, lift augmentation, and drag reduction for more than a decade, exhibiting potential applications to active flow control over various aircraft.24,25 Fig. 1 shows a single DBD plasma actuator which consists of an exposed electrode and a covered electrode with a layer of dielectric placed in between to insulate them. The actuator is driven by an Alter- nating Current (AC) voltage source which could be sinusoidal, squared, triangle, pulsed, etc. in a voltage wave form. The col- lision of ions in the plasma resulting from discharge transfers momentum to the boundary layer, inducing an air flow accel- erating and moving away from the exposed electrode as shown in Fig. 1. This accelerating effect can be regarded to be equiv- alent to an imaginary volumetric force acting on the fluid. Besides, DBD plasma actuators have obvious thermal effects, and can heat the dielectrics below the exposed electrodes and the ambient air. The thermal characteristics of DBD plasma have been studied by researchers, which indicated that the rotational temperature of the weakly ionized plasma can reach as high as 200 C.26–29 This paper applied DBD plasma actuation to anti-icing on an airfoil in an icing wind tunnel to verify its effectiveness, and explored its mechanism of anti-icing. The experimental process Fig. 1 Schematic illustration of a single DBD plasma actuator in active flow control.PDF Image | Experimental study of an anti-icing method over an airfoil based on pulsed dielectric barrier discharge plasma
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