**1. Introduction**

Aircraft icing is widely recognized as one of the most serious weather hazards to flight safety [1–3]. Ice accumulation has been found to induce large-scale flow

separation over airframe surfaces, thereby, degrading the aerodynamic performance of an airplane significantly [4]. Ice accretion over airframe surfaces can make the aircraft to roll or pitch uncontrollably, and even causes crashes [5]. While considerable research progresses have been made in recent years to provide a better understanding about aircraft icing phenomena, preventing the loss of control due to ice accretion over airframe surfaces still remains an important unsolved problem at the top of National Transportation Safety Board (NTSB)'s most wanted list of aviation safety improvements as highlighted at https://www.ntsb.gov/safety/mwl/ Pages/mwl5-2017-18.aspx.

It should be noted that, while anti-icing refers to the prevention of ice buildup on an airframe surface, de-icing denotes the scenario where ice has already formed on an airframe surface, which is removed subsequently. While a number of anti−/de-icing systems have been developed and implemented for aircraft icing mitigation in recent years, all aircraft anti−/de-icing systems can generally be classified into two categories: active and passive methods. While active methods rely on energy input from an external system for the anti−/de-icing operation, passive methods take advantage of the physical properties of the airframe surfaces (e.g., surface wettability) to prevent/delay ice formation and accretion. Current active anti−/de-icing strategies for aircraft icing mitigation suffer from various drawbacks. For example, spraying aqueous solutions of propylene and ethylene glycol (minimum of 50% concentration) along with other chemical additives are widely used for ground anti−/de-icing at airports before aircraft takeoff. Propylene and ethylene glycol, although readily biodegradable, exert an extremely high biochemical oxygen demand on aquatic systems that result in killing fish and other aquatic creatures due to the depletion of dissolved oxygen [6]. There has been an increasing concern of the environmental impacts from the aircraft de-icing fluid swept away with storm and melt water runoff at airports to ground water and nearby waterways [7]. Pneumatic de-icing systems with rubber boots have been used to break off ice chunks accreted at airfoil/wing leading edge for aircraft in-flight icing protection, but they are usually quite heavy and sometime unreliable [8]. Ultrasonic and mechanical de-icing solutions are not easily integrated into existing aircraft and pose foreign object damage (FOD) hazards to aero-engines [8]. While electric resistant heating l or hot air bleeding systems have been used to melt out ice by heating airframe surfaces, they are usually very inefficient and have demanding power requirements and can also cause damage to composite materials from overheating. Furthermore, the melt water may simply run back and re-freeze at a downstream location to cause uncontrolled ice accretion [8]. Passive anti-icing approaches with hydro−/ice-phobic surface coatings have also been suggested as viable strategies for aircraft icing mitigation [9–11]. However, none of the passive approaches are found to be able to eliminate/prevent ice accretion over airframe surfaces completely, especially in the critical regions (e.g., near the airfoil leading edges) [12, 13]. Thus, it is highly desirable and important to develop novel and effective anti−/de-icing strategies to ensure safer and more efficient operation of aircraft under atmospheric icing conditions.

Dielectric barrier discharge (DBD) plasma actuators, which are fully electronic devices without any moving parts, have been studied extensively in the aerospace engineering community [14–16]. A DBD plasma actuator usually features two electrodes attached asymmetrically on the opposite side of a dielectric barrier layer. When a high voltage (i.e., either in alternating current (AC) or nanosecond pulses), is applied to the electrodes, the air over the encapsulated electrode will be ionized to generate a streak of plasma discharges. For AC-DBD plasma discharge, powered by an AC electric field, ionized air molecules is formed in the discharge region above the covered electrode inducing a fluid velocity adding momentum to

### *An Experimental Investigation on the Thermodynamic Characteristics of DBD Plasma… DOI: http://dx.doi.org/10.5772/intechopen.100100*

the boundary layer [17–20]. For the cases with the applied high voltages in nanosecond pulses (i.e., ns-DBD plasma actuation), it would induce an ultrafast gas heating mechanism, leading to the generation of a shockwave [17–20]. The use of DBD plasma actuators has gained significant interest in the aerospace engineering community as a promising flow control tool to suppress airfoil stall [21–23] and eliminate separation of laminar boundary layer flows [24, 25] for improved aerodynamic performances. It should be noted that, even though DBD plasma actuators have been widely used for various flow control applications [26, 27], the electromechanical efficiency of DBD-plasma- based approach (e.g., the ratio of the energy used to induce ionic wall jet flows for flow control to the total energy consumed by the actuator) was found to be usually very low (i.e., no more than 0.20%) [28], and majority of the energy consumed by the plasma actuators would be dissipated via gas heating and dielectric heating [29].

As revealed clearly by Stanfield et al. [30] and Dong et al. [31], the rotational temperature of the gas above the grounded electrode of a DBD actuator during the plasma actuation can be increased up to 200°C, while the vibrational temperatures were observed to be an order of magnitude higher than the rotational temperature. It was also found that the primary mechanism for the heating of the dielectric layer is through heat transfer from the plasma, i.e., through direct injection, convection, and radiation. To further characterize the thermal effects of DBD plasma discharges, Tirumala et al. [32] used an infrared thermography technique to measure the surface temperature over a DBD plasma actuator, and found that the predominant mechanism of dielectric heating is due to the heat transfer from the plasma to the gas, which then heats up the dielectric surface through forced convection. The increase of the surface temperature was found to have linear relationship with both the applied voltage and the input frequency. By adopting the significant thermal effects of DBD plasma actuators, Cai et al. [33] conducted an explore study to demonstrate the feasibility of using plasma-induced thermal effects for anti−/de-icing operations by embedding an AC-DBD plasma actuator on an ice accreting cylinder model. The thermal effects of AC-DBD plasma actuation were found to be effective for both anti-icing and de-icing operations.

For the flow control applications on aircraft, DBD plasma actuators are usually designed to be mounted in the aerodynamically delicate regions where the aerodynamic characteristics would alter greatly as incoming flow changes (e.g., leading edges of wings and inlet lips of aeroengines) [14, 34]. It should be noted that, such aerodynamically delicate regions are usually also the preferential sites for ice formation and accretion [33]. Since DBD plasma actuation has been found to induce significant surface heating effects along with the ionic wind generation [30, 31], DBD plasma actuators can also be used promising candidates for aircraft icing mitigation. By leveraging icing research tunnels to generate icing conditions to simulate the dynamic ice accretion process over airfoil/wing surfaces, a series of experimental studies were conducted recently to demonstrate the feasibility of utilizing the plasma-induced thermal effects to suppress dynamic ice accretion process over the surfaces of airfoil/wing models for aircraft icing mitigation [29, 35–39].

In the present study, we report the research progress made in our research efforts to utilize the plasma-induced thermal effects to suppress dynamic ice accretion process over the surfaces of airfoil/wing models for aircraft icing mitigation. In the context that follows, while the fundamental mechanism of thermal energy generation in DBD plasma actuation is introduced briefly, the significant differences in the working mechanism of the plasma-based surface heating approach from those of conventional resistive electric heating methods for aircraft anti−/de-icing applications are highlighted. By leveraging the unique Icing Research Tunnel available at Iowa State University (*i.e.*, ISU-IRT), a comprehensive experimental campaign is

conducted to quantify the thermodynamic characteristics of an DBD plasma actuator embed over the surface an airfoil/wing model exposed to frozen cold incoming airflow with significant convective heat transfer in the context of aircraft anti−/de-icing. By embedding both a DBD plasma actuator and a conventional electrical film heater onto the surface of the same airfoil/wing model, an experimental investigation is also conducted to provide a side-by-side comparison between the DBD plasma actuator and the electrical film heater in preventing ice formation and accretion over the airfoil surface under a typical icing condition. While a high-speed camera is used to capture the transient details of the dynamic ice accretion and water transport processes over the airfoil surface, an infrared thermal imaging system is utilized to map the surface temperature evolutions during the dynamic ice accretion process or anti−/ de-icing process with the AC-DBD plasma and the electrical film heater turned on. The temporally-synchronized-and-resolved IR thermal imaging results are correlated with the acquired ice accretion images to elucidate the underlying physics for a better understanding of the fundamentals of the DBD plasma-based approach for aircraft icing mitigation.
