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being frozen into solid ice, the supercooled droplets were heated up immediately after impacting onto the plasma-on side of the airfoil surface. Similar to what was described by Zhang et al. [26], the surface water mass on the plasma-on side was found to form a thin- film flow at first in the region near the airfoil leading edge, and then run back along the airfoil surface as driven by the boundary-layer airflow over the airfoil/wing model. As shown clearly in the snapshot image given in Fig. 3b, as the water film advanced downstream, isolated water rivulets were found to form further downstream, resulting in isolated water transport channels to transport the surface water mass collected at the leading edge from the continuous impingement of the water droplets. Eventually, the impinged surface water mass was found to shed from the airfoil trailing edge. As time went by, more and more supercooled water droplets carried by the incoming airflow would impinge onto the airfoil surface. As shown clearly in the acquired images given in Figs. 3c and 3d (i.e., the snapshots captured at the time instances of t 35 s and t 195 s), the ice layers accumulated on the plasma-off side of the airfoil surface were found to become thicker and thicker. The corresponding surface temperature distributions given in Figs. 4a and 4d also confirmed the existence of ice layers accreted over the plasma-off side surface by having the temperatures lower than the frozen temperature of water (i.e., Tw < 0°C). However, as shown quantitatively in Figs. 4c and 4d, the surface temperatures on the plasma-on side of the airfoil were still found to be well above the frozentemperatureofwater(i.e.,Tw >0°C).Asaresult,theplasma- on side of the airfoil surface was always found to be free of ice over the duration of the entire ice-accretion experiments (i.e., up to 300 s), as revealed clearly in Fig. 3. In summary, the present study demonstrates clearly that, in addition to being widely used as an effective flow control method to suppress large-scale flow separation and airfoil stall, DBD plasma actuators can be used as a promising anti-/deicing tool for aircraft icing mitigation. B. Evolution of Airfoil Surface Temperature Under Glaze-Icing Conditions Based on the time sequences of the acquired IR thermal imaging measurement results, the evolution characteristics of the surface temperature distributions over the airfoil surface during the ice- accretion experiment can be evaluated quantitatively. As shown clearly in Fig. 4, due to the released latent heat of fusion associated with the phase changing of the impinged supercooled water droplets, the surface temperatures on the plasma-off side of the airfoil/wing model were found to change only slightly during the ice-accretion process, which was analyzed in detail in the previous study of Liu et al. [6]. The focus of the present study is on the plasma-on side of the airfoil surface to gain further insight into the underlying physics pertinent to using the thermal effects induced by plasma generation for aircraft icing mitigation. Figure 5 shows the extracted surface temperature profiles along the chordwise direction at the centerplane of the plasma-on side of the airfoil/wing model (i.e., along the line of A-A, as shown in Fig. 5) under the test conditions of U∞ 40 m∕s, T∞ −5°C, and LWC 1.50 g∕m3 at the time instances of t 0.3, 10, 20, 40, and 80 s, respectively. The locations of the exposed electrodes of the DBD plasma actuators were also shown in the plot for comparison. It can be seen clearly that the airfoil surface temperatures were found to vary significantly along the chordwise direction of the airfoil/wing model, featured by the irregular sawtoothlike structures in the profiles. As shown clearly in Fig. 5, the appearance of the sawtoothlike structures was found to be correlated well with the locations of the interfaces between the exposed electrodes and the embedded electrodes of the DBD plasma actuators. It indicates that the DBD plasma generation at the electrode interfaces would induce a maximum local temperature, which agrees well with the findings reported in the previous study of Joussot et al. [22]. The measurement results given in Fig. 5 also reveal clearly that, due to the thermal effect induced by the DBD plasma generation, the surface temperatures in the regions near the plasma actuators were found to be always well above the frozen temperature of water (i.e., Tw > 0°C), Fig. 5 the plasma-on side of the airfoil surface under the glaze-icing conditions ofU∞ 40m∕s,T∞ −5°C,andLWC1.50g∕m3. which can effectively prevent the formation of ice over the airfoil surface during the entire duration of the ice-accretion experiment. Although the same electric voltage was applied to all the DBD plasma actuators, the surface temperatures in the regions near the airfoil leading edge (i.e., near the first DBD plasma actuator) were found to be much lower than those at the downstream locations. It is believed to be closely related to the enhanced convective heat transfer near the airfoil leading edge due to the direct impingement of the supercooled water droplets along with the frozen-cold incoming airflow. By comparing the surface temperature profiles at different time instances, it can be seen clearly that, with more and more heat added onto the airfoil surface via DBD plasma generation, the surface temperature of the airfoil model was found to increase continuously with the time in general. Although the initial surface temperature in the downstream regions without DBD plasma actuators (i.e., in the downstream regions of X∕D > 0.35) was found to be below the frozen temperature of water (i.e., as shown in the profile at the time instance of t 0.3 s), their values were to increase rapidly and becameabovethefrozentemperatureofwater(i.e.,Tw >0°C)about 20 s later due to the runback of the warm surface water from the “hot” upstream locations into the cold downstream regions. Figure 6 shows the variations of the measured surface temperatures as functions of time at three preselected points over the airfoil surface (i.e., the points of A, B, and C as shown in Fig. 6, located at ∼ 5, 25, and 55% chord, respectively), which can be used to reveal the evolution characteristics of the surface temperature over the airfoil/ wing model more clearly and quantitatively. As described previously, because the DBD plasma actuators were switched on about 10 s before turning on the water spray system of ISU-IRT, the initial surface temperatures (i.e., the temperature at the time instance of t 0) at point A (i.e., T#A;t0 2.0°C) and point B (i.e., T#B;t0 9.0°C) were found to be well above the frozen temperature of water (i.e., Tw > 0°C), due to the thermal effect induced by DBD plasma generation. The initial surface temperature at point C (i.e., T#C;t0 −2.0°C) was also found to be slightly higher than the temperature of the incoming airflow (i.e., T∞ −5.0°C) caused by the convective heat transfer from hot upstream regions. As shown clearly in Fig. 6, the surface temperature at selected point A was found to decrease rapidly, and it even became negative (i.e., below the frozen temperature of water) at the beginning stage of the ice-accretion experiment (i.e., in the stage of t < 5 s). This was caused by the rapid cooling associated with the impingement of the first group of supercooled water droplets onto the airfoil surface near the leading edge. With the continuous heating effect induced by the DBD plasma generation, the surface temperature at point A was found to increase gradually and became greater than the frozen temperature of water ZHOU ET AL. 1101 Extracted profiles of surface temperature at the centerplane of Downloaded by IOWA STATE UNIVERSITY on October 5, 2018 | http://arc.aiaa.org | DOI: 10.2514/1.J056358PDF Image | Utilization of Thermal Effect Induced by Plasma Generation for Aircraft Icing Mitigation
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