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1102 ZHOU ET AL. Fig. 6 points T∞ −5°C, and LWC 1.50 g∕m3. Time evolutions of the surface temperatures at three preselected during the ice-accretion experiment with U∞ 40 m∕s, again at the time instance of t ≈ 35 s. As time went by, the surface temperature at point A was found to reach a relatively thermal stable state eventually, with the surface temperature being about 2.0°C for the rest of the ice-accretion experiment (i.e., up to t ≈ 300 s). As shown schematically in Fig. 6, point B is located at the top of the third plasma actuator (i.e., at ∼25% chord). Due to the heating effect induced by the DBD plasma generation from the third plasma actuator and the relatively weaker convective heat transfer at this downstream location, the surface temperature at point B was found to be always much higher than the frozen temperature of water (i.e., T#B >>0°C).AlthoughthesurfacetemperatureatpointBdecreased slightly at the initial stage of the ice-accretion experiment (i.e., in the stage of t < 3 s), the temperature value was found to recover much faster and reach a relatively thermal stable state at about 15 s after starting the ice-accretion experiment. After reaching a relatively thermal stable state, the surface temperature at point B was found to be maintained at ∼11.0°C for the rest of the ice-accretion experiment (i.e., up to t ≈ 300 s) for this test case. The airfoil/wing model used in the present study was made of polymer-based material, which has very low thermal conductivity. Because the location of point C was away from the DBD plasma actuators, evolution characteristics of the surface temperature at point C were mainly affected by the convective heat transfer process over the airfoil surface. As shown clearly in Fig. 6, due to the heating effect associated with the runback of warm surface water from the hot upstream region, the surface temperatures at point C were found to increase continuously during the ice-accretion experiment. Although the temperature at point C was found to be below the frozen temperature of water initially (i.e., T#C;t0 −2.0°C), it became greater than the frozen temperature of water quickly (i.e., at t > 20 s) due to the heating effect associated with the runback of the warm water. Since t > 80 s, the surface temperature at point C was found to be maintained almost constantly at about 2.0°C for the rest of the ice-accretion experiment (i.e., up to t ≈ 300 s). C. Effects of Plasma Generation on Ice Accretion Under Rime-Icing conditions To further explore the anti-/deicing performance of the DBD plasma actuators for aircraft inflight icing mitigation under a typical rime-icing condition, an experimental study was also conducted with the airflow temperature in the ISU-IRT decreased to T∞ −15°C and LWC 1.0 g∕m3. Figure 7 shows the typical snapshots to reveal the dynamic ice-accretion process over the airfoil surface under such rime-icing conditions (i.e., U∞ 40 m∕s, T∞ −15°C, and LWC 1.0 g∕m3 ). It is should be noted that the power inputs applied Typical snapshots of the dynamic ice-accretion process over the Fig. 7 airfoil surface under the rime-icing conditions of U∞ 40 m∕s, T∞ −15°C, and LWC 1.0 g∕m3. to the DBD plasma actuators were kept at the same levels as those under the glaze-icing condition during the ice-accretion experiment. As revealed clearly in Fig. 7, on the plasma-off side of the airfoil surface, the supercooled water droplets were found to ice up immediately as they impinged onto the airfoil surface due to the much colder temperature for this test case. No obvious rivulet formation or surface water runback were observed during the ice-accretion process, as expected for a typical rime-ice-accretion process. However, on the plasma-on side of the airfoil surface, although no obvious ice accretion could be identified at the front portion of the airfoil surface (i.e., approximately twice the area of the region covered by the DBD plasma actuators), a large chunk of ice was found to accrete eventually over the rear portion of the airfoil surface. This could be explained by the fact that, after impacted onto the warm surface near the airfoil leading edge (i.e., the region protected by the DBD plasma actuators), supercooled water droplets would be heated up and form a thin water film/rivulet flow near the airfoil leading edge, as revealed clearly by Zhang et al. [26]. The unfrozen surface water film would run back, as driven by the boundary-layer airflow over the airfoil surface. Due to the intensive heat transfer between the runback surface water and the boundary- layer airflow over the airfoil surface at a frozen-cold temperature of −15°C, the surface water was found to be frozen gradually as it ran back. As a result, ice accretion was found to take place in the downstream region beyond the area protected by the DBD plasma actuators. As time went by, with the continuous impinging of the supercooled water droplets onto the airfoil surface, more and more surface water mass would be collected and eventually frozen into ice over the rear portion of the airfoil/wing model, i.e., in the downstream region beyond the area protected by the DBD plasma actuators, which was revealed clearly in Fig. 7. In summary, although the thermal effects induced by the DBD plasma generation were demonstrated to be effective in preventing ice formation and accretion over the protected area under the rime- icing condition with a much colder ambient temperature, the runback 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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