# Experimental Study on Anti-Icing Performance of NS-DBD Plasma Actuator
## 3 Experimental Results
### Anti-Icing Performance
It can be found that the surface temperature sharply rises after igniting the plasma actuation at t = 0 s. About 25 s after that, the temperature does not change anymore (thermal equilibrium was reached). After opening the spray system when t = 100 s, the surface temperature decreases. But there are differences between the two different discharge conditions. When applied  
Figure 14: Time variation of the surface temperature distribution during the anti-icing process when applied with LV-HF discharge. Figure 15: Time variation of the surface temperature distribution during the anti-icing process when applied with HV-LF discharge. with LV-HF discharge, another thermal equilibrium was reached when t = 150 s. Compared to the former state, the surface temperature decreases. The surface temperature of point A decreased by about 23 \({}^{\circ}\)C, that of point B by 18 \({}^{\circ}\)C, and 12 \({}^{\circ}\)C for point C. For the case of HV-LF discharge, the surface temperature continues to decline until it is below zero. The heating sources of the NS-DBD plasma actuator can be divided into two parts: one is the gas heating near the edge of exposed electrodes; the other is the heating of the dielectric barrier [30]. Of course, there was a convective heat transfer process between the heated gas and dielectric surface. It is worth noting that only a thin layer of air (less than 1 mm) was heated. Discharge happens at both the rising and falling edge of the applied voltage, which induces two successive ultrafast heating processes corresponding to the two current peaks which occur during one single pulse (shown in Figure 6) [23]. The temperature of air contained within the discharge volume rises several orders of magnitude in less than 1 \(\upmu\)s [38]. After that, as Figure 17 shows, the actuator will keep \"silent\" for \(\tau_{silent}=1/f_{p}-\tau_{heat}\). [PERSON] et al. [21] studied the effect of the NS-DBD actuator on a flat plate laminar boundary layer, and found that the \"hot spot\" (a region of high temperature) convects downstream, becoming elongated into a streak under the condition of a 5 m/s flow. Here, the heated gas at the front part of airfoil model flows downstream along with cold airflow. In the meantime, convective heat transfer with the dielectric surface and heat conduction with cold airflow occurs, which means that the heat is quickly dissipated. NS-DBD plasma actuation seems to wrap a layer of hot air over the surface of the airfoil model; each pulse individually contributes to the thermal expansion. The supercooled water droplets need to pass through a hot thin air film before they hit the wing surface. Part of input energy of the plasma discharge is directly injected into the heated air and used to warm the supercooled water droplets. Higher voltage induces a relatively larger heating area, and single pulse energy increased as well. When a HV-LP discharge occurred, single pulse energy increased, and more energy was used for gas  
Figure 16: Time variations of surface temperature at the location of selected points: (**a**) LV-HF discharge; (**b**) HV-LF discharge. Figure 17: Time series diagram of NS-DBD discharge. heat during one single pulse, while it was quickly dissipated due to the long \"silent\" period. It is suggested that there is a threshold frequency, corresponding to the voltage amplitude of the electric actuation signal and the incoming flow condition, to achieve effective anti-icing performance. Apart from thermal effects, aerodynamic effects should also be taken into consideration. Previous study has revealed that the mixture of gas and supercooled water droplets is in a non-equilibrium state. When a shock wave propagates in such a mixture, it would cause the relaxation processes of momentum-, heat- and mass transfer [39; 40]. Pressure waves induced by ultrafast gas heating of the NS-DBD plasma actuator may also affect the movement trajectory of supercooled water droplets, and then influence the water collection coefficient.