An Experimental Investigation on the Thermodynamic Characteristics of DBD Plasma Actuations for Aircraft Icing Mitigation

We report the research progress made in our research efforts to utilize the thermal effects induced by DBD plasma actuation to suppress dynamic ice accretion over the surface of an airfoil/wing model for aircraft icing mitigation. While the fundamental mechanism of thermal energy generation in DBD plasma discharges were introduced briefly, the significant differences in the working mechanisms of the plasma-based surface heating approach from those of conventional resistive electric heating methods were highlighted for aircraft anti−/de-icing applications. By leveraging the unique Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT), a comprehensive experimental campaign was conducted to quantify the thermodynamic characteristics of a DBD plasma actuator exposed to frozen cold incoming airflow coupled with significant convective heat transfer. By embedding a DBD plasma actuator and a conventional electrical film heater on the surface of the same airfoil/wing model, a comprehensive experimental campaign was conducted to provide a side-by-side comparison between the DBD plasma-based approach and conventional resistive electrical heating method in preventing ice accretion over the airfoil surface. The experimental results clearly reveal that, with the same power consumption level, the DBD plasma actuator was found to have a noticeably better performance to suppress ice accretion over the airfoil surface, in comparison to the conventional electrical film heater. A duty-cycle modulation concept was adopted to further enhance the plasma-induced thermal effects for improved anti−/de-icing performance. The findings derived from the present study could be used to explore/optimize design paradigm for the development of novel plasma-based anti−/de-icing strategies tailored specifically for aircraft icing mitigation

Quantifying the pathways of latent heat dissipation during droplet freezing on cooled substrates

When a liquid droplet freezes on a cooled substrate, the portion of latent heat released by ice formation that is not immediately absorbed by the supercooled liquid droplet is transferred to the solid substrate below the droplet and the surrounding air. It is important to quantify heat dissipation through these two pathways because they govern the propagation of frost between multiple droplets. In this paper, infrared (IR) thermography measurements of the surface of a freezing droplet are used to quantify the fraction of latent heat released to the substrate and the ambient air. These IR measurements also show that the crystallization dynamics are related to the size of the droplet, as the freezing front moves slower in larger droplets. Numerical simulations of the solidification process are performed using the IR temperature data at the contact line of the droplet as a boundary condition. These simulations, which have good agreement with experimentally measured freezing times, reveal that the heat transferred to the substrate through the base contact area of the droplet is best described by a time-dependent temperature boundary condition, contrary to the constant values of base temperature and rates of heat transfer assumed in previous numerical simulations reported in the literature. In further contrast to the highly simplified descriptions of the interaction between a droplet and its surrounding used in previous models, the model developed in the current work accounts for heat conduction, convection, and evaporative cooling at the droplet-air interface. The simulation results indicate that only a small fraction of heat is lost through the droplet-air interface via conduction and evaporative cooling. The heat transfer rate to the substrate of the droplet is shown to be at least one order of magnitude greater than the heat transferred to the ambient air

Asymmetric Solidification During Droplet Freezing in the Presence of a Neighboring Droplet

A supercooled liquid droplet that freezes on a cold substrate interacts with the local surroundings through heat and mass exchange. Heat loss occurs to the substrate via conduction and at the droplet interface via evaporative cooling, diffusion, and convection. In a group of many droplets, these interac- tions are believed to be responsible for inter-droplet frost propagation and the evaporation of supercooled neighboring droplets. Furthermore, interactions between a standalone freezing droplet and its surround- ings can lead to the formation of condensation halos and asymmetric solidification induced by exter- nal flows. This paper investigates droplet-to-droplet interactions via heat and mass exchange between a freezing droplet and a neighboring droplet, for which asymmetries are observed in the final shape of the frozen droplet. Side-view infrared (IR) thermography measurements of the surface temperature for a pair of freezing droplets, along with three-dimensional numerical simulations of the solidification process, are used to quantify the intensity and nature of these interactions. Two droplet-to-droplet interaction mech- anisms causing asymmetric freezing are identified: (1) non-uniform evaporative cooling on the surface of the freezing droplet caused by vapor starvation in the air between the droplets; and (2) a non-uniform thermal resistance at the contact area of the freezing droplet caused by the heat conduction within the neighboring droplet. The combined experimental and numerical results show that the size of the freez- ing droplet relative to its neighbor can significantly impact the intensity of the interaction between the droplets and, therefore, the degree of asymmetry. A small droplet freezing in the presence of a large droplet, which blocks vapor from freely diffusing to the surface of the small droplet, causes substan- tial asymmetry in the solidification process. The droplet-to-droplet interactions investigated in this paper provide insights into the role of latent heat dissipation during condensation frosting

Investigation of transient phase change phenomena of water droplets for marine icing applications

Icing reduces the stability, reliability, productivity and safe operation of offshore exploration vessels, icebreakers and marine structures. A better understanding of the fundamentals of ice accumulation on variable surfaces will be helpful for developing solutions to reduce or prevent ice accretion on offshore vessels and marine structures. The main way that ice accretes on vessels and offshore structures is the solidification of wave-generated saltwater spray. Additionally, in cold climate conditions, vessels and offshore structures can also experience atmospheric or fresh water icing. This research investigates the primary cause of both ice accretion processes. Results were analyzed in terms of pre-impact and post-impact processes. The pre-impact results of a single water droplet suggest the droplet is not in thermal equilibrium with ambient air. Additionally, the nucleation process can occur at higher temperatures than its equilibrium freezing point. Furthermore, it predicts that the nucleation temperature is controlled by the droplet’s volume and the atmospheric temperature. Larger sized droplets have a higher nucleation temperature than smaller sized droplets. Moreover, internal circulation can enhance heat transfer from the droplet to the surrounding air and can accelerate nucleation in cold environmental conditions, as well as influence the fragmentation process. It was observed that the drag force is not only a function of droplet size, but also of ambient temperature and internal circulation. The post-impact results show that for a droplet impacting on a semi-infinite medium, the thermal penetrated depth is minimal. Therefore, during the post-impact study, the substrate can be considered as an isothermal surface. Furthermore, the droplet solidification process is primarily affected by droplet size, spreading area, surface temperature and pre-impact velocity. The lower the object temperature, the faster the cooling rate. Additionally; the larger the droplet size, the more time it takes to solidify. Experimental studies on droplet impact behaviour on both bare and coated substrates were analyzed. The experiments suggest that a lower spreading and longer freezing time occur more frequently on coated substrates than on uncoated substrates. The main reasons for this are surface roughness, contact angle hysteresis, surface energy, surface properties, the thickness of the coatings and droplet-substrate surface tension. The new experimental data and numerical predictions presented in this thesis will help to develop superior and more accurate ice prediction and prevention models

Experimental investigations on thermodynamic characteristics of DBD plasma and applications for aircraft icing mitigation

Aircraft icing is widely recognized as a significant hazard to aircraft operations. When an airplane flies in a cold climate, some of the super-cooled droplets will impact and freeze on exposed airframe surfaces to form ice shapes. Ice accumulation can degrade the aerodynamic performance of an airplane significantly by increasing drag while decreasing lift. In moderate to severe conditions, an airplane can become so iced up that continued flight is impossible. While a number of anti-/de-icing systems have been developed for aircraft inflight icing mitigation, current anti-/de-icing strategies suffer from various drawbacks, including being too complex, too heavy or draw too much power to be effective. Very recently, dielectric barrier discharge (DBD) plasma actuation has been suggested as a promising, alternative anti-/de-icing method, by leveraging the thermal effects induced by DBD plasma generation. In the present study, a comprehensive study was conducted to examine the thermodynamic characteristics of DBD plasma with the intention to explore its potential as an effective anti-/de-icing method for aircraft icing mitigation. The experimental study was performed in the unique Iowa State University Icing Research Tunnel (i.e., ISU-IRT). A NACA 0012 airfoil/wing model embedded with DBD plasma actuators was designed and installed in ISU-IRT under typical glaze-/rime icing conditions pertinent to aircraft inflight icing phenomena. During the experiments, the dynamic ice creation process and corresponding surface temperature over the airfoil surface were captured by using a high-speed imaging system and an infrared (IR) thermal imaging system. The thermal effects of Alternative Current DBD (i.e., AC-DBD) plasma actuators were compared quantitatively with conventional electric film heater, and the AC-DBD plasma-based anti-icing methods were found to be at least as effective as, if not better than the conventional electrical heaters in preventing ice formation and accretion over the surface of the airfoil/wing model. In addition, thermal characteristics and anti-icing performance of nanosecond-pulsed DBD (NS-DBD) plasma actuator were also investigated under different icing situations. Surface heating during NS-DBD plasma actuation was found to be strongly affected by environmental and operational conditions. The anti-icing performance of NS-DBD plasma actuation would be improved with increasing pulse repetition frequency. Furthermore, configuration of the plasma-based anti-icing system was optimized to improve efficiency of the icing mitigation. Streamwise employed plasma actuators can increase the heat dissipation to downstream of the airfoil to reduce the rivulet formation. Additionally, a hybrid anti-icing approach was introduced by combining NS-DBD plasma actuators and superhydrophobic surface. NS-DBD plasma actuator employed on an airfoil surface can successfully prevent ice formation at the leading edge, while superhydrophobic coating avoids runback water to freeze on surface and form ice rivulets. The findings derived from the present study are very helpful to elucidate the underlying physics and to explore/optimize design paradigms for the development of effective and robust plasma-based anti-/deicing strategies to ensure safer and more efficient operation of aircraft in cold weather

Droplet Dynamics Under Extreme Ambient Conditions

This open access book presents the main results of the Collaborative Research Center SFB-TRR 75, which spanned the period from 2010 to 2022. Scientists from a variety of disciplines, ranging from thermodynamics, fluid mechanics, and electrical engineering to chemistry, mathematics, computer science, and visualization, worked together toward the overarching goal of SFB-TRR 75, to gain a deep physical understanding of fundamental droplet processes, especially those that occur under extreme ambient conditions. These are, for example, near critical thermodynamic conditions, processes at very low temperatures, under the influence of strong electric fields, or in situations with extreme gradients of boundary conditions. The fundamental understanding is a prerequisite for the prediction and optimisation of engineering systems with droplets and sprays, as well as for the prediction of droplet-related phenomena in nature. The book includes results from experimental investigations as well as new analytical and numerical descriptions on different spatial and temporal scales. The contents of the book have been organised according to methodological fundamentals, phenomena associated with free single drops, drop clusters and sprays, and drop and spray phenomena involving wall interactions

Plasma Science and Technology

Plasma science and technology (PST) is a discipline investigating fundamental transport behaviors, interaction physics, and reaction chemistry of plasma and its applications in different technologies and fields. Plasma has uses in refrigeration, biotechnology, health care, microelectronics and semiconductors, nanotechnology, space and environmental sciences, and so on. This book provides a comprehensive overview of PST, including information on different types of plasma, basic interactions of plasma with organic materials, plasma-based energy devices, low-temperature plasma for complex systems, and much more