Fuel economy is one of the biggest challenges facing the aviation industry. To overcome these challenges, researchers are working on next generation aviation systems.
Next generation aircraft will be either hybrid power, or all-electric power, which would help with fuel consumption. But electric aircraft present challenges in other areas such as the prevention of ice formation. Ice formation on aircraft can degrade the aerodynamic performance significantly by reducing lift while increasing drag. Tech Briefs Media Group (TBMG) editor, Billy Hurley, interviewed researcher Afaq Ahmed Abbasi of Northwestern Polytechnical University’s Department of Fluid Mechanics to learn more about these challenges.
TBMG: What are the traditional methods of preventing ice formation on aircraft?
Abbasi: There are numerous methods used for ice prevention on an aircraft surface. The methods can be classified into three main categories: liquid-based, such as weeping wings;mechanical-based, such as pneumatic boots;thermal-based, such as hot-air and electro-thermal systems.
TBMG: Why can’t these methods be used on all-electric aircraft?
Abbasi: Actually, there are many reasons not to use these methods on all-electric aircraft. With technology breakthroughs in the areas of aerodynamics, materials, and power, next-generation aviation systems will use a large number of composite materials, all electric engine technology, and the latest aerodynamic technologies. The reasons conventional anti-icing methods cannot be used in next generation aviation includes, for example, the fact that the application of natural laminar flow technique requires extremely smooth and precisely finished surfaces, while the current mechanical-based and hot-air anti/de-icing techniques have surface gaps and steps which can provoke laminar-turbulent transition not suitable for use on all-electric aircraft.
Another example – the bleed hot air system is the method most used by jet aircraft to keep flight surfaces above the freezing temperature required for ice to accumulate. But this system will not be suitable for next-generation aviation, which is advancing towards all-electric and composite materials.
There is no hot air system for all-electric aircraft, and a hot air system is not beneficial for the protection of composite structures because of the materials’ temperature limitations.
TBMG: What is your approach to preventing ice formation, and what is innovative about it?
Abbasi: We proposed a concept of plasma icing control using Dielectric Barrier Discharge (DBD) plasma actuators utilizing both aerodynamic and thermal effects. Actually, plasma actuators were recently used for flow control using their aerodynamic effect (induced wind). We have shown that the temperature effect (thermal effect) can be used for icing control. The idea is to achieve flow control and icing control using the same set of equipment, i.e. plasma actuators. The actuators are used for icing control in icing conditions and for flow control in non-icing environments.
Plasma icing control technology almost satisfies all the icing control requirements of next generation aircraft. First, it can make use of both the aerodynamic and thermal effects of the plasma, meaning it can utilize most of the power consumption of the plasma actuation. Second, it can be smoothly set on any surface, i.e. parts of the aircraft, to keep the natural laminar flow. Third, the temperature of the AC-SDBD discharge has a limited temperature increase due to its stable self-limiting character in atmospheric pressure, so it will protect the composite structures during anti-/de-icing. Lastly, it can be easily settled on all-electric aircraft as a full electric-based icing control technique.
Over the past year, we have been working to explore the mechanism of plasma icing control through experimental research. Through the plasma actuator characterization in still air at atmospheric pressure (PIV and surface temperature measurements) and wind tunnel experiments (surface temperature measurements and high-speed camera records), we found that the performance of plasma icing control is directly related to the design of the plasma actuators, based on the coupled aerodynamic and thermal effects. Such novel findings provide an important basis for system optimization of plasma icing control.
TBMG: What are plasma actuators, and how do they play a role in your method? What inspired the use of these plasma actuators?
Abbasi: Plasma actuators are actually a set of electrodes placed with each other and powered using high voltage. The plasma actuator is composed of two electrodes separated by a dielectric material arranged in an asymmetric fashion. Application of a sufficiently high-voltage AC signal between the electrodes weakly ionizes the air over the dielectric covering the encapsulated electrode. The ionization of the air is a dynamic process within the AC cycle. The ionized air, in the presence of the electric field, results in a body force vector that acts on the ambient air, inducing a velocity field. Such induced air flow can be modulated to achieve active aerodynamic control. It has the advantages of non-mechanical parts, zero reaction time, broad frequency bandwidths and relatively low energy consumption. Most importantly, the plasma actuators can be conveniently arranged on the surface of the vehicle parts or the wind turbine.
This plasma discharge induces the adjacent air towards the actuator surface producing downstream acceleration, known as ionic wind (aerodynamic effect). This ionic wind of AC-SDBD plasma actuators is generally several meters per second. Many researchers use this ionic wind for flow control applications. But induced aerodynamic effect only consumes a small part of total electrical power, and a large part is converted into heat energy. Therefore, plasma actuators have an obvious thermal effect. We can utilize this thermal effect for icing control.
It all started with a simple but historic experiment; an ice cube taken out of the refrigerator was placed in the discharge area of the plasma actuator. We were surprised to see the ice cube melted into water within a few seconds, without causing any short circuit. This ice cube test was our first inspiration to use these actuators for icing control.
After that, we used a cylindrical model, and placed the model in an icing wind tunnel. In photos taken in the icing wind tunnel, it can clearly be seen that when the actuator is on, ice is prevented/removed from the surface.
TBMG: How did you test this out, and what did the tests determine?
Abbasi: We carried out icing tests in the university’s icing wind tunnel. It provides an icing environment felt by an aircraft at a specific altitude. We could vary different icing parameters and wind speeds in the wind tunnel to see how our model would behave for specific conditions/ environment.
Three different types of actuators were designed to generate the induced air flow in different directions to the incoming flow. Two sets of plasma actuators were placed on the same surface of the airfoil, and the anti-icing effects of the plasma were observed by comparing each set. One set of actuators was turned on, while the other one was on during both quiescent air and wind tunnel tests. The actuators were characterized using two parameters: Particle-Image-Velocimetry (2D-PIV) and the surface temperature distribution measurements. The plasma icing control over the airfoil was studied in the icing wind tunnel, discussed, and conclusions were drawn based on experimental results in the quiescent air and wind tunnel. The tests determined that these actuators can be efficiently used for de-icing.
TBMG: So what’s next? How do you envision this technology being used in the future?
Abbasi: Our work is just beginning in this field. Based on the conclusion, the methods and materials used for this research are sufficient, but still further research is required for the detailed cause and effect relationship between the plasma discharge and the icing.
The most important part of future work is to establish a direct connection between the plasma aerodynamic effect and the thermal effect based on experimental research or numerical simulation. Furthermore, the effects of plasma actuation (both thermal and aerodynamic effects) on the flow around the airfoil in the icing environment should be studied, and the changes in the spatial velocity field and the surface pressure field with plasma actuation need to be studied. There is still a lot to do in this field, although we hope that this technology will replace the conventional methods for icing control in the future.