Power Generation Aboard Aircraft Using MHD  
Why is power generation needed aboard an aircraft?
Power must be generated aboard an aircraft to operate the flight controls, the navigational equipment, as well as the aerodynamic flow control devices utilizing energy deposition (such as the DBD plasma actuators or the MHD accelerator for instance). On military aircraft, it may also be necessary to generate power to feed an energy weapon such as a megawatt-class laser or microwave beam.
Why can't conventional generators be used?
Electrical power can be particularly difficult to generate efficiently aboard an aircraft especially at high supersonic speeds when a ramjet or scramjet engine is in use. Indeed, ramjets and scramjets are characterized by the absence of a mechanical compressor and its associated rotating shaft. The absense of rotating machinery makes it problematic to generate power using a conventional generator.
What are the power generation methods that can be used at high speeds?
One alternative to the conventional generator are fuel cells. Fuel cells would be well suited as long as the amount of power generated is small. For the large amounts of power needed to operate an energy weapon or the aerodynamic flow control devices using energy deposition, fuel cells would not be well suited due to their prohibitive weight.
Another alternative that has received attention in recent years is the MHD generator. The MHD generator generates power by extracting energy from the airflow through the Lorentz force. The Lorentz force is proportional to the current flowing in the air and the applied magnetic field. Contrarily to fuel cells, the MHD generator can output large amounts of power with little weight penalty especially at high Mach number as the maximum amount of power that can be extracted through MHD means corresponds to the enthalpy of the airflow, which becomes particularly high at supersonic or hypersonic speeds.
What is the main difficulty associated with the MHD generator?
Because current can flow only if the air is significantly ionized, and because the temperature of the air around the aircraft or within the engine is not sufficiently high for self-ionization to occur, one of main difficulties associated with the MHD generator is how to ionize the air efficiently.
One way the air can be ionized is by mixing a seed (such as cesium or potassium) with the air. Such has proven to be successful: experimental studies performed recently at the United Technologies Research Center of a Mach 8 scramjet flow seeded with potassium indicate that in the order of 1% of the total enthalpy can be extracted through a MHD generator. Due to the very high kinetic energy of the flow (and hence total enthalpy) at hypervelocities, this would be sufficient to generate several megawatts of power aboard the flight vehicle.
An alternative to alkaline seeds as a means to ionize the flow is electron beams. While the latter require additional power to operate, they have the advantage to produce a more uniform flow conductivity as well as to not inject additional mass. Preliminary studies indicate that electron beams could yield a conductivity as high or higher than the one obtained with alkali seeding. Additionally, since no extra mass needs to be injected in the airflow, an electron-beam-ionized MHD generator could be used in continuous mode throughout the flight in order to produce the necessary electrical power to activate the flight controls and instruments aboard the vehicle.
How You Can Help
We are currently seeking one or more Ph.D. students to perform computational studied comparing the performance of three different types of MHD generators for typical flight conditions of supersonic jets and ramjets: (i) an MHD generator with electron beam ionization, (ii) an MHD generator with alkali-seed ionization, and (iii) an MHD generator with discharge ionization. In particular, the study will assess the impact of three ionization strategies on the generator output and determine which one is best suited depending on the flight conditions. It will be the first time such a study is performed.
The numerical results will be obtained using WARP, which is currently the only CFD code that can integrate in coupled form the plasma and the aerodynamics with aerodynamic-scale integration steplengths. The integration of the plasma with the aerodynamics in coupled form is here critical to obtain high fidelity numerical results for these problems.
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