AAWSAP DIRD, MHD Air Breathing Propulsion and Power for Aerospace Applications, November 2010

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Chapter 2: Aeronautical Applications - Concepts and
System Issues
In this section we will review the following issues and concepts:
•
Basic principles and problems of MHD propulsion, power generation, and flow
control .
•
MHD inlet control.
•
MHD power generation in scramjet flowpath.
•
Plasma-generated virtual surfaces for drag reduction, steering, and virtual
cowl.
•
MHD energy bypass: the Ajax concept.
•
The reverse energy bypass concept.
•
MHD power generation and aerodynamic control for reentry vehicles.
BASIC PRINCIPLES OF MAGNETOHYDRODYNAMICS AND
REQUIREMENTS FOR MHD PERFORMANCE
The basic principles of magnetohydrodynamics (MHD) are understood very well.
When an electrically conducting fluid crosses magnetic field lines, an
electromotive force (Faraday e.m.f., equal to the product of flow velocity u and
the strength of magnetic field B, uB, multiplied by the channel width) is induced
across the fluid and the B field. If then a pair of electrodes is positioned on either
side of the fluid flow and connected via a ballast resistor on the outside, an
electric current will be induced in the circuit, and power will be generated on the
external load. This electric power will represent partial conversion of the flow
enthalpy (consisting of thermal and kinetic energy of the flow) into electricity. At
the same time, the current flowing through the finite-conductivity fluid will
produce Joule heatinga of the flu id that will increase both static temperature and
entropy of the fluid.
The ratio of the extracted electrical power to the Joule dissipation rate is
determined by the ratio of the load resistance to the sum of load and fluid
resistances; this ratio is called the "load factor," k, O<k<l.
The current (current density j) induced in the fluid, being normal to both the
magnetic field B and the flow direction, results in the body force per unit volume
equal to jxB and directed against the flow. This body force, commonly called the
"Lorentz force" (it should be properly called the ampere force or the
ponderomotive force), is directed against the flow in MHD generators, acting to
slow the flow down and reduce its total energy, which is in line with the
electricity extraction.
a Joule heating, given by the expression, Q=l 2Rt, (Q is the beat generated by a constant current, l, flowing
through a conductor of electrical resistance, R, for a time, t), is the process by wbjch the passage of an
electric current through a conductor releases heat. If current, resistance, and time are expressed in amperes,
ohms, and seconds respectively, the unit of Q is the joule. The increase in the kinetic or vibrational
collisional energy of the ions and electrons manifests itself as heat and a rise in the temperature of the
conductor. Rather than a wire, the conductor in this app.lication is an ionized fl uid.
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