Defense Intelligence Reference Document MHD Air Breathing Propulsion And Power For Aerospace Applications

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ram/scramjet combustors if alkali vapor is added to the gas. In those regions,
MHD interactions can be promising for electric power generation or acceleration
of the flow, as well as for flow control. However, at Mach numbers below about
12 (and excluding the combustor or the region just downstream of it), the air is
too cold for a significant thermal ionization even with alkali seeding. The required
level of ionization then has to be created and sustained by nonequilibrium
(nonthermal) means and is associated with a very substantial power budget and
additional heating. Therefore, the efficiency of ionization (which can vary by
orders of magnitude depending on the particular means of ionization) is of first-
order significance for the entire operation and efficiency of the device. Energy
used to ionize and excite the gas molecules can be considered as loss in the
system since this energy is rarely recovered in the form of directed kinetic or
thrust energy. Note that in this regard, ionization by high-energy electron beams
or by repetitive high-voltage nanosecond pulses are promising as the most
energy-efficient means of nonequilibrium ionization. 4, 5,6 ,7
Even with the most efficient ionization techniques, the power budget and
additional heating associated with the ionizer normally limit the achievable level
of ionization. To have a substantial MHD effect,8 , 9 one has to either use a very
strong magnetic field (which is associated with some practical issues) or use the
MHD interaction in a localized and transient regime (e.g., for boundary layer
control).
As for EHD interaction, 10, 11 it relies upon non-neutrality of the plasma and an
electric field to impart momentum to the gas. Although EHD (or "ion wind")
phenomena have been known for many years, the last several years saw a surge
of new interest to this type of interaction. This new boom is due to the
asymmetric dielectric barrier discharge (DBD)-a remarkably simple device that
has been demonstrated to be very effective in delaying and controlling flow
separation and perhaps even laminar-turbulent transition. Although details of the
physics of DBD plasma actuators are still incompletely understood, the simplicity
of these devices, their low power consumption, and the striking effectiveness in
separation control bring these systems to the top of the list of plasma
aerodynamics and plasma-assisted propulsion technologies that have near-term
application prospects.
Combustion
Another area where nonequilibrium (nonthermal) weakly ionized plasmas are
very promising is plasma-assisted combustion. Although heating induced by
plasmas can ignite combustible mixtures, as mentioned above, it is the presence
of "hot" electrons in a cold gas that makes nonequilibrium plasmas quite
interesting for promoting chemical processes such as combustion. Electron-
impact dissociation, excitation, and ionization of molecules can generate
chemically active species such as radicals and excited atoms and molecules, and
those species can initiate or accelerate chemical reactions that would otherwise
be nonexistent or slow at low temperature. A number of novel techniques,
including (but not limited to) high-voltage nanosecond pulses and the so-called
"gliding arc" have been shown to be quite effective in plasma-assisted
combustion. Investigation of detailed mechanisms (often quite complex and
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