Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to magnetohydrodynamic
generators and, more specifically, to an improved gaseous
electrode for such generators.
MHD generators produce electrical power by motion
of a high temperature electrically-conductive gas through a
magnetic field. This movement induces an electromotive
force between opposed electrodes within the generator. m e
rapid motion of the high temperature gases, however, seriously
erodes the generator's electrodes as do internal electric arcs
which connect the MHD generator's main plasma stream to a
load. Further, some MHD generators operate with coal slag
condensed on the interior surface creating a large voltage drop
and thu~ a power loss as the MHD current is conducted through
the coal slag. In this respect, although gaseous electrodes
have been suggested in the past, it is an object of this
invention to provide an improved gaseous electrode using an
electrically conducting gas which does not wear out even
though subjected to high generator current densities.
In accordance ~ith principles of the invention an
electrode's arc i5 caused to move from place to place within
a cavity along one or more openings in the electrode. This
causes the ionized gas to fill the entire cavity and be
forced into the generator's main channel to function as a
gaseo~$ electrode.
In accordance with one embodiment, a MHD system
having an electrode assembly for connecting said system to a
load and duct means for passing a main plasma stream adjacent
to said electrode assembly, said electrode assembly comprises:
a first elongated electrode having first and second ends
thereof and located adjacent said plasma stream, a second
elongated electrode having a first end adjacent said first
end of said first electrode and a second end adjacent said
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second end of said first electrode, and said second electrode
being spaced from said first electrode, means for passing a
gas through the space between the first and said second
electrodes, exit means for permitting said gas to exit from
between said first and second electrodes and into said duct
means for striking an arc.between said first and said second
electrodes for ionizing said gas and electrically connecting
said electrode assembly to said plasma stream, and arc moving
means for causing said arc to move from place to place along
said second elongated electrode between said first and second
ends thereof.
From a different aspect, and in accordance with an
embodiment, a method for operating a gaseous electrode for
an MHD system of the type in which an arc is struck between
first and second elongated electrode elements to ionize a gas
passing therebetween, said method comprising the steps of:
causing said arc to move from place to place in an axial
direction along the surface of at least one of said elec-
trodes.
The foregoing and other objects, features, and
advantages of the invention will be apparent from the following
more particular description of preferred embodiments as
illustrated in the accompanying drawings in which like
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reference characters refer to the same parts throughout the
various views. The drawings are not necessarily to scale,
emphasis instead being placed upon illustrating principles
of the invention.
FIG. 1 is a schematic illustration of a Faraday-
type MHD generator having segmented electrodes.
FIG. 2 is a gchematic pictorial illustration of an
electode embodying the invention.
FIG. 2a is a schematic pictorial illustration of an
alternate embodiment of a portion of the structure illustrated
in FIG. 2.
FIG. 3 is an enlarged cross sectional view of FIG.
2 taken along the lines 3-3 thereof.
FIG. 4 is a cross section of an alternate embodiment
of the electrode illustrated in FIGS. 2 and 3.
FIG. 5 is a schematic pictorial view of an electrode
similar to that of FIG. 2, but including an AC coil wound
about the electrode's body in the plane of the generator's
magnetic flux.
FIG. 6 is an enlarged cross sectional view of FIG.
5 taken along the lines 6-6 thereof.
FIG. 7 is a schematic cross sectional illustration
of a still further embodiment of an electrode embodying the
invention.
FIG. 8 is a bottom view of the FIG. 7 structure taken
along the lines 8-& thereof.
A conventional MHD generator is comprised of a
duct 10 (FIG. 1) which receives a main stream of high
temperature, electrically-conductive plasma at an inlet end
as indicated by arrow 12.
By properly choosing the shape and discharge
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pressure of the duct 10, the plasma can be made to move
through the duct at a substantially constant velocity past
one or more electrodes such as schematically illustrated
segmented electodes 14 and 16 which are placed in circuit
18 with a load 20.
A suitable magnetic flux, represented by an arrow
B, iS placed across the duct in a direction perpendicular to
both the plasma flow 12 and the EMF to be generated between
the electodes 14 and 16.
The electrodes of FIG. 2 is comprised ~f a cylindrical
electrode element 22 uniformly spaced within a cylindrical
cavity 24 of a surrounding elongated electrode element 26.
The upper surface of the element 26 includes a centrally disposed
channel 28 to permit efflux of the electrode's plasma as
will now be described.
A gas injector manifold 30 (FIG. 3) extends within
member 26 and functions to provide a suitable gas -- conven-
tionally an inert ga~ such as argon -- through a passageway
32 into the cavity 24 where it pa~sses around the central
electrode element 22: out of the cha~nel 28, and into the
gonora~or~ itself. In this respect, the central electrode
element 22 is positively biased with respect to electrode
element 26 by a battery 34. In this manner, an arc 36 is
struck between the two electrode elements 22 and 26.
m e generator's main magnetic field B interact~
with the arc 36 to drive the arc in a circular path around
the cavity 24 as indicated by arrow 38 in FIG. 3. The
arc functions to ionize the gas passing through the cavity
24 between the electrode elements 22 and 26 prior to passage
of the resulting plasma out of the channel 28 and into the
generator's main duct thereby forming a gaseous electrode.
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A significant aspect of the above structure is its
"cathode spot" phenomenon. That is, the natural "running"
tendency of the arc 38 causes it to continuously move from
place to place within the cavity between the two electrodes 22
and 26 -- particularly where the central electrode element 22 is
made of copper. In this respect, the arc sometimes oscillates
or "runs" back and forth within the cavity 24 from one end to
.,
the other. Other times, the arc simply jumps about; and, at
still other times, multiple arcs are struck at various locations
along the cylindrical electrodes. But in each case the cathode
~pot itself acts as a means for causing the arc to move from
place to place along the electrodes. Also, as noted, the
interaction between the arc and the main field "B" causes the
arc to rotate about the central electrode 22.
In the illustrated embodiment, the outer electrode
element 26 is provided with passages 40 for a coolant, to reduce
the structure's temperature. This is conventional, however, and
will not be further described.
Although best results have been obtained when the
central electrode is positively biased as noted above, the device
can also be operated when the arc is struck by means of an AC
source such as 42. Similarly, the channel 28 can be replaced
by a series of ports 44 as illustrated in FIG. 2a, and, the gas
introduced through channel 30 can be seeded with materials having
a low ionization potential in order to improve the performance of
the resulting plasma. That is, the gas can be supplemented with
vapors of seed material s~ch as sodium, potasium, cesium, or
compounds thereof. Additionally, the electrode's performance can
be further improved by operating it with a combustible mixture of
gases so that the gaseous electrode is made from a combustion
system augmented by the electric arc 36. When this modification
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is employed, the resulting plasma is hotter and more conductive
so as to make an effective gaseous electrode.
FIG. 4 illustrates an alternate embodiment of the
FIG. 2 and 3 structure. That is, a gas injector manifold 48
is located near the bottom of the cavity 24 so that the gas
enters the cavity tangentially from passageway 50. Also, the
inner electrode element 22 is located eccentrically within the
cavity so that the ratio of the distances a/b is no more than
about 20/1, less then about 1.5/1, and preferrably about 6/1.
It has also been found that the electrode works well when the
width of slot 52 (corresponding to slot 28 in FIG. 2) is between
about twice to ten times the width of dimension "b". This,
however, is a function of many variables and can be considerably
modified without undesira~le effects.
As shown in FIGS. 5 and 6, the motion of the arc
within the cavity 24 can be further controlled by use of a coil
56 energized by an AC source 58. That is, the AC power creates
an alternating magnetic field which drives the arc 36 at the
AC frequency in an oscillating motion along the direction of the
MHD magnetic field ~. In this respect, the inner electrode ele-
ment 22 in FIG. 6 is illustrated as having a cooling passage 58
which, although not illustrated, can also be included in the -
other embodiments of the invention.
In yet another embodiment (FIG. 7) two opposed elect-
rode elements 60 and 62 are located within a cavity 64 (correspond-
ing to 24 above) and energized by an AC source 66 so that an arc
68 is struck between electrodes 60 and 62. A DC coil 70 is
wound about the electrode element 60 and 62 and housed within
an insulating memker 72. The DC field caused by energization of
the coil 70 by battery 74 interacts with the background magnetic
field "B" of the MHD system to cause the arc 68 to oscillate
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back and forth along the rail-type electrodes 60 and 62. Hence,
when gas is introduced into the cavity 64 from a manifold 76 issu-
ing through a plurality of entry holes,78, the gas is ionized by
the arc, the'~unning" of which causes the chamber 64 to be com-
pletely filled with plasma which then exhausts through the channel
28 into the MHD generator's interior.
While the invention has been particularly shown and
described with reference to preferred embodiments thereof, it
will be understood by those skilled in the art that various
alterations in form and detail may be made therein without
departing from the spirit and scope of the invention. For example,
various types of coolants can be employed, materials and gases
can be other than those described, and, in many cases, the
illustrated polarities can be reversed without affecting the
gist of the invention. Also, it will be appreciated that the
above described structure has many attendant advantages. For
example, the high temperature of the gaseous electrode prevents
coal slag from condensing on the electrode opening and maintains
a highly conductive electrical path for the load current.
