Note: Descriptions are shown in the official language in which they were submitted.
1060933
This invention relates to magnetohydrodynamic genera-
tors 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 magne-
tic field. This movement induces an electromotive force between
opposed electrodes within the generator. The rapid motion of
the high temperature gases, however, seriously erodes the genera-
tor's electrodes as do internal electric arcs which connect the
MHD generator's main plasma stream to a load. 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.
Gaseous electrodes have previously been suggested
wherein a~ arc is caused to move from place to place within a
cavity along one or more openings in the electrode, thereby
causing ionized gas to fill the entire cavity and be forced into
the generator's main channel. Such structures have been de-
scribed as employing a cathode spot phenomenon according to which
the MHD magnetic field causes the arc to move about the inner
electrode. It has also been suggested that longitudinal move-
ment of the arc can be controlled by the magnetic field set up
by a coil which is wrapped around the electrode. Such struc-
tures, however, have not necessarily been as simple or reliable
as might sometimes be desired. Hence, it is an object of this
invention to provide a gaseous electrode having controlled ~-
movement of the arc in both circumferential and longitudinal
directions without the requirement of a field-producing coil.
It has also been found that an arc oscillating longi-
tudinally along an inner electrode element of a gaseous electrode
: can cause erosion of the internal electrode element in the area
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1~60933
where the lon~itudinal arc movement is reversed. Consequently,
it is another object of this invention to provide a gaseous
electrode whicll substantially eliminate~ the problem of erosion
on the inner electrode.
In accordance with principles of the invention, an
electrode is positioned at an angle with respect to the MHD
magnetic field, thereby causing the electrode's arc to move in
both circumferential and longitudinal directions. According to
another aspect of the invention, a gas other than the ionized
gas is introduced into the electrode chamber at opposed ends of
the electrode in order to reduce or eliminate erosion.
In accordance with an aspect of the invention there is
provided an improvement in an MHD system of the type having duct
means for passing a plasma therethrough, means for producing a
magnetic field across said duct means, and an electrode assembly
comprising: a first elongated electrode element located adja-
cent said plasma stream, a second elongated electrode element
spaced from said first electrode element, means for passing a
first gas through the space between said first and second elec-
trode elements- exit means for permitting said first gas to
exit from between said first and second electrode elements into
said duct, and, voltage means for striking an arc between said
first and second electrode elements for ionizing said first gas
and electrically connecting said electrode assembly to said
plasma stream: said electrode assembly being positioned such
that the longitudinal axis of at least one of said elongated
electrode elements is at an angle to said magnetic field produced
across said duct means, thereby causing said arc to move from
place to place along the surface of said elongated electrode in
both circumferential and longitudinal directions with respect to
said elongated electrode.
The foregoing and other objects, features, and advan-
,r ~3~
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tages of the invention will be apparent from the following more
particular description of preferred embodiments as illustrated
in the accompanying drawings in which li~e-reference characters
refer to the ~ame part~ throughout the various views. The
drawings are not necessarily to scale, emphasis instead bein~
placed on illustrating principles of the invention.
Figure 1 is a schematic illustration of a Faraday-
type MHD generator having segmented electrodes.
Figure 2 is a schematic pictorial illustration of
an electrode used in the MHD generator of Figure 1.
Figure 3 is a cross-sectional view of Figure 2 taken
along the lines 3-3 thereof.
Figure 4 is a cross-section of Figure 2 taken along
the lines 4-4 thereof.
Flgure S is an illustration of a vectorial analysis -~
of structure according to the present invention.
Figure 6 is a plan view of an electrode wall embodying
the invention.
A conventional MHD generator is comprised of a duct
10 (Figure l) 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 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 electrodes 14 and 16
which are placed in circuit 18 with a load 20.
A suitable magnetic flux, sometimes referred to as a -~
"prime" flux, is represented by an arrow B and placed across the
duct in a direction perpendicular to both the plasma flow 12 and
the EMF to be generated between the electrodes 14 and 16.
The electrode of Figure 2 is comprised of a cylindrical
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1060933
electrode element 22 uniformly spaced by an insulator 23
~Figure 4) within a cylindrical cavity 24 of a surrounding
elongated ~lectrode element 26 provided with passages 27 for a
coolant to reduce the structure's temperature. The upper sur-
face of 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 (Figures 3 and 4) extends
within member 26 and functions to provide a suitable gas --
conventionally an inert gas such as argon -- through passage-
ways 32 into the cavity 24 where it passes around the central
electrode element 22, out of channel 28, and, into the generator
itself. In this respect, the central electrode element 22 is
negatively 'biased with respect to electrode element 26 by a
battery 34. In this manner, an arc 38 is struck between the
two electrode elements 22 and 26, and functions to ionize the
gas passing through the cavity 24 between the electrode elements
22 and 26 pr-or to passage of the resulting plasma out of the
channel 28 and into the generator's main duct thereby forming a
gaseous electrode.
A s-ignifican~ aspect of the above structure is its
"cathode spot" phenomenon. That is, the natural running tend-
~ ency 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 previously -suggested systems, where the magnetic
field vector was parallel to the longitudinal axis of electrode
22, the current density vector of the arc being normal to the
surface of electrode 22, a force acted on the arc accor~ing to
the right-hand rule in a direction tangential to the electrode
surface. This tangential force caused a rotation of the arc
about electrode 22 in a single plane. In order to move the arc
longitudinally along electrode 22, it was suggested that coils
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~060933
be wrapped around the electrode assembly to create a second
magnetic field normal to the first.
The present invention eliminates the need for a second
magnetic field producing means by ~ositioning electrode 22 as
shown in Figure 6 at some angle other than 0 with respect to
the MHD magnetic field.
Figure 5 is an illustration of a vectorial analysis
of the present invention. Letting "X" be the longitudinal axis
of electrode 22, and arbitrarily setting axes "Y" and "Z" normal
10 to axis "X", magnetic field vector "B" is shown in an "Xz" plane
at an angle "a" to the "X" axis. The "X" and "Z" components of
magnetic field "s" are shown as vectors Bx and Bz.
The current density vector "J" of an arc, being at all
times normal to the surface of electrode 22, has components
along the "Y" and "Z" axis only. Hence, a force acts on the
arc in accordance with the left-hand-rule, the magnetic field
Bx and a current density Jz will create a force Fy, that is a
force tangential to the electrode 22; and magnetic field Bz
and a current density Jy will create a force on the arc Fx,
that is a force in the longitudinal direction.
Referring to Figure 5, two points, "E" and "F" have
been chosen as possible locations of an arc to illustrate the
above analysis. At point "E", the current density has a single
component Jz which, with the magnetic field component Bx, creates `;
a force Fc which drives the arc in a circumferential direction. ~ ;
That force is determined by the equation:
Fc = Jz x Bx
At point "F", the current density has a single com-
ponent Jy. According to the left-hand-rule, magnetic field Bz
and current density Jy create a force Fl in the longitudinal
direction. The longitudinal force at this point is
Fi = Jy x Bz
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1~;0933
At the sam~ point "F", magnetic field Bx acts with
current density Jy to create a force Fc
F = J x B
c y x
It should be realized that at any point between "E"
and "F" forces will act on the arc in both the longitudinal and
circumferential directions. As a result of these changing
forces, the cathode spot is caused to follow a path illustrated
by dashed lines in Figure 5. Hence, the arc moves back and forth
along electrode 22 to completely ionize the gas in chamber 24.
It will be realized that longitudinal forces will
exist whenever the màgnetic field vector "B" has components in
the "Y" and/or "Z" directions. Hence, although presently shown
in the X-Z plane, vector "s" may lie in any plane through the
"X" axis.
Figure 6 is a plan view of a series of electrodes
embodying the present invention. The electrodes are shown to
be angles with respect to both the magnetic field and plasma
flow, it being realized that the electrodes could aiternatively
be maintained normal to the plasma flow and angles only with
respect to the magnetic field.
It has been found that at points "E" and "G" (Figure
5) of the cathode spot path, erosion of the electrode 22 may
occur: and according to another aspect of this invention, that
erosion is substantially eliminated by introducing a gas other
than the ionizing gas into either end of the chamber 24.
Accordingly, gas inlets 35 and 36 (Figure 4) are provided.
This second gas is chosen to require a much higher
voltage between electrodes 22 and 26 to maintain an arc there-
between. Generally, diatomic gases serve this purpose, and more
specifically, nitrogen is well suited. As a result of the
higher voltage reguirement, the arc is quenched as it reaches
either end of electrode 22 and restrikes for reverse longitudinal
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1060933
movement, and with the arc quenched at either end, erosion is
substantially eliminated.
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.
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