Experimental data from recent tests of a 45 deg diagonal wall duct are presented and compared with the results of a similar Hall duct. It is shown that while the peak power density of the two devices is approximately equal that the diagonal wall duct produces greater total power output due to its ability to better utilize the available magnetic field

Morgan, J. L.

Smith, J. M.

English

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DOE/NASA/10769-25
NASA TIVI-82864
1
Results and Comparison of
Hall and DW Duct Experiments
(N4SA-'P14 T- 828b4) 	 jtESUL`1S ADD CGMPh ISGN C
HALL AND DW DUCT EXPkBIMLN`iS JNA3A) 14 P
HC: A0.I./Ml A01	 CSCL 201
unc;las
63/75 27953
J. Marlin Smith and J. L. Morgan
National Aeronautics and Space Administration
Lewis Research Center
l
'0 .^^
Work performed for
U.S. DEPARTMENT OF ENERGY
Fossil Energy
y	 Office of Magnetohydrodynarnios
Prepared for
Twentieth Symposium on Engineering
Aspects of Magnetohydrodynamics
Los Angeles, California, June 14-16, 1982
11
DOE/NASA/1 0769-25
NASA TM-82864
I
Results and Comparison of
Hall and DW Duct Experiments
J, Marlin Smith and J, L. Morgan
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Work performed for
U,S, DEPARTMENT OF ENERGY
Fossil Energy
Office of Magnetohydrodynamics
Washington, D.C. 20545
Under Interagency Agreement DE-A 101 -77ET10769
Prepared for
Twentieth Symposium on Engineering
Aspects of Magnetohydrodynamics
Los Angeles, California, June 1416, 1082
11)
RESULTS AND COMPARISON OF HALL AND nW DUCT EXPERIMENTS*
J, Marlin Smith and J, L, Morgan
NASA=Lewis Research Center
Cleveland, Ohio
Abstme1
Rxperimentol data from recent tests of a 45%
diagonal wall duct are presented and compared with
the results of a similar 11411 duct. For the
results obtained here it is shown that while the
peak power density of the two devices is approxi-
mately equal that the diagonal wo l duct produces
greater total power output due tt, its abi ity to
better utilize the available magnetic field,
1. Introduction
In this paper the results of recent tests
utiliZ IN'l a diagonal wall '(Vi) MHD duct with a
diagonaltzation angle of 4b are presented and
compared with the results of a similar ilal l duct,
The experiments wore run in the NASA-Lewis iriglr
magnetic field stronth, liquid-neon-cooled
cry+amagnot facility ?ref, 1), which has been
modified co run under vacuum exhaust through the
addition of a large vacuum tank (ref, 2),
This tank allows evacuation of the system to
approximately 2 psia and has sufficient capacity
to nlailltain ali exhaust pressure of loss than 11 2
atmosphere during tho duration of a 5 sec run.
t i, Duct Construction
Halms
The heat sink hall duct has been described
previously (ref, 2). Briefly, it is made up of 6
modules eirch consisting of 16 circular electrodes
clamped IV. 3 electrically isolated stainless
steel tie bolts between L,
 triangular electrodes
at each end for a total of 20 electrodes/n;odule.
Tile c 
011 
,r electrodes are ,64 cm wide and
electrically insulated from one another by a `nigh
temperature asbestos gasket ,078 cm thick (to
provide the pressure seat), sandwiched between
two .013 cm thick sheets of mica (to provide the
electrical insulation and moisturo barrior),
Lateral movement of tho electrodes is negated by
three fiberglass rods inserted throughthe entire
module. Five with modules are used In the
} resent experiments with t,22 cm thick end
flanges for A total of 102 electrodes having A
total length of a pproximately 78 cm. The duct is
conicaliy bored from A 4.95 cm inlet diameter to
A 1 2. 4 cm outlet diameter,
INS Duct
The construction of the 45" DW duct is
Pill losophically the smite as the Ball duet, i.e.,
modules clamped with stainless roots and shear
movement negaturd by fiberglass rods. The duct
consists of 4 moduilns of 11 electrodes each. The
thickness of the electrodes is 1.25 cm so that
the segmentation pitch is approximately twice
that of the Hall duct, The OW duct was also
restricted by design11o ►itatioAls to a somewhat
smaller overall area ratio than the Null duct and
therefore was conically bored from a 4.95 cm inlet
diameter to a 9491 em diameter at a distance of
57, 8 cm from the entrance, Tiro remainder of the
duct was kept at this dimitter, Yho total len g th
of the duct was 80,2 cm and it was located within
the magnet bore tube so as to best approximate
the area ratio of the Hall duct over the active
region of the I1411 duct. The location of the two
ducts relative to the centerline of the lilagnet
and their internal outlines are shown in figure 1.
III, Powe r Takeoff Location
It is noted from figure I that the power pro-
ducing region for the OW duct is considerably
larger titan that of the Hall duct " This is the
result of the maximum power location of the power
loads for the single load resistance configuration
used in then experiments, In figure 2 the axial
profile of Hall voltage As a function ofdistance
from the magnet centorline is shown, In both
curves the load was connected across the first
and last electrodes, This power takeoff location
loads to regions of power dissipation, as rioted
by "lie negative', slope of the voltage curve At the
front and rear of the ducts, Therofore to obtain
the maximum power output with A single load, the
Pam,
 takeoffs must be located at the front and
rear min-max points of the voltage curve, From
figure 2 it is seen that these points cover a
mt4i larger portion of the magnetic field region
for the DW duct than for the Nall duct.
The roirson forthe above difference is soon
from the equation far the Hall electric field(ref. 3)
1 N . n^	 1 ° ,0 * $ uR
Ex eA(I+dr)	
+0,	 O
where A is the cross sectional area of the duct
acrd a is the local gas velocity, 0 is the
magnetic fiefs! intensity, o is the electrical
conductivity, N is the Ball parameter, A is the
dimensionless effective voltage drop, I is the
load curront and 0 is the tangent of angle
between the direction of uxB and the plane of the
elgrtrode, i,e., 0 x 0 for 1fia11 duct and 1 for
46 OW duct, Therefore,
E	 M 1^ l^ I«ti(1^A)u8
xHall ^^^"^	 (2)
2
ExDW
	
2	 0A1 n (I + il)(I « A)uD	 (3)
WRK bne under DOEINASA Interagency Agreement No. DE»AIOI-77CT10769,
I,r	 ,
E;
t;
1i
The min-max point occurs for for Ex w 0. Com-
paring equations 2 and 3, it is Seim that the
(1 + o) multiplier of the EMF term for the OW duct
is greater than the p multiplier for the Hall
duct. Therefore, other things being .equal ,the
inflection joint for the DW duct will occur at a
lower 8 field than for the Hall duct. Hence a
greater
 used byrthenDW fduc►t witheaisingled region canl
resistance.
Based upon the above considerations, the load
resistance was connected between electrodes 12,
13, 14 to electrodes 42, 43, 44 for the DW duct
and between electrodes 38, 39, 40 to electrodes
90, 91, 92 for the Hall duct,
IV. Load Considerations and Power Output
Figure 3 shows the voltage-current relation-
ships for the Hall and DW duct. The slope of
these VI curves, of course, represent the total
internal impedance of the device. From these
°urves it is found that the internal impedance is
19.8 ohm and 29.9 ohm for the DW duct and the
Hall duct, respectively. This difference in
impedance can be shown to be consistent with
theory. The theoretical value of the impedance
from equation 1 is taken from the first term on
the right of equation 1 to be
	
1	 L .
R
 - rj—j2^o 
	
d 	 (4)
where L is the active length of the duct.
To first order the plasma properties of the two
devices are equal (low power extraction in both
devices i.e., max. enthalpy extraction c, 3.5
percent and therefore from equation 4 their
impedances should ratio approximately like
RHalI M 
Lila II W 2 37.6 cm) „ 1.45
cm	 (5)
where the active lengths were obtained from
figure 1. This result compares favorable with
that obtained from the V-I curves, i.e.,
	
RHal1	 29.9 ohm	
1.51
	
^0W	 ohm
In the discussion to follow, All of the data
was taken with a load resix.tznce of 11.5 ohm while
for peak power output one wants to match the
internal impedance. The reason for the choice of
11.5 ohm was the present voltage isolation limita-
tion upon some of the instrumentation which would
be exceeded atthe upper limits of the magnetic
field strength (5 testa). This clearly favors the
performance of the DW duct since it is a lower
impedance device.
In figure 4 the power output of the two devices
is shown as a function of the square of the
11-field. It is seen, as has been previously
observed over a broad range of Hall duct area
ratios (ref. 4), that the power output of the Hall
duct is linear with the square of the magnetic
field. This is also true of the DW duct Gtr to
approximatel y 4 tesla beyond which the rate if
increase in power output with the square of the
magnetic field dec rpeases. This characteristic
was also observed tor-Hall ducts when the pres-
sure ratio across the duct was insufficient to
provide shock free ;performance throughout the
duct (ref. 4). For the circled points of the DW
duct curve the scatter in the pressure data was
sufficient that the presence of a shock could not
be detected. However, for the square point
labeled "Run 564” (the 4 square points on figure 4
represents runs made at the fuel rich oxygen/fuel
weight ratio of 611) the presence of a shock was
detected.
In figure 5 the time dependence of the Hall
voltage is plotted for Run 564. It is seen that
at a time of between 3.2-3.4 sec there is an
abrupt decrease in the Hall voltage. In figure 6
the axial pressure profile is plotted. The curve
drawn through the circled points represents the
profile Just prior to the Nall voltage decrease.
In this case no abrupt increase in pressure is
observed although the adverse pressure gradient
at the rear of the duct could possitly indicate a
weak shock. The square points represent the
pressure profile after the Hall voltage decrease
and definitely indicates the presence of a shock
in the neighborhood of electrode 24. This can
also be seen from the Hall voltage axial profile
shown in figure 7. In this figure shock location
corresponds to an "abrupt" change in the slope of
the voltage profile in the neighborhood of
electrode 24.
The time dependency for the creation of the
Shock is due to the buildup of back pressure in
the vacuum tank during the run (the vacuum pump
is too small to maintain vacuum pressure while
the combustor is running). In the early stages
of the run, it is suspected that the shock is
located at the exit of the diffuser. When the
back pressure increases to a point where there is
insufficient pressure ratio across the duct then
the shock moves through the constant area diffuser
and downstream portion of the DW duct into the
conical portion of the duct (upstream of electrode
33) where it can stably attach for the remainder
of the run.
The point labled "Run 564" on figure 4 is the
value prior to the shock induced decrease in the
Hall voltage and represents the largest power
extraction yet attained in our facility repre-
senting 275 kW which is 3.5 percent of the input
enthalpy . Also from figure it is noted that the
voltage sustained without electrical breakdown
was 90 V/insulator. This represents a substan-
tial increase over the 50 V/insulator value
observed for a Hall duct with similar segmenta-
tion ratio (ref. 1). The Halluc`ucctdiscussed in
the present paper has a segmentation ratio 1/2
that of the DW duct and no electrical breakdown
has been observed in this duct.
In figure 8 the axial profile ofpower density
for the Hall and DW duct is shown. The interest-
ing point is that the peak power densities are
nearly equal and therefore the higher power out-
g
ut of the OW ct.iannel is primarily due to its
better utilization of the magnetic field in the
end regions.
(6)
I-
V.	 Concluding Remarks
Specific conclusions that m	 be drawn from
the comparison o" the OW and Hall ducts studied
in this paper is that;
1.	 Singly loaded DW ducts can utilize more of
the magnetic field region than can Hall
ducts.
2.	 Hall ducts are higher Impedance devices.
3.	 Although interelectrode electrical break-
down was not specifically studied for the
DW duct voltages of 90 V/insulator were
obtained with no breakdown.	 This is sub-
stantiolly higher than the 50 V/insulator
which was pt ,eviously observed for a Hall
duct of similar segmentation ratio.
References
1. Smith, J. M.:
	 Preliminary Results in the NASA
Lewis 142-02 Combustion MHD Experiment.
Proceedings of the 18th Symposium on the
Engineering Aspects of Magnetohydrodynamics,
Butte, Montana, June 1979, pp. A.2.1-A.2.6.
2, Smith ► 	 J. M.; Morgan, J.	 L,;	 and Wang, S. Y.;
Effects of Vacuum Exhaust Pressure on the
Performance of MHD Ducts at High 8-Field.
AIAA Paper 82-0396, Jan. 1982.
3, Wu ; Y, Cs L.., 	Perfotinance Tbeory of Diagonal
Conducting Wall MHD Venerators.	 AIAA, vol.
14, no. 10, Oct. 1976, pp. 1362-1368.
4. Smith,	 J. M.; W09, S. Y,; and Morgan, J. L.;
High 8-Field, Lane Area Ratio MHD Duct
Experiments.	 1981 IEEE International
Conference on Plasma Science, Santa Fe, New
Mexico, Paper No.	 1F6, May 1981.
f
3
mfi
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ORIGINAL PAGE 13
OF POOR QUALITY
___... HALL DUCT
	
POWER PRODUCING
REGION
-------- DW DUCT
	 HALL DUCT - 6.25IX AR
f
l
POWER PRODUCING REGION - DW DUCT
I	 I	 I	 I	 I	 I_
-30	 -20	 -10	 0	 10	 20	 30 CM
t MAGNET
Figure 1. - Interior crossections of Hall and DW duct,
800
600
0 DW DUCT - RUN 526
	
400	 q HAIL DUCT - RUN 217
200
I	 0
°200
-400
-600
-800
-1 50	 -40	 -30	 -20	 -10	 0	 10	 20	 30
DISTANCE FROM MAGNET CENTERLINE, cm
Figure 2, - Hall voltage profiles for determining power takeoff
location,
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Results and comparison of Hall and DW duct experiments

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