The effect of area ratio variation on the performance of a supersonic Hall MHD duct is investigated. Results indicate that for a given combustion pressure there exists an area ratio below which the power generating region of the duct is shock free and the power output increases linearly with the square of the magnetic field. For area ratios greater than this, a shock forms in the power generating region which moves upstream with increasing magnetic field strength resulting in a less rapid raise in the power output. The shock can be moved downstream by either increasing the combustion pressure of decreasing the exhaust pressure. The influence of these effects upon duct performance is presented

Morgan, J. L.

Smith, J. M.

Wang, S. Y.

English

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Effect of Vacuum Exhaust 
Pressure on the Performance of 
MHD Ducts at High B-Field 
J. Marlin Smith, J. L. Morgan 
and Shih-Ying Wang 
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 
DOE/NASAJ10769-23 
NASA TM-82750 
ASA-TM-82750 19820006035 
L 
MAR 2 6 . ) 
LANGLEY RESEARSH CENT~n 
LIBRARY, NASA 
HAMI-'TON, VIRGINIA 
Under Interagency Agreement DE-AI01-77ET10769 
Twentieth Aerospace Sc iences Meeting 
sponsored by American Institute of Aeronautics 
and Astronautics 
Orlando, Florida, January 11-14, 1982 
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EFFECT OF VACUUM EXHA UST PRE SSURE ON THE PE RFORMANCE OF MHO DUCTS AT HIGH B-FIELD 
J. Marl i n Smith, J. L. Morgan, and Shih-Ying Wang 
NASA Lewis Research Center 
Cleveland , Oh i o 44135 
Abstract 
The effect of area ratio variation on the 
performance of a superson i c Hall MH O duct showed 
that for a given combustion pressure there exists an 
area ratio below whi ch the power generating region 
of the duct is shock free and the power output 
increases linearly with the square of t he magnetic 
field. For area ratios greater than this , a shock 
forms in the power generating region which moves 
upstream with increasing magnetic field strength 
resulting in a less rapid raise in the power 
output. The shock can be moved downstream by either 
increasing the combustion pressure or decreasing the 
exhaust pressure. The influence of these effects 
upon duct performance fs presented in this paper . 
Introduction 
In early experiments in the high magnetic field 
strength, liquid-neoh-cooled cryomagnet facility at 
the Lewis Research Center, a heat sink MHO Hall duct 
was tested at an area ratio of 2.56/1 and then 
rebored and tested at an area ratio of 4/1 . 1- 3 
Besides the resulting increase in power with 
increased area ratio, the significant result has 
been the linear increase in power output with the 
square of the magnetic field. On the basis of these 
results and principally due to the fact that the 
duct was limited by design constraints to a maximum 
area ratio of 4, a new Hall duct was constructed . 
This duct allows a maximum area ratio of 6.25, is 
approximately 20% longer, and has a segmentation 
pitch one-half the size of the original duct. Three 
series of tests have been performec with this duct 
having area ratios of 4, 5, and 6.25, 
respectively. Test data at an area ratio of 4 
agreed with that obtained in the original duct at 
this area ratio. At an area ~atio of 5, the power 
output remained linear with B and the maximum 
power output increased by approximately 30%. 
However, at an area ratio of 6.25, the power no 
longer increased with B2, rising less rapidly at 
higher B-fields. This is the result of the choking 
of the channel which is seen from axial pressure 
profiles to move upstream with increasing B-field . 
In reference 4, the effect of area ratio 
variation on the performance of a supersonic Hall 
MHO duct showed that for a given combustion pressure 
there exists an area ratio below which the power 
generating region of the duct is shock free. In 
this shock-free regime, the power output of the duct 
is found to increase linearly with the square of the 
magnetic field . For area ratios greater than this, 
a shock forms in the power generating region which 
moves upstream with increaSing magnetic field 
strength resulting in a less rapid rise in the power 
output . IncreaSing the combustion pressure moves 
the shock back downstream and results in an increase 
in the performance of the duct up to a point where 
the decrease in electrical conductivity due to the 
increased pressure at the upstream end of the duct 
(fixed inlet area) overcomes the increase in power 
outpu t due to the area ratio increase. 
. The peak performance of the duct fo r this 
series of tests4 occurred at an area ratio of 5/1 
for a combustion pressure of 150 psia . For this 
condition, 180 kW was generated which represents 
approxi mately a 2.5% enthalpy extraction. The 
reason that additional performance was not achieved 
in the 6.25/1 area ratio duct was due to shocks in 
the channel which required a combustion pressure of 
170 psia to remove them. At this pressure, the 
reduction in electrical conductivity was sufficient 
that this operating configuration produced a power 
output somewhat below that obtained with the 5/1 
area ratio duct at a combusti on pressure of 150 
pSia. Therefore. in order to obtain improved 
performance with this duct, removal of the shocks 
without increasing the combustion pressure by 
operation with subatmosp heric exhaust is necessary 
and the results of operation in this mode are the 
topic of this paper. 
MHO Duct Construction 
The original duct used in the 2.56/1 and 4/1 
area ratio experiments is shown on the top half of 
figure 1. It basically consisted of four modules 
each made up of eight circular electrodes clamped 
together with two triangular- shaped end plates by 
three tie rods . The lateral alignment of the module 
was maintained by three fiberglas rods. This 
ten-electrode module has a segmentation pitch of 
1.38 cm. The four modules are bolted together and 
two 2.22 cm triangular end plates are added to 
provide sufficient thickness for bolting to the 
combustor, nozzle and diffuser. The duct therefore 
consisted of 42 electrodes. A picture of this duct 
is shown in figure 2, and other details of the 
design are given in reference 1. 
In the original duct, the area ratio was 
limited to a maximum of 4/1 mainly due to the 
temperature limitation on the fiberglas alignment 
rods and the surface available for the inter-
electrode gaskets . In the new design, this problem 
was overcome by rotating the alignment rods 60 
degrees relative to the tie rod holes, increasing 
the outer diameter of the circular electrodes from 
15 .2 cm to 17.8 cm, and rounding the sides of the 
triangular plates out to the edge of the 17.8 cm 
diameter circular electrodes as shown in the drawing 
on the lower half of figure 1. Also in order to 
reduce the possibility of interelectrode electrical 
breakdown, the segmentation pitch was reduced to 
0 . 74 cm. The duct was then made up of five modules 
consisting of 16 circular electrodes clamped by 
three electrically isolated stainless steel tie 
bolts between two triangular electrodes at each end 
for a total of 20 electrodes/module. The copper 
electrodes are 0.64 cm wide and electrically 
insulated from one another by a high-temperature 
asbestos gasket 0.078 cm thick (to provide the 
pressure seal), sandwiched between two 0.013 cm 
thick sheets of mica (to provide the electrical 
insulation and moisture barrier) . Late ra l movement 
of the electrodes is negated by three fiberglas rods 
inserted through the entire module. Five such 
modules are used in the present experiments with 
~-- --------.=~~------.. ~~~~====----------~~-------------
figures, the shock location is taken from the axial 
pr essure distribution shown in figure 10. In figure 
11, it is seen that for operation without the vacuum 
exhaust that the Hall voltage rises abruptly up to 
the point at which shock occurs and then rises much 
less steeply through the rest of the channel. In 
the case of operation with the vacuum exhaust, the 
shock has moved into the diffuser and no abrupt 
change in slope of the Hall voltage is observed. As 
the B-field is increased to 2.5 tesla (fig. 12), the 
shock moves upstream as does the abrupt slope 
transition point of the Hall voltage. Finally at a 
B-field of 4.5 tesla (fi g. 13), the shock i s driven 
upstream of the upstream power takeoff electrodes, 
and the entire power generating portion of the MHO 
duct is operating subsonically. In this case, no 
abrupt chang~ in the slope of the Hall voltage is 
observed for the run with no vacuum exhaust. 
In figure 14 the effect of vacuum exhaust 
operation at a combustion pressure of 150 pSia is 
shown. In this fiqure t he curve labeled "without 
vacuum" is reproduced from fiqure 5. The circular 
points are the data points fo~ the "with vacuum" 
tests. As previously observed for a combustion 
pressure of 100 psia the effect of operating with 
exahust vacuum is to remove the shocks from the 
channel resulting in a linear power output with the 
square of the B-field. The peak power output in 
these tests was 195 kW, which corresponds to 
approximately 3% of the input enthalpy. 
CONCLUDI NG REMARKS 
Studies of the effect of area ratio variation 
combustion pressure there exi sts an area ratio below 
which the power generating region of the duct is 
shock free. In this shock-free regime, the power 
output of the duct is found to increase linearly 
with the square of the magnetic field. For area 
ratios greater than this, a shock forms in the power 
generating region which moves upstream with 
increasing ma9net ic field strength resulting in a 
less rapid rise in the power output . Increasing the 
combustion pressure moves the shock back downstream 
and results in an increase in the performance of the 
duct up to a po int where the decrease in electrical 
conductivity due to the increased press ure at the 
upstream end of the duct (fixed inlet area) 
overcomes the increase in power output due to the 
area r atio increase. Operation of the system with 
vacuum exhuast moves the shock downstream of the 
power producing region of the duct and again results 
i n a lineor increase i n power output with the square 
of the B-field. 
References 
1. Smith, J . M., "Prel iminary Results in the NASA 
Lewi s H2-02 Combustion MH O Experiment," 
18th Symposium, Engineerinq Aspects 
of Magnetohydrodynamics, Butte, Mon tana , June 
18- 20, 1979, pp. A. 2.1 -A.2. 6. 
2. Smith, J . M., "Results of Duct Area Ratio 
Changes in the NASA Lewis H2- 02 Combustion 
MHO Exper iment," AI AA Paper No. 80-0023, J an . 
1980. 
on the performance of a supersonic Hall MHO duct 3. Smith, J. M., "E xperi me nts on H2- 02 Power 
Generation," Third World Hydrogen Energy 
Co nference, Tokyo, Japan, June 23- 26, 1980. 
have been extended to area ratios of 6.25/1 from the 
previous limit of 4/1 by the design and construction 
of a new duct. It has been shown that for a gj ven 
3 
4. Smith, J . M., Wang, S. Y. , and Morgan, J. L., 
"'iigh B- Field, Large Area Ratio MHO Duct 
Exper iments . " 1981 IEEE International 
Conference on Plasma Science, Santa Fe, New 
Mexico, May 18- 20, 1981 , Paper No . lF6 . 
J 
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Figure L - Comparison of construction between 1. 38 and 0.69 em segmenta tion ducts. 
Figure 2. - 1. 38 Segmentation ratio duct. 
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PEAK B-FIELD tesla 2 
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Figure 5. - Power output versus B-field squared as a function of duct area ratio 
for Pc' 150 psia. 
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t::. 4.0 tesla - RUN 325 
o 5.0 tesla - RUN 245 
40 60 80 
ElECTRODE NUMBER 
Figure 6. - Axial pressure distribution for an AR • 6. 25 and 
Pc' 150 psia as a function of B-field. 
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Figure 9. - Effect of vacuum exhaust upon power output versus B-field 
for AR • 6.25 and Pc' 100 ps ia. 
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Figure 10. - Duct static pressure versus electrode number 
(Pc • 100 psial. 
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Figure 13. - Effect of vacuum exhaust on Hall voltage 
at 4.5 tesla (Pc' 100 psia). 
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Figure 14.- Effect of vacuum exhaust upon pooer output ver-
sus B-field fo r AR : 6.25 and Pc' 150 psia. 
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Effect of vacuum exhaust pressure on the performance of MHD ducts at high B-field

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