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Submitted on 1 Jan 1979
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THE ARC DIAMETER AND RATE OF ROTATION OF A MAGNETICALLY ROTATED ARC WITH
SUPERIMPOSED GAS FLOW
K. Bunting
To cite this version:
K. Bunting. THE ARC DIAMETER AND RATE OF ROTATION OF A MAGNETICALLY RO-
TATED ARC WITH SUPERIMPOSED GAS FLOW. Journal de Physique Colloques, 1979, 40 (C7),
pp.C7-243-C7-244. �10.1051/jphyscol:19797119�. �jpa-00219090�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i Z Z e t 1979, page C7- 243
THE ARC DIAMETER AND RATE OF ROTATION ff A MAGNETICALLY ROTATED ARC WITH SUPEFUMPOSED GAS FLOW
K.A. Bunting.
The E Z e e t r i c i t y C o u n e i Z R e s e a r c h C e n t r e , Capenhursc, Nr. C h e s t e r .
~ b s i r a ~ t --
3. Results A magnetically rotated a r c heater producing a uniformly
heated and highly turbulent hydrogen plasma i s briefly described. T h e effect of nozzle geometry and para- meters affecting the diameter of the a r c and its rate of rotation a r e discussed and some results a r e presented.
It i a shown that steady rotation of the a r c can occur when the externally applied magnetic field is zero. The drag coefficierrt .was determined.
A magnetically rotated a r c heater with hydrogen a s the
plasma working fluid has been developed for use in ffi 7 SCnEWx m HElE.
1- 01 -,I" am arc mtatma ra,e arc o.cm,
I
chemical processing(l,2). A highly turbulent plasma i s necessary to obtain good mixing of the reactants and, in order to satisfactorily control the rate and direction of the chemical reactions, a uniformly heated plasma is required. Both the level of turbulence and the uniformity of heating(3) depend upon the rate of rotation of the a r c and the a r c diameter. It has been shown (1) that cathode erosion is negligible and that anode lifetime is depend- ent upon the rotation rate; a high a r c rotation rate being required to give an acceptable lifetime. Thus, the effect of a r c current, gas flow rate, cathode setback, cathode shape, nozzle geometry and magnetic flux densitv unon the rate of rotation of the a r c have been - - -- - - -
.,
a- - -investigated. The a r c diameter, velocity, anode root Fig. 3 Pholographs of arc, DOA, 160V. 20 I/min. hydrog:.,~.
velocity and the drag coefficient have also been deter- 200,000 p. p. s., B = 0.0026 Tesla, d/A = 1 0 mm ,
1 = 331nm. s = 2 8 mm, 0 = 40°.
mined.
2. Apparatus and procedure
The a r c heater (Figure 1) consisted of a non-transferred.
d. c. plasma torch mounted on the axis of a magnetic field coil. The torch had a cylindrical outer jacket which terminated in a water cooled nozzle (anode). A water cooled rod, tipped with thoriated tungsten served a s the cathode. The plasma working fluid was heated by passage through an a r c generated between the electrodes which was rotated by means of a magnetic field The gas was introduced tangentially to the plenum chamber s o that the direction of swirl was in the same sense a s the rotation produced by the externally applied magnetic field. The a r c rotation rate was measured using differential magnetic search
oils(''^),
the resonant frequency of which was -3 20 Mha The probe output was monitored osciiloscopically and on a frequency counter.A mirror immersed in the plasma and screened with a jet of argon was used to take high speed cine photo- graphs of the arc. The canbination of camera aperture and '@tical filters was chosen s o that. the diameter of the a r c recorded photographically was not reduced by further reductions in the amount of light collected Synchronised photographs of the a r c and the probe out- put were obtained using a pulse circuit to trigger the camera and a pre-framing pulse from the camera to
trigger the oscilloscope.
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797119
4. Discussion
It can be seen fro.4 Figure 2 that increasing the a r c ' velocity and rotatian rate praduced an increase in the convective losses from the a r c and thus a reduced a r c diameter. Similarly, in- creasing the gas flow rate or decreasing the nozzle diameter increased the convective losses and reduced the a r c diameter. A s the a r c current increased the diameter increased approximately a s
1~1~.
The low vis- cosity and density in hydrogen lead to high convective losses a s compared to argon. For example, average values of a r c diameter in the range 0.65mm-l.lmm were measured for I = 90A and 20 I/min. of hydrogen compared t o a typical diameter of 2.5mm under similar conditions in argon. The diameter was found to be in- sensitive to the applied magnetic field.Frame by frame analysis of the high speed cine films (an example is given in Figure 3) confirmed the magnetic probe readings. The rate of rotation of the a r c varied from one revolution to the next but the rate, averaged over a period long compared with the period of revol- ution, was found to be constant. The rate of rotation increased with increasing gas flow rate (Figure 4).
This i s attributed to the increased thermal pinch, produced by the increased gas flow, which gives a narrower a r c that is subject to smaller aerodynamic drag opposing rotation.
The rotation rate in hydrogen a s a function of magnetic flux density i s compared with that in argon in Figure 5.
The rotation rate (na increased with ihcreasing mag- netic flux density (B Tesk) and for hydrogen n(kHz) = 35 + 5400B when 0
<
B 40.003 Tesla and n(kHz) = 56 + 62B when B> 0.02 Tesla witha transition region in between. The curve for argon e-xhibited a similar step but the transition region extended over a greater range of B, the slopes were smaller and the rate of rotation was approximately an order of magnitude less. The rate of rotation decreased with increasing nozzle dia- meter (Figure 6) and increased with increasing throat length ( N gure 7). It was approximately independent of nozzle angle, setback and the angle of the cathode tip.However, for an a r c current d lOOA and 0.058 Tesla (other conditions a s for Figure 5), the rotation rate increased from 53 to 63 kHz a s the radius d curvature of the cathode tip was reduced from 6mm to 4mm. The drag coefficient for the a r c for the range of Reynolds number investigated (640-1530) was found
tb
be 0.95 and was in good agreement with results published for other arcs(5.6. 798).The very high rates of rotation obtained in hydrogen were such a s t o give a uniformly heated plasma despite the small a r c diameter(l1 and a highly turbulent plasma.
The latter was demonstrated by injecting fine carbon particles into the flame.
this effect, which was not observed in argon, is thought to originate in a spatiaL a r c instability, the growth d which i s encouraged by the resulting self-field. Depend- ing upon the swirl direction, the spatial instability may be enhanced or suppressed by the swirl. The effect was strong enough in hydrogen to produce steady rotat- ion rates a s high a s 35 kHz for zero applied field and is the explanation of the rapid initial increase in rotat- ion rate at low magnetic flux density
-
the higher aero- dynamic drag in argon resulting from the hightar density viscosity and diameter suppresses the effect.5. Conclusions
Very high rotation rates ( 80 kHz for B = 0.08 Tesla) have been obtained in hydrogen giving a uniformly heated and highly turbulent plasma and long electrode lifetimes. It is possible to obtain a steady and high rate of rotation in hydrogen with no externally applied magnetic field.
6. References
(1) Bunting, K. A.,'%n a r c heater with hydrogen a s the plasma working fluid,!' E. C. R. C. M554, 1972.
(2) Bunting, K. A.
,
"Magnetically rotated a r c s with superimposed gas flow, " E. C. R. C. M1229, 1979.(3) Chen, D. C. C. and Lawton, J. "Discharge heatern based on magnetically rotated arcs, f l Trans. Inst.
Chem. Eng., B46, No. 9, Dec. 1968.
(4) Humphreys, J. F. and Lawton, J. "Heat transfer from magnetically rotated a r c plasma,
"
E. C. R. C.M317, 1970.
(5) Roman, W. C. and Myers, T. W. A. R. L. 66-0191, Oct. 1966.
(6) Winograd, Y. Y. and Klein, J. F. A. L A. A. J., V7, No. 9, pp 1699-1703, Sept. 1969.
(7) Adams, V. W. Aeronautical Research Council Current Papers No. 743, Parts I and 11, 1964.
(8) Nicolai, L. M. and Keuther, A. M. TpProperties of magnetically balanced arcs,
"
Phys. Fluids. V12, NO. 10, pp 2072-2082, Oct., 1969.7. Acknowledgement
The author wishes to thank Dr A. T. Churchman, Director of The Eleatricity Council Basearch Centre, mpenhurst, for permission to publish this work.
It was noted that the direction of rotation in hydrogen could be with o r against the direction of swirl for magnetic flux densities
<
0.01 Tesla applied in the sense which produce rotation in the swirl direction.The self magnetic field of the a r c is not negligible and
