spECTROSCOPIC AND THERMOMETRIC STUDIES OF CHARGED pARTICLE ENERGY EXCHANGES AROUND HIGH POTENTIAL

ELECTRODES IN FLAME GASES

D E R E K BRADLEY AND SAID M. A. I B R A R I M

Deparbment of Mechanical Engineering, The University, Leeds, England

The emission intensities induced in cimmi-ionised gases around a small spherical electrode have been measured spectroseopic~ly and compared. This Was done with the electrode in the reaction zone of a stoichiometric methane-air flame at low pressure and in other downstream positions. The range of electrode potentials was -650 to -~2000 V. In the reaction zone the electrode voltage had but a small effect upon emissions from excited OII and CH, but a large effect upon emissions from excited N~. In the former case, chemi-excitation predominates, in the latter case field excitation predominates.

There are also differences between anode and cathode emissions. These might be due to electron excitation at the anode and ion excitation at the cathode. A small thermocouple was positioned at different distances from the electrode and gas heathLg effects due to the electrical field were measured. These were most marked at a cathode and were negligible at an anode. The gas lmating increased as the cathode surface was approached. These results are more in ac- cordance with ion heating than with electron heating. I t was possible to fuse the cathode, at a temperature of 2200~ and with a free flame gas temperature of 1700~ by the application of a sufficiently high negative voltage.

The application of negative potential might be a practically effective way of enhancing heat transfer rates at a solid surface. Electrical augmentation of the flame gases occurs in the elec- trostatic boundary layer of the surface and the extra energy supply is localised, where it is most effective.

I, Introduct ion

Energy transfer to solid surfaces from both ac and dc electrically augmented flames has been studied by a number of workers. ~ The electrical discharge impar ts energy to the gas along com- paratively thin arc channels and increased heat transfer rates occur as a result of the increase in gas temperature and dissociation2 '~ Heat t ransfer to a surface also m a y be enhanced by ionic wind effects which are dependent upon ionisation and electrical fields, bu t not necessarily upon electri- cal breakdown, a Energy t ransfer processes in the gas phase, resulting f rom the acceleration of charged particles by electrical fields and their interaction wi th neutral species, are less well understood, despite a number of spectroscopic

studies. In Refs. 5 and 6 emissions were measured when dc fieMs were applied to diffusion flames, while in Refs. 7 and 8 high frequency fields were applied to premixed flarnes.

The purpose of the present paper is to a t t e mp t an explanation of spectroscopic measurements of flame gas emissions and of energy transfer to electrodes hi t e rms of energy exchanges that in- volve both electrons and positive ions? Both anode and cathode glows have been studied, bu t an important differencc f rom more conventional gaseous discharge studies was tha t electrical breakdown was avoided. Ionization a r o ~ pre- dominant ly from flame ehemi-ionization and thus the flame was not completely almlogous to the positive column. A small spherical electrode was traversed through the gases of a premixed

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ELECTRICAL PROPERTIES

/BURNER TUBE

flame and good spatial resolution was obtained of the dc field-induced emissions around the elec- trode, for different electrode positions.

2. Apparatus

The experiments were conducted at a pressure of 44 Torr, with a premixed stoichiometric ~0. methane-air laminar flame on a 3 in diam flat- flame burner that has been described previously. ~~ The burner and electrode system is shown dia~ grammatically in Fig. 1. The burner tube could ~0~ be moved vertically within a fixed combustion chamber while electrodes and thei~mcouples could be traversed radially through housings fastened to side flanges of the combustion chum- ~= ber. This, together with the burner tube and its ~ 0~ matrix, were electrically grounded and the elec- o trical potential was applied between them and a N TM

0.508 mm diam spherical electrode of Pt-40% rhodium using a V.G. Electronics M5K2 power supply. This gave a stable output voltage, adjust- o 0,2 able in increments of 10 V in the range 0 to z ~ 5 kV. The electrode current was measured with a Hewlett Packard 425A microvoltammeter in the high voltage line. Leakage currents were 0 found to be less than 20 pA on open circuit test in air, but with the meter in the grounded line they could be five orders of magnitude greater. The former arrangement also gave a lower noise level.

Relative emission intensities were obtained from the gases between the electrode surface and a radius of up to 26 mm using a Hilger-Engis Monospek 1000 monochromator and an EMI photomultiplier type No. 6256S. The emitted radiation passed horizontally through a quart~ window in the walls of the combustion chamber through two collimating slits, two lenses a~d a chopper disc into the monoehromator. The wave- length intensities measured, with the associated species, are indicated on Fig. 2. No radiation from the electrode could reach the spectrometer.

Gas temperatures were obtained from the t~eadings of silica coated Pt`20%Rh v Pt-40eTvRh thermocouples of 0.001 in diam and 5 mm length. These were traversed radially with the thermo- couple wire horizontal. Temperatures at diffcrent radial distances from the electrode were obtained by measuring the emf on the microvoltammetcr. This was completely floating, in order to give minimal electrical interference from the thermo-

POSITIVE VOLTAGE ,(KV) Fro. 2. Variation of electrode current and emis-

sion intensities with positive pote:~tiM. Electrode height =46.3 nml.

CHARGED PARTICLE ENERGY

couple circuit with the electrical field around the electrode.

Concentrations of chemi-ions and electron temperatures were obtained by using the double spherical probe technique and the theory of Su and Lain, as described previously. ~~ The probe diameters were 0.508 mm and the positive ion collision cross section was taken to be 1X 10 -~ em z, in line with the experimeotal value of mobil- ity obtained by the authors. ~z

3. E x p e r i m e n t a l Resu l t s

The spherical electrode was positioI~ed at a distance of 35 mm from the axis of the combus- tion chamber. The variations of elcetrode current with applied voltage, of both polarities, and with the electrode 46.3 mm above the burner matrix, downstream of the reaction zone, are shown by the broken curves in Figs. 2 and 3. When the electrode potential was positive, the electron cnrrent to tile electrode was limited by the posi- tive ion current to the grom~d~'l matrix. After steady increase of current with voltage, the cur- rent-voltage characteristic exhibited a regime in which there was a smaller rate of increase of

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NEGATIVE VOLTAGE F1o. 3. Variation of electrode current and emis-

sion intensities with negative potential. Electrode height = 46.3 mm.

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FiG. 4. Variation of emission from OH and N=, at electrode voltages of --600, 0, +1000, with elec- trode height. Ion number density in the flame gas is also shown.

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current. Tlds regime is termed the plateau region, and at its upper limit a further increase in voltage resulted in a sharp increase in current prior to sparking, as showal in Fig. 2. Only conditions prior to sparking were investigated throughout the present work. Plateau currents, at a given positive voltage, were invariant with electrode height, as a result of positive ion current satura- tion at the matrix. The voltage at which the plateau commenced increased with electrode height, ranging from 200 V at a height of 3.2 mm to 450 V at a height of 58.2 ram. When the elec- trode potential was negative, the current progres- sively increased with voltage and there was no plateau region, in this case, the electrode current was that due to ion saturation at the electrode and, at a givcn negative voltage, it varied with electrode height in the same way as the chemi-ion number density, shown in I,'ig. 4.

The variations of emission ia~tensities from the gases an)und the electrode with electrode poten- tial, of both signs and for the same electrode position, are shown in Figs. 2 and 3. The in- tensities have been normalized to the intensity of the emission at 3371.3/~ for a positive voltage of 1000. The normalized variations of emission intensities from OH and N2 with electrode height, for voltages of --6(}0, zero and ~ 1000, are shown

1026 ELECTRICAL PROPERTIES

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FJo. 5. Electron heating and ratio of gas tempera- tore with field to that with no field. Eiectrode voltage=-600, for curve A and -500 for curve B, electrode height =33 turn.

in Fig. 4. Figures 2 and 3 show that emissions were stronger for a negative voltage of 500 than for a positive voltage of 1000, although the elec- trode current was greater in the latter case. To the naked eye, the glow around the cathode was much brighter and of greater diameter than that around the anode, even at 1600 V, and its in- tensity could overwhelm that from the flame itself. V~q~en the electrode w~s placed in the position of peak cheml-ionization, spark break- down between electrode and burner matrix oc- curred at +2000 V and --720 V.

I t was observed that an increase in cathode voltage could eventually fuse the electrode, even though the surromlding gas temperature was 1700~ and the fusing temperature was 2200~ It was never, however, possible to fuse a~ anode prior to breakdown. The gas temperature varia- tion with radial distance from the electrode was measured by traversing a thermocouple hori- zontally. Some results are shown by curve A in Fig. 5, for an electrode height of 33 mm and a negative voltage of 600, Curve A gives the ratio of the gas temperature at a point in the presence

of the applied field, to that at the same point but o with no applied voltage. Great care ha~t to be -J exercised to avoid both fusion of the cathode and ~_ contact between electrode mid thermocouple.

The gas heating effect varied with electrode ~ o height and was greater in regions of greater ion

z concentration. No increase in gas temperature was noted around an anode, even in the reaction zone, for an applied potential of +1400 V. ~D

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4. D i s c u s s i o n

4.1. Chemiluminescence, Chemi-Ionizallon~ and Field Emissions

The variations of emission intensities with height shown in Fig. 4 reveal interesting dif- ferences between the transient species OH, created in the reaction zone, and the permanent species N2, the fractional concentration of which changes but little. Within the spectroscopic field of view, in the region of the flame where the ion concentration, n+, is a maximum, the negligible effect of electrode potential upon OH emission indicates that the excitation of the upper energy level must be predominantly by ehemi-excitation, even when the field is quite large. On the other hand, the pronounced effect of electrode potential upon N~ emission indicates that the excitation of this upper level is predominantly by field excita- tion.

Not all of the measured emissions from the transient species in the reaction zone were un- affected by the potential of the electrode. With a cathode potential of --600 V the C~ and CHO emissions were more than doubled. On the other hand, CH emission was closer to that of OH in its behavior. The cathode field excitation of N2 gave emissions approximately proportional to n~, and ionic current whereas the anexte field excitation varied less with height, most probably because the electron current remained ahnost constant as a result of ion saturation at the matrix.

I t is interesting to compare the mechanisms for the field induced emissions with those that obtain hi a glow discharge tube. Whereas in a hydro- carbon flame the ionization is chemical in origin, in a discharge tube it is created by the field. In a flame, in contradistinction to a discharge tube, as a result of chemi-ionization field excitations can occur at fields less than those for the thresh- olds of field ionization. Between flames and dis- charge tubes there can be differences in the mechanisms that lead to anode and negative glows. In a discharge tube both glows arise from excitation by electron collision; at the anode the electrons achieve sufficient energy as they accet-

CHARGED PARTICLE ENERGY

crate towards the electrode while at the cathode the exciting electrons are those emitted from the electrode surface by energetic positive ions. In a flame, as a con~quenec of ehemi-ionization, glows will occur at smaller fields, at which there may be little ion induced electron emission from the c~thode.

In the present studies it is noteworthy that Figs. 2 and 3 demonstrate differences in the voltage order of field excitation for the diffcrent emissions at the anode and cathode. The CO and C~ emissions are scarcely affected by positive potential but are more readily excited by nega- tive potential, while the Ne errdssion seems to be more readily excited by positive potential. This suggests differences at the two electrodes in the excitation mechanisms. Possibly excitations occur by electron collision around the anode and by positive ion collision around the cathode. The implication t,hat there are no significant effects due to cathodic electron emissions is supported by theoretical calculations.

The electron emission depends upon both the impacting ion energy and the nature of the c~thode surface. The electrical field and ion con- centration around the cathode were found from the theory of Su and Lain. u The mean ion energy was calculated by the method adopted for elec- trons but ass~mdng a Maxwellian ion energy distribution? This involved balancing the elmrged partle]e energy gain from the field with the loss in collisions with neutral species. I t was assumed that the ions nmved to the cathode through a series of energetic steady states and that they lost energy only in elastic collisions. Both assump- tions lead to overestimation of the ion energy at the surface, and hence of electrode emission. The work ftmction for the electrode material was taken as 5 eV. The data of MeDanieP 4 on electron yields for mixtures of hydrogen ions on platinum covered with hydrogen molecules were used to form some estimate of the electron emission. I t was estimated that at -- 600 V the current would be increased only by approximately one per cent by ion bombardment. This amount would in- crease with increase in the field. The field emission current from the cathode at 2000~ also was estimated, using the Schottky equation and this too wns relatively small. Although these estimates of electron emission are inevitably very approxi- mate, they indicate that it does not occur on the scale witnessed in discharge tubes.

1027

4.2. Gas Heating Around the Electrodes

There still rercmins the possibility that, even without electron emi~sinn from the cathode sur- face, electrons repelled away from the cathode by

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charge number densities and volumetric excitation rates of 3.1 eV level of CO2 around cathode. Broken curves for electrode potential of -500 V, full curves for electrode potential of -600 V. Gas tempera- lure= 1700~ pressure ffi38 Torr.

the field might be accelerated to energies sufficient to explain both the observed he~ting of tim g~s and the increased emission intensities. This possi- bility also was examined theoretically.

Again the theory of Su and Lain was used. Theoretical profiles around the cathode of elec- trical field, E, ion, n+, and electron, n,, mmlber densities, for cathode potentials of --500 V and --600 V are shown in Fig. 6. The ion and elec- tron numher density remote from the cathode were taken as 10 t~ em ~ , the gas temperature 1700~ and the pressure 38 Tort. The volumetric rate of excitation R/(~.m of species "j" by electron collision from a lower energy level "x" to a higher level "y" is given by g,

R.i(~ m) = kZ~,~)Nzz)n, ,

where n~ is the electron number density, and Nj(=) is the number density of species "j" in the

1028 ELECTRICAL PROPERTIES

"x" energy level. Some valnes of the rate con- stants k~l~,~) as a function of the mean electron energT derived from values of cross sections are given in Ref. 9.

Close to the electrode tlle field is high and the rate constant for the inelastic electron collisions is consequently also high. But becmisc the elec- tron number density, n,, is low the volumetric rate of excitation is low. Some distance away, where n, is high, the field and the associated rate constant are low. Somewhere between these limits the rate of excitation is a maximum and this is shown by the two theoretical excitation rate profiles in Fig. 6. The rate of excitation of the 3.1 eV level of CO~ (level 5 of Ref. 9) is shown for cathode voltages of --500 V and --600 V and a gas temperature of 1700~ at 38 Torr. The curves are typical of those ibr other energy levels in that at, increase in voltage has little effect on the peak rate of excitation, hut it moves the position of this peak radially outwards.

A theoretical estimate of the electron heating of the gas by electron collisions around the cathode was made by assuming that at each radius the energy gained by electrons from the field was equal to that lost by them in inelastic collisions. The latter was found from a summation of tile energy exchange rates for all the different energy levels for all the different species, accord- ing to the procedures previously developed by the authors) The total volumetric rates of gas heat- ing, summed over all energy levels, for the condi- tions appropriate to the measured gas tempera- tures of Fig. 5 but for a cathode voltage of --500 V, are shown by curve 13 in that figure. Compari~m of curves A and B shows that tim measured increases hi gas temperature do not correspond with those to be expected from elec- tron collision theory in that the former were greater nearer to the surface while the latter gave greatest heating semc distance away. With a cathode voltage of --600 V, the maximum of the electron energy loss curve occurred at a radial distance greater than 12 mm. Furthermore, the experhnentally measured electrical power input at the cathode was two orders of magnitude greater than tile theoretical amount to be expected by a volume integration of curve B.

Reference to Fig. 6 shmvs that close to the cathode the field is high and n+ is much greater than n, in the ion sheath region. If a theoretical curve of the ion heating of the gas by ion colli- sions around the electrode were to be constructed, it would be more in harmony with the observed increases in gas temperature than is curve B hi Fig. 5.

Further evidence of the importance of ion heating of the gas around the cathode is obtained from a comparison of both gas heating and field

induced emissions at the anode and cathode. The thermoconple revealed no measurable field heat. ing of the gas around the anode at 1400 V. Yet Figs. 2 and 3 show the associated emission intensi. ties are comparable or greater than those around the cathode at --600 V, at which voltage there was significant field heating of the gas surround. ing the cathode, as shown by curve A in Fig. 5. These differences can be explained by differences in charged particle energy transfer at the two electrodes and in particular by the different relative importance of elastic and inelastic colli- sions. Around the anode inelastic collisions of electrons will be important, while around the cathode elastic collisions of ions will be important. Due to their higher mass, ions are much more effective in exchangbtg their energy in elastic collisions than are electrons.

Earlier studies of the effects of electrical fields have concentrated on the role of electron energy exchanges? The present work indicates that in the region around the cathode ionic energy ex- changes arc probably more significant. Unfortu- nately, it is not possible to quantify these ex- changes in the same way as the electronic ex- changes. The predominant cheml-ion is HsO § (see Ref. 1). Not only are there no quantitative data on different cross sections, but even the ionic reactions that might occur are unknown. There are a large number of possibilities, involv- ing charge transfer, dissociation, excitation and ionization. The chemical changes induced by electrical discharges have been attributed to the presence of positive ionsY ~ Although the influence of electrical fields upon combustion ehcmlstry, through the agency of electrons, has received much attention, probably the role of positive ions is more important.

4.3. Field Heating of the Electrodes

Not only could ions of high energy heat the surrounding gas but they could also heat, and even fuse the catllode, on account of both the increased gas temperature and large flux of ion energy. There was no comparable effect at the anode. The estimates of ion energy at the cathode surface refem'ed to in Sec. 4.1, necessarily based on incomplete data, showed that if the ions were to lose energy only by elastic collisions, they might attain mean energies of 25 eV when accelerated to a cathode at -- 500 V. Such energies explain the intense local heating of electrode and surrounding gas and also suggest the existence of inelastic collisions of ions and associated emis- sions.

From a consideration of the steady state energy balance an approximate estimate was made of the

CITARGED PARTICLE ENERGY 1029

rate of energy transl~r both to the cathode at its melting point of 2200~ and to the gas at a free stream temperature of 1700~ with known velocities and temperature increases due to field heating. I t would seem tha t approximately one half of the input of electrical power was supplied to the cathode and the other half to the gas. In his iuves~tig ations of an argon plasma jet, Kan- zawa found approximately 65 per cent of the Joule heating produced was transferred to the probe36

Because of the spatial localization of the elec- trical power input, only a snmll h';mtion of the total gas flow is heated by it. The application of negative potential might be t~ practically effective way of enhancing heat transfer rates at a solid surface. Electrical augmentat ion of the flame gases occurs in the electrostatic boundary layer aro~md the surface and the extra energy supply is localized where it is most effective. This has advantages, not only over methods of chemical augmentation such as the use of oxygen flames, but also over some other methods of electrical augmentation. Some preliminary experiments on flame cutting of steel plate were carried out by the authors, in which the supports for the l)late were electrieMly insulated, the flame torch electrically grounded, and a negative potential applied to the plate. With propane-oxygen mix- lores, improvements of up to 18c~c, in flame cutting speed were observed, at 1.5 kV and with a ratio of electrical to chemical power of 0.25~v. No doubt, further improvements could be achicvcd by design optintizatlon.

In many configurations, the ionic wind also is important in this context, t A theoretical estimate was made of the wind towards, and at the sur- face of, the spherical cathode u~ing the approach of Lawton and Wcinbcrg 1 and assmning a porons sphere which acted as a sink for the induced gas flow. Such estimates yielded velocities of up to 2000 cm sec -z at the cathode surface. In thc steady state, the energy lost by radiation from the electrode to the cooled walls of the combustion chamber is balanced by tba t gained by convection from the bases. An a t t empt was made to inch,de the ionic wind in the convection heat transfer law and it was cst imated tha t the temperature of the cathode might rise 30~ as a result of it. This cannot explain some of the observed heating effects, particularly the fusing of the cathode at 2200~ with a free s tream gas temperature of 1700~

$. Conclus ions

(1) I n a reaction zone an electrical field around an electrode m ay have but small effect upon emissions of chemi-luminescent species such as

OI t and CH, but large effect upon emissions from species such as N2.

(2) Around an anode field emissions are prin- cipally due to electron collisions.

(3) Around a cathode there can be intense local heating of gas and electrode and these are probably due to collisions of energetic positive ions. The inelastic collisions can give rise to field emissions.

(4) The application of negative potential seems an effective way of enhancing energy transfer rates to a solid surface, as a consequence of the localization of the heating effect.

Acknowledgments

Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support, of this re- search. The authors also express their thanks to their colleague Mr. Manzoor All from whom they have gained much in discussion.

REFERENCES

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2. DAVIES, R. M.: Terdh Symposium (Interna- tional) or~ Combustion, p. 755, The CombuslAon Institute~ 1965.

3. FELLS, I. AND HANKER, J. H.: Combust. Flame 12, 587 (1968).

4. PAYNIm, K. ,~.ND WEINBt;RG, F. J . : Prec. R. See. Lend. A250, 316 (1959).

5. NAKAMURA, J.: Combust. Flame 3, 277 (1959). 6. PI~_~e~:, E. R. AND W~Lr~n~:aG, F. J.: Prec. R.

Soc. Loud. A289, 192 (1965). 7. NESTERI~O, N. A. AND TSIKOP.A, I. L.: Cembast.

Explos. Shock Waves 4, (3), 219 (1968). 8. DEm~ssE, J., Gs F., KLEIN, 8., AND PESKINE,

J.-S.: Gompt. rend. 269, 875 (1969). 9. BltADLEY, D. AND IBRAHIhl, SAm, M. A.: Corn-

bust. Flame 22, 43 (1974). 10. BELL, J. C., BRADLEY, D., AND JEseI~, L. F.:

Thirteenth Symposium (International) on Com- buslior~, p. 345, The Combustion Institute, 1971.

11. BICADI,EY, D. AX-D IBRAHIM, SAID~ M. A.: J. Phys. D: Appl. Phys. 6, 465 (1973).

12. BRADLEY, D. AND IBRAttIM~ SAID, M. A.: J. Phys. D: Appl. Phys., 7, 1377 (1974).

13. Sir, C. II. AND LAM~ S. lI.: Phys. Fluids 6, 1479 (1963).

14. Mel)ANI1,:I~, E. W.: Collisional Phenomena in Ionized Gases, John Wiley, 1964.

15. Lure', Z. E. AND GOLDB~II.(]ER, W. IV[.: ./~.d- vauces in Chemistry Series 80, p. 441, American Chemical Society, 1969.

16. t~NZAWA, A.: Chem. Engng. Japan 36 (9), 1004 (1972).