/
Characterization of atmospheric pressure
gliding arc plasma for the
production of OH and O radicals a
N. C. Roy, M. G. Hafez , and M. R. Talukder Department of Applied Physics and Electronic Engineering, University of Rajshahi, Rajshahi 6205 Bangladesh. a
Department of Applied Mathematics, University of Rajshahi, Rajshahi 6205 Bangladesh. Corresponding author:
[email protected]
Abstract Atmospheric pressure
0/
is generated by a 88
,6
AC power supply. The properties of the produced
plasma are investigated by optical emission spectroscopy (OES). The relative intensity, rotational, vibrational, excitation temperatures and electron density are studied as function of applied voltage, electrode spacing and oxygen flow rate. The rotational and vibrational temperatures are determined simulating the 0 →
∑
′′
0
Π
′
bands with the aid of LIFBASE simulation software. The excitation temperature is
obtained from the
transition taking non-thermal equilibrium condition into account employing intensity
ratio method. The electron density is approximated from the
α
Stark broadening using the Voigt profile fitting
method. It is observed that the rotational and vibrational temperatures are decreased with increasing electrode spacing and O2 flow rate, but increased with the applied voltage. The excitation temperature is found to increase with increasing applied voltage and O2 flow rate, but decrease with electrode spacing. The electron density is increased with increasing applied voltage while it seems to downward trend with increasing electrode spacing and O2 flow rate. 0/
Keywords:
plasma, Optical emission spectroscopy (OES), Broadening mechanism, Reactive Species.
1. Introduction In the recent years, water plasmas have drawn much attraction due to its distinctive features of high oxidationreduction capacity, high enthalpy and above all environment friendly. Due to these features, the water plasmas are applied [1] in environment, biology, hazardous waste treatment, coal gasification and so on [2, 3]. Kim et al. [4] carried out experiment to study the performance of polychlorinated biphenyles waste treatment using 0 steam plasma and found that the steam plasma process yield better efficiency for converting hazardous waste to energy than the conventional burning and air plasma processes. Ni et al.[5] characterized steam dc plasma torch at atmospheric pressure employing electrical and optical emission spectroscopic (OES) diagnostic techniques. They observed that the restrike mode is responsible for fluctuations and instabilities of the steam plasma. Zhu et al.
[6]
demonstrated the gliding arc non-equilibrium plasmas by flowing air at atmospheric
pressure. They studied the effects of air flow rates on the discharge dynamics and ground state
distributions.
They also noted that the air flow rate strongly affects the discharge ignition, maintenance, and emission intensity. Bruggeman et al[7] investigated the electrical and optical properties of dc water discharge plasmas at atmospheric pressure air ,
,
,
and noted that such types of plasmas able to produce a large number of
and other species. The highly reactive
and
radicals are recently used[1,8] for the
destruction of homicide bacteria, virus and fungus. Therefore, for better understanding of the production and
1
control of the active species in the discharge and their interactions with the living tissues are very important. The production of these active species depends on the conditions of the discharge to generate high concentration excited states of the plasma species that have energies above their ground states and can affect the destruction mechanisms of the living tissues. The addition of reactive
and
0 steam discharge can enhance the productions of
in
radicals through different collision processes. Therefore, it is important to determine plasma
parameters (gas temperature, electron temperature, electron density) at different percentage of oxygen in
0
steam plasma under different discharge conditions will help to understand about particle collision processes, plasma reactions and density of the react species in the plasma. OES is a non-perturbing and non-invasive easyto-use plasma diagnostic technique and widely applied to investigate plasma species and to determine gas temperatures. OES is able to identify plasma species such as free radicals, atomic and molecular species and therefore it reveals the insight of the plasma chemical processes[9]. OES also provides valuable information regarding excited atomic or molecular species that enables us to estimate rotational and vibrational temperatures of the species in non-equilibrium The water
0/
/
gliding arc plasma.
gliding arc plasma is characterized by OES method to investigate the production of reactive
species in terms of oxygen fraction in the mixture and operating parameters. The main objectives of this work is to gain insight of the reactive
and
species that influence the plasma reactivity and to obtain better
understanding of the production mechanisms of the species concerned. Experimental setup is discussed in section 2, optical characterization is given in section 3, results and discussions are furnished in section 4, and finally conclusion is drawn in section 5. 2. Experimental Setup Figure 1 shows the schematic illustration of the experimental setup. A 65 1
long, 10
inner diameter and
thick glass tube is used as reactor chamber. The diameter of the nozzle at the exit point is 2
copper electrodes of 10mm long and 1
. Two
diameter two copper electrodes are inserted from the opposite sides
into the discharge tube. Copper electrodes are intentionally used because the distinct
line can be used for
the determination of excitation temperature of the plasma. Experimental measurements are carried out from 2
5
electrode spacing. A small amount of
0 steam with a constant temperature of ~400
(purity 99.65%) is
steam generator is fed with a constant flow rate to the plasma reactor as shown in Fig. 1.
in the entire experimental investigations. Flow rate of
inside the
0 and
are ~400K
115
discharge tube is controlled by a gas flow controller
. The temperatures of 0/
and 300K, respectively, at the entrance of the discharge tube while the point without discharge. The observed temperature difference is ~5 high voltage (maximum 6
for the
, ~25 ) power supply with a frequency 88
mixture is 328K at the tube exit flow rate from 2
07
in combination with a digital oscilloscope
spectra of the produced plasma a 200
08 , and a current
long optical fiber cable is fed to the spectrometer
. A computer is associated to acquire spectral data.
2
.A
1022 . In order to record the emission
The spectrometer has entrance slit of 25 , detector wavelength range of 200 of 1.7
10
is applied across the electrodes. The
waveforms of the discharge voltage and current are recorded with a voltage probe probe
0 steam
flow rates are varied but the
fed to the discharge reactor from the gas cylinder. It is noted that the flow rate remains constant at 0.5
from the
850
2000
1 .
and optical resolution
3. Optical Characterization Species identification Optical emission spectroscopy is a common technique used for species identifications and plasma diagnostics at atmospheric pressure plasmas. Figure 2 shows a typical emission spectrum measured at 5 of 4 Π
11.11%
) for the electrode spacing of 3
′
∑
0 →
′′
0
at 305
320
chemistry due to their highly reactive property 324.32
and 4
4 →4 4
at 486.1 that of
and 656.3
4
, and and
4 →4 4
,
4 →4
°
at
lines are observed due to erosion of the copper
plasmas. The transition of
3 → 2 ) and
, respectively. The emission intensity of
line due to the lower ionization potential of
4 →2
are found
line is observed much stronger than
compared to
and hence more ionization event
occurs with dissipation of the same amount of energy. Therefore, the density of
at 777
°
are the most important species for plasma
. The emissions of
at 326.60
°
0/
electrodes in the highly reactive
[10]
flow rate
. The dominant peaks are found for the band of
0. It is well known that
respectively, in the presence of
with
can be expected higher. The
line will be used for the determination of electron density.
The basic principle of OES method is that the emission intensity from an excited state is proportional to the concentration of the species in that excited state at a particular wavelength[11,12]. At atmospheric pressure, in most cases, the lifetime of the species is large enough where the collisions thermalize the rotational state distribution function with gas temperature intensity of the species can be written as
where ,
, , , , ,
and
and the rotational temperature
The optical emission
[9, 13]
∝
,
1
are the emission line intensity, density of the molecules, Einstein’s transition
probability, Plank’s constant, frequency of transition, quantum numbers of electronic, vibrational and rotational transitions, respectively. The single and double primes indicate the upper and lower levels of transitions, respectively. The
and
radicals are produced in the gap of the electrodes through electron impact
dissociation of
and
molecules, as described in reactions
0.58
2 , respectively. Inserting 1 and
as the typical value found in our experiment, in the rate coefficient equation of reactions
2 , the production rates of 10
1 and
and
2.59
radicals are
, respectively, which indicate that the density of the
10
9.82
and
radical is higher than those of , as can
be seen from Fig. 3. On the other hand, the kinetic energy of the electron is boosted up by the enhanced electric field due to increased applied voltage, hence the collision frequency increases and the production of 1 and
radicals are increased which can be inferred from reactions radicals are also increased with increasing →2 → The rotational temperature
flow rate
[14]
4.2
1.68
can be approximated
[16]
2 , respectively. The density of and
.
10
10
.
exp 1.23
2.19
as the gas temperature
, i.e.
local thermodynamic equilibrium (PLTE) plasmas. The production rate coefficient of 3 is, taking the typical value of the rotational temperature bands,
1.69
10
. It is seen from reaction
3
and
2200
.
1
[14]
2
[15]
for partial radicals of reaction
as determined from the fitting of the
3 and Fig. 3(a) that with increasing with
flow rate the production of
is increased
→
discharge gap.
, radicals
→ The
destruction 1.58
hence more .
10
rate
coefficients
10
and
of
2.06
1.62
radicals
2.07 4 ,
radicals are producing within the 10
10
.
10
through
4
reactions
3
5
and
[18]
are
, respectively, for the typical values of
5 , Figs. 3(a) and 3(b) reveal that the density of
radicals is
flow rate because of lowering gas mixture temperature.
decreasing with increasing
→ →
The loss mechanism of
2.5
10
2.0
10
.
.
4
[18]
5
[18]
at high water concentration is mainly determined by quenching of
and
by the following reaction channels that appear in the discharge zone between the gap of the electrodes inside
the discharge tube. The quenching of excited →
radicals with
→
2200 , are
1.33
4.9
6 and
The quenching rate coefficients of reactions 10
.
10
to 10
.
10
6
[18]
7
[18]
10
, respectively. Due to addition of for the change of
flow rate from
(as can be seen from Fig. 5(a)) and as a result quenching rate is also decreasing. It is to be
noted that the quenching of rate of reaction state density of
7 , estimated considering the same gas temperature 2.03
and
7.5
is approximately 5% [20]
and
, the rotational temperature is being lower from 2450 to 2000 at 5 2
is also an efficient
.
2200 . Observations of reactions
[19]
2.7 [18]
channel for the production
[17]
6 is higher than that of
7 . In order to determine the ground
radicals, laser induced fluorescence spectroscopic (LIF) technique is usually employed
instead of using OES technique due to lack [9,11, 13] of direct relation between the intensity of the emission bands and the ground state density of
-related
radicals.
4. Result and Discussion 4.1 Rotational and vibrational temperatures It is well known [21] that the emitted lines from the plasma are broadened by several broadening mechanisms. These are natural-, Doppler-, instrumental-, pressure- , resonance van der Waals- and Stark broadenings [22, 23]. Attention has been given to determine gas (rotational) temperature due to its key role for biological applications of
radicals. The species used for spectroscopic diagnostics that have significant contributions in the emitted
spectra. The emission intensity of the species produced is dependent on the density of excited species, rotational and vibrational energy distributions, and the volume of the plasma under consideration. In our experiment, radicals produced highest intensity, indicating highest concentration, among other species as shown in Fig. 2. The emission spectra of
radicals are widely used
) and vibrational (
because it facilitates one to determine rotational ( measured spectra. LIFBASE software
[25]
[9, 24]
is used for the determination of
for spectroscopic diagnostics ) temperatures by fitting the
and
applications. The emitted spectral line of Eq.(1) can be modified to the following form
4
because of its versatile [9]
where
the molecules in the electronic state of
function, and
,
,
is a constant, is the Boltzmann constant,
harmonic oscillator term value,
, 2 ,
is the velocity of light,
is the band strength,
is the vibrational term value,
is the density per volume of
is the line strength,
is the
is the vibrational partition
,
is the rotational partition function. The contribution of the Lorentzian function is
,
taken into account considering the non-Gaussian
[9]
measured line shape. The best fitted line shape can be
obtained due to the contributions of Lorentzian and Voigt profiles. Under these conditions, the broadened line intensity profiles can be represented [9] as a function of wavelength ()
where
1
4ln 2
4
1
is the fraction of the Lorentzian contribution to the Voigt profile of the instrumental function,
is the full width at half maximum and
The rotational emission of
radicals in the 306
is the
→
transition wavelength.
318
band is used to determine 0
experimental and simulated spectra are normalized by
0
. Both the
band. The simulated spectrum is
superimposed on the measured spectrum. The best match provides the
and
experimental data measured at a voltage of 5
, and
(11.11%
, 3
, electrode spacing of 3
temperatures. The flow rate of 4
) along with best matched spectrum are shown in Fig. 4 for example. In this case, the instrumental
broadening of 1.7
, Doppler broadening of 0.0028
, collisional broadening of 0.65
are obtained from
the simulated spectrum using LIFBASE software. It is noted that the LIFBASE software includes transition moment, rovibrational wavefunction, rotational Hönl-London factors and the emission coefficient with vibration and rotational quantum numbers that are essential parameters for the spectroscopic computations for the determination of accurate rotational and vibrational temperatures even with low resolution spectroscopic data [25]
.
Figure 5(a) and 5(b) show the effect of voltage and electrode spacing on and
respectively. It is observed from Fig. 5(a) that both with enhanced
flow rate for the electrode spacing of 3
and
for different
flow rate,
are increasing with the increase of voltage . It is noted that the flow of
constant for all measurements. The reasons of the increasing
and
0 steam was kept
arise due to the fact that with
increasing voltage, free electrons are collecting more energy from the increased electric field and transferring this energy to neutral particles through electron-neutral moment transfer process. Fig. 5(b) displays the effect of electrode spacing on and
and
maintaining the discharge voltage of 5
. It is seen from this figure that
are decreasing with increasing electrode spacing. This result can be attributed from the fact that
for a given applied voltage and atmospheric pressure, as the electrode spacing is increased, the electrical field
5
intensity across the electrodes are decreasing, therefore, the mean energy of the electron is reduced and hence less energy from the electrons are transferred to the atoms or molecules. It is also observed from Fig. 5 that are reducing with the increase of
and
carrying Joule heat
flow rate due to the fact that,
plays a significant role in
[26]
out from the discharge region. It is to be mentioned that the fraction of energy is
transferred by electron to
and
molecules by vibrations and rotations. But according to our experimental
conditions, there is no significant effect of
flow rate and mixture temperature on high E/N values [26]. When
flow rate is increased, the collision between molecules is more frequent due to short collision path length. This can be attributed by the fact that the Reynolds number is increasing with increasing gas flow rate in the electrode gap region. The diameter of the discharge tube are different at the entrance (10
) and exit 2
.
So it is reasonable to consider that the gas molecules will be accumulated in the discharge region with increasing
flow rate. Because we can represent the Reynolds number as a function of gas density, the /
Reynolds number
, where
,
,
, and
are the gas density, gas velocity, characteristic
length and dynamic viscosity, respectively. The results obtained agree well with results noted by Zhang et al. [27]
. Due to high collision frequency, the electrons are unable to gain much energy from the applied electric field
and hence transfer less energy to molecules through momentum transfer collision process. On the other hand, the electron transit time becomes shorter within the electrode gap where strong electric field exists. Anghel et al. [28]
5
investigated the effect of operating frequency (MHz range) and gas flow rate (1
) on plasma power
dissipation and revealed that the change of power dissipation is very small with operating frequency and gas flow rate. But in our experiment the source frequency is 88
only, and the effect of discharge power variation
can be considered negligible with gas flow rate and frequency. 4.2 Electron Density Recently, the interest for the spectroscopic diagnostics of atmospheric pressure using increased. The intense intensity of plasma
[29]
line appears when only traces of hydrogen and
. In this experiment, the spectroscopic measurements show that the
intensity instead of
line and hence the
line is considerably 0 are present in the lines produced highest
lines are considered for the plasma diagnosis. Doppler broadening
is produced due to the thermal motion of excited hydrogen atoms. The instrumental broadening (
is
related to the resolution of the spectrometer, slit function and lens error of the optical transmission system. The pressure broadening includes three broadening mechanisms: the Stark broadening [30]
charged particles in plasmas)
, the resonance broadening
According to our experimental conditions, the natural broadening usually negligible
[30]
, (due to the interaction of
and Van der Waals broadening and resonance broadening (
. are
in most plasma experiment because resonance broadening does not induce shift of spectral
line in low density plasmas. However, in order to determine electron density, the spectral line plays an important role to obtain broadening parameters. The broadening mechanisms can be represented by the Gaussian and Lorentzian line profiles. The is given by [31,
FWHM of the Gaussian profile Δ
, 4
Δ while the total FWHM Δ
32]
is directly the sum of the each individual contribution. Δ
, 5
6
The experimental spectrum of the
line is fitted by Voigt profile which is the convolution of Δ
, hence all broadening parameters were
Since the line shapes contain all broadening mechanisms involved
included in the fitting equation as noted in the LIFBASE software. The Voigt fit of 656.34
is shown in Fig. 6, measured at the applied voltage of 5
flow rate of 4
and Δ .
[33]
line shape at wavelength
with electrode spacing of 3
and
). To estimate the value of the broadening, the best fitting are considered,
(11.11%
excluding error by setting three important parameters: the central value, relative intensity and the base line of can be neglected [30, 31] from the contribution of the Lorentzian profile
and
the spectrum. It is noted that
broadening is usually accounts from
due to low resolution of the spectrometer used. But the van der Waals 30% to 35% of the total FWHM of Lorentzian profile 0.69
typical van der Waals broadening
[31]
. Under the experimental conditions considered, the
is obtained. Using Eq. (5) the Stark broadening
obtained from which electron density is determined from the following equation 2.0
10
/
can be
[21, 22]
6
Figures 7(a) and 7(b) show the dependency of electron density
on the applied voltage and electrode spacing
flow rate, respectively. Figure 7(a) indicates that
decreases with increasing electrode spacing
for different
while decreases with increasing
flow rate. Electric field intensity within the gap reduces with increasing
decreases. It is well known that at intense electric field, the electrons gain more
electrode spacing and hence
energy from the enhanced electric fields and hence the ionizing collisions between electrons and neutrals become more frequent. Alternatively, the higher the value of electron temperature, the more effectively ionizes the neutrals causing abundant production of electrons. Therefore, most of the energies of the electrons, as gained species within the electrode gap through ohmic heating
from the electric field, being transferred to the
increases with increasing voltage at constant
and thereby producing more electrons and consequently
is decreased with increasing
rate. Figure 7(b) shows that
flow rate. As increasing
flow rate,
decreasing due to the decrease of water molecule content in discharge region, because of constant and high electron affinity of the become frequent and hence
[33]
molecules. With increasing
flow is flow,
flow rate, the collision of electrons with
decreases with a small amount of
quenching and the electronegativity nature of
[33]
molecule content because of electron
molecules.
4.3 Excitation temperature The excitation temperature (
) may be used as a rough indicator of the electron temperature [34]. Because the
free electrons are responsible for the excitation of atoms or molecules and their energies should be described by the Boltzmann distribution function at a given temperature. Intensity ratio method is the most straightforward method to determine the excitation temperature from the line emission if the population densities of the upper levels of two lines are in partial local thermodynamic equilibrium
[35]
. The intensity of the emission lines
provides the population in the level of atoms that follow the Boltzmann distribution function the relative intensity of two different lines at wavelength
and
can be expressed by following relation [36, 37] , 7
where refers to the intensities of the emission lines, the upper levels,
is the upper level energies and
refers the transition probabilities,
is the Boltzmann constant. Exponential function in Eq. (7)
reveals that the lines should be selected in such a way that the condition
7
is the degeneracy of
satisfies. In the present
analysis, 327.32
lines at the wavelength of 324.12
is determined from the atomic transitions of
, respectively as shown in Fig. 2. The intensities of the
and
lines are taken from the measured
spectrum and the relevant spectroscopic data for the transitions considered are taken from NIST database [38]. Figures 8(a) and 8(b) show the effects of the
is increasing with the increase of
flow rate and electrode spacing on
. It is seen from Fig. 8(a) that
flow rate and applied voltage. The reason of increasing
due to the fact that the presence of electronegative
arises
gas in the plasma causes the increase of sustaining voltage
and the electric field strength. Therefore, the mean kinetic energy is increased and lead to an increase of the excitation temperature. In this range, the contribution of penning ionization decreases due to the reduction of the available
species in the plasmas [39]. The observation of
is in agreement with the results of Joh et al.[34].
With increasing electrode spacing the mean kinetic energy of electrons are reduced, as shown in Fig. 8(b), as discussed earlier. 5. Conclusion /
The atmospheric pressure
gliding arc plasma is produced. The optical emission spectroscopic data as Π
recorded from the plasma discharges reveals that the emission band of atomic transition of
,
are produced. The plasma parameters
,
′
0 → ,
by analysing optical emission spectroscopic data in the voltage range from 4.0 spacing from 2.0
5.0
. The analytical results show that
discharges are in non-thermodynamic equilibrium.
,
,
voltage but decreasing with electrode spacing. On the other hand, flow rate while
and
are decreased.
8
and 5.5
∑
′′
0
and
are determined and the electrode
which means that the plasma and and
are increased with increasing applied are decreased with increasing
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10
Gas flow controller Oxygen Gas
𝐻2 𝑂 Steam
HV power supply
Valve Valve
Oscilloscope Ch-1
Ch-2
Computer Glass tube HV probe Shielding box
Spectrometer Current probe Optical fiber
Water steam
Photograph
Fig. 1. Schematic of the experimental setup with discharge photograph
1400 OH 1200
Measured Spectrum
Intensity[a.u]
1000
H 800
Cu
600 400
O
H
200 0
300
400
500
600
700
800
Wavelength, [nm]
Fig. 2. Emission spectra of steam with 𝑂2 flow rate of 4𝑙𝑝𝑚 (11.11% 𝐻2 𝑂), voltage 5𝑘𝑉, and electrode gap 3𝑚𝑚.
900
Relative Inensity[a.u.]
H2O Density 20% 11.1% 7.7%
1600 1400
5.8% 4.7%
1200 1000 (a)
O radical
800
Relative Inensity[a.u.]
3000 2700 2400
H2O Density 20% 11.1% 7.7%
5.8% 4.7%
2100 1800 1500
(b)
OH radical 4.0
4.5 5.0 Voltage [kV]
5.5
Fig. 3. Relative intensity of 𝑂 and 𝑂𝐻(𝐴 − 𝑋) radicals with voltage for different 𝐻2 𝑂 content for 3𝑚𝑚 electrode spacing.
100
Simulated Measured
Intensity[a.u]
80 60 40 20 0 300
305
310 Wavelength[nm]
315
320
Fig. 4. Experimental and simulated spectra of 𝑂𝐻(𝑋 2 Π(𝑣 ′ = 0) → 𝐴2 + 𝑣 ′′ = 0)) radical emitted from 𝐻2 𝑂 with 𝑂2 flow rate of 4𝑙𝑝𝑚 (11.11% 𝐻2 𝑂), voltage of 5𝑘𝑉 and electrode spacing of 3𝑚𝑚.
2.75
5.8% 4.7%
Trot [kK]
2.50
H2O Density 20% 11.1% 7.7%
2.25
2.00
(a)
7.5
5.8% 4.7%
Tvib [kK]
7.0
H2O Density 20% 11.1% 7.7%
6.5 6.0 5.5 4.0
3.5
4.5 5.0 Voltage [kV]
Trot [kK]
H2O Density 20% 11.1%
3.0
5.5
7.7% 5.8% 4.7%
2.5 (b) 10 H2O Density 20% 11.1%
Tvib [kK]
9 8
7.7% 5.8% 4.7%
7 6 5 2
3 4 Electrode spacing [mm]
5
Fig. 5. Effect on rotational (𝑇𝑟𝑜𝑡 ) and vibrational (𝑇𝑣𝑖𝑏 ) temperatures (a) of voltage determined at 3𝑚𝑚 electrode spacing, and (b) of electrode spacing determined at 5 𝑘𝑉.
1.0
Intensity [Normalized]
0.8 0.6 0.4 0.2 Measured H line Voigt fit
0.0 650
652
654
656
658
660
662
Wavelength [nm]
Fig. 6. Measured (𝑂2 flow rate 4 𝑙𝑝𝑚 (11.11% 𝐻2 𝑂), electrode spacing 3𝑚𝑚 and voltage 5𝑘𝑉) and fitted curves of 𝐻𝛼 line for the determination of electron density from the broadening parameters.
1.6 H2O Density 20% 11.1% 7.7%
1.4
5.8% 4.7%
15
-3
ne[x10 cm ]
1.2
1.0
0.8
(a) 0.6 2
3 4 Electrode Spacing[mm]
5
4 kV 4.5 kV 5 kV 5.5 kV
1.4
15
-3
ne[x10 cm ]
1.2
1.0
0.8
0.6
(b) 80%
84%
88%
92%
96%
[O2]/[O2+ H2O]
Fig. 7. Effect on electron density (ne) of (a) electrode spacing at 5kV and (b) 𝑂2 / 𝑂2 + 𝐻2 𝑂 with electrode spacing of 3 𝑚𝑚.
9
4.0 kV 4.5 kV 5.0 kV 5.5 kV
Tex [kK]
8
7
6
5
(a)
80%
84% 88% [O2]/[O2+ H20]
92%
96%
10
Tex [kK]
8
6
4 (b)
H2O Density 20% 11.1% 7.7% 2
5.8% 4.7%
3 4 Electrode spacing [mm]
5
Fig. 8. Effect on electron excitation temperature (𝑇𝑒𝑥 ) of (a) 𝑂2 / 𝑂2 + 𝐻2 𝑂 determined at 3𝑚𝑚 electrode spacing and (b) electrode spacing determined at 5𝑘𝑉.
