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Combust. Sci. and Tech., 177: 1291–1304, 2005
Copyright Q Taylor & Francis Inc.
ISSN: 0010-2202 print/1563-521X online
DOI: 10.1080/00102200590950476
EFFECTS OF DIRECT CURRENT ELECTRIC
FIELD ON THE BLOWOFF CHARACTERISTICS
OF BLUFF-BODY STABILIZED CONICAL
PREMIXED FLAMES
A. ATA
J. S. COWART
A. VRANOS
B. M. CETEGEN*
Mechanical Engineering Department, University of
Connecticut, Storrs, Connecticut, USA
An experimental study was conducted on the stability enhancement
of conical premixed flames by application of direct current electric
fields. Turbulent conical premixed flames were stabilized at the tip
of a circular cylindrical bluff-body flame holder. An electric field
was set up between a positively charged upper electrode and a
grounded flame holder to determine its effects on the lean limit stability characteristics. In these experiments, the flame blowoff equivalence ratios were determined as a function of mixture velocity,
electric field strength, and the electrode configuration. It was found
that the most pronounced effects were observed at the lowest
mixture velocities in this study of about 5.0 m=s with the influence
of the electric field virtually disappearing at higher velocities of 10
to 15 m=s. The maximum reduction in blowoff equivalence ratios
was 4 to 5% at the low-velocity conditions. These findings are
consistent with the estimates of the ionic wind velocities expected
Received 7 November 2003; accepted 8 December 2004.
Our interest in this problem was stimulated with discussions of BMC and AV with
Dr. H. Calcote. The funding of this work was provided by a seed grant from the University
of Connecticut Research Foundation. A. Ata acknowledges the financial help in the form of
teaching assistantship during part of this study.
*Address correspondence to [email protected]
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in hydrocarbon=air flames and point to the rather weak electric field
effect for applications in high-speed premixed flame stabilization.
Keywords: premixed flames, blowoff, bluff-body, electric field, ionic
wind
INTRODUCTION
Electrical properties of flames have been studied by a number of investigators since the 1950s. These studies have primarily focused on the presence,
identification, and generation mechanisms of ionic species in flames as well
as the interaction of flames with imposed electric fields. Flame ionization
of chemical species has long been recognized and it has been utilized for
measurement of hydrocarbon species. Electrical properties of flames have
been studied extensively by Lawton and Weinberg (1969) and Lawton et al.
(1968). Calcote (1962, 1963), Green and Sugden (1963), van Tiggelen
(1963), Poncelet et al. (1956), and Bulewicz and Padley (1963) have
identified in the neighborhood of 50 possible ionic chemical species
between the mass numbers of 1 to 67 in hydrocarbon flames. Among these
many different chemi-ions, the most likely ones in all flames correspond to
þ
þ
þ
CHþ
3 , H3O , CHO , and C3H3 . The concentration of chemi-ions in flames
has been measured and found to peak near the flame front, where exothermic combustion reactions take place. It has been also indicated by Calcote
et al. (1967) and Green and Sugden (1963) that only a small concentration
of negatively charged chemi-ions are present in hydrocarbon flames with up
to 99% of the negative charge being carried by free electrons. The concentration of the positive chemi-ions has been determined to range between
109 and 1012 ions=cc in premixed hydrocarbon=air flames. The maximum
concentrations are a function of the fuel type and the overall combustion
stoichiometry. It has been reported by Calcote (1962) that the peak
concentration levels are four to five times greater in acetylene flames as
compared to methane, propane, and ethylene flames. The peak concentration dependence on the equivalence ratio resembles that of the flame
temperature in that peak concentrations occur near stoichiometric
conditions and fall off on both fuel-lean and fuel-rich sides.
Many suggestions have been made to explain the high levels of ionization in flames as extensively reviewed by Calcote (1957). The proposed
mechanisms include thermal ionization, ionization due to translational
or electronic excitation, as well as chemi-ionization. Ionization due to
EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF
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collisional energy transfers between high-temperature species have been
deemed unlikely based on the momentum exchange considerations. However, the ionization due to exchanges of electronic excitations has been
suggested to be one of the more likely mechanisms of ion formation. A
third possible mechanism is chemi-ionization, which involves production
of ionic species as a part of the chemical reactions. Another advanced
mechanism concerns the ionization caused by high-energy electrons in
some flames. Although none of these mechanisms are believed to be solely
responsible for the ion production in flames, a combination of the most
likely mechanisms produces the ion concentrations found in flames.
One practical aspect of the presence of positive chemi-ions in the
flame zone is the possibility of affecting flame stability by application
of electric fields on flames. In some of the early work on electric field–
flame interactions, Weinberg and coworkers (Lawton and Weinberg,
1969; Lawton et al., 1968; Browser and Weinberg, 1972) and Calcote
and coworkers (Berman et al., 1991; Calcote, 1949; Calcote and Berman,
1989; Calcote and Pease, 1951) as well as others (Bradley and Nasser,
1984; Noorani and Holmes, 1985) have found a significant effect when
an electric field is applied in the flame stabilization region. For example,
the Bunsen burner flames studied by Calcote and Pease (1951) were stabilized at leaner fuel=air stoichiometries in the presence of an imposed
electric field. The electric potentials in the range of a few to tens of kilovolts have been applied between a positively charged upper electrode and
a grounded burner rim to improve the flame blowoff characteristics.
Enabling flame stabilization at leaner fuel=air stoichiometries produces
beneficial effects of reducing NOx emissions chiefly by the lower peak
flame temperatures. Berman et al. (1991) demonstrated significant
reductions in NOx levels from premixed methane=air Bunsen burner
flames at electric potentials of a few kilovolts. The objective of the
present study was to explore the effects of direct current (DC) electric
fields on the lean limit stability of bluff-body stabilized inverted conical
premixed flames of propane and air. In the remainder of this paper, the
experimental systems are first described followed by the results obtained
for two electrode configurations.
EXPERIMENTAL SYSTEMS
The experiments were performed using an axisymmetric burner, shown
schematically in Figure 1. The burner was made out of brass in the shape
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Figure 1. Schematic of experimental setup.
of a converging cylindrical tube with an inner diameter of 128 mm at the
base and 40 mm at the exit within a total height of 225 mm. At the burner
exit plane, a stainless steel rim with a height of 19 mm was attached to
prevent any damage to the brass burner in case of flame attachment to
the uncooled brass burner rim. A cylindrical rod flame holder with a
diameter of 6.0 mm protruded from the burner exit plane by 2.0 mm.
The flame holder was held in place by a cradle at the base of the burner.
Internal to the burner, a honeycomb flow straightener and a layer of
stainless steel fine-mesh screen were placed before the contraction section to condition the incoming flow into the burner. The fuel and air were
first premixed in a mixing chamber consisting of a cylindrical tube with
two fuel jets entering at right angles to the tube axis. Two sets of perforated plates were placed in the chamber to promote vigorous mixing. The
fuel=air mixture was fed into the burner through eight radial inlets
around the burner body. Air was supplied by a compressor system and
its mass flow rate was measured by a set of critical flow orifices. Fuel flow
was controlled by two electronic mass flow controllers (Porter 202 series
and Tylan FC-280 series). Commercial-grade propane was used as the
fuel in all the reported experiments. The composition was stated by
the supplier to contain a minimum of 96% C3H8 by volume with the
EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF
1295
Figure 2. Burner rim and the electrode configuration details.
remainder being other hydrocarbons and inerts. Mean flow velocities up
to 15 m=s could be attained with this burner configuration and the
capacity of the fuel=air supply system. Based on the uncertainties in
the measurements of the fuel and air flow rates, the uncertainty in the
determination of the equivalence ratio was less than 2%.
Two different electrode configurations and two types of bluff-body
flame holders were used in these experiments. The two upper electrode
configurations are shown schematically in Figure 2. One of them
consisted of a metal ring supported by a holder and it was placed concentrically above the cylindrical bluff-body flame holder as shown. This
upper-ring electrode was elevated to a high positive voltage with respect
to the grounded burner body and the flame holder. In this configuration,
it was intended to influence the chemi-ions in the vicinity of the flame
anchoring region. However, experiments with this configuration yielded
virtually no influence on the blowoff characteristics of the flame with
applied electric potentials up to 8.0 kV and various standoff distances
between the upper electrode and the flame holder. To concentrate the
electric field to the flame anchoring region more effectively, a second
upper electrode configuration was employed as shown in Figure 2. In this
configuration, the upper electrode is a blunt-tip cylindrical stainless steel
rod with a diameter of 3.0 mm. The electrode was placed coincident with
the axis of the flame holder and charged to a high voltage.
The effectiveness of the electric field on the chemi-ions present in
the flame depends not only on the distribution of the electric field intensity but also the electrical properties of the medium occupying the space
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Figure 3. Flame holder tip geometries: (a) right circular cylinder and (b) cylindrical cavity.
between the electrodes. Because the conductivity of the hot flame
products in the recirculation zone behind the bluff-body is typically an
order of magnitude larger than that of the cold mixture, a second
bluff-body flame holder with a tip cavity design was employed as shown
in Figure 3. In this design, the metal part of the flame holder was
recessed back with a ceramic cylindrical cavity above it to alter the characteristics of the medium between the two electrodes. In the following
section, the results from the interaction of the flame with the applied
DC electric fields are presented and discussed.
RESULTS AND DISCUSSION
Experimental results presented in this section are related to the influence
of the DC electric fields on the blowoff characteristics of the turbulent
inverted conical premixed flames. For the cold flow conditions, the
Reynolds numbers based on the bluff-body flame holder diameter and
the gas mixture approach velocity were calculated as 2016, 4200, and
5885 for approach velocities of 5.2, 10.4, and 15.0 m=s, respectively.
These Reynolds number values are reduced by a factor of approximately
25 if the Reynolds number is based on the hot wake temperatures of the
order of 2000 K. The jet Reynolds number at the burner exit range
between 12,700 and 37,000 for the burner exit velocities of 5.2 to 15.0
m=s. The profiles of the mean and turbulent axial velocity were measured
using a hot film anemometer 18 mm upstream of the bluff-body tip. The
measured profiles shown in Figure 4 exhibit a slight skewness of the
ffi
pffiffiffiffiffimean
velocity profile toward the bluff body. The turbulence intensity, u02 =um ,
is found to be less than 2% in the core of the flow and reaches 12% in the
boundary layers near the wall and the bluff-body. Based on these and
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Figure 4. Distribution of mean velocity and turbulence intensity across the nozzle radius
between the nozzle inner wall and bluff-body surface at 18 mm upstream of the bluff-body tip.
other similar measurements at higher approach velocities, it was determined that the flow approaching the flame holder and the anchored conical
flame is spatially uniform and of low turbulence intensity.
Effect of DC Electric Field on the Flame Blowoff Characteristics
The blowoff characteristics of conical premixed flames were mapped by
determining the mixture equivalence ratio at which the flame detaches
and blows off the flame holder at a given approach velocity of the combustible mixture. In these experiments, the fuel flow rate was gradually
reduced until the flame blowoff was realized. The mixture equivalence
ratio and the mixture approach velocity were calculated at the condition
of flame blowoff. It was observed in all cases that the flame lift-off from
the bluff-body and blowoff conditions were essentially the same such
that under no conditions could a lifted flame be stabilized in this
configuration.
The blowoff characteristics are shown in Figure 5 for these inverted
conical flames stabilized at the tip of a circular bluff-body flame holder.
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Figure 5. Blowoff characteristics of turbulent conical premixed flames without and with
electric field interaction for the simple flame holder configuration.
As expected, the blowoff equivalence ratio is a function of the mixture
approach velocity with flame blowoff occurring at higher equivalence
ratios with increasing approach velocity. This finding is a well-established characteristic of premixed flame blowoff curves that have been
presented for different flame holder geometries and stabilization configurations in the literature. It should be noted here that the blowoff data
reported here are for the geometry of the needle-type upper electrode
whose tip was placed 18 mm downstream of the bluff-body flame holder
as shown in Figure 2. In the absence of the electric field, the blowoff
equivalence ratio first increases with increasing mixture velocity, but
its variation becomes stronger at high mixture velocities. This is expected
because the stabilization of the flame at high velocities becomes more
difficult and the flame stabilization region becomes very sensitive to small
velocity or equivalence ratio perturbations. Additionally, determination
of the blowoff point in turbulent flames is not precise in that repeated
determinations of the blowoff equivalence ratio can present significant
variations. In this work, the blowoff data were repeated a number of
times until the limits of the blowoff equivalence ratio were established.
EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF
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Once the blowoff characteristics were established in the absence of the
electric field, the electric field was turned on and the experiments were
repeated. Figure 5 shows the effects of the electric field on the blowoff
characteristics at 4.7 kV and 6.1–7.8 kV voltages. It is seen that a modest
improvement of flame stability (as evidenced by a reduction in blowoff
equivalence ratio) can be achieved at low flow velocities. With increasing
flow velocities, the electric field effect diminishes and it falls within the
limits of fluctuations in the typical measurements. Furthermore, it was
found in these experiments that the application of the higher electric
fields could result in arcing through the high-electrical-conductivity
medium of combustion products. Thus, the magnitude of the applicable
electric field is limited by the electrical properties of the medium between
the electrodes. In all the reported experiments, the shorting of the electric field by arcing between the electrodes was avoided. The data at
higher electric potentials and flow velocities, shown in Figure 5 as
6.1–7.8 kV, thus required adjustment of the maximum applied electric
field between 6.1 and 7.8 kV. Specifically, the applied electric fields
and the corresponding flow velocities were 6.1 kV at 7.5 m=s, 7.0 kV at
8.7 m=s, 7.5 kV at 11.2 m=s, and 7.8 kV at 12.3–14.7 m=s. Although there
appears to be a small systematic reduction in the blowoff equivalence
ratio with increased electric field potential at higher flow velocities, the
effect is small.
A qualitative understanding of these experimental findings can be
realized by estimation of the ionic wind velocity based on the previously
developed theory of Lawton and Weinberg (1969), Lawton et al. (1968),
and Calcote (1962), and comparison of these values with the recirculation zone velocities. In this theory, the migration of positive ionic species
from the flame zone (typically H3O þ in hydrocarbon flames) induces an
ionic wind that can be directed by application of a suitable electric field
to enhance flame stabilization. Specifically, for the bluff-body flame
holder configuration studied here, the ionic wind is directed from the
positively charged upper electrode toward the base of the flame holder
aiding the recirculating flow. The maximum ionic wind magnitude can
be estimated from a momentum balance (see, for example, Lawton and
Weinberg, 1969; Lawton et al., 1968) in the flow where ions drift under
the imposed electric field:
n induced ¼
ja
qk
1=2
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Figure 6. Photographs of the conical V-shaped, bluff-body stabilized flame (a) without
electric field, (b) with electric field at 7.8 kV; Ua ¼ 15.1 m=s, / ¼ 1:0, electrode separation
of 18 mm. Upper positively charged electrode is the glowing circular rod in the flame.
where j ¼ ðE 2 kÞ=ð8paÞ is the current density (A=cm2) expressed in terms
of electric field strength in esu=cm,1 k is the ionic mobility
in cm2=(V sec), a is taken as the electrode spacing in cm, and q is the
gas density in g=cm3. Utilizing these relationships, the theoretical
pffiffiffiffiffiffiffiffi
maximum velocity can be determined from n max ¼ E= 8pq. Evaluating
the gas density at a wake gas temperature of 2000 K for / ¼ 0.8, the
maximum velocities are calculated as 1.3 and 2.2 m=s for the corresponding electric field strengths of 2.6 and 4.3 kV=cm. These values of
the field strength represent 4.7 and 7.8 kV potentials of the upper electrode. They can be compared with the typical maximum velocity in the
recirculation zone toward the flame holder of about 0.3Ua as determined
from particle image velocimetry in the wake of rod-stabilized flames in
the absence of electric field. For the three approach velocities (Ua)
employed in this study, the recirculation zone velocities are 1.6, 3.1,
and 4.5 m=s as compared to the maximum ionic wind velocities calculated earlier. This comparison suggests that the electric field effect is
1
In substitution of electric potential into these equations, the conversion factor of
1 esu ¼ 299.7925 V has to be employed.
EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF
1301
of significance only at low approach velocities and the effect diminishes
with increasing approach velocities. This is consistent with the blowoff
data presented in Figure 5.
Although the maximum decrease in the blowoff equivalence ratio
was about 5% at a flow velocity around 5.0 m=s, the visual changes have
been found to be present at even higher flow velocity conditions, which
exhibited minimal effect on blowoff equivalence ratio. Figure 6 shows
two images of the conical flame. In Figure 6a, the flame is stabilized in
the absence of the electric field. The image in Figure 6b shows the flame
when the electric field was switched on. It is seen that the chemiluminescent flame zone is pulled toward the flame holder, indicating the visual
effect of the application of electric field on the flame stabilization zone.
Figure 7 shows the experimental data obtained for the cavity-type
bluff-body flame holder schematically shown in Figure 3 with the
needle-type upper electrode. In this configuration, the spacing between
the upper electrode and the metal part of the flame holder is increased
by 7.0 mm due to the presence of the ceramic cavity. Thus, application
of higher electric potentials was possible in this configuration. For
Figure 7. Blowoff characteristics of turbulent conical premixed flames without and with
electric field interaction for ceramic cavity-type bluff-body configuration.
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example, the maximum voltage applied to the upper electrode was 11 kV
as opposed to 7.8 kV in the case of the simple circular metal flame
holder. However, this difference is simply due to the increased distance
between the electrodes and the maximum electric field intensity for both
cases was about 4.4 kV=cm. The blowoff data obtained with this configuration are not significantly different than those shown in Figure 5.
In this case, the data indicate that the 4.7 kV potential does not appear
to be sufficient for any stability enhancement. However, there is a progressive trend of reducing the blowoff equivalence ratio with increasing
electric potential, particularly at low mixture velocities. Once again, it is
difficult to quantify the influence of the electric field at higher mixture
velocities due to its small magnitude of the same order as the data scatter.
Finally, the percentage reduction in the blowoff equivalence ratio,
noEF
defined as D/bo ¼ ð/noEF
/EF
is shown in Figure 8. It is found
bo
bo Þ=/bo
that the highest reductions, of the order of 3 to 5%, are obtained at low
mixture velocities around 5 m=s. As the mixture velocity increases, the
effect is reduced and becomes of the same order as the uncertainty in
the determination of the blowoff equivalence ratio.
Figure 8. Percent reduction in blowoff equivalence ratios under the application of electric
field as a function of combustible mixture velocity.
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CONCLUDING REMARKS
An experimental study was conducted to determine the feasibility of
improving the lean limit stability of bluff-body-stabilized inverted conical
flames by application of DC electric fields. For example, it has been
shown in the literature that flame blowoff characteristics could be
improved by electric fields in conical Bunsen burner flames stabilized
around the burner rim. In the flames studied in this work, flames were
stabilized at the tip of a circular cylinder flame holder about which a
DC electric field was setup. The blowoff equivalence ratios were found
to be marginally reduced with the application of electric field intensities
between 2.6 and 4.4 kV=cm. The maximum improvements were limited
to about 5% at mixture velocities of 5 m=s. At higher mixture velocities
up to 15 m=s, the influence of the electric field was found to be of the
same order of magnitude as the uncertainty in the determination of the
blowoff equivalence ratios in these turbulent bluff-body-stabilized
flames. Comparison of the estimated maximum ionic wind velocities
induced by the applied electric field with recirculation zone velocities
suggests that electric field effects are only significant at low approach
velocities (5 m=s). This finding is consistent with the experimental results
reported in this paper. It is thus concluded that the enhancement of
bluff-body flame stabilization by applied DC electric field is limited by
the magnitude of the applied field and the resulting ionic wind. The
maximum applied electric field is in turn limited by the breakdown
voltage at which the arcing ensues in the electrically conductive hightemperature combustion products occupying the space between the electrodes. Finally, cooling of the electrodes placed in the high-temperature
combustion products poses another practical problem which may be
alleviated by a proper cooling scheme.
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