EFFECTS OF DIRECT CURRENT ELECTRIC

FIELD ON THE BLOWOFF CHARACTERISTICS

OF BLUFF-BODY STABILIZED CONICAL

PREMIXED FLAMES

A. ATAJ. S. COWARTA. VRANOSB. M. CETEGEN*

Mechanical Engineering Department, University ofConnecticut, 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 stab-

ility characteristics. In these experiments, the flame blowoff equival-

ence 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 5.0m=s with the influence

of the electric field virtually disappearing at higher velocities of 10

to 15m=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 cetegen@engr.uconn.edu

Combust. Sci. and Tech., 177: 1291–1304, 2005

Copyright Q Taylor & Francis Inc.

ISSN: 0010-2202 print/1563-521X online

DOI: 10.1080/00102200590950476

1291

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 investiga-

tors 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 andWeinberg (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

CHþ3 , H3O

þ, CHOþ, and C3Hþ3 . The concentration of chemi-ions in flames

has been measured and found to peak near the flame front, where exother-

mic 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 concen-

tration 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 concen-

tration 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 ioniza-

tion 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

1292 A. ATA ET AL.

collisional energy transfers between high-temperature species have been

deemed unlikely based on the momentum exchange considerations. How-

ever, 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 sta-

bilized 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 kilo-

volts 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

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1293

of a converging cylindrical tube with an inner diameter of 128mm at the

base and 40mm at the exit within a total height of 225mm. At the burner

exit plane, a stainless steel rim with a height of 19mm 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 6.0mm protruded from the burner exit plane by 2.0mm.

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 sec-

tion 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 perfor-

ated 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

Figure 1. Schematic of experimental setup.

1294 A. ATA ET AL.

remainder being other hydrocarbons and inerts. Mean flow velocities up

to 15m=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 con-

centrically 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 3.0mm. 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 inten-

sity but also the electrical properties of the medium occupying the space

Figure 2. Burner rim and the electrode configuration details.

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1295

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 char-

acteristics 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 15.0m=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 2000K. 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 18mm upstream of the bluff-body tip. The

measured profiles shown in Figure 4 exhibit a slight skewness of the mean

velocity profile toward the bluff body. The turbulence intensity,ffiffiffiffiffiffiu02

p=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

Figure 3. Flame holder tip geometries: (a) right circular cylinder and (b) cylindrical cavity.

1296 A. ATA ET AL.

other similar measurements at higher approach velocities, it was determ-

ined 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 com-

bustible 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.

Figure 4. Distribution of mean velocity and turbulence intensity across the nozzle radius

between the nozzle inner wall and bluff-body surface at 18mm upstream of the bluff-body tip.

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1297

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-estab-

lished characteristic of premixed flame blowoff curves that have been

presented for different flame holder geometries and stabilization config-

urations 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 18mm 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.

Figure 5. Blowoff characteristics of turbulent conical premixed flames without and with

electric field interaction for the simple flame holder configuration.

1298 A. ATA ET AL.

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 elec-

tric 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 7.5m=s, 7.0 kV at

8.7m=s, 7.5 kV at 11.2m=s, and 7.8 kV at 12.3–14.7m=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 recircula-

tion zone velocities. In this theory, the migration of positive ionic species

from the flame zone (typically H3Oþ 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:

ninduced ¼ ja

qk

� �1=2

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1299

where j ¼ ðE2kÞ=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

maximum velocity can be determined from nmax ¼ E=ffiffiffiffiffiffiffiffi8pq

p. Evaluating

the gas density at a wake gas temperature of 2000K for / ¼ 0.8, the

maximum velocities are calculated as 1.3 and 2.2m=s for the corre-

sponding 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 elec-

trode. 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 4.5m=s as compared to the maximum ionic wind velocities calcu-

lated earlier. This comparison suggests that the electric field effect is

1In substitution of electric potential into these equations, the conversion factor of

1 esu ¼ 299.7925V has to be employed.

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 15.1m=s, / ¼ 1:0, electrode separation

of 18mm. Upper positively charged electrode is the glowing circular rod in the flame.

1300 A. ATA ET AL.

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 5.0m=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 chemilumines-

cent 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 7.0mm 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.

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1301

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 con-

figuration 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 pro-

gressive 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,

defined as D/bo ¼ ð/noEFbo � /EF

bo Þ=/noEFbo is shown in Figure 8. It is found

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.

1302 A. ATA ET AL.

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 5m=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 (5m=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 high-

temperature combustion products occupying the space between the elec-

trodes. 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.

REFERENCES

Berman, C.H., Gill, R.J., and Calcote, H.F. (1991) NOx reduction in flames

stabilized by an electric field. ASME Fossil Fuels Combustion Symposium,

PD- 33, 71.

Browser, R.J. and Weinberg, F.J., (1972) The effect of direct electric fields on

normal burning velocity. Combust. Flame, 18, 296.

Bradley, D. and Nasser, S. (1984) Electrical coronal and burner flame stability.

Combust. Flame, 55, 53.

EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1303

Bulewicz, E.M. and Padley, P.J. (1963) A cyclotron resonance study of ionization

in low pressure flames. Proc. Combust. Instit., 9, 638.

Calcote, H.F. (1949) Electrical properties of flames: Burner flames in transverse

electric fields. Proc. Combust. Instit., 3, 245.

Calcote, H.F. (1957) Mechanisms for the formation of ions in flames. Combust.

Flame, 1, 385.

Calcote, H.F. (1962) Ion production and recombination in flames. Proc. Combust.

Instit., 8, 184.

Calcote, H.F. (1963) Ion and electron profiles in flames. Proc. Combust. Instit., 9,

622.

Calcote, H.F. and Berman, C.H. (1989) Increased methane-air stability limits by a

DC electric field. ASME Fossil Fuels Combustion Symposium, PD-25, 25–31.

Calcote, H.F. and Pease, R.N. (1951) Burner flames in longitudinal electric fileds.

Ind. Eng. Chem., 43, 12, 2726.

Calcote, H.F. Kurzius, S.C. and Miller, J. (1967) Negative and secondary ion

formation in low pressure flames. Proc. Combust. Instit., 10, 605.

Green, J.A. and Sugden, T.M. (1963) Some observations on the mechanism of

ionization in flames containing hydrocarbons. Proc. Combust. Instit., 9, 607.

Lawton, J. andWeinberg, F.J. (1969) Electrical Aspects of Combustion, Clarendon

Press, Oxford.

Lawton, J., Mayo, P.J., and Weinberg, F.J. (1968) Electrical control of gas flows

in combusion processes. Proc. R. Soc. Lond. A, 303, 275.

Noorani, R.I and Holmes, R.E. (1985) Effects of electric fields on the blowoff

limits of a methane-air flame. AIAA. J., 23, 9, 1452.

Poncelet, J., Berendsen, R. and van Tiggelen, A. (1956) Comparative study of

ionization in acetylene-oxygen and acetylene-nitrous oxide flames. Proc.

Combust. Instit., 7, 256.

van Tiggelen, A., (1963) Shuler, K.E. and Fenn, J.B. (Eds.) Ionization in High

Temperature Gases, Academic Press, New York, p. 165.

1304 A. ATA ET AL.