Combustion and Flame 157 (2010) 17–24

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flamesin a coflow

M.K. Kim a, S.K. Ryu a, S.H. Won a, S.H. Chung b,*

a School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Koreab Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia

a r t i c l e i n f o

Article history:Received 16 June 2008Received in revised form 29 September 2009Accepted 1 October 2009Available online 30 October 2009

Keywords:Electric fieldsLiftoffBlowoffStabilizationLaminar jet

0010-2180/$ - see front matter � 2009 The Combustdoi:10.1016/j.combustflame.2009.10.002

* Corresponding author.E-mail address: sukho.chung@kaust.edu.sa (S.H. Ch

a b s t r a c t

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow havebeen investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzlewith a single-electrode configuration. The liftoff and blowoff velocities have been measured by varyingthe applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed thatthe AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity.As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowoutvelocity without having electric fields. In such a case, a blowoff occurred directly without experiencinga lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three differentregimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocityof either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC fre-quency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltageand reasonably independent of the AC frequency. At the high voltage regime, the detachment was signif-icantly influenced by the generation of discharges.

� 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

In an effort to develop an advanced combustion system withhigh energy efficiency, reliable ignition, and improved flame stabil-ization, plasma-assisted combustion has been extensively investi-gated and demonstrated that fundamental combustion behaviors,such as ignition, extinction, and flame speed can be significantlyenhanced through the interaction between plasma and combus-tion [1–11].

The major enhancement mechanisms in plasma-assisted com-bustion have been explained based on three distinctive processes.First is overall thermal heating effect caused either by the hot arc-discharge of plasma or by the recombination reaction of active rad-icals produced by plasma [11–13]. In view of overall energy effi-ciency, the thermal heating effect may not be a promising wayfor the enhancement with a plasma system. Second is the kineticenhancement by the interaction with active and electronically ex-cited species produced by plasma [11–13]. Considering that thetypical life time of these excited species is relatively short, the pre-vious studies have implied that the direct in situ production ofthese excited species with combustion-plasma system is necessaryto magnify the kinetic enhancement [11]. Recently, the enhance-

ion Institute. Published by Elsevier

ung).

ment of premixed flame speed in a counterflow burner was ob-served by using a microwave plasma system together with lasertechniques [12]. Third is the hydrodynamic effect from electricfields mainly associated with the ionic wind effect which can in-duce the bulk motion of flow, thus possibly enhance the mixingcharacteristics [14,15].

When plasma is integrated in a combustion system for im-proved performance through the complicated interaction amongthe thermal, kinetic, and hydrodynamic effects, the understandingof the effect of electric fields on flame properties are essential.However, detailed understanding is still rather limited. Frequently,the effects of electric fields have been explained based on thehydrodynamic effect of the ionic wind, which arises from the accel-eration of ions in electric fields by the Lorentz force and subse-quent momentum transfer to neutral particles by randomcollision, resulting in a bulk flow motion [14,15].

Studies on the effect of AC electric fields on flame stabilizationcharacteristics, however, are rather limited. Recently, it has beenshown that the stabilization characteristics of flame reattachment[16] and the propagation of laminar lifted flame edge [17] can besignificantly affected by AC electric fields with relatively smallpower consumption less than O(1 W) by using the single-electrodeconfiguration. The experimental study on the liftoff of nonpre-mixed turbulent jet flames [3] showed that the liftoff velocitycould be increased up to 50% by applying electric fields, thereby

Inc. All rights reserved.

18 M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24

extending the nozzle-attached flame regime appreciably in termsof jet velocity.

The present study is an extension of the previous work [3] tolaminar jet flames by applying both AC and DC electric fields tothe fuel nozzle. The single-electrode configuration has beenadopted as previously [3,16,17] with the emphasis on the effectof electric fields on liftoff and blowoff. Note that even for the casewithout having electric fields, the detailed understanding of liftoffmechanism in nonpremixed jet flames is rather limited, because ofthe complex nature in the mechanism including near-nozzle flowbehavior and heat transfer to fuel nozzle [18,19]. Due to quenchingeffect, a nozzle-attached flame has a edge flame structure experi-encing heat loss [20]. The characteristics of such flames are lessknown as compared to a lifted flame edge with tribrachial (or tri-ple) structure. Combining with the inherent complexities in theliftoff mechanism, the effect of electric fields on liftoff or blowoffis expected to be much more complex to physically characterizein detail at the present stage. Thus, the present study is focusedon extracting systematic experimental data which can serve as afundamental data for future theoretical and modeling works.

2. Experiment

The experimental apparatus consisted of a coflow burner andflow controllers, a power supply system, and a measurement setupas schematically shown in Fig. 1. The coflow burner had a centralfuel nozzle with flush end, as indicated in the inset of Fig. 1, madeof stainless steel with its inner and outer diameters of 0.254 and1.588 mm, respectively. The nozzle length was 10 cm to ensurethe fully developed velocity profile at the nozzle exit in the presentrange of jet velocity. Coflow air passed through meshes, beads anda honeycomb for uniform velocity profile. A concentric acrylic cyl-inder with 90 mm i.d. was installed at the exit of coflow air to sup-press external flow disturbance. The coflow velocity was fixed at3 cm/s to achieve a stable flame, as yet to maintain the resem-blance to the free jet experiment [21]. The whole body of the co-flow burner was made of acetal resin for electrical insulationexcept the fuel nozzle. The fuel was chemically-pure grade pro-pane. The flow rates of fuel and air were controlled by mass flow

Fig. 1. Schematic of ex

controllers calibrated with a bubble meter and a wet-test gas me-ter. Direct images of lifted/nozzle-attached flames were taken witha digital camera, and the liftoff height was determined as a dis-tance between the flame base and the nozzle tip.

A DC and AC power supply (Trek, 10/10B-FG) was utilized as anelectrical source and the frequency of AC was controlled in therange of 60–1000 Hz by a function generator to obtain sinusoidalwave pattern of AC voltage. The applied voltage was varied up to7 kV in the RMS value. The high voltage terminal of the power sup-ply was connected to the fuel nozzle, thus the fuel nozzle served asa high voltage electrode. The other terminal was connected to thebuilding ground such that the system can be regarded as an openelectric circuit [3,16,17]. In this case, the electric fields can be as-sumed to be formed between the nozzle electrode and infiniteground far away from the nozzle [17]. The intensity of electricfields will be proportional to applied voltage. Because the intensityspreads out from high voltage electrode into the space, the inten-sity is higher near the electrode and lower as moving away fromthe nozzle. Actual electric field intensity exerted on a flame, how-ever, will be much more complex by the existence of ions and elec-trons in the flame zone, which is typically in the order of 109–1012 cm�3 [1,2]. Moreover, the charges in a flame zone can distortthe electric fields, especially near the quenching zone of the flamebase. Therefore, the data will be presented in terms of the experi-mental variables of applied voltage and frequency.

The electric power consumption was monitored at various ap-plied voltages and frequencies by using a current probe (Tektronix,TCPA300) and an oscilloscope (Tektronix, TDS1012B). The behaviorof flame was observed by varying the jet velocity for specified elec-tric field conditions. To visualize the change in flame structure byelectric fields, a planar laser-induced fluorescence (LIF) techniquefor OH radicals was adopted. Details of the LIF setup have been re-ported previously [22].

3. Results and discussion

In a laminar jet with small jet velocity, a nozzle-attached flamecan be formed at the exit of fuel nozzle. As the jet velocity in-creases, the flame length increases linearly with jet velocity. When

perimental setup.

Fig. 2. Direct photographs of nozzle-attached and lifted flames for: (a) (U0 [m/s],Va [kV]) = (9.52, 0); (b) (9.65, 0); (c) (9.65, 0.2); (d) (10.47, 0.2); (e) (10.67, 0.2) atf = 60 Hz (dotted line: position of nozzle exit).

160 00.10.20.3m

m]

Voltage [kV] BlowoutDF

RPF

80

120 0.40.5

eigh

t HL [m

0.4

0.5 kVLPFTribrachial

point

40

80

Lifto

ff he

0110.2

0.3

Blowoff

08 9 10 11 12 13 14

0 0.

0.6

Blowoff

Jet velocity U0 [m/s]

Fig. 3. Liftoff height with jet velocity for f = 60 Hz at various AC voltages.

M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24 19

the jet velocity reaches a liftoff velocity, the flame base detachessuddenly from the nozzle and a stationary lifted flame can beformed [20–24]. Such behavior without having electric fields isexhibited in Fig. 2a and b for the fuel jet velocities ofU0 = 9.52 m/s and 9.65 m/s corresponding to nozzle-attached andstationary lifted flames, respectively.

When the AC voltage of Va = 0.2 kV and the frequency off = 60 Hz is applied, Fig. 2c for U0 = 9.65 m/s shows that the flamemaintains as a nozzle-attached, as compared to the lifted flame(b) at the same jet velocity. The nozzle-attached flame persistsup to U0 = 10.47 m/s (d). At further increased jet velocity, a liftoffoccurs and a stationary lifted flame is stabilized, as demonstratedat U0 = 10.67 m/s (e). We have also tested the DC cases and foundthat the effect of DC electric fields on liftoff velocity from a noz-zle-attached flame was negligible as compared to the case withouthaving electric fields. This behavior is in accordance with the casefor the turbulent nonpremixed jet flames reported previously [3].In the following, we will focus on the effect of AC electric fields.

The liftoff velocity from a nozzle-attached flame and liftoffheight behavior are plotted in terms of jet velocity in Fig. 3 forf = 60 Hz by varying the applied voltage. Without applying voltage,the liftoff from a nozzle-attached flame occurs at U0 = 9.65 m/s asmarked by the vertical arrow and then the liftoff height HL in-creases nonlinearly with the jet velocity. As the jet velocity be-comes excessive, a blowout occurs at U0 = 13.26 m/s. Thisnonlinear dependence has been investigated extensively basedon the structure of laminar lifted flame edge having the tribrachialstructure [20–24]. It consists of a lean premixed flame (LPF) and arich premixed flame (RPF) wings and a trailing diffusion flame (DF),all extending from a tribrachial point, as schematically shown inthe inset in Fig. 3.

The coexistence of three different types of flames indicates thatthe tribrachial point is located along the stoichiometric contour inthe mixing layer of the jet [25]. The existence of premixed flamewings implies that it has a propagation speed. Hence, for a liftedflame edge to be stationary, the propagation speed of tribrachial

edge should balance with the local flow velocity. By applying thestoichiometry and the balance mechanism to the similarity solu-tions of momentum and fuel concentration in laminar free and co-flow jets [20–26], it has been derived that the liftoff height in termsof jet velocity has a functional relation as HL 1 Un

0 for free jet withn = (2Sc � 1)/(Sc � 1), where Sc is the Schmidt number of fuel. Thebest fit indicated as the dotted line in Fig. 3 has n = 4.602, corre-sponding to Sc = 1.384. This value is in good agreement with thatcalculated from thermodynamic tables of Sc = 1.376 [23]. Theblowout velocity has also been predicted from the similarity solu-tions, over which the balance mechanism and the stoichiometrycannot be simultaneously satisfied for the fuel with Sc > 1 [22].

The liftoff condition, however, cannot be predicted from thesimilarity solutions [20–24], since the solutions are singular atthe origin and it is necessary to account for the heat loss to the noz-zle, the wall termination of radicals, and the air partial-premixingin a quenching zone. These effects have not been clearly character-ized yet. Phenomenologically, the previous experimental data [23]indicated that the boundary velocity gradient [18] can be corre-lated with the liftoff behavior.

When AC voltage is applied, the results in Fig. 3 show that theliftoff velocity increases sensitively to the applied AC voltage, asmarked by the vertical arrows. Although the liftoff velocity is influ-enced appreciably by the applied voltage, the liftoff height afterliftoff with the voltage is not influenced much as shown in thecomparison to the case without having electric fields (solid circles).This can be attributed to the electric field intensity acting on theflame base. A representative electric field intensity will be inver-sely proportional to the distance from the nozzle electrode to theflame base [16,17]. For nozzle-attached flames, the distance be-tween the flame base and nozzle tip is typically O(1 mm), whilethe liftoff height is typically O(10–100 mm). Consequently, theelectric field intensity acting on the flame base for lifted flame willbe one or two order smaller than that for nozzle-attached flame atthe same applied voltage. Note that the applied voltage of the pres-ent liftoff study is one-order less than that for the reattachment oflifted flame [16]. Furthermore, in the previous study on the propa-gation speed of tribrachial flames [17], the region where the ap-plied electric field affects the propagation speed was smallerthan about 40 mm from the nozzle exit. As can be seen in Fig. 3,the liftoff heights were larger than about 40 mm, such that theelectric fields could not influence much the propagation speed oftribrachial flame, so does to the liftoff height.

For Va > 0.6 kV, a blowoff occurs directly from a nozzle-attachedflame, without experiencing a stationary lifted flame. This is be-

16f = 60 Hz

14

yU

0 [m/s

]

Blowout Blowoff

12

t vel

ocity

LiftoffNozzle attached flame

Lifted flame

8

10Jet Nozzle-attached flame

80 0.2 0.4 0.6 0.8 1

Applied voltage Va [kV]

Fig. 4. Liftoff, blowout, and blowoff velocities for f = 60 Hz at various AC voltages.

20 M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24

cause the detachment velocity is already larger than the blowoutvelocity without having applied voltage.

The effectiveness of applying AC voltage on the liftoff and blow-off velocities is further demonstrated in Fig. 4, where the jet veloc-ities at the critical conditions of liftoff, blowout and blowoff areplotted in terms of AC voltage for the cases with low Va less than1 kV. The liftoff and blowout velocities for Va = 0 kV is marked atU0 = 9.65 and 13.26 m/s, respectively. As the applied voltage in-creases, the liftoff velocity increases reasonably linearly. ForVa > 0.6 kV, the blowoff velocity becomes larger than the blowoutvelocity without applying voltage of U0 = 13.26 m/s, such that theflame blows off directly from a nozzle-attached flame withoutexperiencing a stationary lifted flame. The extension of the noz-zle-attached flame regime clearly demonstrates the effectivenessof applying AC voltage to the nozzle for improved jet flame stabil-ization. In the following we will adopt the terminology of detach-ment to cover the liftoff and blowoff.

3.1. Effect of AC voltage

The detachment velocity has been further analyzed up to highervoltage. The results are plotted in Fig. 5 for f = 60 Hz in terms of thenormalized velocity, which is scaled with the liftoff velocity Uo

0jLO

at Va = 0 kV. The result exhibits that there exists three distinct re-

3

U0o | LO

Intermediate HighLow

2

2.5

city

U0 /

U

1.5

2

Detachment limitized

vel

oc

PA

0.5

1 Partially-attached (PA)

Pointing edge (PE)

Nor

mal

i

PE

0 50 1 2 3 4 5 6 7

Applied voltage Va [kV]

Fig. 5. Normalized detachment velocity with AC voltage for f = 60 Hz.

gimes in terms of applied voltage: (1) low voltage regime up toVa � 4 kV; (2) intermediate voltage regime up to Va � 6 kV; (3) highvoltage regime for Va > 6 kV. When the applied voltage is low, thedetachment velocity increases monotonically with Va. A satisfac-tory linear correlation of detachment velocity with the appliedvoltage was obtained as Uo

0jLO¼1:059þ0:421�Va½kV�ðf ¼60 HzÞup to Va � 4 kV, with the correlation coefficient of R = 0.99.

In this regime, the transition from a nozzle-attached flame to itsdetachment occurs consistently and the flame base maintains axi-symmetric nature prior to the detachment. This behavior is dem-onstrated in Fig. 6a–e, where the direct photographs for thenozzle-attached flames just prior to the detachment, having axi-symmetric base, are exhibited. Note that the increase in the flamelength at the detachment condition in the low voltage regime is anindication of the extension of nozzle-attached flame regime interms of jet velocity by the influence of AC electric fields, sincethe jet flame length in the laminar regime increases linearly withjet velocity [23].

When the applied voltage becomes larger than 4 kV, the detach-ment velocity exhibits markedly different behavior as compared tothe low voltage regime, in such a way that the detachment velocitystarts to decrease linearly with the AC voltage. In this intermediatevoltage regime, for example Va = 5 kV in Fig. 5, the base of the noz-zle-attached flame still maintains axisymmetric by increasing thejet velocity up to Uo=Uo

0jLO ¼ 1:6. At further increased jet velocityup to the detachment limit, the base of the nozzle-attached flamebecomes slanted such that the base is only partially-attached (PA),as exhibited in Fig. 6f. Even though the edge is partially-attached,the edge shape is relatively smooth without having a cusp-likebehavior. The partially-attached point sometimes rotates ran-domly along the nozzle rim. At further increased jet velocity, even-tually the detachment occurs at Uo=Uo

0jLO ¼ 2:0. A mild hissingsound was accompanied right before the detachment in this par-tially-attached regime.

The generation of hissing sound, which is an indication ofacoustic pressure fluctuation locally, is an indication of corona dis-charge. Note that pressure fluctuation leads to flow disturbanceand fluctuation. The previous visualization of flow fields [16] byadopting the schlieren technique demonstrated a disturbance inflow fields even for the cold flow at very high voltage. Hence, thedecrease in the detachment velocity with applied voltage in theintermediate voltage regime can be partially attributed to the flowdisturbance associated with corona discharge. Detailed flow fieldmeasurement will be a subject of future study.

In the high voltage regime, the flame can be either partially at-tached or lifted depending on applied voltage and the flame baseshows a pointing-edge (PE) with a cusp-like shape as demon-strated in Fig. 6g and h. The pointing-edge appears intermittentlywhen the flame base is partially lifted and the position movesaround the nozzle. A pronounced sizzling noise was generated.This behavior is similar to the case for the previous turbulent jetflames with AC electric fields [3]. From the visualization of N2

emission at 337 nm, the cusp-like pointing-edge had been attrib-uted to the generation of streamer discharge. The detachmentvelocity again decreases with the applied voltage, even thoughthe decrease is weak, as compared to the cases in the intermediatevoltage regime. Note that for Va = 7 kV, the pointing-edge appearsfrom Uo=Uo

0jLO � 0:5 and the detachment occurs at Uo=Uo0jLO � 1.

This implies that the enhancement effect in the detachment veloc-ity by the applied AC electric fields is significantly deteriorated inthe high voltage regime as compared to the low voltage regime.

Once the flame base is partially lifted, the yellow luminositydue to soot production decreases appreciably by the effect of airentrainment into the fuel region, in such a way that the flamehas a partially-premixed nature. This is exhibited in Fig. 6a andh, whose jet velocities are comparable. In the intermediate and

Fig. 6. Direct photographs of nozzle-attached flames just prior to nozzle-detachment at various AC voltages for f = 60 Hz: (a) (U0 [m/s], Va [kV]) = (9.51, 0); (b) (14.83, 1); (c)(17.28, 2); (d) (21.84, 3); (e) (27.15, 4); (f) (19.39, 5); (g) (12.44, 6); (h) (9.24, 7).

M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24 21

high voltage regimes near the detachment, the overall flame issomewhat tilted from the jet axis, caused by the uneven airentrainment into jet core by the nature of non-axisymmetric edgenear the nozzle. This is especially pronounced for the cases withpointing edges, as demonstrated in Fig. 6g and h.

For a fixed Va in the low voltage regime, as the jet velocity in-creases with a step, a blowoff occurs at a certain jet velocity. Insuch a case, when the fuel jet was re-ignited, a nozzle-attachedflame can be stabilized again. This process has been repeated byincreasing jet velocity until the re-ignition cannot be achieved.Without adopting this procedure, the transition between the lowand intermediate voltage regimes was gradual than the sharp tran-sition depicted in Fig. 5.

3.2. Effect of AC frequency

The effect of AC frequency on the detachment velocity has beeninvestigated and the results are shown in Fig. 7. As the frequencyincreases in the low voltage regime, the normalized detachmentvelocity increases, such that the enhancement effect of AC isimproved. For the case with f = 1000 Hz, Uo=Uo

0jLO becomes evenlarger than four, indicating that the jet flame persists as nozzle-

4.0

4.5

601500o | LO

Frequency [Hz]

3.0

3.5

1503005008001000

city

U0 /

U0

2.0

2.5

zed

velo

c

1.0

1.5

Nor

mal

iz

0 1 2 3 4 5 6 7Applied voltage V

a [kV]

Fig. 7. Normalized detachment velocity with AC voltage at various frequencies.

attached over four times of the liftoff velocity without havingelectric fields. Concerning the characteristics in the detachmentvelocity with applied voltage in the low voltage regime, the behav-ior at f = 60 Hz shows that the increasing trend in Uo=Uo

0jLO with ap-plied voltage somewhat slows down for 1 < Va < 2 kV, as exhibitedin Fig. 7. As the frequency increases, the linearity improves in thelow voltage regime.

In the intermediate and high voltage regimes, the detachmentvelocity deteriorates with the increase in the applied voltage. Ascompared to the low voltage regime, the variation in the detach-ment velocity with frequency in the intermediate and high voltageregimes is relatively weak.

The transition between the low and intermediate voltage re-gimes, corresponding to the maximum in the detachment velocityfor a specified frequency in Fig. 7, occurs at lower voltage as thefrequency increases. This behavior is represented in Fig. 8, wherethe transition voltage, marked as the solid circles, is plotted interms of frequency. The transition voltage decrease with AC fre-quency, which can be understood since the electrical dischargeor streamers are expected to be sensitive to the time rate of changein voltage [27]. The best fit of the present transition voltage is Va

[kV] = 6.33 � 1.33 � log (f [s�1]) with R = 0.98. The threshold volt-age will be further discussed later.

7.0

8.0U

0= 7.00 m/s

U0= 9.04 m/s

V] Partially-attached base

5.0

6.0Transition voltageThreshold voltage

age

V a [kV Partially-attached base

Axisymmetric base

3.0

4.0

plie

d vo

lta

Intermediate-voltage regime

1 0

2.0

3.0

App Low-voltage regime

1.0100 1000

Frequency f [Hz]

Fig. 8. Flame regimes in terms of AC voltage and frequency.

0.20

0.35

0.4currentpowerselected datacurrent

0.15

0.25

0.3

selected data

ms)

[mA

] Active phase

0.10

0.15

0.2ur

rent

(rm

power [W

Time

voltagedifference

0.05

0.05

0.1Cu

W]

thresholdvoltage

0.00 00 1 2 3 4 5 6 7

Applied voltage (rms) [kV]

Fig. 9. Measured current and calculated active power with AC voltage for f = 500 Hz.

22 M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24

Below this transition voltage criterion, the linearity of thedetachment velocity with AC voltage is maintained. This transitionvoltage is also an indication for the maximum attainable enhance-ment in the liftoff velocity.

For a fixed U0, as the applied voltage increases, an axisymmetricflame base could change to non-axisymmetric base, as can be seenin Fig. 5. The electric fields exerted on a non-axisymmetric flamebase will be distorted from an axisymmetric one. This voltage fora fixed U0 corresponds to the maximum possible voltage to main-tain axisymmetric electric fields to the flame base. The conditionsof the voltage from axisymmetric to partially-attached edge areplotted in Fig. 8 for U0 = 7.0 and 9.04 m/s. The result shows thatthe overall behavior has a similar trend as the transition betweenthe low and intermediate voltage regimes, in that Va decreaseswith the logarithmic dependence on the frequency.

The dependence of frequency on the transition between theintermediate and high voltage regime is somewhat ambiguous.However, the criterion for the high voltage regime for the detach-ment can be reasonably approximated as Va � 5.5 kV from the re-sult in Fig. 7.

Fig. 10. Visualization of OH concentration with planar LIF measurement with fixedU0 = 7 m/s by varying applied voltage; (a) Va = 0, (b) 4 kV with f = 60 Hz andsubtraction image (c) between (a) and (b), and by varying AC frequency at fixedVa = 3 kV; (d) f = 60, (e) 500 Hz and subtraction image (f) between (d) and (e).

3.3. Electric power measurement

To investigate the role of electric power consumption on the re-gimes of detachment, the current through the high voltage elec-trode was measured at various AC voltages and frequencies byusing the current probe and oscilloscope. The measured rms cur-rent Ia for f = 500 Hz is shown in Fig. 9 with applied AC voltage.The current increases monotonically with the voltage. Consideringthat the detachment velocity has a transition between the low andintermediate voltage regimes at Va = 2.60 kV for f = 500 Hz in Fig. 8,the measured current alone cannot reflect the transition from thelow to intermediate voltage regimes.

Since there exists phase difference between the current and volt-age as shown in the inset of Fig. 9, the active power was calculatedbased on the definition of Pa[W] = Va[kV] � Ia[mA] � cos (h), whereh is the phase difference between the voltage and current. The resultof active power in Fig. 9 for f = 500 Hz shows that the active power isvery small (<0.01 W) for the applied voltage lower than 2 kV andstarts to increase reasonably linearly at higher voltage. This resultimplies the existence of a threshold voltage to active power, whichcan be determined from the extrapolation of the linear fitting of ac-tive power in the relatively higher voltage regime. The thresholdvoltage thus obtained for f = 500 Hz is 2.45 kV from the linear fitting

of the data marked as the closed squares. This value is in good agree-ment with the transition voltage of 2.60 kV for f = 500 Hz shown inFig. 7. The threshold voltages at various frequencies were repre-sented in Fig. 8 as the open diamonds. The result shows that the tran-sition voltage, determined from the maximum detachment velocity,agrees well with the threshold voltage determined from the activepower. This result indicates that the transition from the low to inter-mediate voltage regime can be attributed to the discharge character-istics. After the transition, the detachment velocity deteriorates bythe discharge, since the detachment velocity of laminar jet flamesis sensitive to flow perturbation [18–20] which is accompanied withthe discharge.

3.4. OH PLIF measurement

To demonstrate the effect of AC electric fields on flame stabil-ization, planar LIF images for OH radicals were taken for a fixedflow condition of U0 = 7 m/s in Fig. 10, which corresponds to very

4.0

U0o | LO

3.0city

U0 /

U

2.060150300

Frequency [Hz]

ized

Vel

oc

1.0

3005008001000N

orm

ali

0 5 10 15 20 25 30 35

Va X f 0.38 [kV-s–0.38]

Fig. 11. Correlation of detachment velocity with applied voltage and frequency inthe low voltage regime.

4.0 60150U

0o | LO

Frequency [Hz]

3.0

1503005008001000

city

U0 /

U

2.0ized

vel

oc

1.0Nor

mal

i

2 2.5 3 3.5 4 4.5 5 5.5 6

Applied voltage Va [kV]

Fig. 12. Correlation of detachment velocity with applied voltage and frequency inthe intermediate voltage regime.

M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24 23

stable edge structure. The images were the accumulation of 100realizations to enhance the signal to noise ratio.

The case without applying voltage (a) shows a typical OH distri-bution, having the maximum intensity along the stoichiometriccontour and the effect of quenching near the nozzle rim, exhibitingedge structure. For Va = 4 kV and f = 60 Hz (b), the image showsthat the quenching distance has been increased and the heightand width of the flame are reduced slightly. The subtraction image(c) between (a) and (b) exhibits clearly this change. This can beunderstood based on enhanced entrainment of air into fuel streamby the increase in the quenching distance.

As the frequency increases from 60 to 500 Hz (Fig. 10d–f), onlythe region very near the rim has been influenced. Although thepresent visualization provides limited information, the appliedvoltage and frequency could significantly alter the edge structureof nozzle-attached flame, which could affect the detachment veloc-ity. Detailed quantification of the effects of electric fields, heattransfer to the nozzle, and near-nozzle flow on detachment shouldbe a future study.

3.5. Correlation of detachment velocity

To further characterize the effect of applied voltage and fre-quency in the linear regime, the rate of increase in the detachmentvelocity with the applied voltage at various AC frequencies hasbeen determined. The frequency dependence was found to bedU0/dVa � f0.38. By utilizing this relation along with the lineardependence of the detachment velocity on the applied voltage,the experimental results in the low voltage linear regime are re-plotted in Fig. 11 as a function of applied voltage and frequency.The result demonstrates the linear correlation between the nor-malized detachment velocity and Va � f0.38 with the best fit ofU0=Uo

0jLO ¼ 1:040þ 0:094� Va½kV� � f 0:38½s�0:38� with R = 0.99. Thisrelation is valid up to the transition voltage shown in Fig. 8.

Table 1Effect of voltage and frequency on various jet flame behaviors.

Flame behavior (flame types) Experimental conditions

Detachment velocity (nozzle-attached flame) Laminar (present) Low voltage Va <Intermediate voltHigh voltage Va >

Turbulent [3] Low Va

Reattachment velocity (tribrachial flame) Laminar [16]Propagation speed (tribrachial flame) Laminar [17]

As mentioned previously, the detachment velocity has weakdependence on AC frequency in the intermediate voltage regime.In this regard, Fig. 12 shows the correlation between the detachmentvelocity and the applied voltage in the intermediate voltage regime.A reasonable correlation independent of frequency is obtained withthe best fit of Uo=Uo

0jLO = 5.967 � 0.845 � Va[kV] with R = 0.98. Thisrelation is valid for the voltage larger than the transition voltage rep-resented in Fig. 8 and the voltage approximately smaller than 5.5 kV.In the high voltage regime, we have only limited data because of theAC power supply up to 7 kV at 1000 Hz. However, note that theenhancement of the detachment velocity is weak as demonstratedin Fig. 7, meaning that Uo=Uo

0jLO is close to unity.

4. Various electric field effects

As mentioned previously, the effect of electric fields, especiallyfor AC, has not been fully understood yet. Depending on flame typeand phenomena, the dependence on the voltage and frequency var-ies significantly. For the future study to understand the detailedphysical mechanism of electric fields, we have summarized the ef-fect of AC and DC on the flame characteristics in nonpremixed jetswith the single-electrode configuration, including the present lam-inar detachment, the turbulent liftoff [3], the laminar reattachmentfrom a lifted flame to a nozzle-attached flame [16], and the propa-gation speed of tribrachial edge [17]. The result is summarized inTable 1. Here, the detachment is from a nozzle-attached edgeincluding liftoff and blowoff. The reattachment is from a liftedflame to a nozzle-attached flame, where the lifted flame has a tri-brachial structure. The propagation speed of tribrachial edge flamehas been measured in a laminar jet from a transient propagation oftribrachial edge flame after ignition, where x is the distance be-tween the high-voltage nozzle electrode and the edge position.Thus, Va/x can be regarded as a representative electric fieldintensity.

AC voltage AC frequency DC voltage

2–4 kV Linear increase Increase �f0.38 Minimalage Va (2–4 � 5.5 kV) Linear decrease Minimal Minimal5.5 kV Weak dependence Weak dependence Minimal

Linear increase Increase �log (f) MinimalLinear increase Decrease �1/f0.37 MinimalIncrease �Va/x Minimal Increase �Va/x

24 M.K. Kim et al. / Combustion and Flame 157 (2010) 17–24

As listed in Table 1, the effect of voltage is generally linear forthe various phenomena listed. The effect of frequency, however,depends sensitively on the phenomena, for example, positive effecton the laminar detachment in the low voltage regime and negativeeffect on the reattachment velocity. The DC voltage has minimal ef-fects for stationary flames, including the detachment and reattach-ment, while it has positive effect for the unsteady propagatingflames, which has been explained based on the variation of theelectric field intensity from the variation in x as the flame propa-gates. Further research is needed in the future to understand de-tailed underlying physics.

5. Concluding remarks

The enhancement of jet flame stabilization in the laminar co-flow has been investigated experimentally by observing the liftoffand blowoff behaviors of nonpremixed propane jet flames byapplying the electric fields. The detachment velocities have beenmeasured by varying the applied AC voltage and frequency inthe single-electrode configuration. The results showed that thedetachment velocity could be extended appreciably by applyingthe AC voltage to the central fuel nozzle. In the low voltage re-gime, the detachment velocity increased linearly with the appliedvoltage and has its maximum value at the transition voltage. Theincrease in the AC frequency improved the stabilization in thenozzle-attached flame mode. In the intermediate voltage regime,the detachment velocity decreased with the applied voltage andthe dependence on the frequency was weak. From the electricpower measurement, it has been revealed that the transition fromthe low to intermediate voltage regimes can be attributed to dis-charge characteristics. The OH PLIF measurement indicated thatthe detachment velocity can be significantly enhanced due tothe change in flame edge structure caused by the applied AC volt-age in the low voltage regime. In the high voltage regime, thedetachment velocity was appreciably influenced by the genera-tion of streamers, such that the base of flame exhibited pointingedge.

Acknowledgments

This work was supported by HKCRC through IAMD and CCRCfrom KAUST. The authors would like to thank Prof. Yiguang Juand Dr. Timothy Ombrello for their helpful suggestions.

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