ELECTRIC FIELD ASSISTED DOWNWARD …...Cite this Article: Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu Baweja and Herambraj Nalawade, Electric

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 11, November 2017, pp. 596–615, Article ID: IJMET_08_11_062

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

ELECTRIC FIELD ASSISTED DOWNWARD

SPREADING FLAME

Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra,

Chakshu Baweja and Herambraj Nalawade

Department of Aerospace Engineering, SRM University, Chennai, India

ABSTRACT

In flame spread research, an important area of concern is establishing operating

criteria in presence of an external energy source. The work is motivated by the need to

understand the flame behavior in presence of an external electric influence to produce

unique combustion results that are not simple interpolations. Through systematic

experimentation, the effect of an external electric field on a downward spreading

flame is investigated. An experimental setup was upraised and related energy

interactions between the flame and the electric source are explored under diverse

conditions to respond to the unique aspect of combustion. The role of controlling

parameters viz., separation distance, electrodes symmetry, number of electrodes and

arc impingement on flame and the pilot fuel were evaluated in terms of flame spread

rate variation. Results shows that the presence of an external electric field

significantly affects spreading of flame and the governing interaction mechanics

between an electric and heat energy source strongly depends on the separation

distance. As a potential energy source, the external electric field stimulates the role of

a heat source and heat sink under varying conditions. Amalgamated configurations

with increased number of electrodes were found to exhibit the counter-balancing

features limiting the spreading flame behavior. The resultant flame behavior and

extent of change in the spreading rate substantiates with the altered thermochemistry

and related losses. The results direct in developing technological application for

better fire safety provisions and efficient combustion.

Keywords: Opposed flow flame spread, Electric Field, Flame spread rate, Forward

heat transfer, Fire safety.

Cite this Article: Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan,

Vinayak Malhotra, Chakshu Baweja and Herambraj Nalawade, Electric Field Assisted

Downward Spreading Flame, International Journal of Mechanical Engineering and

Technology 8(11), 2017, pp. 596–615.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11

Electric Field Assisted Downward Spreading Flame


1. INTRODUCTION

Fire is one of the very powerful and useful resources of nature. Its diverse nature finds its

applications from daily household needs to a million-dollar space mission. However, the same

tends to be the cause of biggest disasters if not controlled properly. Fire control is required for

both efficient combustion as well as fire-safety applications. Noticeable research works have

proven the fact that it is impossible to eliminate all ignition sources. Thus, fire inhibition is

achieved through use of fire resistant materials and external resources to eliminate excessive

spread. One of the solution sought was use of an external energy sources viz., sound,

magnetic, light with related inter energy interactions leading to improved fire control. One of

the prominent forms of fire disasters have been noted to arises from electrical sources. Major

accidents, air crashes, industrial and domestic fires etc. are responsible for magnificent

hazards and loss of resources, mankind and every year enormous financial spend on its

control.

A small spark can evolve into a massive fire in no time causing great deal of monetary

loss and fatalities. Selected cases include exposed wiring, overloaded outlets, extension cords,

overloaded circuits, static discharge being primary source of electrical fires. Appreciable

research efforts have been tended to understand the inter-relation between an electric arc and

transition into vast fires resulting in present fire safety equipments and standards. However,

momentous technological and engineering advancements have proven significant deficient in

these systems owing to prevent fires accidents owing to limited understanding of fire

behavior. This has necessitated active research efforts to magnify the physical domain of

electric field interaction with fires for enhanced safety. Fires essentially are studied on a solid

fuel in the scaled down form of a spreading flame (figure 1). The intended investigations are

thoroughly worked upon to gain essential physical insight into the fire spreading and control.

Flame spread represents diffusion flame propagating parallel to the solid fuel surface. The rate

at which flame spread is referred to as flame spread rate which is a function of instantaneous

heat transfer from burning to unburnt surface. The phenomenon is represented as a subject of

continuous heat and mass transfer (solid and gas phase). The studies characterize spreading

flame behavior under diverse conditions.

Figure 1 Schematic of (a) opposed flow flame spread, (b) concurrent flow flame spread.

The flame spread combustion phenomenon is predominantly studied as (a) Opposed flow

flame spread (b) Concurrent flow flame spread. In concurrent flow flame spread, the flame

spread in the direction of flow and is thus assisted with the enhanced heat transfer to the

unburnt fuel. Whereas, in opposed flow flame spread, spreading of flames is obstructed by

opposed flow direction. An aspect deeply associated is fire generation from an electric source

however, the main concern is to build up efficient preventive measures to suppress any slipup

as electrical fires are one of the uncontrollable phenomenon. Present work focuses on physical

insight into the flame-electric energy interactions under diverse conditions by studying the

flame spreading behavior. The work is motivated by the better combustion applications and

fire safety (figure 2).

Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu

Baweja and Herambraj Nalawade


Flame and electric field Interaction had been an active area of interest since last century.

Classical work by Thomson [2] revaluated the significance of combustion with electrons.

Brande [1] and Malinowski [3] adjudged the effect of high voltage on flames. Lewis [4]

conducted experiments by applying electric field longitudinally in the direction of flow of

gases. Air and flammable gases were led separately between electrodes. Important

explorations included a notable flame deflection in the direction of positive ion flow and

singularity of flame extinguishment with increase in potential. The work surfaced way for the

speeding up and slowing down of flame speed with respect to the negative electrode placed

upstream or downstream. Opposed jet diffusion flame extinction under applied electric field

of varying intensity and polarity was explored by Heinsohn [5].

Figure 2 Schematic of an electric arc and downward spreading flame interaction

Alteration in concentration combustion chemistry and order of reaction were determined

to be the key parameters with varying polarity and intensity. Electric field was found to

support flames with greater flame strength than without a field. The experimentation was

followed by Jaggers and Engel [7] estimation using experimental and numerical approach

with DC, AC and hf electric field to get persuasive clarification. Important declaration of

work detailed a DC field of 0.5 kV/cm increases the burning velocity by a factor of 2.

Additionally, the results were expounded with modeling and estimation of changes in flame

temperature. Floating flame under both transverse and longitudinal electric field was seen

with Ionic wind effects eliminated during this course of experiments. An electric field

interaction with a fire and its extinguishing mechanisms was investigated by Call and

Schwartz [8]. The work detailed preliminary investigations into various electrode/flame

configurations & evident physical effects on flame. The results were comparable to those

instituted by Lewis [4]. An important advancement was identification of high voltage to be

more influential in diffusion flame heat source than premixed flame heat source by Belsham

[9].

In last decade, the emphasis was restructured to understand the role of an electric field on

flame stability and efficient combustion with remarkable reviews like Zake et. al., [10], Ata

et. al., [12]. Effects of pulsed and continuous DC electric fields on reaction zone of premixed

propane-air flames was explored by Marcum and Ganguly [11]. The study reported systematic

characterization of the electric-field-induced changes of the shape and size of the inner cone

along with associated changes in the flame temperature profiles with equivalence ratios

between 0.8 and 1.7. The resulting electric pressure was noted to decrease the Lewis numbers



of the ionic species and drove the effective flame Lewis number below unity. A steady

influence of pressure difference across a flame due to ionic wind induced diffusion effect was

observed to result in increased flame speed and strong negative curvature of wrinkled laminar

flame.

An empirical relation was provided based on the changes observed as:

)1()11

(−

−+

∆µµ

α jp

Where, “∆�” is change in pressure; “�±” represents mobilities of the respective charge

carriers and “�” is current density.

A mathematical model to verify the stabilization due to ionic wind and prediction of flame

behavior was presented by Belhi et. al., [15]. The work relied on transport equation and

Arrhenius solution to reproduce the tendencies of experimental observations. This led to

Reshetnikov et. al., [16] focusing on experimental dependencies of position of maximal heat

release regions and electric charge localization zones on the inert agent nature and oxidizer

excess coefficients in diffusion flames. A novel numerical model to simulated the behavior

and motion of candle flame in electric field was presented by Anbarafshan et. al., [17]. In

recently, advancement in modern scientific equipment have repositioned studies on electrical

control of combustion. The contributions can be found in several reviews like Drews et. al.,

[18], Kuhl et. al., [19], Chien and Dunn-Rankin [20], Weinberg et. al., [21], Li [23], Barmina

et. al., [24], Fang, et. al., [25] in understanding electric field controlled combustion

characteristics. The presence of DC electric field on behavior of small scale diffusion ethanol

flame was patterned by Gan et. al., [26]. The findings were analyzed with measurement and

variation in flow rates, flame temperature and flame shapes. Similar study was conducted by

Xu et. al., [27] using liquid biobutanol diffusion micro flame. The explorations comprised

both experimental investigations and numerical simulation and were found to be consistent.

Shrivastava et. al., [28] explored the external influences on downward spreading flame

assisted by magnetic presence. The work investigated the interdependence of two diverse

phenomenon and related implications. Magnetic effect on flames was investigated in the form

of number of magnets, separation distance, intensity and polarity of magnets. Results detailed

that magnetic presence mostly augments the spreading of flames and is reluctant to the heat

sink effect. in the preceding part of work [29], the transitional combustion phenomenon from

smoldering to flaming on the downward spreading flames was investigated in the assistance

of an external heat source. Incense sticks were used as potential fuel and the study primarily

aimed at understanding the feasibility and spontaneity of transition owing to an external heat

source. Forward Heat transfer was noted to significantly deviate and intensify with varying

separation distance and number of external heat sources. With practical considerations,

external heat sources arrangement and orientation were stated to source demanding

consequences on the combustion process. It is important to note that combustion in electrical

presence have been appreciably investigated in diverse situations. However, topical reports of

unwanted accidents signify the deficit in present understanding and thus necessitates active

research efforts to acutely recognize the governing mechanism under diverse conditions. The

work is motivated by the need for efficient combustion and fire safety. Present study aims at

providing both qualitative and quantitative facets of the spreading flame behavior due to an

electric field.




The specific objectives of the study are to:

1. Understand the interactions between electric field and spreading flame.

2. Investigate the effect of controlling parameter on flame spreading viz., separation

distance, number of electrodes and configurations, symmetry, orientation.

2. EXPERIMENTAL SETUP AND SOLUTION METHODOLOGY

An experimental apparatus was upraised for the present study. It comprises of a) rectangular

glass enclosure with top face open and base with four open slits b) Iron stand to support the

enclosure (c) Paraffin wax candle as fuel (d) Molding clay to support the candle (please see

figure 3) and (e) Complete electric setup for arc (figure 3).

(a) (b) (c) (d)

Figure 3 Experimental Setup base (a) schematic (All dimensions are in mm) (b) front view and (c)

Top view (d) marked candle (external flaming source).

The molecular formula for paraffin is CnH2n+2, [13] where the value of n ranges from 19 to

36 and the average value is 25. Stoichiometric equation of the same can be yielded as:

�� 38�� 3.76�� → 25�� 26�� 142.88��

The candle sticks were made into cuboidal shape with 4 mm width, 4 mm thickness and

67 mm height. The fuel sticks were marked in two parts viz., opening 8 mm for combustion

stabilization followed by three regular intervals of 10 mm to tack the ignition front

propagation with time. To produce an arc, electrical circuit (figure 5) is devised using 85W

CFL circuitry and flyback transformer (18P EF22432). Four electrodes were made using pins

of electrical plugs. Electrodes were supported with wooden stands which could be moved

horizontally and vertically manually with minimal disturbance. Electrical AC input of 220-

230V was given to the CFL which ensured a stable arc of 20 mm. Flow of charge takes place

from left-hand side (positive) electrode to right hand-side (negative) one and a prior check

was made for electric field detection using a paper sheet that gets attracted towards electrodes.

It is important to note that the experimental setup affords visible arc till electrodes separation

distance of 0.50 cm transiting to an intermittent arc at 0.75 cm - 1 cm. Further separation in

electrodes vanishes arc parting electric field. The fuel is fixed at center and electrodes were

moved along the slits accordingly to vary the distance for all the configurations. Electrode tip

were placed such to impinge at the reaction zone of the flame. Necessary provision was made

to move the electrodes vertically along with flame as the wax melts and flame moves

downwards. The experiments were carried out in a quiescent room under normal gravity

conditions. Ignition was done at the apex of the candle for all configurations by exposing to a

pilot flame. Selected time interval of 10 minutes was taken in order to facilitate uniformity in

each reading and bring room to normalcy. Stopwatch was used to measure the lap time across

the marks. Entire experimentation was duly video graphed using Nikon D7000 DSLR at 1/60



shutter speed with Iso 1600-2000 and F1.8 and F3.5 aperture settings for better imagery of

each zone of flame.

(a) (b) (c)

(d) (e) (f) (g)

Figure 4 Electrical system and components (a) 85W CFL circuit (b) Flyback Transformer (c) Whole

Experimental system (d)-(e) Electrodes and wooden electrode holders (f) Arc generation (f) Electrodes

and flame.

Flame spread rate was articulated as the division of average distance burnt with average time taken.

Therefore, to ensure flame spread rate (Vf ) does not go out of bound, linear method is used as:

)2(av

sf

t

lV =

Where, “��” is the standard length of fuel taken (here, 1 cm) and “��” is the average time

taken for all three marked distances. From classical theory of ignition spread, assuming unity

width of fuel, the flame spread rate (Vf ) is defined by energy balance as:

)3()( aSurfacesss

netf

TTc

qV

−=

∫τρ

Where, “� !"#” = Net integrated heat transfer per unit time per unit area to the unburnt

fuel (Forward heat transfer). “$�” is solid-phase specific heat, “%�” is solid fuel thickness, “&�”

is solid fuel density, “'�()*�+"” is the surface temperature and “',” denotes the ambient

temperature.

The governing physics of a spreading flame is experimentally stated in terms of the heat

feedback to the unburnt fuel. Flame spread rate is preconized as a dependable variable of the

ignition which is development to a self-sustained reactive combustion through reaction rates.

This transition is reflected by an imbalance between the heat production and heat loss which

relates to the heat feedback mechanism as:




Forward Energy Transfer = Energy Production−Energy Loss

)4(LPF

qqq −=

The energy production is based on an Arrhenius approximation as:

)5(* RT

E

iC

a

PeAVCHq

−

∆=

The energy loss is buoyant convection taken by assuming constant concentration of

reactants indicating a uniform temperature:

)6()( ∞−= TThAqL

Where, “F

q ” is the Forward energy transfer, “P

q ” is the Energy production, “L

q ”

represents Energy loss, “V” is the volume, “T” is the temperature and “t” represents time, “

CH∆ ” is the heat of combustion, “iC ” represents the Concentration of reactants,“ *A ” is the

Pre-exponential factor, “aE ” is the Activation energy, “ R ” is the Universal gas constant., “

h ” is the convective heat transfer coefficient.

It is important to note that the experimental results presented, represent extensively

patterned repeatability of the third order for all the studies carried out.

3. RESULTS AND DISCUSSION

Systematic experiments were carried out on downward spreading candle flame in the purely

convective atmosphere under normal gravity conditions. According to the classical heat

transfer theory, a part of exothermic energy is supplied to the unburnt fuel (forward heat

transfer) and controls the flame spreading (equation 3). Flame spread is quantified in terms of

recurring energy production comprising of the energy loss and the related heat feedback

mechanism. An external influence viz., external electric field is likely to alter this heat

feedback mechanism by affecting the ignition process. Any variation in the energy generation

process is reflected in altered flame spread rate.

Prior to the main study, a base reading was taken to evaluate the external electric field

effect. The electrode separation distance was varied without an external electric field. A

quiescent steady flame was observed (figure 5) propagating uniformly along the solid fuel

acknowledging no external influence. The downward flame spread rate was found out to be

0.05787 mm/sec (figure 6).

Figure 5 Pictorial view of downward flame Figure 6 Flame spread rate variation with

spread without external electric influence. separation distance without an electric source.



Electric fields are well known to be effective within a closed region. First, the effect of

separation distance (‘ds’) on a spreading flame was investigated. The operational

configuration consisted of electrodes placed diametrically opposite directing impingement on

the flame center of a spreading flame in between them. It is important to note that for the

present study, the flame center was fixed as the origin and the height of electrodes was

adjusted concurrently to consistently match directed impingement. To simplify the

investigation, the spreading rate changes are quantified in a separation distance based zones

viz., nearby (ds= 0-2.5 cm), intermediate (ds= 2.5-7.5 cm) and faraway (ds= >7.5 cm).

Figure 7 shows the flame spread rate variation with the separation distance for origin

impingement. Looking at the plot one can note that, flame spread rate exhibits a non-

monotonic trend with reduction in separation distance. In the faraway zone, gradual rise till

peak was noted at ds= 7.5cm (~10% rise) whereas, in the Intermediate zone, the extent of the

external effect drops linearly to w/o external source till the nearby zone. The separation

distance reduction in the nearby zone results significant increase till ds= 1 cm (representing

intermittent arc (0.75cm & 1cm)) with a rise of 67.55% at 0.75cm and 10.01% at 1cm

respectively. At ds= 0.5 cm, presence of a continuous impinging arc on a spreading flame

results in notable spreading rate upsurge in with 90.95% rise.

Figure 7 Variation of flame spread rates with half separation distance in presence of electric field.

The spreading rate changes in the presence of an external electric field corroborates the

electric field effect on a spreading flame. It is worthwhile to note that the variation in

separation distance alters spread rate however, the effect never drops below that of a single

flame (without electric field). The enhanced flame spread rates indicates the heat source

features of an impinging electric field which can be attributed to the enhanced forward heat

transfer. A distinction in the extent of external electric field effect can be noted with and

without the presence of an arc. Electric field without an arc results in low enhancement in

flame spread rate however, with an arc (here, ds< 1 cm), results in a drastic rise. An external

electric field with arc represents a classic case of energy interactions with the changes owing

to an external electric field and accompanied arc being replicated in the flame spread

behavior.




(a) (b) (c)

(d) (e) (f)

Figure 8 Pictorial views of flame and electric field interaction along the half separation distance (a)

With arc(ds=0.5cm), (b) Intermittent arc(ds=1cm), (c)-(f) No arc (ds= 2.5cm, 5cm, 7.5cm, 10cm).

To understand the flame and electric field interaction, we look at the experimentation

images for cases in figure 7. Figure 8 shows the visible changes in flame at different

distances. Primary changes with external electric field influence was noted with sudden

changes in the flame balance. The flame was observed to be attracted towards downstream

electrode and becoming highly luminous and sooty. The arc occurrence is noted with the

curling flame diverted to the downstream electrode and remains strongly attached to it

throughout with the continuous sparking sound.

As the power supply was turned off, the flame regains its original state with proportionate

increase in blue color region. Enhanced electric field effect was noted with significant

changes in blue and yellow flame regions. With arc, the blue region extended to almost ½ of

the flame and then again reduces to 1/3 after few seconds of turning off thus suggesting that

the impinging arc brings in the chemical changes in the ongoing combustion reaction. In the

faraway regime, the flame stretches and slightly bends towards downstream electrode. Figure

9 shows the images taken at an interval of 40 seconds for selected cases of with arc(ds=0.5cm)

and Intermittent arc(ds=1cm).

0.5cm: 40seconds 0.5cm: 80seconds



1cm: 40seconds 1cm: 80seconds

Figure 9 Pictorial views of spreading flame disparity with impinging arc for ds=0.5cm and 1 cm in

successive 40seconds.

The governing physics is based on combustion reactions inside the flame involving latent

energy exchange mechanism between the electric field and the flaming source. The extent of

energy transfer from the flame or to the flame is determined by the flow of charges across and

around the flame. The external influence and flame energy interactions lead to two distinct

type of interactions viz., external to internal type and internal to external type. The external to

internal type characterizes alteration of localized field around the flame by an external

influence. This as a consequence alters the internal thermochemistry in terms of chemical

reaction rates and thus the combustion and heat transfer features due to the induced effect.

The effect is intensely distinguished in the modified energy production mechanism and thus

in forward heat transfer which is replicated in as changed flame spread rates. Electric field

without arc in the vicinity alters the localized field of a flame and thus the flame spread rates

represents external to internal energy interactions. Secondary type viz., internal to external

signifies the direct alteration of chemical reaction rates which varies combustion and heat

transfer features. This type represents abrupt variation in thermochemistry resulting in

significant changes in combustion characteristics with an altered forward heat transfer and

thus spreading flame rate.

Further investigation was carried out by varying the symmetry of the electrodes. Now the

electrodes were placed in perpendicular symmetry (figure 10). Non-monotonic variation of

spread rates due to symmetry effect can be noted from the plot. Significant rise can be noted

at 0.5cm & 2cm only.

Figure 10 Effect of electrodes symmetry on flame spread rates.




(a) (b) (c)

(d) (e) (f)

Figure 11 Pictorial views of the perpendicular electrodes placement and flame variation (a) ds=0.5cm,

(b) ds=1cm, (c)-(f) ds= 2.5cm, 5cm, 7.5cm, 10cm.

The effect of diagonally impinging arc is to reduce the rise of spread rates to 26.23% at

0.5cm in comparison to 90.95% obtained in previous result thus giving a difference of

51.27% with respect to electrodes placed diametrically opposite. At 2cm rise of 24.87% was

obtained. Also, a drop of 5.4% in spread rates can be noted at 1cm with relatively small flame

bended towards and attached to one of the electrodes. However now there is no peculiar rise

at 7.5cm but slight rise at 5cm of 6.6%. Flame tends to be insensitive at ds ≥7.5cm. This

indicates that symmetry of electrodes plays an important role in influencing the flame spread

along with separation distance. Figure 11 shows the variation in spreading flame along the

distance due to changed symmetry. Further, figure 11 shows the pictorial view of flame in

every 40 seconds at selected separation distance. Arc can be seen to be impinging diagonally

at 0.5cm. Noticeable change in flame shape, size and luminosity can be observed at a distance

of 1cm in the frame interval of 40seconds each. Characteristic variation in flame at 1cm

reduces the heat feedback content thus decreasing the spread rates. Remarkable variation in

flame spread rates in the presence of arc arouse the need to understand it further therefore

next study investigates the role of continuous non-impinging arc on spreading flame.

In order to envisage the effects of continuous arc in the vicinity of flame, arc was formed

at the origin while varying the distance of candle from the flame. Figure 12 shows the varying

spread rates with the change in separation distance. Again, a non-monotonic behavior of

spread rate was noted. With arc not directly impinging at the flame but just present aside the

flame spread rates were noted to be more than that obtained for diametrically placed

electrodes except for 0.5cm, 2.5cm and 5cm. This gives maximum rise of 21.44% at 2cm and

rise of 20.51% at 7.5cm and negligible drop at 2.5cm & 5cm respectively. Interesting to note

flame doesn’t show much visible effect other than at ds<2cm (figure 13). This clearly depicts

the effect of impinging and non-impinging arc on flame. Impinging arc contributes more

towards increasing spread rates thus providing more heat to the flame whereas an arc just

present in the vicinity of flame tends to be less effective when placed parallel very near to

flame. This also highlights the relative variation obtained in spread rates in previous two cases

with directly impinging arc and diagonal arc which barely impinges on flame.



Figure 12 Flame spread rate variation with half separation distance in presence of a continuous arc.

(a) (b) (c)

(d) (e) (f)

Figure 13 Pictorial view of spreading flame in the presence of Continuous arc (a)-(h) with arc (ds=

0.5cm, 1cm, 2.5cm, 5cm, 7.5cm, 10cm).

Changes in flame in frame interval of 40 seconds at 0.5 cm and 1cm shows the relative

insensitiveness of flame shape and size to arc in the vicinity. This variation in spread rates can

be owed to the changes brought into the electrochemical reactions occurring at the flame due

to impinging arc. Heating effect of arc causes the spread rates to rise whereas the energy

interaction between electric field and flame with changing distance varies the net heat transfer

to and from the flame thus changing the spread rates accordingly. Looking at the results

obtained so far one can predict that the presence of continuous impinging arc and electric field

induces the changes in the electrochemistry of the flame. To validate the same next study was

done by impinging the arc on wax instead of a flame. The variation in spread rate can be seen

from figure 14. The magnificent rise at 0.5cm clearly validates the heating effect of arc.

The reason for this can be attributed to rapid pyrolysis of the fuel which gives a

remarkable rise of 2416.5% compared to that of single flame. Also, it gives a rise of 35.39%,

16.28%, 17.16% and 21.93% at distances 1cm, 1.5cm, 2cm and 2.5cm respectively. The

effect of wax impinging arc reduces at far field and intermediate distances.




Figure 14 Flame spread rate variation with arc impinging on wax.

(a) (b) (c)

(d) (e) (f)

Figure 15 Pictorial view of effect of arc impinging on wax, (a) ds=0.5cm, (b)-(h) (ds=1cm, 2.5cm,

5cm,7.5cm, 10cm).

Figure 15 shows the flame at different distances. The flame can be noted to be more

stretched than that at two electrodes impinging arc on flame. The pictorial view of flame at

selected separation distance of 0.5cm, 1cm and 1.5cm. Frames were obtained for 0.5cm in

10seconds as the flame spreads rapidly. The lengthy flame can be observed at this distance.

At a distance of 1cm one can note the flame becoming slightly concave over an electrode but

not coming in contact with it. Effect due to field of upstream (positive) electrode alone is

investigated next (figure 16). Variation in spread rates was obtained only for the nearby

regime. Not much significant change was noticed whereas the maximum spread rate lies at the

center of the regime at 1.5cm giving rise of 7.3%. Pictorial view of the same is shown in

figure 17, flame can be seen slightly repelled from the upstream electrode.



Figure 16 Flame spread rate variation with half separation distance for single electrode interaction.

(a) (b)

Figure 17 Pictorial view of variation of flame spread rates with single electrode (a) ds=0.5cm (b) ds=

2cm.

Figure 18 Comparison of flame spread rate variation under diverse electric field conditions.




(a) (b) (c)

(d) (e) (f)

Figure 19 Pictorial view of variation of flame spread rates under various conditions (a) Single Flame,

(b) Impinging arc (c) One Electrode field (d) Perpendicular symmetry (e) Wax Impinging (f)

Continuous Arc.

The comparative effect on flame under diverse electric field configurations can be

visualized from figure 18. This plot inter-relates all the previously seen effects with each

other. And one can summarize how the flame behaves in each condition by looking at figure

19. Difference between one electrode, two electrodes placed opposite and perpendicular, wax

impingement and continuous arc can be noted. It can be clearly seen that the maximum spread

rates obtained was in case of wax impinging arc and found to be 1217.92% more than that

obtained with arc impinging on flame. Spread rate of perpendicular electrodes is 51.27%

more than that obtained with oppositely placed ones at 0.5cm. Continuous arc follows a

difference of 41.86% with impinging arc at 0.5cm. Presence of impinging arc causes

significant changes in spreading flame at nearby distance rather than continuous non-

impinging arc. Peculiar rise at 7.5cm in some cases owes to the inter-convertibility of flame

heat energy and electric energy. Presence of electric field tends to alter the flow field of the

flame which in turns varies the effective utilization of flame’s heat energy provided that the

heat losses from flame remain constant. However, the contribution of electric field in

increasing the spread rates is not as much as due to impinging arc which also interferes with

flame chemistry along with the flow field. Electric field changing the flow field indirectly

varies the reactivity of the flame which varies the heat transfer and thus the flame spread

rates.



Figure 20 Variation in flame spread rates with number of varying electrodes configurations.

(a) (b) (c)

(d) (e) (f)

Figure 21 Pictorial view of variation in flame spread rates with number of electrodes and their

configuration (a) P-N (b) P-N-P(ds= 0.5cm) (c) P-N-P-N(ds= 0.5cm) (d) P-P-N-N(ds= 0.5cm) (e) P-P-

N-N(ds= 0.5cm), (f) P-P-N-N(ds= 0.5cm).

Figure 22 Schematic of varying electrodes configurations

Effect of electric field due to different number of electrode on flame was investigated

next. The configuration formed (figure 20) were two electrode (PN), three electrode (PNP)

where two upstream electrodes were placed at 90 degree and one downstream at diametrically

opposite end, in the same way PNPN configuration is obtained by just introducing one more

downstream electrode, both of these configuration forms a diagonal as well as straight

impinging arc between any two electrodes of opposite polarity at a time. In PPNN upstream




and downstream electrodes were placed opposite to each other respectively forming a

diagonal arc only at any of two opposite polarity electrodes.

Seeing figure 20 one can note the non-linearity in spread rates with increasing as well as

decreasing rates. Maximum rise with respect to single flame is obtained in P-N configuration

of 90.95% at 0.5cm and Maximum drop occurs at PPNN configuration of 13.53% at 1cm.

Following each configuration individually no drop occurs at PN configuration whereas PNP

follows a rise of 7.71% at 2cm and drop of 5.89% at 10cm. Similarly, PNPN has a drop of

12.84% at 1.5cm and rise of 5.7% only at 2cm in contrast to this PPNN configuration gives

a rise of 24.31% at 0.5cm. It is important to note that the maximum rise in each case pertains

to nearby regime whereas maximum drop is also followed in nearby regime with four

electrode configurations. However, the three-electrode configuration not tends to rise in

intermediate and faraway regimes. Another interesting thing to note is that using four

electrodes but in two different configurations causes drastic changes in spread rates. As can be

noted PPNN is more effective in decreasing spread rates at intermediate regime while PNPN

is equally effective for producing same effect at nearby regime. Difference of maximum rise

given by both is 18.61% and that of maximum drop is 0.69%. In comparison to the PN

configuration PNP gives a rise of 5.4% and drop of 7.27% and PPNN gives a rise of 51.27%

and drop of 27.22% at respective distance of their maximum rise and maximum drop in PNP

and PPNN configurations. However, PNPN gives a difference in drop of 22.16% and that in

rise of 3.4% with respect to PN electrode. At the distance of 7.5cm the spread rates of PNP

and PNPN merges. Pictorial view of each configuration is shown in figure 21 with detailed

schematic in figure 22. Difference in flame luminosity and shape can be noted for PNP

configuration. For PNPN case, it can be noted that at the distance of 1.5cm flame becomes

very small and more prone to external influence however; reducing the distance to 1cm makes

it even smaller and more disturbed. This external effect deprives the flame of its heat energy

and thus explains the reduction in spread rates at these distances. PPNN configuration

represents visible effect on flame at 1cm determines it yielding maximum drop. From the

results discussed above it can be seen that the effect of two electrodes is far more than the

three and four electrodes placed in different configurations owing to the non-uniform

distribution of the electric field in the vicinity of flame. This alters the flow field such that the

effective utilization of flame decreases. The relative changes in heat energy with the

indulgence of electric energy vary with different configurations along with the separation

distance. Direct arc impingement causes the electrochemical changes in the flame hence

increasing its spread rates whereas the electric field alone offers weak effect on the flame

spread rates.

4. CONCLUSION

Assistance of electric field in downward spreading of flame was investigated through

experimental study. The effect of controlling parameters viz. separation distance, number of

electrodes, electrode symmetry, impinging arc & continuous arc and wax impinging arc was

explored with respect to the spread rate of a single flame. The study focused on understanding

the interaction of arc and electric field with the flame. Based on the controlling parameters,

following conclusions can be drawn:

1. Presence of electric field in the vicinity of flame waivers significant changes in the

spread rates.

2. Separation distance found to be the key controlling parameter along which presence

and absence of arc determines the extent of change in spread rate of flame. At distance

<0.5cm consistent presence of arc causes a sudden rise in spread rates. Magnitude of

rise varies with the electrodes placement around the flame. Peculiar rise can be noted



at a distance of 7.5cm in selected cases viz. two electrodes diametrically opposite and

continuous arc.

3. Varying the symmetry of electrodes from diametrically opposite to perpendicular

imparts qualitative and quantitative changes in the spreading flame. Proportionate

increase in blue region of the flame signifies efficient combustion phenomenon.

4. Impinging arc acts as a heat source thus effectively increasing the spread rates. It tends

to disturb the chemical reactivity of flame which increases the effective utilization of

flame heat energy thus reducing losses.

5. Non-Impinging continuous arc doesn’t show much change on the flame in lateral

direction. Unusual increase at 7.5 cm results due to the interconvertibility between

heat energy and electric energy.

6. Impingement of arc on wax validates it changing the flame chemistry and altering the

changes in the pyrolysis occurring at fuel surface. This results in remarkable

enhancement of spread rates.

7. Varying number of electrodes tends to suppress the overall effect with the

redistribution of electric field and flow of charges thus altering the corresponding flow

field around flame. This in turns results in decreasing spread rates as well owing to the

decrease in the heat generation capability of the flame.

8. Overall work provides a comprehensive understanding of the interaction mechanism

between flame and external electric source and thus can be used as an effective way to

control fires and for efficient combustion.

9. Applications of the work: Present work offers a perceptive understanding of an

external electric field effect on a spreading flame. The effective changes in flame are

investigated to occur owing to combined effects chemical alteration inside the flame

and of ionic wind. These effects could result in flame stabilization or flame

extinguishment under varying conditions and configurations. The physical insight

from the present work would be very useful for better combustion and fire safety

applications. Utility in selected cases of fire safety includes, devising efficient

controlling provisions for Domestic and Industrial fires, controlling fire spreading in

Propulsive systems viz., Aircrafts and Utilization of recent technological advances in

designing, testing and validating efficient fire suppression methods. Alongside, insight

of better combustion application includes, reduction in hazardous gaseous emissions

from incomplete combustion, addressing low regression rate problems in hybrid

rocket motors, thrust increment in advanced propulsion systems viz., magnetic &

electric, studying microgravity flame spreading for deep space missions, Supersonic

combustion.

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ELECTRIC FIELD ASSISTED DOWNWARD …...Cite this Article: Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu Baweja and Herambraj Nalawade, Electric