ELECTRIC FIELD ASSISTED DOWNWARD …...Cite this Article: Poorva Shrivastava, Vinoth Kumar...
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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
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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
http://www.iaeme.com/IJMET/index.asp 598 [email protected]
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
Electric Field Assisted Downward Spreading Flame
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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.
Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu
Baweja and Herambraj Nalawade
http://www.iaeme.com/IJMET/index.asp 600 [email protected]
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
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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:
Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu
Baweja and Herambraj Nalawade
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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.
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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.
Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu
Baweja and Herambraj Nalawade
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(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
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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.
Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu
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(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.
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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.
Poorva Shrivastava, Vinoth Kumar Annamalai, Vikram Ramanan, Vinayak Malhotra, Chakshu
Baweja and Herambraj Nalawade
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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.
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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.
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(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.
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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
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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
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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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