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Effect of AC electric fields on flame spread over electrical wire
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 33 (2011) 1145–1151
www.elsevier.com/locate/proci
Effect of AC electric fields on flame spread over electrical wire M.K. Kim a, S.H. Chung a,⇑, O. Fujita b a
Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia b Division of Mechanical and Space Engineering, Hokkaido University, Sapporo, Japan Available online 29 September 2010
Abstract The effect of electric fields on the characteristics of flame spread over insulated electrical wire has been investigated experimentally by varying AC voltage and frequency applied to the wire in the normal gravity condition. The polyethylene (PE) insulated electrical wire was placed horizontally on electrically non-conducting posts and one end of the wire was connected to the high voltage terminal. Thus, the electrical system is the single electrode configuration. The wire was ignited at one end and the flame spread rate along the wire has been measured from the images using a video camera. Two distinct regimes existed depending on the applied AC frequency. In the low frequency regime, the flame spread rate decreased with the frequency and voltage. While in the high frequency regime, it decreased initially with voltage and then increased. At high frequency, the spread rate was even over that without applying electric fields. This result implies that fire safety codes developed without considering the effect of electric fields may require modifications. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Wire flame; Electric fields; Spread rate; Ionic wind effect; Extinction
1. Introduction Flame spread over an electrical wire is one of the key issues for the safety and survivability in electrical fires. In previous studies, various factors such as the size of wire, the type of insulation, gravity levels, pressure, and the condition of ambient flow and dilution, have been investigated to resolve the issues of fire-safety pertinent in both ground and space environments [1–8]. Fujita et al. [9] investigated the influence of external flow on wire flame in a microgravity test, ⇑ Corresponding author.
E-mail address: [email protected] (S.H. Chung).
including oxygen concentration, geometrical effect, and thermal effect. Nakamura et al. [10] suggested a thermal balancing mechanism to explain the opposed-wind effect on flame spread of electrical wire in sub-atmospheric pressure. Various heat transfer mechanisms related to wire flames were discussed and the thermal balance of these heat transfers was proposed as a controlling mechanism of the spread rate. A numerical study has also been conducted including detailed physical modeling for spreading flame [11]. The role of conduction heat transfer through wire was elucidated. In an actual fire on electrical wires, there is a possibility of electromagnetic fields induced by the voltage and/or current passing though the wire. In the reaction zone of flame spread, there exist abundant charge carriers which could interact with
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.155
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
electric fields [12], influencing flame spread rate. In spite of this possibility, the effect of electric fields on the characteristics of flame spread over insulated wire has not been investigated. In the present study, the effect of AC electric fields on flame spread over electrical wire was investigated experimentally. This ground-based experiment can be a preliminary test in studying the spread rate over electrical wire in a microgravity environment. The frequency of AC can vary depending on systems, e.g., 50–60 Hz for utility power generation and typically about 400 Hz in aircrafts, although it can vary 360–720 Hz and even over 1 kHz at full throttle. AC voltage can also vary significantly depending on applications in general. In the present experiment these ranges will be covered. We will limit our study for a single wire configuration. Experiments on multi-wires, which are common in practice, will be reported later. 2. Experiment The apparatus consisted of a sample wire, an AC power supply, and a video camera as schematically shown in Fig. 1. Polyethylene (PE) insulated nichrome (NiCr) wires were tested, having the insulator with o.d. 0.8, i.d. 0.5, and 320 mm in length. The wire was installed on a holder made of non-conductive acetal-resin. One end of the wire was attached to a fixture and the other end was connected to a spring to maintain it straight with tension to prevent bending by thermal expansion during flame propagation. The flame was initiated by a hot-wire igniter, which was placed on an air-cylinder. To minimize the interaction between the ignition system and applied electric fields, the igniter was retreated away after ignition. A programmable logic controller was utilized to control the time sequences of experiment. First, electrical power was supplied to the igniter for 8 s. After ignition, the air-cylinder was activated to move the igniter away. Then, the AC power supply was turned on with specified AC voltage and frequency. Subsequently, the camera was triggered to take images of spreading
Function generation
Backlight
Power supply R
Spread
Flame
Wire
Spring
High Voltage terminal
X Igniter
Voltage probe Oscilloscope
Air cylinder
flame at 30fps with backlight illumination. The length of the flame spread zone was 120 mm after the igniter. The spread rate can be influenced by AC frequency and natural convection. In the present, average spread rate is typically of the order of 1 mm/s. The variations in spread rate are represented as error bars. The AC power supply (Trek, 10/10B-FG) applies electric fields to the wire. The AC frequency fac was varied in the range of 10–1000 Hz by using a function generator and the AC voltage Vac was varied up to 7 kV (RMS). The frequency and voltage were monitored by an oscilloscope through a 1000:1 probe. One end of the wire was directly connected to the high voltage terminal of the AC power supply and the other terminal to a building ground, such that the system can be regarded as an open circuit. In this configuration, the induced electric fields can be assumed to be distributed between the high potential wire and imaginary infinite ground [13]. The applied electric fields can be regarded as axisymmetric along the wire, since the length of wire is much longer than the diameter of the wire. The electric field intensity is expected to decay in radial direction. Near the end of wire, the convergence of electric flux can increase electric fields. To exclude this effect, together with initial transient, spread data were analyzed in the range of 10–60 mm from the ignition point and averaged. 3. Results and discussion Figure 2 shows the variation in the flame position X with time t at various frequencies for Vac = 3 kV, together with the baseline case without applying AC. Here, X is defined as the distance from the igniter and t = 0 is set when the flame reaches X = 10 mm. The result exhibits that the rate of change in flame position is influenced by AC electric fields, such that the position increases faster for fac = 1000 Hz and slower for fac = 60 and 400 Hz, as compared to that for the baseline case. 60
Po osition off flame ffront X [m mm]
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50
40 0kV 60 Hz, 3kV 400 Hz, 3kV 1000 Hz, 3kV
30
20
GND
Wire holder
10 PLC Video camera
Fig. 1. Schematic of experimental setup.
0
5
10
15
20
25
Time t [s]
Fig. 2. Position of flame front with time.
30
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151 2kV
3kV
4kV
5kV
w
60 Hz
3.5
Spread rate S [[mm/s]
1kV
400 Hz
1147 Frequency [Hz] 10 200 400 20 30 600 60 800 80 1000 100
Extinction (1000Hz)
3.0
2.5
2.0
1.5
Extinction (100Hz)
1.0 0
Fig. 3. Instantaneous flame images at various applied AC voltages and frequencies.
The instantaneous images, obtained at X = 30 ± 1 mm, are shown in Fig. 3 at various AC electric fields. It is interesting to note that the flame shape is appreciably modified. As the voltage increases, the flame leans toward the burnt side. For fac = 60 Hz, the height and length of the flame decrease monotonically with voltage. For fac = 400 Hz, non-monotonic behavior is exhibited, especially the flame width decreases first and then increases. For Vac = 3 kV and fac = 400 Hz, the flame size increases appreciably such that the portions covering both unburned and burnt sides of wire are extended in the horizontal direction. At excessively large voltage, the flame was extinguished after initial transient, e.g., at 5 kV for 60 Hz and 4 kV for 400 Hz. The molten PE inside the flame becomes darker with AC voltage and the molten PEs for fac = 400 Hz are relatively darker than those for fac = 60 Hz. This can be attributed to the deposit of soot produced in the flame zone by electrostatic force. Although the flame at the baseline case produces soot by exhibiting yellow luminous flame, most of the soot generated is oxidized upon exiting the upper part of the flame and soot deposition to molten PE and on the surface of burnt wire was negligible. As a result, the molten PE is clear and the burnt region of the wire is free from soot deposition. With AC fields, however, appreciable amount of soot was deposited on the burnt wire, which can be confirmed from the observation of the burnt wire which is coated with soot after flame spread. 3.1. Spread rate with AC electric fields The spread rate was linear fitted during the time history of flame position shown in Fig. 2. Note that the change of flame position is reasonably linear within the range. Figure 4 shows the spread rate Sw = dX/dt as a function of voltage with 1 kV increment at various frequencies. As the voltage increases, the spread rate generally decreases with voltage for the frequency range of fac = 10–200 Hz marked as dotted lines, except
1
2
3
4
5
Applied voltage V
ac
6
7
[kV]
Fig. 4. Spread rates of wire flame with applied voltages Vac for various frequencies.
for fac = 10 Hz, where Sw starts to increase over 4 kV and then decrease again over 6 kV. At further increased voltage, the flame is extinguished after initial transient, as marked for 100 Hz. The spread rate decreases with the frequency. The standard deviations are marked as error bars, which is amplified with voltage as can be seen for fac = 10 Hz. In cases of fac = 400–1000 Hz marked as solid lines, the spread rate behavior changes to nonmonotonic to the voltage, such that it decreases initially and then increases appreciably for Vac > 2 kV. Also, the spread rate increases with the frequency. Note that for Vac = 3 kV and fac = 1000 Hz, the spread rate is even larger than that of the baseline case (0 kV). This result suggests that the spread rate of wire flame is significantly influenced by AC electric fields and under certain conditions, the spread rate can be larger than that for the baseline case. This is an important fact that may require consideration in establishing a fire safety code on electrical wires. For relatively low voltages, the variation of spread rate with applied voltage demonstrates reasonably linear decreasing trend, exhibiting a linear regime. The decreasing slope of spread rate can be determined by linear fitting for the data in the linear regime. The absolute value of the rate of change in the spread rate jdSw/dVacj is plotted in Fig. 5 with AC frequency in log–log scale. The slope initially increases up to about 400 Hz and then decreases. Dotted lines show the power fittings for both increasing (regime I) and decreasing (regime II) regimes. The correlations between AC frequency and the absolute rate of change in spread rate have the best fits of jdSw/dVacj [mm/s/kV] = 0.10 fac0.284 [s0.284] in regime I and jdSw/dVacj [mm/ s/kV] = 3.08 fac0.278 [s0.278] in regime II, where the correlation coefficients R are 0.98 and 0.94, respectively. These two fitting lines intersect at fac = 440 Hz, which can be defined as a transition frequency. This transition is consistent with the
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|dSw/dV Vac| [mm//s/kV]
Regime I Regime II 0.5 0.4 0.3
0.2
Transition 0.1 10
100
1000
Applied AC frequency fac [Hz]
Fig. 5. Rate of change in spread rate with applied AC voltage as function of AC frequency.
change in the behavior from monotonic to nonmonotonic in the spread rate with applied voltage (Fig. 4). 3.2. Effect of AC electric fields on flame shape To further elucidate the interaction between electric fields and spread rate, detailed observations have been performed. Figure 6 shows the instantaneous images obtained at X = 30 mm at various frequencies for Vac = 3 kV together with the baseline case. The flame height (H) and width (W) are defined as marked. The flame-shape changes appreciably with AC frequency. As the frequency increases, the flame leans toward the burnt region with exposed wire. Considering quenching distance, a flame can be located closer to the wire than unburned region. Then, the flame in bare-wire region can experience stronger electric fields. Note that the radial gradient of electric field intensity is high near wire surface. The distance of the flame edge in the unburned side from the molten PE generally decreases with the frequency. At higher frequency over 400 Hz, however, the distance appreciably increases. Previously, the effects of forced convection, ambient pressure, and conductivity of electric wire
on the spread rate of wire flames were investigated [10]. It has been proposed that a thermal balance among various heat transfer rates is controlling spread characteristics, including heat transfer from flame to unburned (Q1) and burnt (Q3) wire, heat supplies to molten PE through the wire (Q2) and to the insulator (Q4) and heat loss to ambient (Qloss), as schematically shown in Fig. 7. When one (or more) of these heat transfer characteristics is changed by electric fields, the balance of heat transfers will be modified, and then the spread rate should be adjusted to re-balance the heat transfers. This implies that the spread rate could be influenced by various ways when electric fields are applied. Charged particles existed in a reaction zone can be accelerated by the Lorentz force, thus the associated diffusion flux can be enhanced. Accelerated ions can transfer momentum to neutral particles by random molecular collisions such that bulk flow can be generated, resulting in the ionic wind effect. Through these effects, the flame shape could be changed. Once the height and width of flame are changed, not only the distance between wire and reaction zone but also the length of wire covered by the flame will be changed. Chemical reaction rates associated with charged particles can also be influenced by the applied electric fields. The major portion of the flame will have a nonpremixed flame nature, such that the effect of electric fields on chemical reaction may play a weak role since a nonpremixed flame is controlled by diffusion process. However, the edge flames near the wire could exhibit a premixed flame nature by quench zone. In such a case, the effect of electric fields on chemical reaction rate may play an important role. Moreover, the electric field intensity is expected to be large near the wire. In such a case, heat transfer from flame to wire and molten PE will be directly influenced. Consequently, the generation and evaporation of molten PE could be altered due to modified thermal balance. Soot particles generated in the flame will be influenced by electric fields through electrostatic force and electrophoresis. Soot deposition Electric fields
Electric fields Flame Qloss
Spread
Q1
Q3 Q4
Q2 Wire
Molten PE Electric fields
Fig. 6. Instantaneous images at various frequencies for fixed voltage of Vac = 3 kV.
Qloss
Fig. 7. Schematic of spreading flame.
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
increased with applied voltage and frequency as mentioned previously. This implies that soot particles migrated toward the wire by electric fields. Soot deposition on electrodes was enhanced for both positive and negative DC voltages in electrostatic precipitators and was linearly proportional to DC voltage [14–16]. Resulting soot deposition on wire with AC could affect radiation heat transfer from wire to ambient, thereby the thermal balancing process by altering heat transfer characteristics of PE and wire together with material properties such as viscosity and surface tension of molten PE. Edge flame behavior especially near solid surface is also a complex phenomenon, which has not been accurately quantified. Therefore, respective influences on wire flame behavior cannot be fully characterized at this stage and will be a future study. The spread rate with frequency for Vac = 3 kV is plotted in Fig. 8a to compare with the variation in flame structure in Fig. 6. In regime I, the rate decreases with AC frequency. The flame leans toward the burnt downstream and simultaneously, the flame height and width decrease with frequency. Figure 8b shows the variations in flame height and width with AC frequency for Vac = 3 kV. The flame is flickering during spread due to buoyancy effect typically with 4–8 Hz, so as its height and width. The flame oscillation can occur due to not only by the buoyancy effect but also interaction with applied AC. Detailed analysis of flame oscillation is beyond the scope of present paper and will be investigated in the future. Here, the mean height (Hm) and width (Wm), averaged over X = 20–40 mm, are exhibited with error bars. The result shows that the flame height in regime I is quite smaller than that without having electric fields ðH om ¼ 16:5 mmÞ, marked on the vertical axis, and decreases with frequency up to 100 Hz. The leaning of the flame toward the burnt side wire can be conceived to be similar to the effect of outer convective flow. In such a case, the heat
Sw [mm//s]
3.5 3.0
Sw
2.5
I
o
II
2.0
Hm and Wm [m mm]
1.5
Sw
1.0 16
Hm
o
14
(a)
Hm Wm
12 10 8
Wm
6 10
o
(b) 100
1000
Applied AC frequency fac [Hz]
Fig. 8. Spread rate of wire flame Sw, mean flame height Hm and width Wm with applied AC frequency for fixed voltage of Vac = 3 kV.
1149
transfer from flame to unburned PE (Q1) can be decreased, which reduces the spread rate, as was observed in the outer convective flow experiment [10]. With applied AC, however, the flame is not only leaned but also shortened its size, such that the heat transfer from flame to burnt wire (Q3) will be decreased. Consequently, the heat conduction toward unburned side (Q2, Q4) will also be decreased. Then, both the production and evaporation rates of molten PE will be decreased, resulting in the decrease in flame size. The decrease in flame size will reduce the total heat release from flame surface and enhance the cooling effect (Qloss) due to increased temperature gradient [17,18]. In regime II, the flame length and width increased and recovered to the values without having electric fields as AC frequency increase. As compared to regime I, the luminous flame enlarges with AC electric fields (Fig. 6). A possible process for this enhancement may be explained as follows. If the reaction rate near flame edge is enhanced, the length of front flame covering unburned wire could be increased, which can be partly observed in Fig. 6 for the cases of fac = 600 and 1000 Hz. In such a case, the generation of molten PE will be promoted due to enhanced heat transfer (Q1), leading to the increase in flame size on burnt wire region. Then, the heat conduction (Q2) will be enhanced due to increase of heat transfer (Q3). Again, it will lead to the amount of molten PE larger, thereby the flame size. Therefore, the spread rate could be recovered and even becomes larger than that for the baseline case. This enhancement process is similar to the explanations for the enhancement by increasing wire conductivity [10]. Further investigation will be required to fully understand the electric field effects on wire flame. It was discussed that the change in flame size is related to the variation of spread rate. Then, some inconsistencies are encountered in the relation between flame spread rate and flame size. In Fig. 8, e.g., the spread rates are slower than that without having electric fields of S ow ¼ 2:78 mm=s, except for fac = 1000 Hz. In case of flame width, however, not only the case of fac = 1000 Hz but also several other cases have larger flame width. In case of the flame height, the heights are always smaller than that without having electric fields of H om . And the minimum height was 6.148 mm at fac = 100 Hz. This is not consistent with the minimum spread rate of Sw = 1.49 mm/s at fac = 200 Hz. This suggests that the length scales need to be considered in terms of 3D aspect instead of 2D. For fac = 200 Hz, the flame height and width becomes almost the same with 6.39 and 6.24 mm, respectively, having near spherical shape. Note that a sphere has minimum surface area with the same volume. If heat generation in unit area of flame surface is constant based on diffusion flame nature, the
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M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
I
II
500
400 o
As 300
200
100 10
100
Extinction 6
5
4
Spread
3
1000
10
A li d ffrequency fac [Hz] Applied [H ]
100
1000
Applied frequency fac [Hz]
Fig. 9. Spheroidal surface area of wire flame As with applied AC frequency for fixed voltage of Vac = 3 kV.
minimum surface area means that the total heat generation from the flame will be the minimum. To test the 3D aspect of flame surface, the flame is assumed to be a spheroid. Then, the surface area As becomes 2p [b2 + (a2b)/(a2 b2)0.5 arcsin{(a2 b2)0.5/a}], where a and b are the longer and shorter radii, respectively. The estimated spheroidal surface area of wire flame is plotted in Fig. 9 with AC frequency. The result shows that the variation of surface area with applied frequency is quite similar to the change in the spread rate (Fig. 8a). This indicates that, even though it is a rough approximation, the 3D aspect of flame surface area is reasonable in representing the variation in spread rate with AC electric fields. Figure 10 shows the spread rate with the surface area for several cases of applied AC electric fields, demonstrating satisfactory correlation. The best fit is Sw [mm/s] = 0.9482 + 0.0056 As [mm2] with R = 0.946. In this correlation, the data for the case of fac = 1000 Hz, Vac = 3 kV was excluded, since the flame surface is highly corrugated (Fig. 6) such that the spheroid assumption could appreciably be underestimated.
Fig. 11. Extinction voltage Vac with applied frequency fac.
3.3. Extinction behavior The wire flame was extinguished at excessive voltages. Figure 11 shows the extinction voltage Vext with frequency. The result shows that the extinction voltage decreases with frequency in regime I having the best fit of V ext ½kV ¼ 8:94 fac0:149 ½s0:149 with R = 0.936. In regime II, it becomes nearly constant of about 3.5 kV. To further elucidate the extinction phenomena, the transient variations in the flame and molten PE positions were examined. Figure 12 shows the timehistories of the front positions of flame and molten PE for Vac = 5 kV and fac = 60 Hz (regime I) with the snap-shot images. The result exhibits that the front position of flame keeps on increasing with time while that of the molten PE follows the flame initially and then maintains at a certain constant position. Consequently, the distance between the two front positions Dfm maintains a near constant value (0.6 mm) and then increases with time. Finally, the flame extinguished. In regime I, the spread rate is inversely proportional to both voltage and frequency as shown in
3
10
Dfm
2.8 2.6 2.4 2.2 2
0kV 3kV (fixed) 60Hz (fixed) 400Hz (fixed) Fitting
1.8 1.6 1.4 50
Dista ance Dfm [mm]
8
40
150
200
250
300
350
400
450
(c)
6
4
35
(c)
(b) Molten PE Flame Distance
(a) Ext
30
2 25
0 0
100
A
(b)
(a)
Front positions s [mm]
Spread rate Sw [mm/s]
II
I
7
Extinction voltage Vext [kV]
2
Spherroidal su urface arrea As [m mm ]
8 600
1
2
3
4
5
6
Time [sec]
2
Spheroidal surface area As [mm ]
Fig. 10. Spread rate Sw with spheroidal surface area As with various applied AC voltages and frequencies.
Fig. 12. Front positions of flame and molten PE and those distance with relative time for applied AC voltage Vac = 3 kV and frequency fac = 60 Hz.
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
Fig. 4. And it has also been shown that the change in spread rate is relative to the change in flame size by applied AC electric fields. As the surface area enclosing molten PE decreases, the heat transfer from flame to the molten PE will be reduced. If the reduction becomes excessive, then not enough heat to generate molten PE, resulting in the lack of fuel to sustain the flame. As a result, the flame front and molten PE are separated prior to the extinction as shown in Fig. 12 marked as A. In regime II, the flame extinction occurs at a constant voltage even though the surface area increases as compared to that for regime I. In such cases, it was observed that a certain amount of molten PE with high deposit of soot was dripping from the wire occasionally. This can be partly attributed to the radiation absorption to molten PE such that the surface tension of it could be decreased, then the molten PE will drop. Note that the viscosity of molten PE will decrease as the temperature increase by the radiation absorption. This sudden loss of molten PE could be attributed to the extinction in regime II. In this regard, microgravity experiments need to be conducted in the future to exclude such effect and then the extinction characteristics may be modified in regime II. Finally, the present experiment created many unanswered questions regarding the effect of electric fields on the spread of wire flames, which requires future studies both ground-based and microgravity experiments to understand the underlying detailed mechanisms. 4. Concluding remarks The flame spread rate over polyethylene (PE) insulated electrical wire has been investigated experimentally by applying AC electric fields in the normal gravity condition. With the single electrode configuration, the spread rate of wire flame has been measured by varying applied AC voltage and frequency in the range of 10–1000 Hz and 0– 7 kV, respectively. The result showed that the spread rate was appreciably modified by the voltage and over a certain critical voltage the flame was extinguished. For a fixed applied voltage, two distinct regimes existed depending on the frequency. In the low frequency regime, the flame spread rate decreased with the frequency. In the high frequency regime, the flame spread rate increased with the frequency, even over the spread rate without having electric fields. The variation of wire-flame structure with applied electric fields was investigated by analyzing the flame height, width, and surface area. The results showed that the surface area correlated well with the variation in spread rate with electric fields. The detailed mechanisms leading to these phenomena will be a future study.
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In case of actual fire for electric wires, there could be a possibility of electrical voltage applied. In such a case, the flame spread rate could be modified from that without having electric fields. The relevant fire-safety code both in normal and microgravity conditions developed based on without applying electric fields may require to be modified by considering the situation of fire for electric wires experiencing electric fields. Acknowledgements This work was supported by AEA Project/ KAUST and by “Ground-Based Research Program for Space Utilization” by Japan Space Forum (2006–09). References [1] L. Orloff, J. de Ris, G.H. Markstein, Proc. Combust. Inst. 15 (1974) 183. [2] A.C. Fernandez-Pello, Combust. Flame 31 (1978) 135–148. [3] K. Saito, J.G. Quintiere, F.A. Williams, in: Proceedings of the 1st International Symposium on Fire Safety Science, Hemisphere, Washington, DC, 1986, p. 75. [4] K.B. McGrattan, T. Kashiwagi, H.R. Baum, S.L. Olson, Combust. Flame 106 (1996) 377. [5] S.L. Olson, Combust. Sci. Technol. 76 (1991) 233. [6] T. Kashiwagi, S.L. Olson, O. Fujita, M. Kikuchi, K. Ito, Proc. Combust. Inst. 26 (1996) 1345. [7] S. Bhattacharjee, R.A. Altenkirch, Combust. Flame 84 (1991) 160. [8] R.A. Altenkirch, L. Tang, K. Sacksteder, S. Bhattacharjee, M.A. Delichatsios, Proc. Combust. Inst. 27 (1998) 2515. [9] O. Fujita, K. Nishizawa, K. Ito, Proc. Combust. Inst. 29 (2002) 2545–2552. [10] Y. Nakamura, N. Yoshimura, T. Matsumura, H. Ito, O. Fujita, JTST 3 (2008) 430–441. [11] A. Umemura, M. Uchida, T. Hirata, J. Sato, Proc. Combust. Inst. 29 (2002) 2535–2543. [12] J. Lawton, F. Weinberg, Electrical Aspect of Combustion, Clarendon Press, 1969. [13] S.H. Won, S.K. Ryu, M.K. Kim, M.S. Cha, S.H. Chung, Effect of electric fields on the propagation speed of tribrachial flames in coflow jets, Combust. Flame 152 (2008) 496–506. [14] K. Yamashita, S. Karnani, D. Dunn-Rankin, Combust. Flame 156 (2009) 1227–1233. [15] A. Jaworek, A. Krupa, T. Czech, J. Electrostat. 65 (2007) 133–155. [16] E. dela Cruz, J.S. Chang, A.A. Berezin, D. Ewing, J.S. Cotton, M. Bardeleben, J. Electrostat. 67 (2009) 128–132. [17] J. Park, C.B. Oh, K.T. Kim, J.S. Kim, A. Hamins, 5th ASPACC, 2005. [18] K. Maruta, M. Yoshida, H. Guo, Y. Ju, T. Nioka, Combust. Flame 112 (1998) 181–187.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 33 (2011) 1145–1151
www.elsevier.com/locate/proci
Effect of AC electric fields on flame spread over electrical wire M.K. Kim a, S.H. Chung a,⇑, O. Fujita b a
Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia b Division of Mechanical and Space Engineering, Hokkaido University, Sapporo, Japan Available online 29 September 2010
Abstract The effect of electric fields on the characteristics of flame spread over insulated electrical wire has been investigated experimentally by varying AC voltage and frequency applied to the wire in the normal gravity condition. The polyethylene (PE) insulated electrical wire was placed horizontally on electrically non-conducting posts and one end of the wire was connected to the high voltage terminal. Thus, the electrical system is the single electrode configuration. The wire was ignited at one end and the flame spread rate along the wire has been measured from the images using a video camera. Two distinct regimes existed depending on the applied AC frequency. In the low frequency regime, the flame spread rate decreased with the frequency and voltage. While in the high frequency regime, it decreased initially with voltage and then increased. At high frequency, the spread rate was even over that without applying electric fields. This result implies that fire safety codes developed without considering the effect of electric fields may require modifications. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Wire flame; Electric fields; Spread rate; Ionic wind effect; Extinction
1. Introduction Flame spread over an electrical wire is one of the key issues for the safety and survivability in electrical fires. In previous studies, various factors such as the size of wire, the type of insulation, gravity levels, pressure, and the condition of ambient flow and dilution, have been investigated to resolve the issues of fire-safety pertinent in both ground and space environments [1–8]. Fujita et al. [9] investigated the influence of external flow on wire flame in a microgravity test, ⇑ Corresponding author.
E-mail address: [email protected] (S.H. Chung).
including oxygen concentration, geometrical effect, and thermal effect. Nakamura et al. [10] suggested a thermal balancing mechanism to explain the opposed-wind effect on flame spread of electrical wire in sub-atmospheric pressure. Various heat transfer mechanisms related to wire flames were discussed and the thermal balance of these heat transfers was proposed as a controlling mechanism of the spread rate. A numerical study has also been conducted including detailed physical modeling for spreading flame [11]. The role of conduction heat transfer through wire was elucidated. In an actual fire on electrical wires, there is a possibility of electromagnetic fields induced by the voltage and/or current passing though the wire. In the reaction zone of flame spread, there exist abundant charge carriers which could interact with
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.155
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
electric fields [12], influencing flame spread rate. In spite of this possibility, the effect of electric fields on the characteristics of flame spread over insulated wire has not been investigated. In the present study, the effect of AC electric fields on flame spread over electrical wire was investigated experimentally. This ground-based experiment can be a preliminary test in studying the spread rate over electrical wire in a microgravity environment. The frequency of AC can vary depending on systems, e.g., 50–60 Hz for utility power generation and typically about 400 Hz in aircrafts, although it can vary 360–720 Hz and even over 1 kHz at full throttle. AC voltage can also vary significantly depending on applications in general. In the present experiment these ranges will be covered. We will limit our study for a single wire configuration. Experiments on multi-wires, which are common in practice, will be reported later. 2. Experiment The apparatus consisted of a sample wire, an AC power supply, and a video camera as schematically shown in Fig. 1. Polyethylene (PE) insulated nichrome (NiCr) wires were tested, having the insulator with o.d. 0.8, i.d. 0.5, and 320 mm in length. The wire was installed on a holder made of non-conductive acetal-resin. One end of the wire was attached to a fixture and the other end was connected to a spring to maintain it straight with tension to prevent bending by thermal expansion during flame propagation. The flame was initiated by a hot-wire igniter, which was placed on an air-cylinder. To minimize the interaction between the ignition system and applied electric fields, the igniter was retreated away after ignition. A programmable logic controller was utilized to control the time sequences of experiment. First, electrical power was supplied to the igniter for 8 s. After ignition, the air-cylinder was activated to move the igniter away. Then, the AC power supply was turned on with specified AC voltage and frequency. Subsequently, the camera was triggered to take images of spreading
Function generation
Backlight
Power supply R
Spread
Flame
Wire
Spring
High Voltage terminal
X Igniter
Voltage probe Oscilloscope
Air cylinder
flame at 30fps with backlight illumination. The length of the flame spread zone was 120 mm after the igniter. The spread rate can be influenced by AC frequency and natural convection. In the present, average spread rate is typically of the order of 1 mm/s. The variations in spread rate are represented as error bars. The AC power supply (Trek, 10/10B-FG) applies electric fields to the wire. The AC frequency fac was varied in the range of 10–1000 Hz by using a function generator and the AC voltage Vac was varied up to 7 kV (RMS). The frequency and voltage were monitored by an oscilloscope through a 1000:1 probe. One end of the wire was directly connected to the high voltage terminal of the AC power supply and the other terminal to a building ground, such that the system can be regarded as an open circuit. In this configuration, the induced electric fields can be assumed to be distributed between the high potential wire and imaginary infinite ground [13]. The applied electric fields can be regarded as axisymmetric along the wire, since the length of wire is much longer than the diameter of the wire. The electric field intensity is expected to decay in radial direction. Near the end of wire, the convergence of electric flux can increase electric fields. To exclude this effect, together with initial transient, spread data were analyzed in the range of 10–60 mm from the ignition point and averaged. 3. Results and discussion Figure 2 shows the variation in the flame position X with time t at various frequencies for Vac = 3 kV, together with the baseline case without applying AC. Here, X is defined as the distance from the igniter and t = 0 is set when the flame reaches X = 10 mm. The result exhibits that the rate of change in flame position is influenced by AC electric fields, such that the position increases faster for fac = 1000 Hz and slower for fac = 60 and 400 Hz, as compared to that for the baseline case. 60
Po osition off flame ffront X [m mm]
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50
40 0kV 60 Hz, 3kV 400 Hz, 3kV 1000 Hz, 3kV
30
20
GND
Wire holder
10 PLC Video camera
Fig. 1. Schematic of experimental setup.
0
5
10
15
20
25
Time t [s]
Fig. 2. Position of flame front with time.
30
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151 2kV
3kV
4kV
5kV
w
60 Hz
3.5
Spread rate S [[mm/s]
1kV
400 Hz
1147 Frequency [Hz] 10 200 400 20 30 600 60 800 80 1000 100
Extinction (1000Hz)
3.0
2.5
2.0
1.5
Extinction (100Hz)
1.0 0
Fig. 3. Instantaneous flame images at various applied AC voltages and frequencies.
The instantaneous images, obtained at X = 30 ± 1 mm, are shown in Fig. 3 at various AC electric fields. It is interesting to note that the flame shape is appreciably modified. As the voltage increases, the flame leans toward the burnt side. For fac = 60 Hz, the height and length of the flame decrease monotonically with voltage. For fac = 400 Hz, non-monotonic behavior is exhibited, especially the flame width decreases first and then increases. For Vac = 3 kV and fac = 400 Hz, the flame size increases appreciably such that the portions covering both unburned and burnt sides of wire are extended in the horizontal direction. At excessively large voltage, the flame was extinguished after initial transient, e.g., at 5 kV for 60 Hz and 4 kV for 400 Hz. The molten PE inside the flame becomes darker with AC voltage and the molten PEs for fac = 400 Hz are relatively darker than those for fac = 60 Hz. This can be attributed to the deposit of soot produced in the flame zone by electrostatic force. Although the flame at the baseline case produces soot by exhibiting yellow luminous flame, most of the soot generated is oxidized upon exiting the upper part of the flame and soot deposition to molten PE and on the surface of burnt wire was negligible. As a result, the molten PE is clear and the burnt region of the wire is free from soot deposition. With AC fields, however, appreciable amount of soot was deposited on the burnt wire, which can be confirmed from the observation of the burnt wire which is coated with soot after flame spread. 3.1. Spread rate with AC electric fields The spread rate was linear fitted during the time history of flame position shown in Fig. 2. Note that the change of flame position is reasonably linear within the range. Figure 4 shows the spread rate Sw = dX/dt as a function of voltage with 1 kV increment at various frequencies. As the voltage increases, the spread rate generally decreases with voltage for the frequency range of fac = 10–200 Hz marked as dotted lines, except
1
2
3
4
5
Applied voltage V
ac
6
7
[kV]
Fig. 4. Spread rates of wire flame with applied voltages Vac for various frequencies.
for fac = 10 Hz, where Sw starts to increase over 4 kV and then decrease again over 6 kV. At further increased voltage, the flame is extinguished after initial transient, as marked for 100 Hz. The spread rate decreases with the frequency. The standard deviations are marked as error bars, which is amplified with voltage as can be seen for fac = 10 Hz. In cases of fac = 400–1000 Hz marked as solid lines, the spread rate behavior changes to nonmonotonic to the voltage, such that it decreases initially and then increases appreciably for Vac > 2 kV. Also, the spread rate increases with the frequency. Note that for Vac = 3 kV and fac = 1000 Hz, the spread rate is even larger than that of the baseline case (0 kV). This result suggests that the spread rate of wire flame is significantly influenced by AC electric fields and under certain conditions, the spread rate can be larger than that for the baseline case. This is an important fact that may require consideration in establishing a fire safety code on electrical wires. For relatively low voltages, the variation of spread rate with applied voltage demonstrates reasonably linear decreasing trend, exhibiting a linear regime. The decreasing slope of spread rate can be determined by linear fitting for the data in the linear regime. The absolute value of the rate of change in the spread rate jdSw/dVacj is plotted in Fig. 5 with AC frequency in log–log scale. The slope initially increases up to about 400 Hz and then decreases. Dotted lines show the power fittings for both increasing (regime I) and decreasing (regime II) regimes. The correlations between AC frequency and the absolute rate of change in spread rate have the best fits of jdSw/dVacj [mm/s/kV] = 0.10 fac0.284 [s0.284] in regime I and jdSw/dVacj [mm/ s/kV] = 3.08 fac0.278 [s0.278] in regime II, where the correlation coefficients R are 0.98 and 0.94, respectively. These two fitting lines intersect at fac = 440 Hz, which can be defined as a transition frequency. This transition is consistent with the
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M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151 1
|dSw/dV Vac| [mm//s/kV]
Regime I Regime II 0.5 0.4 0.3
0.2
Transition 0.1 10
100
1000
Applied AC frequency fac [Hz]
Fig. 5. Rate of change in spread rate with applied AC voltage as function of AC frequency.
change in the behavior from monotonic to nonmonotonic in the spread rate with applied voltage (Fig. 4). 3.2. Effect of AC electric fields on flame shape To further elucidate the interaction between electric fields and spread rate, detailed observations have been performed. Figure 6 shows the instantaneous images obtained at X = 30 mm at various frequencies for Vac = 3 kV together with the baseline case. The flame height (H) and width (W) are defined as marked. The flame-shape changes appreciably with AC frequency. As the frequency increases, the flame leans toward the burnt region with exposed wire. Considering quenching distance, a flame can be located closer to the wire than unburned region. Then, the flame in bare-wire region can experience stronger electric fields. Note that the radial gradient of electric field intensity is high near wire surface. The distance of the flame edge in the unburned side from the molten PE generally decreases with the frequency. At higher frequency over 400 Hz, however, the distance appreciably increases. Previously, the effects of forced convection, ambient pressure, and conductivity of electric wire
on the spread rate of wire flames were investigated [10]. It has been proposed that a thermal balance among various heat transfer rates is controlling spread characteristics, including heat transfer from flame to unburned (Q1) and burnt (Q3) wire, heat supplies to molten PE through the wire (Q2) and to the insulator (Q4) and heat loss to ambient (Qloss), as schematically shown in Fig. 7. When one (or more) of these heat transfer characteristics is changed by electric fields, the balance of heat transfers will be modified, and then the spread rate should be adjusted to re-balance the heat transfers. This implies that the spread rate could be influenced by various ways when electric fields are applied. Charged particles existed in a reaction zone can be accelerated by the Lorentz force, thus the associated diffusion flux can be enhanced. Accelerated ions can transfer momentum to neutral particles by random molecular collisions such that bulk flow can be generated, resulting in the ionic wind effect. Through these effects, the flame shape could be changed. Once the height and width of flame are changed, not only the distance between wire and reaction zone but also the length of wire covered by the flame will be changed. Chemical reaction rates associated with charged particles can also be influenced by the applied electric fields. The major portion of the flame will have a nonpremixed flame nature, such that the effect of electric fields on chemical reaction may play a weak role since a nonpremixed flame is controlled by diffusion process. However, the edge flames near the wire could exhibit a premixed flame nature by quench zone. In such a case, the effect of electric fields on chemical reaction rate may play an important role. Moreover, the electric field intensity is expected to be large near the wire. In such a case, heat transfer from flame to wire and molten PE will be directly influenced. Consequently, the generation and evaporation of molten PE could be altered due to modified thermal balance. Soot particles generated in the flame will be influenced by electric fields through electrostatic force and electrophoresis. Soot deposition Electric fields
Electric fields Flame Qloss
Spread
Q1
Q3 Q4
Q2 Wire
Molten PE Electric fields
Fig. 6. Instantaneous images at various frequencies for fixed voltage of Vac = 3 kV.
Qloss
Fig. 7. Schematic of spreading flame.
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
increased with applied voltage and frequency as mentioned previously. This implies that soot particles migrated toward the wire by electric fields. Soot deposition on electrodes was enhanced for both positive and negative DC voltages in electrostatic precipitators and was linearly proportional to DC voltage [14–16]. Resulting soot deposition on wire with AC could affect radiation heat transfer from wire to ambient, thereby the thermal balancing process by altering heat transfer characteristics of PE and wire together with material properties such as viscosity and surface tension of molten PE. Edge flame behavior especially near solid surface is also a complex phenomenon, which has not been accurately quantified. Therefore, respective influences on wire flame behavior cannot be fully characterized at this stage and will be a future study. The spread rate with frequency for Vac = 3 kV is plotted in Fig. 8a to compare with the variation in flame structure in Fig. 6. In regime I, the rate decreases with AC frequency. The flame leans toward the burnt downstream and simultaneously, the flame height and width decrease with frequency. Figure 8b shows the variations in flame height and width with AC frequency for Vac = 3 kV. The flame is flickering during spread due to buoyancy effect typically with 4–8 Hz, so as its height and width. The flame oscillation can occur due to not only by the buoyancy effect but also interaction with applied AC. Detailed analysis of flame oscillation is beyond the scope of present paper and will be investigated in the future. Here, the mean height (Hm) and width (Wm), averaged over X = 20–40 mm, are exhibited with error bars. The result shows that the flame height in regime I is quite smaller than that without having electric fields ðH om ¼ 16:5 mmÞ, marked on the vertical axis, and decreases with frequency up to 100 Hz. The leaning of the flame toward the burnt side wire can be conceived to be similar to the effect of outer convective flow. In such a case, the heat
Sw [mm//s]
3.5 3.0
Sw
2.5
I
o
II
2.0
Hm and Wm [m mm]
1.5
Sw
1.0 16
Hm
o
14
(a)
Hm Wm
12 10 8
Wm
6 10
o
(b) 100
1000
Applied AC frequency fac [Hz]
Fig. 8. Spread rate of wire flame Sw, mean flame height Hm and width Wm with applied AC frequency for fixed voltage of Vac = 3 kV.
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transfer from flame to unburned PE (Q1) can be decreased, which reduces the spread rate, as was observed in the outer convective flow experiment [10]. With applied AC, however, the flame is not only leaned but also shortened its size, such that the heat transfer from flame to burnt wire (Q3) will be decreased. Consequently, the heat conduction toward unburned side (Q2, Q4) will also be decreased. Then, both the production and evaporation rates of molten PE will be decreased, resulting in the decrease in flame size. The decrease in flame size will reduce the total heat release from flame surface and enhance the cooling effect (Qloss) due to increased temperature gradient [17,18]. In regime II, the flame length and width increased and recovered to the values without having electric fields as AC frequency increase. As compared to regime I, the luminous flame enlarges with AC electric fields (Fig. 6). A possible process for this enhancement may be explained as follows. If the reaction rate near flame edge is enhanced, the length of front flame covering unburned wire could be increased, which can be partly observed in Fig. 6 for the cases of fac = 600 and 1000 Hz. In such a case, the generation of molten PE will be promoted due to enhanced heat transfer (Q1), leading to the increase in flame size on burnt wire region. Then, the heat conduction (Q2) will be enhanced due to increase of heat transfer (Q3). Again, it will lead to the amount of molten PE larger, thereby the flame size. Therefore, the spread rate could be recovered and even becomes larger than that for the baseline case. This enhancement process is similar to the explanations for the enhancement by increasing wire conductivity [10]. Further investigation will be required to fully understand the electric field effects on wire flame. It was discussed that the change in flame size is related to the variation of spread rate. Then, some inconsistencies are encountered in the relation between flame spread rate and flame size. In Fig. 8, e.g., the spread rates are slower than that without having electric fields of S ow ¼ 2:78 mm=s, except for fac = 1000 Hz. In case of flame width, however, not only the case of fac = 1000 Hz but also several other cases have larger flame width. In case of the flame height, the heights are always smaller than that without having electric fields of H om . And the minimum height was 6.148 mm at fac = 100 Hz. This is not consistent with the minimum spread rate of Sw = 1.49 mm/s at fac = 200 Hz. This suggests that the length scales need to be considered in terms of 3D aspect instead of 2D. For fac = 200 Hz, the flame height and width becomes almost the same with 6.39 and 6.24 mm, respectively, having near spherical shape. Note that a sphere has minimum surface area with the same volume. If heat generation in unit area of flame surface is constant based on diffusion flame nature, the
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I
II
500
400 o
As 300
200
100 10
100
Extinction 6
5
4
Spread
3
1000
10
A li d ffrequency fac [Hz] Applied [H ]
100
1000
Applied frequency fac [Hz]
Fig. 9. Spheroidal surface area of wire flame As with applied AC frequency for fixed voltage of Vac = 3 kV.
minimum surface area means that the total heat generation from the flame will be the minimum. To test the 3D aspect of flame surface, the flame is assumed to be a spheroid. Then, the surface area As becomes 2p [b2 + (a2b)/(a2 b2)0.5 arcsin{(a2 b2)0.5/a}], where a and b are the longer and shorter radii, respectively. The estimated spheroidal surface area of wire flame is plotted in Fig. 9 with AC frequency. The result shows that the variation of surface area with applied frequency is quite similar to the change in the spread rate (Fig. 8a). This indicates that, even though it is a rough approximation, the 3D aspect of flame surface area is reasonable in representing the variation in spread rate with AC electric fields. Figure 10 shows the spread rate with the surface area for several cases of applied AC electric fields, demonstrating satisfactory correlation. The best fit is Sw [mm/s] = 0.9482 + 0.0056 As [mm2] with R = 0.946. In this correlation, the data for the case of fac = 1000 Hz, Vac = 3 kV was excluded, since the flame surface is highly corrugated (Fig. 6) such that the spheroid assumption could appreciably be underestimated.
Fig. 11. Extinction voltage Vac with applied frequency fac.
3.3. Extinction behavior The wire flame was extinguished at excessive voltages. Figure 11 shows the extinction voltage Vext with frequency. The result shows that the extinction voltage decreases with frequency in regime I having the best fit of V ext ½kV ¼ 8:94 fac0:149 ½s0:149 with R = 0.936. In regime II, it becomes nearly constant of about 3.5 kV. To further elucidate the extinction phenomena, the transient variations in the flame and molten PE positions were examined. Figure 12 shows the timehistories of the front positions of flame and molten PE for Vac = 5 kV and fac = 60 Hz (regime I) with the snap-shot images. The result exhibits that the front position of flame keeps on increasing with time while that of the molten PE follows the flame initially and then maintains at a certain constant position. Consequently, the distance between the two front positions Dfm maintains a near constant value (0.6 mm) and then increases with time. Finally, the flame extinguished. In regime I, the spread rate is inversely proportional to both voltage and frequency as shown in
3
10
Dfm
2.8 2.6 2.4 2.2 2
0kV 3kV (fixed) 60Hz (fixed) 400Hz (fixed) Fitting
1.8 1.6 1.4 50
Dista ance Dfm [mm]
8
40
150
200
250
300
350
400
450
(c)
6
4
35
(c)
(b) Molten PE Flame Distance
(a) Ext
30
2 25
0 0
100
A
(b)
(a)
Front positions s [mm]
Spread rate Sw [mm/s]
II
I
7
Extinction voltage Vext [kV]
2
Spherroidal su urface arrea As [m mm ]
8 600
1
2
3
4
5
6
Time [sec]
2
Spheroidal surface area As [mm ]
Fig. 10. Spread rate Sw with spheroidal surface area As with various applied AC voltages and frequencies.
Fig. 12. Front positions of flame and molten PE and those distance with relative time for applied AC voltage Vac = 3 kV and frequency fac = 60 Hz.
M.K. Kim et al. / Proceedings of the Combustion Institute 33 (2011) 1145–1151
Fig. 4. And it has also been shown that the change in spread rate is relative to the change in flame size by applied AC electric fields. As the surface area enclosing molten PE decreases, the heat transfer from flame to the molten PE will be reduced. If the reduction becomes excessive, then not enough heat to generate molten PE, resulting in the lack of fuel to sustain the flame. As a result, the flame front and molten PE are separated prior to the extinction as shown in Fig. 12 marked as A. In regime II, the flame extinction occurs at a constant voltage even though the surface area increases as compared to that for regime I. In such cases, it was observed that a certain amount of molten PE with high deposit of soot was dripping from the wire occasionally. This can be partly attributed to the radiation absorption to molten PE such that the surface tension of it could be decreased, then the molten PE will drop. Note that the viscosity of molten PE will decrease as the temperature increase by the radiation absorption. This sudden loss of molten PE could be attributed to the extinction in regime II. In this regard, microgravity experiments need to be conducted in the future to exclude such effect and then the extinction characteristics may be modified in regime II. Finally, the present experiment created many unanswered questions regarding the effect of electric fields on the spread of wire flames, which requires future studies both ground-based and microgravity experiments to understand the underlying detailed mechanisms. 4. Concluding remarks The flame spread rate over polyethylene (PE) insulated electrical wire has been investigated experimentally by applying AC electric fields in the normal gravity condition. With the single electrode configuration, the spread rate of wire flame has been measured by varying applied AC voltage and frequency in the range of 10–1000 Hz and 0– 7 kV, respectively. The result showed that the spread rate was appreciably modified by the voltage and over a certain critical voltage the flame was extinguished. For a fixed applied voltage, two distinct regimes existed depending on the frequency. In the low frequency regime, the flame spread rate decreased with the frequency. In the high frequency regime, the flame spread rate increased with the frequency, even over the spread rate without having electric fields. The variation of wire-flame structure with applied electric fields was investigated by analyzing the flame height, width, and surface area. The results showed that the surface area correlated well with the variation in spread rate with electric fields. The detailed mechanisms leading to these phenomena will be a future study.
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In case of actual fire for electric wires, there could be a possibility of electrical voltage applied. In such a case, the flame spread rate could be modified from that without having electric fields. The relevant fire-safety code both in normal and microgravity conditions developed based on without applying electric fields may require to be modified by considering the situation of fire for electric wires experiencing electric fields. Acknowledgements This work was supported by AEA Project/ KAUST and by “Ground-Based Research Program for Space Utilization” by Japan Space Forum (2006–09). References [1] L. Orloff, J. de Ris, G.H. Markstein, Proc. Combust. Inst. 15 (1974) 183. [2] A.C. Fernandez-Pello, Combust. Flame 31 (1978) 135–148. [3] K. Saito, J.G. Quintiere, F.A. Williams, in: Proceedings of the 1st International Symposium on Fire Safety Science, Hemisphere, Washington, DC, 1986, p. 75. [4] K.B. McGrattan, T. Kashiwagi, H.R. Baum, S.L. Olson, Combust. Flame 106 (1996) 377. [5] S.L. Olson, Combust. Sci. Technol. 76 (1991) 233. [6] T. Kashiwagi, S.L. Olson, O. Fujita, M. Kikuchi, K. Ito, Proc. Combust. Inst. 26 (1996) 1345. [7] S. Bhattacharjee, R.A. Altenkirch, Combust. Flame 84 (1991) 160. [8] R.A. Altenkirch, L. Tang, K. Sacksteder, S. Bhattacharjee, M.A. Delichatsios, Proc. Combust. Inst. 27 (1998) 2515. [9] O. Fujita, K. Nishizawa, K. Ito, Proc. Combust. Inst. 29 (2002) 2545–2552. [10] Y. Nakamura, N. Yoshimura, T. Matsumura, H. Ito, O. Fujita, JTST 3 (2008) 430–441. [11] A. Umemura, M. Uchida, T. Hirata, J. Sato, Proc. Combust. Inst. 29 (2002) 2535–2543. [12] J. Lawton, F. Weinberg, Electrical Aspect of Combustion, Clarendon Press, 1969. [13] S.H. Won, S.K. Ryu, M.K. Kim, M.S. Cha, S.H. Chung, Effect of electric fields on the propagation speed of tribrachial flames in coflow jets, Combust. Flame 152 (2008) 496–506. [14] K. Yamashita, S. Karnani, D. Dunn-Rankin, Combust. Flame 156 (2009) 1227–1233. [15] A. Jaworek, A. Krupa, T. Czech, J. Electrostat. 65 (2007) 133–155. [16] E. dela Cruz, J.S. Chang, A.A. Berezin, D. Ewing, J.S. Cotton, M. Bardeleben, J. Electrostat. 67 (2009) 128–132. [17] J. Park, C.B. Oh, K.T. Kim, J.S. Kim, A. Hamins, 5th ASPACC, 2005. [18] K. Maruta, M. Yoshida, H. Guo, Y. Ju, T. Nioka, Combust. Flame 112 (1998) 181–187.