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Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents
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Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents Hao He a , Qixing Zhang a,∗ , Ran Tu b , Luyao Zhao a , Jia Liu a , Yongming Zhang a,∗ a b
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China College of Mechanical Engineering and Automation, Huaqiao University, Xiamen, Fujian 361021, PR China
h i g h l i g h t s • • • •
Overload current effects on the transient wire temperature profile were predicted. A relationship between molten loss and volume variation was proposed. The dripping frequency was obtained theoretically and experimentally. The flame width, height and spreading velocity presented different behaviors.
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Article history: Received 31 March 2016 Received in revised form 15 July 2016 Accepted 29 July 2016 Available online xxx Keywords: Dripping behavior Molten insulation Molten drip Overload currents Wire fire
a b s t r a c t The dripping behavior of the molten thermoplastic insulation of copper wire, induced by flame spread under overload currents, was investigated for a better understanding of energized electrical wire fires. Three types of sample wire, with the same polyethylene insulation thickness and different core diameters, were used in this study. First, overload current effects on the transient one-dimensional wire temperature profile were predicted using simplified theoretical analysis; the heating process and equilibrium temperature were obtained. Second, experiments on the melting characteristics were conducted in a laboratory environment, including drop formation and frequency, falling speed, and combustion on the steel base. Third, a relationship between molten mass loss and volume variation was proposed to evaluate the dripping time and frequency. A strong current was a prerequisite for the wire dripping behavior and the averaged dripping frequency was found to be proportional to the square of the current based on the theoretical and experimental results. Finally, the influence of dripping behavior on the flame propagation along the energized electrical wire was discussed. The flame width, bright flame height and flame spreading velocity presented different behaviors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The incidence of electrical fires has increased with urbanization and electrification. In China, a total of 115,599 electrical fires occurred in 2013, resulting in 745 deaths, 538 injuries and ¥1.9 billion in direct economic loss; 60.0% of the fires involved the failure of electrical wires [1]. In the United States, an estimated 25,900 residential building electrical fires were reported annually from 2009 to 2011; these fires caused an estimated 280 deaths, 1125 injuries and $1.1 billion in property damage. Electrical wire and cable insulation is the leading factor, igniting first in about 30% of incidents
∗ Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Zhang).
[2]. From accident statistics, most electrical fires are caused by short circuit, overheating and worn wire with the ignition of the insulation attached to the wires. Once ignited, fire propagates along the wire, releasing heat, soot particles, and toxic gases. If the wire burns fiercely with a fast propagation speed, the molten insulation accumulates over time. When the volume of molten insulation reaches a certain limit, dripping can occur. The hot molten or burning polymer may ignite nearby combustibles, expand combustion range and increase the fire risk. Various standards and tests have been developed to evaluate the fire performance of electrical wires [3]. For example, NASA’s standard test was used to study the effects of wire gauge and insulation thickness, internal wire temperature, and sample orientation on the wire insulation flammability [4]. Hasegawa et al. experimentally studied the ignition delay time and the upward flame-spread rate over the surface of insulated electrical cables under an externally
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Please cite this article in press as: H. He, et al., Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.07.070
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Nomenclature d ı A ˛ c ˙ m q˙ I T f g B Nu r t
v w ε
Diameter (m) Insulation thickness (m) Area (m2 ) Thermal diffusion coefficient (m2 /s) Density (kg/m3 ) Thermal conductivity (W/m/K) Specific heat (J/kg/K) Mass loss rate (g/s) Heat release rate per unit length (kW/m) Current (A) Temperature (K or ◦ C) Density (kg/m3 ) Frequency (Hz) Gravity (m/s2 ) Mass transfer number Nusselt number Radius (m) Time (s) Flame spreading velocity (m/s) Bright flame width (m) Emissivity Stefan-Boltzmann constant
Subscripts c Core p Insulation g Gas o Outer surface Flame f ∞ Ambient
applied radiation heat flux [5]. Leung et al. formulated a mathematical model to study the effect of the inert central core on thermal pyrolysis of the insulation layer, without flame, during the heating process [6]. Based on the heat transfer between the hot gases and burning surface, Umemura et al. proposed a physical model to explain self-sustained burning in flame propagation of electrical wires in microgravity [7]. For fire safety in space, Fujita and Nakamura et al. have focused on flame propagation along electrical wires in microgravity. They conducted a series of experiments to study the influence of both internal and external parameters, including wire initial temperature, core diameter, ambient oxygen concentration [8], low external flow [9], opposed-wind [10], subatmospheric pressure [11], and dilution gas on wire combustion under normal gravity and microgravity. They also studied the effect of alternating current (AC) electrical fields on the flame spread over electrical wire [12] and the ignition of electrical wire with short-term excess electric current [13]. Huang developed a model to explain ignition and the following transition to spread [14]. Takahashi performed several tests to examine the influence of flow velocity on the dependent volume change of molten insulation under varying external opposed flow conditions in microgravity [15]. Hu explored the effect of a high thermal conductivity metal core at different inclinations on flame spread over electric wire [16]. Lim et al. investigated the effect of electrical fields on the characteristics of flame spread along a polyethylene (PE) insulated electrical wire by varying the AC frequency and voltage [17]. Recently, several works examined the thermoplastic properties of the dripping melt. Zhang et al. explored the effect of the melting behavior of thermoplastic polymers on the mass loss rates during the steady burning stage [18]. Y. Wang investigated the thermal stabilities of eight thermoplastic materials (PE etc.) as well as their melting
Fig. 1. Schematic of the experimental apparatus.
Fig. 2. Photo of the wire sample holder.
drops generated under the UL 94 vertical burning test conditions [19]. Kandola et al. presented a methodology to record the real-time melting, burning, and dripping behavior of thermoplastic polymers [20]. A heat-transfer model was developed to compute the surface temperature of the polymer samples at various furnace temperatures [21]. N. Wang conducted several medium-scale experiments on the thermal hazard induced by melting and dripping of thermoplastics [22]. Xie et al. studied the loop mechanism between the wall fire and pool fires induced by the melting and dripping of thermoplastic based on a T-shape trough [23], and by comparing the polymers’ faster flowing burning the results suggested that the fire hazard of polyethylene (PE) is clearly higher than polystyrene (PS) [24]. In general, although previous works have focused on the ignition and flame spread of electrical wire and the thermoplastic properties, there are limited studies of the dripping of molten insulation especially that induced by strong electrical currents, which are more common and could be more dangerous. Therefore, in this paper, the effect of electrical current on the molten dripping insulation was investigated. Two theoretical relationships were developed to determine the temperature and dripping behavior of molten insulation under strong electrical currents. An experimental study has been performed using several wires, to validate these theoretical relationships. The results of this study could be useful for the fire safety design of electrical wires. 2. Experimental Fig. 1 shows the experimental apparatus, which consisted of three parts: a wire sample holder, two constant current sources (CCS), and an ignition part. As shown in Fig. 2, the sample holder consisted of a base, holder, Bakelite plates, compression spring, wiring terminal, coil heater, and sample wire. The dimensions of the base were 300 mm (L) × 60 mm (W) × 15 mm (H). The base and
Please cite this article in press as: H. He, et al., Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.07.070
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Therefore, the transient one dimensional wire heating process is established as follows:
∂Tp ∂Tp ∂ 1 q˙ + × (p r ) = cp r ∂r ∂r ∂t
(1)
within the insulation: q˙ = 0 (rc < r < ro )
(2)
and the boundary conditions: I2 dTc − c Ac cc q˙ = 1 (r = rc ) Ac dt
(3)
q˙ = q˙ loss = do h(Tp − T∞ ) (r = ro )
(4)
Fig. 3. Wire heating process and temperature profile.
holder were made of stainless steel, and the Bakelite plates were used as an insulation material to avoid a short circuit. The compression spring on the left kept the wire sample straight and tight during the whole process. The coil heater, which was made of Nicochrome, could function as an ignitor when energized. The ignition part was free on the base, and it was set to maximize the available wire length. CCS 1, had a measuring range of 0–21 A with an accuracy 0.1 A, and was used to adjust the current of the sample wire. CCS 2 (measuring range 0–7 A with an accuracy 0.01 A), was used to precisely control the current imposed on the coil heater. A front-view video camera (SONY NEX-5R, 50 fps) and a high-speed photography (ASO TRI-VIT) were used to record the process. The characteristic parameters of the flame propagation and the dripping of molten insulation were obtained by image processing. Three types of polyethylene (PE)-coated copper wires (Table 1) with different diameters but the same insulation thickness were used. The ratio of core section area (representing the electrical resistance) was about 1:2.5:5. To avoid the effect of air flow on flame propagation and insulation dripping behavior, all of the experiments were performed under quiescent conditions. After the current was selected, a self-preheating process started, which lasted for at least 30 s to make sure the wire reached a thermal balance. Then, CCS 2 was turned on to ignite the wire. To avoid extra heat transfer from the coil heater to the burning part, CCS 2 was turned off after the flame passed through the ignition zone. All of the experiments were conducted between 20 and 25 ◦ C ambient temperature and 70–80% relative humidity. Generally, at least five repeated tests were conducted under the same conditions to reduce error. 3. Theoretical analysis of the wire heating and dripping process 3.1. Wire heating process To calculate the wire temperature, which changes over time under an electrical current, a simplified numerical relationship was established; the schematic is shown in Fig. 3. Four assumptions were adopted as follows: (1) the thermal properties of the solids were assumed to be temperature independent, they took the temperature-averaged value, which was kept constant throughout pyrolysis; (2) the wire was straight and infinitely long, the temperature distribution occurring in the radial direction; (3) the copper core (maximum diameter, 1.1 mm) maintained a constant temperature as the thermal conductivity was greater than that of the thermoplastic insulation p << c ; (4) the thermal contact resistance between the core and the insulation was ignored.
Here, q˙ loss is the heat loss flux from the outer surface of wire; 1 is the electrical resistivity for the mental core; h is the heat transfer coefficient, which consists of convection and radiation in two parts, as h = hconv + hrad . The convective heat transfer can be calculated by hconv = Nug /d [25] and the radiation heat transfer can be 2 ) [26]. calculated by hrad = ε(To + T∞ )(To2 + T∞ 3.2. Dripping process of the molten insulation With flame propagation along the wire, the mass of insulation ˙ mlt , represents gradually reduces. The mass of molten insulation, m ˙ p , and the the relationship between the mass-loss of insulation, m ˙ f . They can be calculated as follows [27,28]: burning rate, m ˙ mlt = m ˙ p−m ˙ f; m ˙ p = p Ap vf ; m ˙ f = CPo wf m
kg ln[1 + B] cp ıf
(5)
˙ f is an eigenvalue that satisfies a given set of The burning rate m environmental and fuel property conditions. The symbols Po , ıf and kg in Eq. (5) represent the outer perimeter of the wire, the boundary layer thickness of the flame, and the gas conductivity, respectively. The coefficient C = 1.80 accounts for the enhanced gas-to-surface heat transfer because of the cylindrical curvature. The mass transfer number is B≡
(1 − r )YO2 ,∞ ( Hp / ) − cg (Tig − T∞ ) Lp
(6)
where r is the radiative loss fraction, YO2 ,∞ is the oxygen mass fraction in air, Hp is the combustion heat of PE, is the stoichiometric fuel-to-air mass ratio, and Lp is the latent heat of vaporization; the subscript ig represents the ignition point. In addition, Lp consists of the latent heat of pyrolysis and any other heat loss from the insulation surface, and it decreases with increasing current. The mass transfer number B contains both environmental and fuel property factors. Eq. (6) expresses the ratio of chemical energy released to energy required to vaporize the fuel (per unit mass of fuel). It is clear that it must be greater than 1 to achieve sustained burning. As the amount of molten insulation increases with flame propagation, drops would form and fall as a result of accumulation. The volume of a drop of the dripping insulation relates to the surface tension, which can be calculated as follows [29]: 3
Vlim =
3 fc 2 4 ( ) 3 2p g
(7)
Here, is the liquid surface tension. fc is the correction factor required because the drop is not a perfect sphere, but an elongated ellipsoid with a spot of residue left at the bottom of wire. Then the frequency of dripping can be expressed as: f =
˙ mlt m p Vlim
(8)
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4 Table 1 Configuration of wires used in this study. Type
d c Core diameter (mm)
ıp Insulation thickness (mm)
d 0 Sample diameter (mm)
Ratio of core section area Ac /A0 (%)
Ratio of insulation section area Ap /A0 (%)
Rated current (A)
A B C
0.50 0.80 1.10
0.15 0.15 0.15
0.80 1.10 1.40
39.06 52.89 61.73
60.94 47.11 38.27
0.78 1.96 3.80
Fig. 4. Temperature profile of wire for various currents.
perature; types B and C required more time because of their lower resistance and heat production. As shown in Fig. 4, the equilibrium temperature reached the melting point and exceeded the pyrolysis temperature of PE insulation under currents of 8 A and 16 A, respectively. For wire type A under a current of 7 A, flame propagation remained stable and no dripping occurred. With an increased current of 8 A (10 times the rated current), the volume of molten insulation increased, and drops occurred. The molten insulation reached pyrolysis at about 15 A, with white smoke and volatile compounds released above the wire. When the current was raised over 17 A, the insulation would pyrolyze, and volatize completely in several seconds, resulting in the wire core being exposed to the air. For type B and C wires, the accumulation of molten insulation increased with current, and drops formed at 14 A. However, even at a current of 20 A, the maximum available, the equilibrium temperature could not reach the pyrolysis point of PE, and no decomposition phenomenon was observed.
4. Results and discussion 4.1. Temperature profile for the wire under overload currents
4.2. Dripping behavior of molten insulation
Fig. 4 shows the wire temperature curve varying with time, based on Eqs. (1)–(4), for wire type A, for currents of 8 A, 10 A, 12 A, 14 A, and 16 A (10–20 times the rated current), respectively. The equilibrium temperature is defined as the stable temperature to which the wire has risen. For wire type A, it took 30 s for the wire temperature to rise from room temperature to the equilibrium tem-
The molten insulation accumulated with flame propagation under overload currents. Drops occurred when this accumulation reached a certain limit. Fig. 5 shows the typical time-sequence images of the molten insulation dripping process, with an interval of 20 ms. The change of flame shape with time is also presented. The maximum of flame length in the vertical orientation above wire
Fig. 5. Images for the flame before and after a drop, captured by video camera.
Fig. 6. Images of the dripping process, captured by high-speed photography.
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core is regarded as flame height, and the bright flame width is the maximum of flame length in horizontal orientation. As shown in Fig. 5, a slow process of accumulation was observed before a drop fell, which occurred suddenly after the volume reached to a certain amount. When the drop occurred, the flame height rapidly changed, i.e., from 23.1 mm to 10.0 mm, a reduction of 56.8%. The bright flame width changed slightly, by approximately 9.0 mm during the dripping process. Fig. 6 shows the dripping process from a drop beginning to occur, to the flame on the base extinguishing. These images show the flame shape slightly change during the falling stage. The dripping front position, which is defined as the distance from the wire core the falling melt drop traveled, is shown as black dots in Fig. 7. By using the methods of binomial fitting to obtain, first- and secondorder differentials, the speed and acceleration of the molten drop is achieved. When the drop fell onto the steel base, it continued to combust for a short time, as shown in Fig. 8. This process lasted about 0.1 s, from the falling edge coming into contact with the base, until flame extinction. The molten drop spread rapidly at the moment of contact, resulting in the expansion of the combustion area. The flame height first decreased and then increased as with gasification of combustible materials decreased. The flame area then reduced gradually to extinction because of the cooling effect of the steel
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Fig. 7. Dripping front position, speed and acceleration of the molten drop.
base. It is convinced that the molten drop continue burning for quite a while after falling, even on a noncombustible material with high thermal conductivity.
Fig. 8. Combustion of the molten drop on the base.
Fig. 9. Dripping time and frequency on overload currents.
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Fig. 10. Flame shape changing with continuous drops, for wire type A under 15 A.
(1) A simplified theoretical relationship is presented to describe the temperature profile for the wire under overload currents. The effect of overload current on the temperature curve and the equilibrium temperature of the wires are obtained. (2) The formation of dripping molten insulation is analyzed, along with the falling speed and acceleration. The molten drop continues to burn on the steel base for about 0.1 s. (3) The relationship between the overload currents and the dripping frequency is developed for the dripping process. The calculation results are consistent with the experimental results; the dripping frequency and the square of the current are found to be in positive linear correlation. (4) The effect of dripping behavior on the flame propagation is also discussed. The flame height is reduced by 50% upon the occurrence of a drop and then increases gradually with the accumulation of molten insulation, presenting a certain periodicity. The bright flame width has slight vibrations and the flame propagation velocity is not affected.
4.3. Time interval and frequency of dripping molten insulation
Acknowledgements
Here, dripping time T is defined as the adjacent time intervals for the drops and dripping frequency f is the reciprocal of the dripping time. Fig. 9(a) shows the dripping time vs. overload current with a fit of 0.9913 for a diameter of 0.5 mm, 0.9812 for a diameter of 0.8 mm, and 0.9942 for a diameter of 1.1 mm, respectively. The experimental and numerical values of dripping frequency for the type A wire are shown in Fig. 9(b); the two values are in good agreement. The fitting curve is:
This work was supported by the Joint Funds of the National Natural Science Foundation of China under Grant Nos. U1233102 and 51506059, the National Program on Key Basic Research Project of China under Grant No. 2012CB719702, and the Collaborative Innovation Center of City Public Safety of Anhui Province. The authors gratefully acknowledge all of this support.
f = 0.0016I 2 − 0.0507, R2 = 0.9998
References (9)
The dripping frequency is nearly proportional to the square of the current. Considering vf ∼I 2 , as reported in previous research work [30], and the limited influence of current on wf , it can be ˙ p ∼vf ∼I 2 and m ˙ f ∼ ln(1 + B). The change in the logadeduced that m rithmic form of mass transfer number is very small with increasing current. Therefore, in the experimental range, the dripping frequency and the square of current are in a positive linear correlation. 4.4. Effect of the drop behavior on flame propagation Because of the combustible loss caused by the molten drop, the flame shape of the wire fire changed before and after a drop, as shown in Fig. 10; the bright flame width, the flame height, and the flame leading edge position all changed with time. For the flame height, there was a rapid increase before a drop occurred, and at the moment of a molten drop separating from the wire, it jumped from the maximum (e.g., 29.48 mm) to the minimum (e.g., 12.54 mm), a reduction of 57.5%. After the drop, the flame height gradually increased with the accumulation of molten insulation until the next drop occurred; this behavior was periodic. Conversely, the bright flame width, wf , remained unchanged during the process, with only a slight vibration during the drop. Using a linear fit of the leading edge position, the solid line shown in Fig. 10 is achieved. The dripping behavior had a limited effect on the flame propagation velocity. 5. Conclusions In this study, the dripping behavior of molten wire insulation (PE insulated copper core), and flame spread under overload currents were examined experimentally. The purpose of this work is to gain a better understanding of the fire risk and reduce the occurrence of more disasters, for the thermoplastic widely used in electrical engineering. The main conclusions are as follows:
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents Hao He a , Qixing Zhang a,∗ , Ran Tu b , Luyao Zhao a , Jia Liu a , Yongming Zhang a,∗ a b
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China College of Mechanical Engineering and Automation, Huaqiao University, Xiamen, Fujian 361021, PR China
h i g h l i g h t s • • • •
Overload current effects on the transient wire temperature profile were predicted. A relationship between molten loss and volume variation was proposed. The dripping frequency was obtained theoretically and experimentally. The flame width, height and spreading velocity presented different behaviors.
a r t i c l e
i n f o
Article history: Received 31 March 2016 Received in revised form 15 July 2016 Accepted 29 July 2016 Available online xxx Keywords: Dripping behavior Molten insulation Molten drip Overload currents Wire fire
a b s t r a c t The dripping behavior of the molten thermoplastic insulation of copper wire, induced by flame spread under overload currents, was investigated for a better understanding of energized electrical wire fires. Three types of sample wire, with the same polyethylene insulation thickness and different core diameters, were used in this study. First, overload current effects on the transient one-dimensional wire temperature profile were predicted using simplified theoretical analysis; the heating process and equilibrium temperature were obtained. Second, experiments on the melting characteristics were conducted in a laboratory environment, including drop formation and frequency, falling speed, and combustion on the steel base. Third, a relationship between molten mass loss and volume variation was proposed to evaluate the dripping time and frequency. A strong current was a prerequisite for the wire dripping behavior and the averaged dripping frequency was found to be proportional to the square of the current based on the theoretical and experimental results. Finally, the influence of dripping behavior on the flame propagation along the energized electrical wire was discussed. The flame width, bright flame height and flame spreading velocity presented different behaviors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The incidence of electrical fires has increased with urbanization and electrification. In China, a total of 115,599 electrical fires occurred in 2013, resulting in 745 deaths, 538 injuries and ¥1.9 billion in direct economic loss; 60.0% of the fires involved the failure of electrical wires [1]. In the United States, an estimated 25,900 residential building electrical fires were reported annually from 2009 to 2011; these fires caused an estimated 280 deaths, 1125 injuries and $1.1 billion in property damage. Electrical wire and cable insulation is the leading factor, igniting first in about 30% of incidents
∗ Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Zhang).
[2]. From accident statistics, most electrical fires are caused by short circuit, overheating and worn wire with the ignition of the insulation attached to the wires. Once ignited, fire propagates along the wire, releasing heat, soot particles, and toxic gases. If the wire burns fiercely with a fast propagation speed, the molten insulation accumulates over time. When the volume of molten insulation reaches a certain limit, dripping can occur. The hot molten or burning polymer may ignite nearby combustibles, expand combustion range and increase the fire risk. Various standards and tests have been developed to evaluate the fire performance of electrical wires [3]. For example, NASA’s standard test was used to study the effects of wire gauge and insulation thickness, internal wire temperature, and sample orientation on the wire insulation flammability [4]. Hasegawa et al. experimentally studied the ignition delay time and the upward flame-spread rate over the surface of insulated electrical cables under an externally
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Please cite this article in press as: H. He, et al., Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.07.070
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Nomenclature d ı A ˛ c ˙ m q˙ I T f g B Nu r t
v w ε
Diameter (m) Insulation thickness (m) Area (m2 ) Thermal diffusion coefficient (m2 /s) Density (kg/m3 ) Thermal conductivity (W/m/K) Specific heat (J/kg/K) Mass loss rate (g/s) Heat release rate per unit length (kW/m) Current (A) Temperature (K or ◦ C) Density (kg/m3 ) Frequency (Hz) Gravity (m/s2 ) Mass transfer number Nusselt number Radius (m) Time (s) Flame spreading velocity (m/s) Bright flame width (m) Emissivity Stefan-Boltzmann constant
Subscripts c Core p Insulation g Gas o Outer surface Flame f ∞ Ambient
applied radiation heat flux [5]. Leung et al. formulated a mathematical model to study the effect of the inert central core on thermal pyrolysis of the insulation layer, without flame, during the heating process [6]. Based on the heat transfer between the hot gases and burning surface, Umemura et al. proposed a physical model to explain self-sustained burning in flame propagation of electrical wires in microgravity [7]. For fire safety in space, Fujita and Nakamura et al. have focused on flame propagation along electrical wires in microgravity. They conducted a series of experiments to study the influence of both internal and external parameters, including wire initial temperature, core diameter, ambient oxygen concentration [8], low external flow [9], opposed-wind [10], subatmospheric pressure [11], and dilution gas on wire combustion under normal gravity and microgravity. They also studied the effect of alternating current (AC) electrical fields on the flame spread over electrical wire [12] and the ignition of electrical wire with short-term excess electric current [13]. Huang developed a model to explain ignition and the following transition to spread [14]. Takahashi performed several tests to examine the influence of flow velocity on the dependent volume change of molten insulation under varying external opposed flow conditions in microgravity [15]. Hu explored the effect of a high thermal conductivity metal core at different inclinations on flame spread over electric wire [16]. Lim et al. investigated the effect of electrical fields on the characteristics of flame spread along a polyethylene (PE) insulated electrical wire by varying the AC frequency and voltage [17]. Recently, several works examined the thermoplastic properties of the dripping melt. Zhang et al. explored the effect of the melting behavior of thermoplastic polymers on the mass loss rates during the steady burning stage [18]. Y. Wang investigated the thermal stabilities of eight thermoplastic materials (PE etc.) as well as their melting
Fig. 1. Schematic of the experimental apparatus.
Fig. 2. Photo of the wire sample holder.
drops generated under the UL 94 vertical burning test conditions [19]. Kandola et al. presented a methodology to record the real-time melting, burning, and dripping behavior of thermoplastic polymers [20]. A heat-transfer model was developed to compute the surface temperature of the polymer samples at various furnace temperatures [21]. N. Wang conducted several medium-scale experiments on the thermal hazard induced by melting and dripping of thermoplastics [22]. Xie et al. studied the loop mechanism between the wall fire and pool fires induced by the melting and dripping of thermoplastic based on a T-shape trough [23], and by comparing the polymers’ faster flowing burning the results suggested that the fire hazard of polyethylene (PE) is clearly higher than polystyrene (PS) [24]. In general, although previous works have focused on the ignition and flame spread of electrical wire and the thermoplastic properties, there are limited studies of the dripping of molten insulation especially that induced by strong electrical currents, which are more common and could be more dangerous. Therefore, in this paper, the effect of electrical current on the molten dripping insulation was investigated. Two theoretical relationships were developed to determine the temperature and dripping behavior of molten insulation under strong electrical currents. An experimental study has been performed using several wires, to validate these theoretical relationships. The results of this study could be useful for the fire safety design of electrical wires. 2. Experimental Fig. 1 shows the experimental apparatus, which consisted of three parts: a wire sample holder, two constant current sources (CCS), and an ignition part. As shown in Fig. 2, the sample holder consisted of a base, holder, Bakelite plates, compression spring, wiring terminal, coil heater, and sample wire. The dimensions of the base were 300 mm (L) × 60 mm (W) × 15 mm (H). The base and
Please cite this article in press as: H. He, et al., Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.07.070
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Therefore, the transient one dimensional wire heating process is established as follows:
∂Tp ∂Tp ∂ 1 q˙ + × (p r ) = cp r ∂r ∂r ∂t
(1)
within the insulation: q˙ = 0 (rc < r < ro )
(2)
and the boundary conditions: I2 dTc − c Ac cc q˙ = 1 (r = rc ) Ac dt
(3)
q˙ = q˙ loss = do h(Tp − T∞ ) (r = ro )
(4)
Fig. 3. Wire heating process and temperature profile.
holder were made of stainless steel, and the Bakelite plates were used as an insulation material to avoid a short circuit. The compression spring on the left kept the wire sample straight and tight during the whole process. The coil heater, which was made of Nicochrome, could function as an ignitor when energized. The ignition part was free on the base, and it was set to maximize the available wire length. CCS 1, had a measuring range of 0–21 A with an accuracy 0.1 A, and was used to adjust the current of the sample wire. CCS 2 (measuring range 0–7 A with an accuracy 0.01 A), was used to precisely control the current imposed on the coil heater. A front-view video camera (SONY NEX-5R, 50 fps) and a high-speed photography (ASO TRI-VIT) were used to record the process. The characteristic parameters of the flame propagation and the dripping of molten insulation were obtained by image processing. Three types of polyethylene (PE)-coated copper wires (Table 1) with different diameters but the same insulation thickness were used. The ratio of core section area (representing the electrical resistance) was about 1:2.5:5. To avoid the effect of air flow on flame propagation and insulation dripping behavior, all of the experiments were performed under quiescent conditions. After the current was selected, a self-preheating process started, which lasted for at least 30 s to make sure the wire reached a thermal balance. Then, CCS 2 was turned on to ignite the wire. To avoid extra heat transfer from the coil heater to the burning part, CCS 2 was turned off after the flame passed through the ignition zone. All of the experiments were conducted between 20 and 25 ◦ C ambient temperature and 70–80% relative humidity. Generally, at least five repeated tests were conducted under the same conditions to reduce error. 3. Theoretical analysis of the wire heating and dripping process 3.1. Wire heating process To calculate the wire temperature, which changes over time under an electrical current, a simplified numerical relationship was established; the schematic is shown in Fig. 3. Four assumptions were adopted as follows: (1) the thermal properties of the solids were assumed to be temperature independent, they took the temperature-averaged value, which was kept constant throughout pyrolysis; (2) the wire was straight and infinitely long, the temperature distribution occurring in the radial direction; (3) the copper core (maximum diameter, 1.1 mm) maintained a constant temperature as the thermal conductivity was greater than that of the thermoplastic insulation p << c ; (4) the thermal contact resistance between the core and the insulation was ignored.
Here, q˙ loss is the heat loss flux from the outer surface of wire; 1 is the electrical resistivity for the mental core; h is the heat transfer coefficient, which consists of convection and radiation in two parts, as h = hconv + hrad . The convective heat transfer can be calculated by hconv = Nug /d [25] and the radiation heat transfer can be 2 ) [26]. calculated by hrad = ε(To + T∞ )(To2 + T∞ 3.2. Dripping process of the molten insulation With flame propagation along the wire, the mass of insulation ˙ mlt , represents gradually reduces. The mass of molten insulation, m ˙ p , and the the relationship between the mass-loss of insulation, m ˙ f . They can be calculated as follows [27,28]: burning rate, m ˙ mlt = m ˙ p−m ˙ f; m ˙ p = p Ap vf ; m ˙ f = CPo wf m
kg ln[1 + B] cp ıf
(5)
˙ f is an eigenvalue that satisfies a given set of The burning rate m environmental and fuel property conditions. The symbols Po , ıf and kg in Eq. (5) represent the outer perimeter of the wire, the boundary layer thickness of the flame, and the gas conductivity, respectively. The coefficient C = 1.80 accounts for the enhanced gas-to-surface heat transfer because of the cylindrical curvature. The mass transfer number is B≡
(1 − r )YO2 ,∞ ( Hp / ) − cg (Tig − T∞ ) Lp
(6)
where r is the radiative loss fraction, YO2 ,∞ is the oxygen mass fraction in air, Hp is the combustion heat of PE, is the stoichiometric fuel-to-air mass ratio, and Lp is the latent heat of vaporization; the subscript ig represents the ignition point. In addition, Lp consists of the latent heat of pyrolysis and any other heat loss from the insulation surface, and it decreases with increasing current. The mass transfer number B contains both environmental and fuel property factors. Eq. (6) expresses the ratio of chemical energy released to energy required to vaporize the fuel (per unit mass of fuel). It is clear that it must be greater than 1 to achieve sustained burning. As the amount of molten insulation increases with flame propagation, drops would form and fall as a result of accumulation. The volume of a drop of the dripping insulation relates to the surface tension, which can be calculated as follows [29]: 3
Vlim =
3 fc 2 4 ( ) 3 2p g
(7)
Here, is the liquid surface tension. fc is the correction factor required because the drop is not a perfect sphere, but an elongated ellipsoid with a spot of residue left at the bottom of wire. Then the frequency of dripping can be expressed as: f =
˙ mlt m p Vlim
(8)
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4 Table 1 Configuration of wires used in this study. Type
d c Core diameter (mm)
ıp Insulation thickness (mm)
d 0 Sample diameter (mm)
Ratio of core section area Ac /A0 (%)
Ratio of insulation section area Ap /A0 (%)
Rated current (A)
A B C
0.50 0.80 1.10
0.15 0.15 0.15
0.80 1.10 1.40
39.06 52.89 61.73
60.94 47.11 38.27
0.78 1.96 3.80
Fig. 4. Temperature profile of wire for various currents.
perature; types B and C required more time because of their lower resistance and heat production. As shown in Fig. 4, the equilibrium temperature reached the melting point and exceeded the pyrolysis temperature of PE insulation under currents of 8 A and 16 A, respectively. For wire type A under a current of 7 A, flame propagation remained stable and no dripping occurred. With an increased current of 8 A (10 times the rated current), the volume of molten insulation increased, and drops occurred. The molten insulation reached pyrolysis at about 15 A, with white smoke and volatile compounds released above the wire. When the current was raised over 17 A, the insulation would pyrolyze, and volatize completely in several seconds, resulting in the wire core being exposed to the air. For type B and C wires, the accumulation of molten insulation increased with current, and drops formed at 14 A. However, even at a current of 20 A, the maximum available, the equilibrium temperature could not reach the pyrolysis point of PE, and no decomposition phenomenon was observed.
4. Results and discussion 4.1. Temperature profile for the wire under overload currents
4.2. Dripping behavior of molten insulation
Fig. 4 shows the wire temperature curve varying with time, based on Eqs. (1)–(4), for wire type A, for currents of 8 A, 10 A, 12 A, 14 A, and 16 A (10–20 times the rated current), respectively. The equilibrium temperature is defined as the stable temperature to which the wire has risen. For wire type A, it took 30 s for the wire temperature to rise from room temperature to the equilibrium tem-
The molten insulation accumulated with flame propagation under overload currents. Drops occurred when this accumulation reached a certain limit. Fig. 5 shows the typical time-sequence images of the molten insulation dripping process, with an interval of 20 ms. The change of flame shape with time is also presented. The maximum of flame length in the vertical orientation above wire
Fig. 5. Images for the flame before and after a drop, captured by video camera.
Fig. 6. Images of the dripping process, captured by high-speed photography.
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core is regarded as flame height, and the bright flame width is the maximum of flame length in horizontal orientation. As shown in Fig. 5, a slow process of accumulation was observed before a drop fell, which occurred suddenly after the volume reached to a certain amount. When the drop occurred, the flame height rapidly changed, i.e., from 23.1 mm to 10.0 mm, a reduction of 56.8%. The bright flame width changed slightly, by approximately 9.0 mm during the dripping process. Fig. 6 shows the dripping process from a drop beginning to occur, to the flame on the base extinguishing. These images show the flame shape slightly change during the falling stage. The dripping front position, which is defined as the distance from the wire core the falling melt drop traveled, is shown as black dots in Fig. 7. By using the methods of binomial fitting to obtain, first- and secondorder differentials, the speed and acceleration of the molten drop is achieved. When the drop fell onto the steel base, it continued to combust for a short time, as shown in Fig. 8. This process lasted about 0.1 s, from the falling edge coming into contact with the base, until flame extinction. The molten drop spread rapidly at the moment of contact, resulting in the expansion of the combustion area. The flame height first decreased and then increased as with gasification of combustible materials decreased. The flame area then reduced gradually to extinction because of the cooling effect of the steel
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Fig. 7. Dripping front position, speed and acceleration of the molten drop.
base. It is convinced that the molten drop continue burning for quite a while after falling, even on a noncombustible material with high thermal conductivity.
Fig. 8. Combustion of the molten drop on the base.
Fig. 9. Dripping time and frequency on overload currents.
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Fig. 10. Flame shape changing with continuous drops, for wire type A under 15 A.
(1) A simplified theoretical relationship is presented to describe the temperature profile for the wire under overload currents. The effect of overload current on the temperature curve and the equilibrium temperature of the wires are obtained. (2) The formation of dripping molten insulation is analyzed, along with the falling speed and acceleration. The molten drop continues to burn on the steel base for about 0.1 s. (3) The relationship between the overload currents and the dripping frequency is developed for the dripping process. The calculation results are consistent with the experimental results; the dripping frequency and the square of the current are found to be in positive linear correlation. (4) The effect of dripping behavior on the flame propagation is also discussed. The flame height is reduced by 50% upon the occurrence of a drop and then increases gradually with the accumulation of molten insulation, presenting a certain periodicity. The bright flame width has slight vibrations and the flame propagation velocity is not affected.
4.3. Time interval and frequency of dripping molten insulation
Acknowledgements
Here, dripping time T is defined as the adjacent time intervals for the drops and dripping frequency f is the reciprocal of the dripping time. Fig. 9(a) shows the dripping time vs. overload current with a fit of 0.9913 for a diameter of 0.5 mm, 0.9812 for a diameter of 0.8 mm, and 0.9942 for a diameter of 1.1 mm, respectively. The experimental and numerical values of dripping frequency for the type A wire are shown in Fig. 9(b); the two values are in good agreement. The fitting curve is:
This work was supported by the Joint Funds of the National Natural Science Foundation of China under Grant Nos. U1233102 and 51506059, the National Program on Key Basic Research Project of China under Grant No. 2012CB719702, and the Collaborative Innovation Center of City Public Safety of Anhui Province. The authors gratefully acknowledge all of this support.
f = 0.0016I 2 − 0.0507, R2 = 0.9998
References (9)
The dripping frequency is nearly proportional to the square of the current. Considering vf ∼I 2 , as reported in previous research work [30], and the limited influence of current on wf , it can be ˙ p ∼vf ∼I 2 and m ˙ f ∼ ln(1 + B). The change in the logadeduced that m rithmic form of mass transfer number is very small with increasing current. Therefore, in the experimental range, the dripping frequency and the square of current are in a positive linear correlation. 4.4. Effect of the drop behavior on flame propagation Because of the combustible loss caused by the molten drop, the flame shape of the wire fire changed before and after a drop, as shown in Fig. 10; the bright flame width, the flame height, and the flame leading edge position all changed with time. For the flame height, there was a rapid increase before a drop occurred, and at the moment of a molten drop separating from the wire, it jumped from the maximum (e.g., 29.48 mm) to the minimum (e.g., 12.54 mm), a reduction of 57.5%. After the drop, the flame height gradually increased with the accumulation of molten insulation until the next drop occurred; this behavior was periodic. Conversely, the bright flame width, wf , remained unchanged during the process, with only a slight vibration during the drop. Using a linear fit of the leading edge position, the solid line shown in Fig. 10 is achieved. The dripping behavior had a limited effect on the flame propagation velocity. 5. Conclusions In this study, the dripping behavior of molten wire insulation (PE insulated copper core), and flame spread under overload currents were examined experimentally. The purpose of this work is to gain a better understanding of the fire risk and reduce the occurrence of more disasters, for the thermoplastic widely used in electrical engineering. The main conclusions are as follows:
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Please cite this article in press as: H. He, et al., Molten thermoplastic dripping behavior induced by flame spread over wire insulation under overload currents, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.07.070