Limiting oxygen concentration (LOC) of burning polyethylene insulated wires under external radiation

Fire Safety Journal 86 (2016) 32–40

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Limiting oxygen concentration (LOC) of burning polyethylene insulated wires under external radiation ⁎

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Kyosuke Miyamotoa, Xinyan Huangb, , Nozomu Hashimotoa, Osamu Fujitaa, , Carlos Fernandez-Pellob a b

Division of Mechanical and Space Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA

A R T I C L E I N F O

A BS T RAC T

Keywords: Flammability limit Wire fire LDPE/HDPE Copper core In-depth radiation Dripping

Electrical cables and harnesses have been identified as a potential source of fire in the spacecraft cabin. Future space missions may require spacecraft cabin environments to have elevated oxygen concentrations and reduced ambient pressures which could change the wire fire behaviors. In this work, a group of experiments is conducted to measure the flammability limit of polyethylene (PE) insulated wires under varying oxygen concentration and external radiation. Wires with different insulation dimensions, core conditions (with and without copper core) and insulations (LDPE, HDPE and black LDPE) are examined. Experiments show that external radiation extends the burning limit of the wire insulation to a lower limiting oxygen concentration (LOC) in a linear manner for all wire configurations. Comparison also reveals that the copper core acts as a heat sink to reduce the wire flammability, similar to its role in the ignition of wire insulation, while different from the heat source found in flame spread over the wire insulation. It is also observed that with the external radiation, LDPE insulated wire become less flammable than HDPE and black LDPE insulated wires, in contrast to the result without external radiation. A simple theoretical analysis shows that (1) the in-depth radiation through the semi-transparent LDPE to the copper core acts as an additional cooling to weaken the external radiative heating, and (2) the easier dripping of molten LDPE reduces its flammability. The results of this work provide valuable information about the fire risk of electrical wires under variable oxygen concentration and external heating from an adjacent fire. Thus, it may be useful toward upgrading the fire safety design and standards of future space missions.

1. Introduction Future space missions may require spacecraft cabin environments to be different from those currently used in the International Space Station (ISS). These cabin environments may even vary between exploration vehicles depending on mission purpose and duration, among other factors [1,2]. Environmental variables such as flow velocity, oxygen concentration, ambient pressure, presence of an external radiant flux, partial or microgravity (μg), may increase or decrease the material flammability and fire dynamics for any particular set of environmental conditions [3]. There is a need to understand how these environmental variables affect the parameters normally used to assess the flammability of a particular material. In particular, electrical cables and harnesses have been identified as a potential source of fires in a spacecraft, often initiated by poor contact, short circuit and external heating [1]. For this reason, fire behavior of wire insulation under potential spacecraft cabin environments have received consider-

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able attention lately. A typical electrical wire consists of potentially flammable polymer insulation and a metal core. The metal core can either promote or prevent the wire fire [4,5]. Once the wire is ignited, fires may continue to spread along the insulation and other nearby combustibles, generating heat, smoke and toxic gases. Many studies have investigated fire behavior on thin wires (diameter ~1 mm) under various environmental variables [4–10]. Kikuchi et al. [6] found that the flame spread rate in Ethylene-tetrafluoro-ethylen (ETFE) insulated wires increases with decreasing wire diameter. Fujita et al. [7] discovered that the flame spread rate over polyethylene (PE) insulated wires peaked at a flow velocity around 0.10 m/s in microgravity. Nakamura et al. [4] revealed that the flame-spread rate over PE insulated wires increases with decreasing pressure and increasing core conductance (both size and thermal conductivity). Wang et al. [10] found that the flame spread rate over wire insulation can significantly increase with internal joule heating of the metal core.

Corresponding authors. E-mail addresses: [email protected] (X. Huang), [email protected] (O. Fujita).

http://dx.doi.org/10.1016/j.firesaf.2016.09.004 Received 5 May 2016; Received in revised form 28 September 2016; Accepted 30 September 2016 0379-7112/ © 2016 Elsevier Ltd. All rights reserved.

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Nomenclature

A B c d E ΔH k ṁ ′ ′ MW n N P q̇′ ′ r T X Y Z

ρ ϕ χ

cross-section area mass transfer (or B) number specific heat diameter activation energy heat of reaction slope

Subscripts

0 a b c dr e f g in O2 m o p s, r

mass flux

molecular weight reaction order number of tests probability heat flux

radius temperature volume fraction mass fraction pre-exponential factor

Greeks

δ λ ω̇′ ′′ *

enthalpy equivalence ratio radiative heat loss fraction

no external radiation ambient burning core dripping external flame gas in-depth radiation oxygen melting outer pyrolysis or polymer insulation surface re-radiation

Superscripts thickness thermal conductivity volumetric reaction rate

* min

critical minimum value

insulation burning limit as a function of external radiation and oxygen concentration. The experimental setup has been designed and constructed in UC Berkeley in line with the ASTM limiting oxygen test [11,12]. A diagram of the experimental apparatus is presented in Fig. 1(a). It consists of a cylindrical flow duct, infrared heaters, and supporting instrumentation. The test section itself consists of a 254 mm long Pyrex cylinder with an inner diameter of 70 mm and wall thickness of 2 mm. Outside the Pyrex cylinder, there are three IR lamps equidistantly placed at 120 degree intervals. These radiant heaters are quartz infrared halogen lamps with a lit length of 127 mm and a total length of 220 mm, fitting into parabolic strip heaters. Irradiation to the fuel sample is measured by a Schmidt-Boelter radiometer (MEDTHERM Co.) and calibrated to be approximately uniform with an overall incident flux of up to 43 kW/ m2. Below the test section, a flow chamber allows for controlled flows of modified air to pass through borosilicate beads, homogenizing the flow and evenly mixing the gases. The air flow uses a prescribed oxygen concentration of between 11% and 21% (balancing nitrogen) at a fixed velocity of 4 cm/s at 1 atm. The gas velocity is selected following the ASTM limiting oxygen test [11]. Because of buoyancy, the actual velocity encountered by the flame is larger than 4 cm/s. The tested wires are each 100 mm long and have one of three different PE insulations: (1) semi-transparent low-density polyethylene (LDPE), (2) white opaque high-density polyethylene (HDPE), and (3) black LDPE (B-LDPE), as shown in Fig. 1(c). Their physicochemical parameters are listed in Table 1. The B-LDPE is produced by doping 5% wt. carbon black particles into the pure LDPE, and has a little higher density and thermal conductivity. Mechanically, LDPE has a lower crystallinity, stiffness and density than HDPE. Visually, because of a relatively loose molecular structure, pure LDPE is semi-transparent (see Fig. 1(c)). Thermally, LDPE has a lower temperature/heat of both melting and pyrolysis, and a smaller thermal inertia ( ρλc [18]) than HDPE. Basically, all these physicochemical parameters suggest that the LDPE insulated wire should be more flammable, and have a higher fire risk than the HDPE insulated wire. Two wire dimensions are selected:(I) dc / do =3.5/8.0 mm and (II) dc / do =5.5/9.0 mm, as shown in Fig. 1(b) and Table 2. In order to study

The next generation of space station and exploration vehicles are designed to have elevated oxygen concentrations and reduced pressure [2]. Such new spacecraft cabin atmosphere designs have brought attention to the flammability limit of wire [5,8,9]. The limiting oxygen concentration (LOC) is defined as the minimum oxygen volumetric fraction that supports a candle like flame [11]. It is expected that the standard of fire resistant materials need to be redefined in oxygenenriched atmospheres, which are currently defined with LOC > 21%. Previously, Huang et al. [5] showed that as the oxygen concentration increases, the ignition of wire and the following transition to flame spread becomes easier, even in the reduced pressure. Takahashi et al. [8] observed that microgravity results in a lower LOC for the burning of PE insulated wires because of the elimination of natural convective cooling. Moreover, in real fire scenarios wire is likely to be heated by an adjacent fire or heat source, while few studies have addressed the wire fire behaviors under the external heating. In the previous work [9], external radiation was found to reduce the value of LOC for flame spread over ETFE insulated wires in both normal gravity and microgravity. Also, few studies have looked into the fire behavior in wires with relatively thick insulation (δp > 1 mm) under new spacecraft environments. Once a thick insulation is ignited, fire becomes more intensive for a longer duration, and more difficult to suppress, indicating a greater fire hazard. Additionally, to the best of the authors’ knowledge, no study has explored the influence of insulation transparency and micro-structure on the flammability limit and fire behavior of insulated wire. In this work, the flammability of wire insulation under varying oxygen concentration and external radiation is investigated experimentally. Wires with different dimensions (8 and 9 mm), core conditions (with and without copper core), and insulations (LDPE, HDPE and black LDPE) are tested. Their flammability limits are measured, compared and discussed with a simplified theoretical analysis.

2. Experimental setup A group of experiments are conducted to measure the wire 33

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Fig. 1. Schematic of (a) the experimental apparatus, (b) two wire dimensions, and (c) photo for tested PE insulated wires.

Experiment aims to determine whether a candle like flame [11] can present under a given set of ambient oxygen and external radiation. Reducing the oxygen concentration and external radiation eventually leads to a change from “burn” to “no burn”, thus defining the LOC of the wire under variable external radiation. A digital video camcorder (Canon PC1742, 24 fps) is used to record flame behaviors through the Pyrex cylinder. Because of the complexity and randomness in experiment, near the flammability limit 4–7 repeating runs were conducted under the same test condition to quantify the probability of each experimental outcome.

the influence of core, two core conditions are compared: (1) a solid copper core, and (2) a thin hollow stainless steel tube (0.2 mm wall thickness) used to simulate the case in absence of a core, while preventing the bending of soft PE tube once heated. The wire is supported at its bottom by an aluminum cylinder of the same diameter. For ignition, each wire sample is first preheated under the tested external radiation and opposed air flow for 1 min. Then, a small premixed methane flame is placed on the top of wire for another 2 min. Table 1 Density ( ρ ), heat conductivity ( λ ), specific heat (c ), melt point (Tm ), heat of melting (ΔHm ), pyrolysis temperature (Tp ), and heat of pyrolysis (ΔHp ) of LDPE and HDPE. The physical properties of copper (Cu), stainless steel (SS) and air are given as reference. Thermal properties are evaluated near room temperature, and ΔH > 0 represents endothermic.

ρ [13] (kg/ m3)

λ [13] (W/ m K)

c [13] (kJ/ kg K)

Tm [14] (°C )

ΔHm [15] (MJ/ kg)

Tp [14] (°C )

ΔHp [14] (MJ/kg)

LDPE B-LDPE HDPE

927a 929a 944a

0.23b 0.24b 0.32b

1.55

0.50

387

1.8

0.81

404

2.3

Cu [16] SS [16] Air [16]

8954 8000 1.18

398 13.8 0.026

0.400 0.384 1.07

105– 110 130– 135 – – –

– – –

– – –

– – –

a b

2.00

3. Experimental results In the experiment, two distinct outcomes have been found: (1) flame extinguishes shortly after the ignition (no-burn), and (2) the entire 100 mm insulation is consumed by flame (burn). Fig. 2 shows the snapshots of flame sustained in 8 mm copper wires with three different PE insulations. Three types of burning processes have been identified: (i) downward flame spread over the surface of the insulation; (ii) melting and subsequent dripping of the molten insulation which may be burning in some cases; and (iii) mass burning of the insulation in the stagnation region created by a stationary flame. Both LDPE and B-LDPE wires show more molten PE dripping downward along the wire due to gravity (see Fig. 2). Especially, more dripping was observed for LDPE near the flammability limit when the flame is weak. This is likely because compared to HDPE, (1) LDPE has a lower melting temperature and heat (see Table 1), and (2) the molten

Measured in experiment ( ± 2 kg/m3). Measured by 1-D reference bar method ( ± 5%) [17] at 50 °C .

Table 2 Configurations of four PE insulated wires (100 mm long), and the measured characteristics of wire flammability limit, qe′̇ ′ *=−k (XO*2 −XO*2,0 ) .

dO (mm)

dc (mm)

δp (mm)

9.0

5.5

1.75

8.0

3.5

2.25

Ac /Ao (mm)

37% Cu 5% SS 19% Cu 4% SS

k (kW/m2)

XOmin 2 (%)

XO*2,0 (%)

LDPE

HDPE

B-LDPE

LDPE

HDPE

B- LDPE

LDPE

HDPE

B- LDPE

630 720 630 640

410 680 440 680

420 670 450 670

20.6 17.3 18.8 17.4

21.0 17.4 19.1 17.4

21.0 17.3 19.3 17.5

14.5 11.5 14.5 12.5

14.5 12.5 14.5 12.5

14.0 11.8 14.0 11.8

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Fig. 2. Snapshots of flame sustained in 8 mm copper wires with (a) LDPE, (b) HDPE, and (c) B-LDPE insulation under air (XO2=21%) and without the external radiation.

insulated wire [9]. This is understandable because PE is easier to degrade and produce more flammable pyrolysate than ETEF. Therefore, the composition and degradation chemistry of polymer insulation plays an important role in wire flammability. For a better comparison, the LOC boundaries in term of the external radiation for all wire configurations are re-plotted in Fig. 6. As expected, the LOC boundaries of all six no-core wires are very similar (also see their fitting coefficients in Table 2), and only HDPE shows a slightly higher flammability (maybe a result of weak melting and dripping, discussed more in Section 4.1.3). Compared to the no-core wires, the LOC boundaries as well as the value of XOmin 2 for copper wires are appreciably higher. Moreover, the LOC for wire with a large copper core (dc / do =5.5 mm/9.0 mm) is also substantially higher than that with a small core (dc / do =3.5 mm/ 8.0 mm). It is also observed that in no-burn cases, because of greater cooling effects, the PE insulation left on the copper core is much thicker than that on the hollow stainless steel core. As such the copper core acts as a heat sink, and its cooling effect increases with the core thermal conductance (both size and thermal conductivity). Similar heat sink effect of the metal core was found in normal-gravity and microgravity flammability experiments of about 1 mm PE wire [8], and in ignition experiments [5]. However, this result is contrary to the previously found heat source effect during the flame spread over wire insulation [4]. Comparison of the LOC boundaries of the wires with different PE insulations shows that without the external radiation (q̇e′ ′=0 ), LOC0 (LDPE ) < LOC0 (Black LDPE )≤LOC0 (HDPE ), as seen in Table 2. Thus, the rank of wire flammability (or fire risk) is

LDPE is less viscous. The melting and dripping conditions may affect the measured value of LOC [19], and is discussed more in Section 4.1.3. In order to find the LOC of the insulation as a function of the external radiation, the oxygen concentration (denoted by the oxygen volume fraction, XO2 ) and external radiation (q̇e′ ′) of a no-burn experiment are increased in steps of 0.5% and 5 kW/m2, respectively until a burn situation occurs. Near the LOC boundary, the flame spread becomes very slow or stationary with the flame burning like a candle. Under these conditions, the burn probability is evaluated by the ratio of number of burn cases (Nb ) to the total number of repeating runs (Ntot ):

Pb=

Nb ×100% Ntot

(1)

The measured burn probabilities for all wire configurations are presented in Fig. 3 (LDPE), 4 (HDPE) and 5 (B-LDPE). The plotted burn probability helps identify the boundaries of LOC. Then, the flammability limit is defined at the 50% chance of burn, as a function of critical oxygen concentration (XO*2 ≡LOC ) and critical external radiation (q̇e′ ′ *). Clearly, the external radiation extends the wire flammability limit towards a lower LOC. Note that the measured LOC here is a specific result based on the ASTM limiting oxygen standard [11], which may vary, given any modification in the experimental setup and condition [12,19]. A linear correlation is used to produce the LOC boundary variation with the external radiation:

qe′̇ ′ *=−k (XO*2 −XO*2,0 )

(2)

LDPE wire > B − LDPE wire ≥ HDPE wire

in which XO*2,0≡LOC0 is the critical oxygen concentration without external radiation, which is given in Table 2 for each wire configuration. These boundaries of LOC variation with external radiation are also plotted in Figs. 3–5. Excellent linearity (R2 >0.97) is found for all wire configurations, and their slopes (k ) are summarized in Table 2. Such linear boundary was also found in previous experiments on a thinner (1.1 mm) ETFE insulated wire under both microgravity and normal gravity environments [9]. The above results also show that there is a minimum oxygen concentration (XOmin 2 ) for all wires, below which flame will extinguish no matter the external radiation level. For all copper wires, a XOmin 2 ≈ 14.5% is found, regardless of the type of PE insulation. In spite of larger copper cores, the measured XOmin 2 of PE insulted wire is much lower than the previously found XOmin 2 =27% for a thinner 1.1 mm ETFE

(qe′̇ ′=0)

(3a)

This rank is consistent with the thermal-physical properties of the insulation shown in Table 2 where LDPE has a lower resistance to melting and pyrolysis as well as a smaller thermal inertia. However, under external radiation (q̇e′ ′ >0 ), it can be seen from Fig. 6 that LOC (LDPE ) > LOC (Black LDPE ) >LOC (HDPE ). In other words, the rank of wire flammability (or fire risk) becomes

LDPE wire < B − LDPE wire < HDPE wire

(qe′̇ ′ >0)

(3b)

which is in contrast to the result without external radiation. Therefore, the material flammability should not only depend on their thermalphysical properties. Instead, it can be significantly altered by the external heating condition. 35

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Fig. 3. Measured burn probability and flammability limit of semi-transparent LDPE insulated wires (a) dc /do =5.5/9.0 [mm], and (b) dc /do =3.5/8.0 [mm] with copper core (upper) and without core (lower).

′ ′ and Hm are the radiation; ṁ b′ ′ * is the critical burning mass flux; and ṁdr mass flux and enthalpy of the molten PE dripping from the burning region.

4. Discussions 4.1. LOC under external radiation In order to provide a phenomenological description of the burning process of the wire insulation, Fig. 7 illustrates (a) a diagram for flame attaching to the cylindrical wire insulation under an small opposed air flow (ua ), (b) the temperature profile of core which acts as a heat sink, and (c) the heat exchange on the wire outer surface. The flame needs to transfer heat to the insulation, pyrolyze the polymer and ignite the pyrolysate to sustain the stationary flame (i.e. burning). There are two mechanisms can lead to flame extinction: (1) quenching due to the cooling of insulation, and (2) blow off due to the slow flame chemistry. For the quenching due to the cooling of insulation, a critical mass transfer number (B*) [20] may be used to quantify the limiting burning condition before quenching. Considering the new boundary condition on the wire (or insulation) outer surface (Fig. 7(c)), it can be modified as

B*=

4.1.1. The influence of core and dimension Assume that at the LOC, B* has a constant minimum value to sustain a diffusion flame. If XO*2 is decreased, then q̇e′ ′ * has to increase (Eq. (4)), which agrees with the linear LOC boundary seen in Fig. 6. If the copper core is removed (q̇c′ ′=0 ), in order to keep B* constant, a lower q̇e′ ′ * is required for the same XO*2 , that is, the wire becomes more flammable. With a copper core and in-depth radiation, the cooling through the insulation can be calculated from a simplified one-dimensional heat transfer equation in the cylindrical coordinate:

⎞⎤ 1 d ⎡ ⎛ dT ⎢r ⎜λ p +q ′̇ ′⎟ ⎥=0 r dr ⎣ ⎝ dr in ⎠ ⎦

(5)

q̇in′ ′

is the in-depth radiation. As illustrated in Fig. 7(c), the inwhere depth radiation acts as an additional cooling on the outer surface. Part of the in-depth radiation transmitted through the insulation may be further reflected by copper core, so for simplicity, q̇in′ ′ is assumed to be constant within the thin insulation. With two boundary conditions of Tr = rp=Tp and Tr = rc=Tc , the cooling flux within the insulation can be solved as

YO*2 (∆Hf / ϕ)(1−χf ) − cg (Tp − Ta ) ′ ′ Hm]/ ṁ b′ ′ * ΔHp + [(qs′̇ ,′r + qc′̇ ′ − qe′̇ ′ *) + ṁdr

(4)

where χf presents the flame radiative heat loss; YO*2 =XO*2 (MWO2 / MW ) is the critical oxygen mass fraction; ∆Hp and ∆Hf are the heat of pyrolysis for the polymer and the heat of combustion of the pyrolysate, respectively; ϕ is the equivalence ratio; Ta and Tp are the ambient temperature and characteristic pyrolysis temperature of the insulation, respectively; q̇s′,′r is the surface re-radiation; q̇c′ ′ >0 is the cooling through the insulation to the copper core which acts as a heat sink; q̇′ ′ * is the critical external

qc′̇ ′=−λp

dT dr

+qin′̇ ′= rp

λp (Tp − Tc ) + qin′̇ ′ / δp rp ln(rp / rc )

where δp=rp−rc is the thickness of PE insulation.

e

36

(6)

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Fig. 4. Measured burn probability and flammability limit of opaque HDPE insulated wires (a) dc /do =5.5/9.0 [mm], and (b) dc /do =3.5/8.0 [mm] with copper core (upper) and without core (lower).

LDPE should be more flammable than the white HDPE, but it disagrees with comparison between Fig. 6(b) and (c). One possible reason is their different melting and dripping behaviors among the three PE insulations. The complex influence of material melting and dripping on LOC has been noticed previously, e.g. in [19,21], but has not been well analyzed yet. In a normal flame spread scenario, the dripping of molten polymer insulation may facilitate flame spread and increase the fire hazard. Also, the flame is often found to attach to the moving drips. However, for the burning of insulation near the LOC (flammability boundary), the dripping behaves as a cooling effect to remove the available fuel from burning zone, and flame is unlikely to attach to the drips. As illustrated in Fig. 7(a), such dripping cooling effect may be quantified ′ ′ Hm ) into the denominator in Eq. by adding the dripping heat flow (ṁdr (4). For both pure and black LDPE wires, more significant dripping is observed than HDPE (see Fig. 2). For this reason, HDPE wires with the least dripping cooling effect should show the highest flammability, even more flammable than the most opaque B-LDPE, agreeing with the results shown in Fig. 6(a-c).

Calculation shows that for the 9 mm wire with a 5.5 mm copper ⎡ ⎛ rp ⎞ ⎤ core, 1/ ⎢rp ln ⎜ r ⎟ ⎥ is 0.45 mm−1, larger than 0.30 mm−1 for the 8 mm ⎝ c ⎠⎦ ⎣ wire with a 3.5 mm copper core. Therefore, q̇c′ ′ is larger for the 9 mm copper wire, thus, it requires a higher q̇e′ ′ * under the same XO*2 for all PE insulations. This agrees with the experimental results, as shown in Fig. 6. 4.1.2. The influence of in-depth radiation Because of the doping by carbon particles, the B-LDPE is almost opaque under both visual and infrared light. Extra experimental measurements confirm that less than 2% incoming radiation can be transmitted through the ~2 mm B-LDPE insulation. Therefore, for BLDPE insulation, there is negligible in-depth radiation (q̇in′ ′→0 ), leading to a weaker heat sink in Eq. (6). For this reason, under the same external radiation, B-LDPE insulated wires become more flammable than the semi-transparent LDPE insulated wires, as compared between Fig. 6(a) and (c). For the white HDPE, it is less transparent to the halogen lamp radiation because of its high crystallinity and low degree of molecular branching. Measurements shows that the transmitted radiation through HDPE is about half of that through LDPE (see detailed measurements in Appendix). Therefore, the white HDPE has a weaker in-depth radiative cooling (q̇in′ ′) than the semi-transparent LDPE, showing a lower flammability, as compared between Fig. 6(a) and (b).

4.2. Minimum oxygen concentration That producing a sufficient amount of flammable pyrolysate and mixing it with air is necessary, while is not sufficient, to sustain a burning flame. As the oxygen concentration decreases, eventually the gas mixture may not be ignited because the reaction is too weak to be sustained or to generate enough heat transfer from the flame to the insulation. Alternatively, the mixture could be outside of its flamm-

4.1.3. The influence of melting and dripping Based on the analysis of in-depth radiation, the most opaque B37

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Fig. 5. Measured burn probability and flammability limit of B-LDPE insulated wires (a) dc /do =5.5/9.0 [mm], and (b) dc /do =3.5/8.0 [mm] with copper core (upper) and without core (lower).

see Fig. 7(a)) depends on the local flow condition; and q̇l′ ′ is the heat loss to ambient. The reaction rate can be expressed by the Arrhenius Law as

ability limits. Therefore, there is a minimum oxygen concentration ( XOmin 2 ), below which the flame will extinguish despite of the external radiation level. The heat transfer critical condition can be analyzed through the energy balance in the gas phase, where the critical rate of heat generation (q̇r′ ′ *) within the gas mixture should be equal to the rate of heat loss as

qr′̇ ′ *=ω̇′ ′′ * ∆Hf δg=ql′̇ ′+qc′̇ ′

⎛ E ⎞ min nO2 nf ω̇′ ′′ *=Z exp ⎜ − ⎟ (XO2 ) X f ⎝ RT ⎠

(8)

where Z , E and n are the pre-exponential factor, activation energy and reaction order, respectively. Clearly, the overall combustion rate increases with the oxygen concentration. Therefore, the additional cooling from wire core (q̇c′ ′) requires a higher XOmin 2 to ignite the gas

(7)

where ∆Hf is the heat of flaming combustion; the flame thickness (δf ,

Fig. 6. Comparison of flammability limit for different wire configurations and insulation transparencies of (a) semi-transparent LDPE, (b) opaque HDPE, and (c) black LDPE.

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Fig. 7. Diagram of (a) the burning process of wire, (b) the temperature profile of core, and (c) heat exchange on the wire (or insulation) outer surface.

wires, in contrast to its behavior without external radiation. A simple phenomenological analysis shows that (1) the in-depth radiation through the semi-transparent LDPE to the copper core acts as an additional cooling to weaken the external radiative heating, and (2) the easier dripping of molten LDPE reduces its flammability. This is the first time that the burning of wire insulation under varying oxygen concentration, external radiation, core and insulation configurations have been investigated. Thus the results may have implications toward upgrading the fire safety design and standards of future space missions.

mixture, which agrees with the experimental observations in Fig. 6. Because little radiation can be absorbed by gas, XOmin 2 is insensitive to the external radiation or the type of PE insulation (including the indepth radiation and dripping conditions).

5. Conclusions The limiting oxygen concentration (LOC) in terms of an external radiant source and the critical external radiation for wires with different dimensions, core conditions (copper core and no core) and insulation materials (LDPE and HDPE) were measured. Results show that the external radiation extends the burning of insulation to a lower LOC, and there is a linear correlation between LOC and critical external radiation for all wire configurations. Comparison of different core conditions shows that the copper core acts as a heat sink to reduce the wire flammability. This heat sink effect of metal core have also been observed the wire ignition, while it is different from the heat source effect found in flame spread over wire. With external radiation, the semi-transparent LDPE insulated wires become less flammable than the white HDPE and black LDPE insulated

Acknowledgements This research is supported by NASA Grant NNX14AF01G and JAXA as a candidate experiment for the third stage use of JEM/ISS titled “Evaluation of gravity impact on combustion phenomenon of solid material towards higher fire safety” (called as “FLARE”). The authors thank Dr. Shmuel Link, Dr. Daniel Murphy, Meng Qu, Andrew Mikhail, Connie Lee, and Dr. Menglong Hao (UC Berkeley) for their comments and assistance in experiments.

Fig. A1. Measured radiation fraction transmitted through (a) semi-transparent LDPE, and (b) white HDPE insulations.

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Appendix Under the tested radiation provided by the same halogen lamp, the transparencies of pyrex glass and different PE insulations are measured using a Schmidt-Boelter radiometer (MEDTHERM Co.). For the 2 mm thick pyrex glass cylinder, 97 ± 1% incoming radiation is transmitted. For the 1.75 mm and 2.25 mm black LDPE insulation samples, less than 1% incoming radiation through the pyrex glass is transmitted, indicating that black LDPE insulation is almost opaque. The measurements of transmitted radiation through semi-transparent LDPE and white HDPE are showed in Fig. A1. Results shows that the radiation transmitted through visually semi-transparent LDPE is almost twice that through the white HDPE. Therefore, the in-depth radiation in HDPE wire is stronger, while for LDPE wire more radiation reaches core directly. References [1] R. Friedman, Fire safety in spacecraft, Fire Mater. 20 (5) (1996) 235–243. [2] K.E.Lange, et al., Bounding the spacecraft atmosphere design space for future exploration missions, 2005. [3] O. Fujita, Solid combustion research in microgravity as a basis of fire safety in space, Proc. Combust. Inst. 35 (3) (2015) 2487–2502. [4] Y. Nakamura, et al., Flame spread over electric wire in sub-atmospheric pressure, Proc. Combust. Inst. 32 (2) (2009) 2559–2566. [5] X. Huang, Y. Nakamura, F.A. Williams, Ignition-to-spread transition of externally heated electrical wire, Proc. Combust. Inst. 34 (2) (2013) 2505–2512. [6] M. Kikuchi, et al., Experimental study on flame spread over wire insulation in

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