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Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity
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Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity Longhua Hu a,b, Yong Lu a, Kosuke Yoshioka b, Yangshu Zhang a, Carlos Fernandez-Pello c, Suk Ho Chung d, Osamu Fujita b,∗ a State
Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, China b Division of Mechanical and Space Engineering, Hokkaido University, Sapporo, Japan c Department of Mechanical Engineering, University of California, Berkeley, CA 94720, United States d Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia Received 1 December 2015; accepted 20 September 2016 Available online xxx
Abstract Materials, such as electrical wire, used in spacecraft must pass stringent fire safety standards. Tests for such standards are typically performed under normal gravity conditions and then extended to applications under microgravity conditions. The experiments reported here used polyethylene (PE)-insulated (thickness of 0.15 mm) Nichrome (NiCr)-core (diameter of 0.5 mm) electrical wires. Limiting oxygen concentrations (LOC) at extinction were measured for upward spreading flame at various forced opposed-flow (downward) speeds (0−25 cm/s) at several inclination angles (0−75°) under normal gravity conditions. The differences from those previously obtained under microgravity conditions were quantified and correlated to provide a reference for the development of fire safety test standards for electrical wires to be used in space exploration. It was found that as the opposed-flow speed increased for a specified inclination angle (except the horizontal case), LOC first increased, then decreased and finally increased again. The first local maximum of this LOC variation corresponded to a critical forced flow speed resulted from the change in flame spread pattern from concurrent to counter-current type. This critical forced flow speed correlated well with the buoyancy-induced flow speed component in the wire’s direction when the flame base width along the wire was used as a characteristic length scale. LOC was generally higher under the normal gravity than under the microgravity and the difference between the two decreased as the opposed-flow speed increases, following a reasonably linear trend at relatively higher flow speeds (over 10 cm/s). The decrease in the difference in LOC under normaland microgravity conditions as the opposed-flow speed increases correlated well with the gravity acceleration
∗
Corresponding author. Fax: +81 11706 7841. E-mail address: [email protected] (O. Fujita).
http://dx.doi.org/10.1016/j.proci.2016.09.021 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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component in the wire’s direction, providing a measure to extend LOC determined by the tests under normal gravity conditions (at various inclination angles and opposed-flow speeds) to LOC under microgravity conditions. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Limiting oxygen concentration (LOC); Extinction; Electrical wire; Inclination angle; Microgravity
1. Introduction Fire safety in space exploration is an important issue such that materials used in a spacecraft must pass stringent fire safety standards. Tests for such standards are typically performed under normal gravity and then extended to applications under microgravity. The US National Aeronautics and Space Administration (NASA) designed a test (Test 4 of NASA-STD-6001B) [1,2] to evaluate the fire safety of insulation materials for electrical wires. In this test, an insulation material is judged to be “safe” if a spreading flame is self-extinguished within a limited spreading distance of less than 15 cm and if the dripped flaming debris does not ignite a piece of K-10 paper placed 20 cm below the sample. This test is performed without external air flow at a positive inclination angle of 75° from the ground under normal gravity and ambient atmospheric oxygen concentration. Because of the buoyancy exerted on burnt gas, an upward draft of air is generated during the experiment such that the flow configuration is an upward concurrent spreading flame. This pass/fail nature of the test limits the applicability of the results. For broader applicability, a quantitative scale measuring the degree of a material’s flammability would be useful. A recently proposed alternative and quantitative fire safety test that scales flammability parameters is the limiting oxygen concentration (LOC) method [3]. A minimum oxygen concentration in which a spreading flame can be sustained under specified conditions (e.g., sample thickness, external forced flow speed, or pressure) is adopted as the scale. The LOC method has an advantage of providing quantitative limiting values, instead of pass/fail criteria, thus it is suggested [3] that the evaluation based on the LOC method can be a promising potential future standard. However, LOC experiments are typically performed under normal gravity, whereas space applications require microgravity. The discrepancy between LOC under microgravity and LOC under normal gravity should be properly formulated for the LOC method to be acceptable as a fire safety standard for space applications [3]. Here, we focus on the behavior of spreading flames over electrical wires and the extinction LOC at various inclination angles under normal gravity. We also make com-
parisons with such behaviors under microgravity to quantify their differences. Flame spread behaviors of various insulation and core metal materials on electrical wires under microgravity (without effect from the wire inclination angle) along with external air flow speeds were previously investigated to determine LOCs [4] as well as ignition limits [5,6], effects of AC electric fields and insulation melting behavior [7-10]. Some of these results were compared with a horizontal wire configuration under normal gravity conditions [4-11]. Nakamura et al. [12] investigated the effect of pressure on flame spread for horizontal electrical wires. Bhattacharjee et al. [13] predicted that there was a critical fuel thickness in flame extinction in a quiescent microgravity environment. Osorio et al. [14] studied LOC of horizontal, ethylene tetrafluoroethylene (ETFE)-insulated copper wires exposed to an external radiant flux under normal gravity and microgravity conditions. Concerning the effect of the inclination angle, Hu et al. [15] experimentally studied recently the flame spread rate for both upward (concurrent flow) and downward (countercurrent) propagations over electrical wires. However, as far as the authors aware, there is no published work reporting on the combined effects of the inclination angle and opposed-flow speed on LOCs of electrical wires under normal gravity conditions and how to link these effects to microgravity conditions. Combined effects add complexity to the problem, but they are important to be considered in flammability evaluations. The experiments conducted here used polyethylene (PE)-insulated Nichrome (NiCr)-core electrical wires. Extinction LOCs were measured at various opposed-flow speeds at several inclination angles under normal gravity conditions and the differences from those obtained under microgravity conditions [16] were quantified and correlated to provide a reference for the development of fire safety test standards for electrical wires used in space exploration.
2. Experiment A schematic of the experimental setup is presented in Fig. 1 (details had been reported previously 7-12,16]). The experimental chamber is 0.5 m long with a square cross-section of 0.26 × 0.26 m. A
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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a
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b
Oxygen detector
Combustion chamber
Combustion chamber Wire
Fan
Wire
Flame Air flow
Igniter
Glass window Honeycombs
Flow duct
Absorbent
Triangular blocks Fig. 1. Experimental setup.
rectangular flow duct of 0.38 m long with a crosssection of 0.14 (height) × 0.15 m (width) is positioned inside the chamber. A suction fan is positioned at one end of the duct to induce a flow in a speed range of Ua = 0-25 cm/s, a comparable range of a typical ventilation flow in spacecraft [1,3,17]. Honeycombs are positioned at both ends of the duct for the flow to be nearly uniform. Polyethylene (PE) insulated electrical wire (NiCr; nickel-chrome core) with a core diameter (dc ) of 0.50 mm and an insulation thickness (δ p ) of 0.15 mm is positioned along the centerline of the duct. A possible movement of the melted polyethylene along the inclined electric wire depends on the inclination angle in normal gravity, while this effect is absent in microgravity. Thus, in this work, this behavior is suppressed by using a thin insulation thickness to examine the difference in gas phase processes in normal gravity and microgravity. The effective length of the sample wire is 160 mm. The wire sample is ignited by a coil heater (heated about 10 s) at the fan side of the test section such that flame spread with an opposed-flow is realized. As soon as a flame starts to propagate, the ignition current is shut off. The upward flame spread is designed for a relatively conservative test. The effect of the inclination angle of the electrical wire under normal gravity is studied by varying the overall inclination angle of the chamber (Fig. 1b), ranging from θ = 0–75° using two triangular blocks. The oxygen concentration (balanced by nitrogen) and pressure (maintained at about 100 kPa with less than 1 kPa variation) inside the chamber are monitored in real time. The dynamic process of flame spread is recorded by a CCD video camera (2 megapixels, 1 pixel = 0.13274 mm; 30 fps), which is fixed to the chamber. Flame shape characteristics, as well as the flame front position with time, are
obtained through post-processing the images. LOC was defined as the condition when the flame was self-extinguished at a spread distance of less than 10 cm, which was determined as a function of the inclination angle and the opposed-flow speed under normal gravity conditions. LOC under microgravity can be found in [16]. Similar to the method in [16], our experiments were conducted by setting initial O2 concentration and monitoring the flame extinction as well as its corresponding temporal O2 concentration to determine LOCs. Oxygen sensor (Jikco JKO-25LII, Galvanic cell type oxygen analyzer) was used to monitor oxygen concentration in the chamber with the resolution of 0.1% and accuracy of ±0.5% (range: 0–25%). The sensor position is very close to the inlet of the duct and far from the exit of the duct, thus the O2 concentration measured by the sensor is that at the inlet of the combustion duct. After extinction, the O2 concentration reading did not change. The sensor data were synchronized with video data, as described in details in [10]. 3. Results and discussion 3.1. Variation in LOC with opposed-flow air speed Measured LOCs in relation to opposed-flow speeds at several inclination angles under normal gravity are shown in Fig. 2, along with microgravity data [15] available up to Ua = 20 cm/s. Here, the error bars are from the three repeats of experiments, which show good repeatability as indicated by the small uncertainties as compared to the overall variation of LOC on opposed-flow speed. The result shows that LOCs in general are lower under microgravity (more flammable) than under normal gravity. The difference in LOCs for θ = 0° between the normal- and microgravity conditions
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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Fig. 2. LOC against opposed-flow speed at several inclination angles under normal gravity (three regions recognized) and comparison with microgravity (Wf flame base width; Ua ∗ critical opposed-flow speed when flame is perpendicular to wire). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
remains reasonably constant for Ua = 6−20 cm/s. For θ > 0° cases, the difference generally decreases as opposed-flow speed increases. When θ is large (45°, 60° and 75°), LOCs under normal gravity and
microgravity become comparable at Ua = 20 cm/s (marked with green circles in Fig. 2). Under normal gravity with θ = 0°, LOC initially decreases and then increases with U0 . The
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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initial small decrease in LOC as the opposed-flow speed increases, which is similar to that shown previously at low flow speed [16], can be attributed to the increased heat supply toward the PE, as the main body of the flame is deflected by the flow to be closer to the wire. When the opposed-flow speed increases to a relatively higher level, it is convinced that the extinction of the flame is mainly caused by the Damköhler number (Da) effect (blow-off) in the gas phase [16]. This can be characterized by the ratio of a residence time (τ res ) to a chemical reaction time (τ chem ), where the Damköhler number was defined as Da ≡ tres /tchem = (αg /Ua2 )ρgYO A exp{−E/RTf }. Here, α g , ρ g , YO , A, E, R, and Tf are the thermal diffusivity and density of gas phase, oxygen mass fraction, pre-exponential factor, activation energy, universal gas constant, and flame temperature, respectively. Note that in the blowoff-dominant regime, extinction Da was found to be constant [16]. When the opposed-flow speed becomes larger, LOC needs to be larger to sustain a flame, since the flame is weakened with relatively strong opposed-flow such that a higher oxygen concentration is required. It is clear that the extinction in microgravity in this opposed flow speed range is dominated by blow-off effect that the LOC just increases with increase in opposed flow speed [16]. When θ = 15° and 30°, LOC initially increases, then decreases and finally increases again with Ua . When θ > 30°, LOC increases and then decreases up to the tested Ua of 25 cm/s. Behaviors at other inclination angles suggest for θ > 30°, LOC may increase again as Ua further increases. Although the microgravity data are available only up to Ua = 20 cm/s, we also anticipate that, when Ua > 20 cm/s for large inclination angles (θ > 30°; for example 75°), LOC may be larger under microgravity than under normal gravity. This can be attributed to the fact that under strong opposed-flow speed, flame extinction is controlled by the Damköhler number effect [3,16], which leads to an increase in LOC as the opposed-flow speed increases. However, under such opposedflow (blowoff) dominant extinction conditions, the buoyancy-induced flow under normal gravity conditions could counteract the opposed-flow. Thus, LOC under normal gravity conditions can be smaller than LOC under microgravity conditions. In the inclined wire cases, a local maximum LOC (marked with dotted red circles in Fig. 2) exists when Ua is small and the corresponding critical Ua ∗ generally increases as θ increases. This can be attributed to the buoyancy-induced upward flow, UB , as schematically shown in Fig. 3, where the velocity component parallel to the wire direction of UBw could counteract the opposed-flow, Ua . When Ua is small, the intensity of the buoyancy-induced flow component can be larger than Ua . In such a case, the flame spread resembles a concurrent flame spread. As Ua increases, UBw becomes relatively
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Fig. 3. Physical interpretation of local maximum LOC based on balance of buoyancy-induced flow in the wire’s direction with opposed-flow speed.
Fig. 4. LOC and flame spread rate against inclination angle without having opposed-flow.
smaller than Ua , such that the flame spread turns into a counter-current pattern. The observed local maxima behavior can therefore be explained based on the balance of the buoyancy-induced flow with the opposed-flow along the wire’s direction. For the local maximum condition, the flame is nearly perpendicular to the wire, similar to the case without having opposed-flow (see inset photos in Fig. 2). In general, the flame spread rate under the concurrent condition (with low opposed-flow air speed) should be higher than that under the counter-current condition (with relatively stronger opposed-flow speed) because of heating to unburned PE. For a given θ, as Ua increases, the flame spread rate should first decrease, exhibiting opposite trend to LOC behavior. This is because, when the flame spread rate is relatively high, its corresponding LOC should be relatively low. This is evident in Fig. 4 where variations in LOC and flame spread rate measured in [15] (with same electrical wires used in this study at the initial standard ambient oxygen concentration of 21%) are shown in relation to the inclination angle for the case without having opposed-flow. As the inclination angle
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increases, the flame spread rate increases while LOC decreases. For a given inclination angle, as opposedflow speed increases (but smaller than Ua ∗ ), the flame tends to be perpendicular to the wire and the heat feedback from flame to insulation material decreases, thus LOC should increase until the opposed-flow speed reaches Ua ∗ . When the opposed-flow speed is larger than Ua ∗ , the flame inclines to the burnt-side bare core, heating the bare core more effectively. Part of heat is transferred to insulation through metal core, so LOC decreases slightly as shown in Fig. 2. The flame heat feedback to the electrical wire depends on its relative position to the wire, which can be changed by the inclination angle or the flow. Without flow, the heat feedback should just increase with the increase in the angle. However, with an opposed flow and inclination, this heat feedback should first decrease and then increase with flow speed. When the flow speed is relative low, the flame is pushed away from the wire (the flame is initially more close to the inclined wire without opposed flow) that the heat feedback decreases until the flame is perpendicular to the wire. And then further increasing the opposed flow speed will make the flame tilt toward the wire again that the heat feedback should increase, as shown by the flame positions in Fig. 2(g). As the opposed-flow speed increases further, flame extinction is mainly caused by the Damköhler number effect such that LOC increases with increasing opposed-flow speed. Based on the balance of the two velocity components at the local maximum (when the flame is nearly perpendicular to the wire), the flame spread rate is relatively much smaller (∼1 cm/s) than that of the buoyancy-induced flow or the opposed-flow (∼10 cm/s), as such one can write ∗ UBw = Ua∗
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(1)
Note that the buoyancy-induced flow speed can be expressed in terms of the gravitational acceleration g as ρ ∗ UBw ∼ W f g sin θ (2) ρ∞ where the flame base width (Wf ) is used as a characteristic length scale, since it reasonably represents the diffusion flame size that is responsible for the burning rate, ρ ∞ is the ambient density, ρ is the difference in densities between the ambient and burnt gas (a fixed flame temperature of 1200 K is assumed here by taking the data from [12]). Because the flame base width (Wf ) in the wire’s direction is adopted as the length scale to represent the acceleration distance, the gravitational acceleration component in the wire’s direction, gsinθ , is used. The measured base width (Wf ) at the corresponding local maximum LOC is shown in Fig. 5. For θ ≥ 15°, the widths at the local maxima remain reasonably constant irrespective of the inclination
Fig. 5. Variation of measured Wf and LOC at the local maximum against inclination angle.
Fig. 6. Scaling of critical opposed-flow air speed at local maximum with buoyancy-induced flow speed in wire direction based on inclination angle.
angle, while they are appreciably smaller than the horizontal case without opposed-flow. The corresponding LOCs at the local maxima exhibit small variations (about 0.86%). The balance of the opposed-flow and the buoyancy-induced flow component in Eqs. (1) and (2) are shown in Fig. 6. The results show that there is a strong linear correlation. Although for the horizontal case, the corresponding Wf is quite different from those of the inclined cases as indicated in Fig. 5, this correlation is still applicable for horizontal case because the buoyancy-induced flow component along the wire is zero (sinθ = 0) and the corresponding critical opposed-flow speed is also zero (Fig. 2a). The best fit is ρ ∗ Ua = 1.015 gW f sin θ (3) ρ∞ Note that for this experiment, UBw = 20 sin θ [cm/s] can be a first simple approximation, assuming a typical buoyancy-induced burnt gas flow speed of 20 cm/s. However, Eqs. (2) and (3) can be
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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Fig. 7. Limiting oxygen concentration against inclination angle at several opposed-flow speeds under normal gravity (solid line) and microgravity (dashed line) conditions.
more general considering the dependence of UBw on flame geometry and density differences. As was shown in Fig. 2 under normal gravity conditions, as the opposed-flow speed increases after the local maximum, LOC of a flame follows an opposed spreading pattern; decreases and then increases. This change is similar to the initial LOC variation in the horizontal condition (Fig. 2a), where LOC first decreases and then increases with Ua , as similarly shown in [16] for horizontal case. In summary, as indicated in Fig. 2, the variation in LOC with opposed-flow speed in upward spreading flames over inclined electrical wires in normal gravity can be classified into three regimes. There exists a local maximum in LOC (red dash-line circles in Fig. 2), which corresponds to a balance of opposed-flow air speeds (Ua ∗ ) at a specified inclination angle with a buoyancy-induced upward flow component along the wire. 3.2. Difference in LOC under normal- and microgravity conditions Due to difficulties in conducting experiments under microgravity relevant to fire safety for space applications, LOC data under normal gravity are required to extrapolate to those under microgravity. In this regard, we provide a methodology to link LOC data under these two conditions. Figure 7 plots LOC in relation to inclination angle at several opposed-flow speeds under normal- and micro-gravity conditions. Generally, under normal gravity, LOC decreases as the inclination angle increases, except for Ua = 6 cm/s, where LOC first increases and then decreases. This is more obvious as the opposed-flow speed increases. As discussed previously, LOC is smaller under microgravity than under normal gravity. This difference seems to decrease as the opposed-flow speed increases (up to 20 cm/s and after that there could be a reversal as indicated by the green circles in Fig. 2).
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Fig. 8. Difference in LOC under normal and microgravity conditions against opposed-flow speed and their correlations at relatively higher flow speeds (10−20 cm/s) at several inclination angles (κ = [LOCμg −LOC1g ]/ Ua ).
Since flammability tests for applications used under microgravity are usually conducted under normal gravity, a practical relationship in quantifying their differences in relation to opposed-flow speed and inclination angle is desirable and is discussed in the following. Figure 8 plots the absolute values of the difference in LOC under normal- and micro-gravity conditions [LOCμg −LOC1g ] in relation to opposedflow speed at several inclination angles. In a space station or spacecraft, weak ventilation convection flow in the range of 10−20 cm/s [1,3,17] is common. The result shows that in this flow speed range, the difference [LOCμg −LOC1g ] is nearly constant regardless of the opposed-flow speed in the horizontal case. At 10 cm/s, the difference is about −3% and independent of inclination angle. The linearly correlated slope (indicating the rate of change in LOC difference between microgravity and normal gravity conditions with respect to the opposed-flow speed, (κ = [LOCμg −LOC1g )/Ua ) increases as the inclination angle increases. Note that if Da at extinction is unchanged for different gravitational conditions, the definition of Da implies YO ∼Ua 2 ideally (as indicated in Fig. 2 of [16]). However, there could still be heat loss effect, which can be coupled non-negligibly with Da effect. The experimental data shown in Fig. 1 of [16] actually indicated that in this opposed-flow speed range, YO was reasonably proportional to Ua , especially for NiCr wire. Additional complication is that it is still unclear whether the extinction Da is the same between normal- and micro-gravity conditions. In this regard, the linear correlation κ applied here in Fig. 8 is characterizing the LOC differences between normal- and micro-gravity. The variation in the correlated slope κ can be physically attributed to the difference in gravity, or more specifically to the gravity acceleration compo-
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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Fig. 9. Correlation of the rate of change in LOC difference between microgravity and normal gravity conditions with respect to opposed-flow speed against the gravitational acceleration component in wire direction.
nent in the wire’s direction, which counteracts the opposed-flow. For this reason, κ is correlated with the inclination angle through the gravitational acceleration component in the wire direction, gsinθ , as shown in Fig. 9. The result shows that they correlate reasonably as a linear function in general (although there seems to be some leveling-off at relative large gsinθ ). The best linear fit is in terms of the inclination angle and opposed-flow speed (for 10 ≤ Ua ≤ 20 cm/s) by taking the LOC difference at 10 cm/s to be −3.0% on average as: LOCμg − LOC1g = −3.0 + 0.034(Ua [cm/s] − 10 )g sin θ [%] (4) This simple and practical formula links LOCs measured under normal gravity conditions to those under microgravity conditions by varying the inclination angle and opposed-flow speed. The correlation does not work for the concurrent case with small Ua (less than 10 cm/s). This is because, under concurrent conditions, the flame extinction mechanism is relatively more complex and the external forced flow is not the dominant factor, while concurrent heating might be dominant. This simplified correlation should be limited to counter-current conditions at relatively higher opposed-flow speeds. 4. Conclusions This paper presents limiting oxygen concentrations for extinction of upward spreading flames over thin PE insulated NiCr electrical wires with varying inclination angles (0−75°) and an opposedflow (0−25 cm/s) under normal- and micro-gravity conditions. Major findings include: (1) For a specified inclination angle (except the horizontal case, 0°), the LOC first increases, then decreases and finally increases again as
the opposed-flow air speed increases. The first local maximum in the LOC variation corresponds to a critical air-flow speed resulting from the change in the flame spread pattern from concurrent to countercurrent. This critical air flow speed correlates well with the buoyancy-induced flow speed component in the wire’s direction at inclination angles when the flame base width along the wire is used as a characteristic length scale. (2) LOC is generally higher under normal gravity conditions than under microgravity conditions and the difference between the two decreases as the air flow speed increases, following a mostly linear trend at relatively higher air flow speeds (over 10 cm/s). (3) The decrease in the difference in LOC under normal- and micro-gravity conditions as the air flow speed increases (their correlated linear slope) correlates well with the gravity acceleration component in the wire’s direction (gsinθ), which provides a measure to extend LOC measured by the tests under normal gravity conditions (at various inclination angles and opposed-flow air speeds) to LOC under microgravity conditions. As mentioned previously, however, this extrapolation method is developed for relatively thin PE insulators to minimize the flowing and dripping of molten PE along the wire (the proposed correlation is more likely suitable for nonmelting insulator) and for NiCr wire having small conductivity. Such that the generalization requires a caution and requires further study including the effects of materials (core, insulation) of wire and their geometries.
Acknowledgment This work was supported by Key Project of National Natural Science Foundation of China (NSFC) under Grant No. 51636008, the Excellent Young Scientist Fund of the National Natural Science Foundation of China (NSFC) under grant no. 51422606, Newton Advanced Fellowship (NSFC: 51561130158; RS: NA140102), Key Research Program of Frontier Sciences, CAS under Grant No. QYZDB-SSW-JSC029, the Fok Ying-Tong Education Foundation under grant no. 151056, Fundamental Research Funds for the Central Universities under Grant Nos. WK2320000035, and JSPS Fellowship (P12360) to Longhua Hu, by JAXA to Osamu Fujita 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”, and by King Abdullah University of Science and Technology (KAUST) to Suk Ho Chung.
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Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity Longhua Hu a,b, Yong Lu a, Kosuke Yoshioka b, Yangshu Zhang a, Carlos Fernandez-Pello c, Suk Ho Chung d, Osamu Fujita b,∗ a State
Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, China b Division of Mechanical and Space Engineering, Hokkaido University, Sapporo, Japan c Department of Mechanical Engineering, University of California, Berkeley, CA 94720, United States d Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia Received 1 December 2015; accepted 20 September 2016 Available online xxx
Abstract Materials, such as electrical wire, used in spacecraft must pass stringent fire safety standards. Tests for such standards are typically performed under normal gravity conditions and then extended to applications under microgravity conditions. The experiments reported here used polyethylene (PE)-insulated (thickness of 0.15 mm) Nichrome (NiCr)-core (diameter of 0.5 mm) electrical wires. Limiting oxygen concentrations (LOC) at extinction were measured for upward spreading flame at various forced opposed-flow (downward) speeds (0−25 cm/s) at several inclination angles (0−75°) under normal gravity conditions. The differences from those previously obtained under microgravity conditions were quantified and correlated to provide a reference for the development of fire safety test standards for electrical wires to be used in space exploration. It was found that as the opposed-flow speed increased for a specified inclination angle (except the horizontal case), LOC first increased, then decreased and finally increased again. The first local maximum of this LOC variation corresponded to a critical forced flow speed resulted from the change in flame spread pattern from concurrent to counter-current type. This critical forced flow speed correlated well with the buoyancy-induced flow speed component in the wire’s direction when the flame base width along the wire was used as a characteristic length scale. LOC was generally higher under the normal gravity than under the microgravity and the difference between the two decreased as the opposed-flow speed increases, following a reasonably linear trend at relatively higher flow speeds (over 10 cm/s). The decrease in the difference in LOC under normaland microgravity conditions as the opposed-flow speed increases correlated well with the gravity acceleration
∗
Corresponding author. Fax: +81 11706 7841. E-mail address: [email protected] (O. Fujita).
http://dx.doi.org/10.1016/j.proci.2016.09.021 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
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component in the wire’s direction, providing a measure to extend LOC determined by the tests under normal gravity conditions (at various inclination angles and opposed-flow speeds) to LOC under microgravity conditions. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Limiting oxygen concentration (LOC); Extinction; Electrical wire; Inclination angle; Microgravity
1. Introduction Fire safety in space exploration is an important issue such that materials used in a spacecraft must pass stringent fire safety standards. Tests for such standards are typically performed under normal gravity and then extended to applications under microgravity. The US National Aeronautics and Space Administration (NASA) designed a test (Test 4 of NASA-STD-6001B) [1,2] to evaluate the fire safety of insulation materials for electrical wires. In this test, an insulation material is judged to be “safe” if a spreading flame is self-extinguished within a limited spreading distance of less than 15 cm and if the dripped flaming debris does not ignite a piece of K-10 paper placed 20 cm below the sample. This test is performed without external air flow at a positive inclination angle of 75° from the ground under normal gravity and ambient atmospheric oxygen concentration. Because of the buoyancy exerted on burnt gas, an upward draft of air is generated during the experiment such that the flow configuration is an upward concurrent spreading flame. This pass/fail nature of the test limits the applicability of the results. For broader applicability, a quantitative scale measuring the degree of a material’s flammability would be useful. A recently proposed alternative and quantitative fire safety test that scales flammability parameters is the limiting oxygen concentration (LOC) method [3]. A minimum oxygen concentration in which a spreading flame can be sustained under specified conditions (e.g., sample thickness, external forced flow speed, or pressure) is adopted as the scale. The LOC method has an advantage of providing quantitative limiting values, instead of pass/fail criteria, thus it is suggested [3] that the evaluation based on the LOC method can be a promising potential future standard. However, LOC experiments are typically performed under normal gravity, whereas space applications require microgravity. The discrepancy between LOC under microgravity and LOC under normal gravity should be properly formulated for the LOC method to be acceptable as a fire safety standard for space applications [3]. Here, we focus on the behavior of spreading flames over electrical wires and the extinction LOC at various inclination angles under normal gravity. We also make com-
parisons with such behaviors under microgravity to quantify their differences. Flame spread behaviors of various insulation and core metal materials on electrical wires under microgravity (without effect from the wire inclination angle) along with external air flow speeds were previously investigated to determine LOCs [4] as well as ignition limits [5,6], effects of AC electric fields and insulation melting behavior [7-10]. Some of these results were compared with a horizontal wire configuration under normal gravity conditions [4-11]. Nakamura et al. [12] investigated the effect of pressure on flame spread for horizontal electrical wires. Bhattacharjee et al. [13] predicted that there was a critical fuel thickness in flame extinction in a quiescent microgravity environment. Osorio et al. [14] studied LOC of horizontal, ethylene tetrafluoroethylene (ETFE)-insulated copper wires exposed to an external radiant flux under normal gravity and microgravity conditions. Concerning the effect of the inclination angle, Hu et al. [15] experimentally studied recently the flame spread rate for both upward (concurrent flow) and downward (countercurrent) propagations over electrical wires. However, as far as the authors aware, there is no published work reporting on the combined effects of the inclination angle and opposed-flow speed on LOCs of electrical wires under normal gravity conditions and how to link these effects to microgravity conditions. Combined effects add complexity to the problem, but they are important to be considered in flammability evaluations. The experiments conducted here used polyethylene (PE)-insulated Nichrome (NiCr)-core electrical wires. Extinction LOCs were measured at various opposed-flow speeds at several inclination angles under normal gravity conditions and the differences from those obtained under microgravity conditions [16] were quantified and correlated to provide a reference for the development of fire safety test standards for electrical wires used in space exploration.
2. Experiment A schematic of the experimental setup is presented in Fig. 1 (details had been reported previously 7-12,16]). The experimental chamber is 0.5 m long with a square cross-section of 0.26 × 0.26 m. A
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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a
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b
Oxygen detector
Combustion chamber
Combustion chamber Wire
Fan
Wire
Flame Air flow
Igniter
Glass window Honeycombs
Flow duct
Absorbent
Triangular blocks Fig. 1. Experimental setup.
rectangular flow duct of 0.38 m long with a crosssection of 0.14 (height) × 0.15 m (width) is positioned inside the chamber. A suction fan is positioned at one end of the duct to induce a flow in a speed range of Ua = 0-25 cm/s, a comparable range of a typical ventilation flow in spacecraft [1,3,17]. Honeycombs are positioned at both ends of the duct for the flow to be nearly uniform. Polyethylene (PE) insulated electrical wire (NiCr; nickel-chrome core) with a core diameter (dc ) of 0.50 mm and an insulation thickness (δ p ) of 0.15 mm is positioned along the centerline of the duct. A possible movement of the melted polyethylene along the inclined electric wire depends on the inclination angle in normal gravity, while this effect is absent in microgravity. Thus, in this work, this behavior is suppressed by using a thin insulation thickness to examine the difference in gas phase processes in normal gravity and microgravity. The effective length of the sample wire is 160 mm. The wire sample is ignited by a coil heater (heated about 10 s) at the fan side of the test section such that flame spread with an opposed-flow is realized. As soon as a flame starts to propagate, the ignition current is shut off. The upward flame spread is designed for a relatively conservative test. The effect of the inclination angle of the electrical wire under normal gravity is studied by varying the overall inclination angle of the chamber (Fig. 1b), ranging from θ = 0–75° using two triangular blocks. The oxygen concentration (balanced by nitrogen) and pressure (maintained at about 100 kPa with less than 1 kPa variation) inside the chamber are monitored in real time. The dynamic process of flame spread is recorded by a CCD video camera (2 megapixels, 1 pixel = 0.13274 mm; 30 fps), which is fixed to the chamber. Flame shape characteristics, as well as the flame front position with time, are
obtained through post-processing the images. LOC was defined as the condition when the flame was self-extinguished at a spread distance of less than 10 cm, which was determined as a function of the inclination angle and the opposed-flow speed under normal gravity conditions. LOC under microgravity can be found in [16]. Similar to the method in [16], our experiments were conducted by setting initial O2 concentration and monitoring the flame extinction as well as its corresponding temporal O2 concentration to determine LOCs. Oxygen sensor (Jikco JKO-25LII, Galvanic cell type oxygen analyzer) was used to monitor oxygen concentration in the chamber with the resolution of 0.1% and accuracy of ±0.5% (range: 0–25%). The sensor position is very close to the inlet of the duct and far from the exit of the duct, thus the O2 concentration measured by the sensor is that at the inlet of the combustion duct. After extinction, the O2 concentration reading did not change. The sensor data were synchronized with video data, as described in details in [10]. 3. Results and discussion 3.1. Variation in LOC with opposed-flow air speed Measured LOCs in relation to opposed-flow speeds at several inclination angles under normal gravity are shown in Fig. 2, along with microgravity data [15] available up to Ua = 20 cm/s. Here, the error bars are from the three repeats of experiments, which show good repeatability as indicated by the small uncertainties as compared to the overall variation of LOC on opposed-flow speed. The result shows that LOCs in general are lower under microgravity (more flammable) than under normal gravity. The difference in LOCs for θ = 0° between the normal- and microgravity conditions
Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021
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Fig. 2. LOC against opposed-flow speed at several inclination angles under normal gravity (three regions recognized) and comparison with microgravity (Wf flame base width; Ua ∗ critical opposed-flow speed when flame is perpendicular to wire). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
remains reasonably constant for Ua = 6−20 cm/s. For θ > 0° cases, the difference generally decreases as opposed-flow speed increases. When θ is large (45°, 60° and 75°), LOCs under normal gravity and
microgravity become comparable at Ua = 20 cm/s (marked with green circles in Fig. 2). Under normal gravity with θ = 0°, LOC initially decreases and then increases with U0 . The
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initial small decrease in LOC as the opposed-flow speed increases, which is similar to that shown previously at low flow speed [16], can be attributed to the increased heat supply toward the PE, as the main body of the flame is deflected by the flow to be closer to the wire. When the opposed-flow speed increases to a relatively higher level, it is convinced that the extinction of the flame is mainly caused by the Damköhler number (Da) effect (blow-off) in the gas phase [16]. This can be characterized by the ratio of a residence time (τ res ) to a chemical reaction time (τ chem ), where the Damköhler number was defined as Da ≡ tres /tchem = (αg /Ua2 )ρgYO A exp{−E/RTf }. Here, α g , ρ g , YO , A, E, R, and Tf are the thermal diffusivity and density of gas phase, oxygen mass fraction, pre-exponential factor, activation energy, universal gas constant, and flame temperature, respectively. Note that in the blowoff-dominant regime, extinction Da was found to be constant [16]. When the opposed-flow speed becomes larger, LOC needs to be larger to sustain a flame, since the flame is weakened with relatively strong opposed-flow such that a higher oxygen concentration is required. It is clear that the extinction in microgravity in this opposed flow speed range is dominated by blow-off effect that the LOC just increases with increase in opposed flow speed [16]. When θ = 15° and 30°, LOC initially increases, then decreases and finally increases again with Ua . When θ > 30°, LOC increases and then decreases up to the tested Ua of 25 cm/s. Behaviors at other inclination angles suggest for θ > 30°, LOC may increase again as Ua further increases. Although the microgravity data are available only up to Ua = 20 cm/s, we also anticipate that, when Ua > 20 cm/s for large inclination angles (θ > 30°; for example 75°), LOC may be larger under microgravity than under normal gravity. This can be attributed to the fact that under strong opposed-flow speed, flame extinction is controlled by the Damköhler number effect [3,16], which leads to an increase in LOC as the opposed-flow speed increases. However, under such opposedflow (blowoff) dominant extinction conditions, the buoyancy-induced flow under normal gravity conditions could counteract the opposed-flow. Thus, LOC under normal gravity conditions can be smaller than LOC under microgravity conditions. In the inclined wire cases, a local maximum LOC (marked with dotted red circles in Fig. 2) exists when Ua is small and the corresponding critical Ua ∗ generally increases as θ increases. This can be attributed to the buoyancy-induced upward flow, UB , as schematically shown in Fig. 3, where the velocity component parallel to the wire direction of UBw could counteract the opposed-flow, Ua . When Ua is small, the intensity of the buoyancy-induced flow component can be larger than Ua . In such a case, the flame spread resembles a concurrent flame spread. As Ua increases, UBw becomes relatively
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Fig. 3. Physical interpretation of local maximum LOC based on balance of buoyancy-induced flow in the wire’s direction with opposed-flow speed.
Fig. 4. LOC and flame spread rate against inclination angle without having opposed-flow.
smaller than Ua , such that the flame spread turns into a counter-current pattern. The observed local maxima behavior can therefore be explained based on the balance of the buoyancy-induced flow with the opposed-flow along the wire’s direction. For the local maximum condition, the flame is nearly perpendicular to the wire, similar to the case without having opposed-flow (see inset photos in Fig. 2). In general, the flame spread rate under the concurrent condition (with low opposed-flow air speed) should be higher than that under the counter-current condition (with relatively stronger opposed-flow speed) because of heating to unburned PE. For a given θ, as Ua increases, the flame spread rate should first decrease, exhibiting opposite trend to LOC behavior. This is because, when the flame spread rate is relatively high, its corresponding LOC should be relatively low. This is evident in Fig. 4 where variations in LOC and flame spread rate measured in [15] (with same electrical wires used in this study at the initial standard ambient oxygen concentration of 21%) are shown in relation to the inclination angle for the case without having opposed-flow. As the inclination angle
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increases, the flame spread rate increases while LOC decreases. For a given inclination angle, as opposedflow speed increases (but smaller than Ua ∗ ), the flame tends to be perpendicular to the wire and the heat feedback from flame to insulation material decreases, thus LOC should increase until the opposed-flow speed reaches Ua ∗ . When the opposed-flow speed is larger than Ua ∗ , the flame inclines to the burnt-side bare core, heating the bare core more effectively. Part of heat is transferred to insulation through metal core, so LOC decreases slightly as shown in Fig. 2. The flame heat feedback to the electrical wire depends on its relative position to the wire, which can be changed by the inclination angle or the flow. Without flow, the heat feedback should just increase with the increase in the angle. However, with an opposed flow and inclination, this heat feedback should first decrease and then increase with flow speed. When the flow speed is relative low, the flame is pushed away from the wire (the flame is initially more close to the inclined wire without opposed flow) that the heat feedback decreases until the flame is perpendicular to the wire. And then further increasing the opposed flow speed will make the flame tilt toward the wire again that the heat feedback should increase, as shown by the flame positions in Fig. 2(g). As the opposed-flow speed increases further, flame extinction is mainly caused by the Damköhler number effect such that LOC increases with increasing opposed-flow speed. Based on the balance of the two velocity components at the local maximum (when the flame is nearly perpendicular to the wire), the flame spread rate is relatively much smaller (∼1 cm/s) than that of the buoyancy-induced flow or the opposed-flow (∼10 cm/s), as such one can write ∗ UBw = Ua∗
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(1)
Note that the buoyancy-induced flow speed can be expressed in terms of the gravitational acceleration g as ρ ∗ UBw ∼ W f g sin θ (2) ρ∞ where the flame base width (Wf ) is used as a characteristic length scale, since it reasonably represents the diffusion flame size that is responsible for the burning rate, ρ ∞ is the ambient density, ρ is the difference in densities between the ambient and burnt gas (a fixed flame temperature of 1200 K is assumed here by taking the data from [12]). Because the flame base width (Wf ) in the wire’s direction is adopted as the length scale to represent the acceleration distance, the gravitational acceleration component in the wire’s direction, gsinθ , is used. The measured base width (Wf ) at the corresponding local maximum LOC is shown in Fig. 5. For θ ≥ 15°, the widths at the local maxima remain reasonably constant irrespective of the inclination
Fig. 5. Variation of measured Wf and LOC at the local maximum against inclination angle.
Fig. 6. Scaling of critical opposed-flow air speed at local maximum with buoyancy-induced flow speed in wire direction based on inclination angle.
angle, while they are appreciably smaller than the horizontal case without opposed-flow. The corresponding LOCs at the local maxima exhibit small variations (about 0.86%). The balance of the opposed-flow and the buoyancy-induced flow component in Eqs. (1) and (2) are shown in Fig. 6. The results show that there is a strong linear correlation. Although for the horizontal case, the corresponding Wf is quite different from those of the inclined cases as indicated in Fig. 5, this correlation is still applicable for horizontal case because the buoyancy-induced flow component along the wire is zero (sinθ = 0) and the corresponding critical opposed-flow speed is also zero (Fig. 2a). The best fit is ρ ∗ Ua = 1.015 gW f sin θ (3) ρ∞ Note that for this experiment, UBw = 20 sin θ [cm/s] can be a first simple approximation, assuming a typical buoyancy-induced burnt gas flow speed of 20 cm/s. However, Eqs. (2) and (3) can be
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Fig. 7. Limiting oxygen concentration against inclination angle at several opposed-flow speeds under normal gravity (solid line) and microgravity (dashed line) conditions.
more general considering the dependence of UBw on flame geometry and density differences. As was shown in Fig. 2 under normal gravity conditions, as the opposed-flow speed increases after the local maximum, LOC of a flame follows an opposed spreading pattern; decreases and then increases. This change is similar to the initial LOC variation in the horizontal condition (Fig. 2a), where LOC first decreases and then increases with Ua , as similarly shown in [16] for horizontal case. In summary, as indicated in Fig. 2, the variation in LOC with opposed-flow speed in upward spreading flames over inclined electrical wires in normal gravity can be classified into three regimes. There exists a local maximum in LOC (red dash-line circles in Fig. 2), which corresponds to a balance of opposed-flow air speeds (Ua ∗ ) at a specified inclination angle with a buoyancy-induced upward flow component along the wire. 3.2. Difference in LOC under normal- and microgravity conditions Due to difficulties in conducting experiments under microgravity relevant to fire safety for space applications, LOC data under normal gravity are required to extrapolate to those under microgravity. In this regard, we provide a methodology to link LOC data under these two conditions. Figure 7 plots LOC in relation to inclination angle at several opposed-flow speeds under normal- and micro-gravity conditions. Generally, under normal gravity, LOC decreases as the inclination angle increases, except for Ua = 6 cm/s, where LOC first increases and then decreases. This is more obvious as the opposed-flow speed increases. As discussed previously, LOC is smaller under microgravity than under normal gravity. This difference seems to decrease as the opposed-flow speed increases (up to 20 cm/s and after that there could be a reversal as indicated by the green circles in Fig. 2).
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Fig. 8. Difference in LOC under normal and microgravity conditions against opposed-flow speed and their correlations at relatively higher flow speeds (10−20 cm/s) at several inclination angles (κ = [LOCμg −LOC1g ]/ Ua ).
Since flammability tests for applications used under microgravity are usually conducted under normal gravity, a practical relationship in quantifying their differences in relation to opposed-flow speed and inclination angle is desirable and is discussed in the following. Figure 8 plots the absolute values of the difference in LOC under normal- and micro-gravity conditions [LOCμg −LOC1g ] in relation to opposedflow speed at several inclination angles. In a space station or spacecraft, weak ventilation convection flow in the range of 10−20 cm/s [1,3,17] is common. The result shows that in this flow speed range, the difference [LOCμg −LOC1g ] is nearly constant regardless of the opposed-flow speed in the horizontal case. At 10 cm/s, the difference is about −3% and independent of inclination angle. The linearly correlated slope (indicating the rate of change in LOC difference between microgravity and normal gravity conditions with respect to the opposed-flow speed, (κ = [LOCμg −LOC1g )/Ua ) increases as the inclination angle increases. Note that if Da at extinction is unchanged for different gravitational conditions, the definition of Da implies YO ∼Ua 2 ideally (as indicated in Fig. 2 of [16]). However, there could still be heat loss effect, which can be coupled non-negligibly with Da effect. The experimental data shown in Fig. 1 of [16] actually indicated that in this opposed-flow speed range, YO was reasonably proportional to Ua , especially for NiCr wire. Additional complication is that it is still unclear whether the extinction Da is the same between normal- and micro-gravity conditions. In this regard, the linear correlation κ applied here in Fig. 8 is characterizing the LOC differences between normal- and micro-gravity. The variation in the correlated slope κ can be physically attributed to the difference in gravity, or more specifically to the gravity acceleration compo-
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Fig. 9. Correlation of the rate of change in LOC difference between microgravity and normal gravity conditions with respect to opposed-flow speed against the gravitational acceleration component in wire direction.
nent in the wire’s direction, which counteracts the opposed-flow. For this reason, κ is correlated with the inclination angle through the gravitational acceleration component in the wire direction, gsinθ , as shown in Fig. 9. The result shows that they correlate reasonably as a linear function in general (although there seems to be some leveling-off at relative large gsinθ ). The best linear fit is in terms of the inclination angle and opposed-flow speed (for 10 ≤ Ua ≤ 20 cm/s) by taking the LOC difference at 10 cm/s to be −3.0% on average as: LOCμg − LOC1g = −3.0 + 0.034(Ua [cm/s] − 10 )g sin θ [%] (4) This simple and practical formula links LOCs measured under normal gravity conditions to those under microgravity conditions by varying the inclination angle and opposed-flow speed. The correlation does not work for the concurrent case with small Ua (less than 10 cm/s). This is because, under concurrent conditions, the flame extinction mechanism is relatively more complex and the external forced flow is not the dominant factor, while concurrent heating might be dominant. This simplified correlation should be limited to counter-current conditions at relatively higher opposed-flow speeds. 4. Conclusions This paper presents limiting oxygen concentrations for extinction of upward spreading flames over thin PE insulated NiCr electrical wires with varying inclination angles (0−75°) and an opposedflow (0−25 cm/s) under normal- and micro-gravity conditions. Major findings include: (1) For a specified inclination angle (except the horizontal case, 0°), the LOC first increases, then decreases and finally increases again as
the opposed-flow air speed increases. The first local maximum in the LOC variation corresponds to a critical air-flow speed resulting from the change in the flame spread pattern from concurrent to countercurrent. This critical air flow speed correlates well with the buoyancy-induced flow speed component in the wire’s direction at inclination angles when the flame base width along the wire is used as a characteristic length scale. (2) LOC is generally higher under normal gravity conditions than under microgravity conditions and the difference between the two decreases as the air flow speed increases, following a mostly linear trend at relatively higher air flow speeds (over 10 cm/s). (3) The decrease in the difference in LOC under normal- and micro-gravity conditions as the air flow speed increases (their correlated linear slope) correlates well with the gravity acceleration component in the wire’s direction (gsinθ), which provides a measure to extend LOC measured by the tests under normal gravity conditions (at various inclination angles and opposed-flow air speeds) to LOC under microgravity conditions. As mentioned previously, however, this extrapolation method is developed for relatively thin PE insulators to minimize the flowing and dripping of molten PE along the wire (the proposed correlation is more likely suitable for nonmelting insulator) and for NiCr wire having small conductivity. Such that the generalization requires a caution and requires further study including the effects of materials (core, insulation) of wire and their geometries.
Acknowledgment This work was supported by Key Project of National Natural Science Foundation of China (NSFC) under Grant No. 51636008, the Excellent Young Scientist Fund of the National Natural Science Foundation of China (NSFC) under grant no. 51422606, Newton Advanced Fellowship (NSFC: 51561130158; RS: NA140102), Key Research Program of Frontier Sciences, CAS under Grant No. QYZDB-SSW-JSC029, the Fok Ying-Tong Education Foundation under grant no. 151056, Fundamental Research Funds for the Central Universities under Grant Nos. WK2320000035, and JSPS Fellowship (P12360) to Longhua Hu, by JAXA to Osamu Fujita 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”, and by King Abdullah University of Science and Technology (KAUST) to Suk Ho Chung.
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Please cite this article as: L. Hu et al., Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.021