Ignition limits of short-term overloaded electric wires in microgravity

doi:10.1016/j.proci.2012.06.064

Proceedings of the Combustion Institute

Volume 34, Issue 2, 2013, Pages 2665-2673

https://doi.org/10.1016/j.proci.2012.06.064 Get rights and content

Abstract

Ignition phenomena of electric wires carrying short-term excess electric currents were investigated in microgravity with experiments and calculations. Microgravity experiments were conducted in 100 m and 50 m drop towers and calculations were carried out with a one dimensional cylindrical coordinate system. The experimental results showed that the limiting oxygen concentration (LOC) under a given electric current was much lower in microgravity than that in normal gravity except for extremely large electric current overload cases. According to the calculations, the supplied electric current, the Joule energy supplied to the wire, determined the amount of pyrolysis gas from the insulation and the resulting thickness of the gaseous fuel layer around the sample in gas phase increased. The increased fuel layer thickness resulted in a longer ignition delay, which leads to lower LOC. The changes in the estimated LOC changed as a function of supplied energy and agreed well with the experimental results. Further, the minimum ignition energy causing ignition (ignition limit) is nearly constant under a constant oxygen concentration, which supports experimental findings in previous research.

Introduction

A most likely cause of fires in space is combustion of the wire harness of spacecraft [1], [2], and such fires are generally started with short-circuiting or overloading of electric wires. Therefore, it is important to know the ignition characteristics of overloaded wires in microgravity to improve fire safety in space. To know the electric current needed for ignition of electric wires is important in the design of circuit breakers. As carbon dioxide extinguishers are used in the International Space Station [3], limiting oxygen concentration for ignition is essential to design extinguishers based on asphyxiating effects and to establish the ambient oxygen concentration in spacecraft.

Knowledge of the ignition characteristics of solid components in microgravity is a basic subject which must be studied to prevent the breaking out of fires, and a number of theoretical and experimental studies have been conducted. One example of solid ignition research in microgravity is piloted ignition of PMMA plates [4], [5] and others are non-piloted ignition of thin cellulosic sheets [6], [7], [8], [9] or PMMA sheets [10] heated by external radiant sources. However, there are few studies except from our research group [11], [12] about spontaneous ignition of electric wires caused by Joule heat generated in the wire core.

In the previous research [11], the authors reported dramatic extensions of ignition limits in terms of supplied electric current under microgravity. In that research, excess electric current was continuously supplied until ignition occurred. In other research [12], the authors investigated the ignition characteristics with short-term excess electric currents. There, the length of the current supply was selected as the main test parameter to simulate the status of circuit breaker activation shortly after overloading happens, and delayed ignition of electric wires after short-term overloading was observed in microgravity. In the results with short-term electric current supply, the ignition delay may be longer than with a continuous current supply. The electric current is smaller in microgravity than that on the ground as in the continuous current case. The established findings [11], [12] indicate the probability of ignition increases in microgravity. One matter that has not been established in the previous work is the limiting oxygen concentration, below which wire ignition does not occur even with excess electric current. A detailed knowledge of this basic parameter is essential for fire safety in space.

The present study, investigates the ignition limit; the minimum value of the electric energy supply causing ignition, and LOC; limiting oxygen concentration, below which ignition does not occur at a given electric current supply for short-term excess electric currents under microgravity, by experiments and calculations. The mechanism to determine the ignition limit and the LOC is discussed based on the numerical results.

Section snippets

Experiments

All experiments were performed in the apparatus similar to that used in previous research [12]. An outline of the experimental set-up is shown in Fig. 1. It is composed of a combustion chamber with the sample wire inside, a constant current supply system, and an image recording system. The tested sample in Fig. 2 is a nickel–chrome core wire coated with polyethylene. The outer diameter of the sample is 0.8 mm and the inner core diameter is 0.5 mm. The effective length of the sample is 70 mm; the

Limiting oxygen concentration (LOC) by experiments

Figure 4 shows a photo of wire combustion immediately after ignition in microgravity. The ignition is initiated at some point and then propagates along the wire. This figure indicates the flame has a cylindrical two-dimensional shape surrounding the sample wire in the absence of buoyancy induced convection.

Figure 5 shows Mach-Zehnder interference images for the ignition phenomena in microgravity. The supplied energy is 6.7 J/cm with the current applied for 0.5 s at an oxygen concentration of 40%,

Temperature distributions with and without ignition

Figure 7 shows temperature distributions for two cases calculated with different amounts of energy supplied. (a) is a case with ignition, 7.120 J/cm and (b) is a case without ignition, 7.119 J/cm. The current was applied for 1.0 s and the ambient oxygen concentration 21%. In both cases, at the cessation of the current supply, 1.0 s, the maximum temperature is at the wire surface. At 1.4 s, the high temperature region has spread outward, as enough pyrolysis gas is evolved into the gas phase and the

Conclusions

Ignition phenomena for electric wires with short-term excess electric current supply were investigated by microgravity experiments and numerical calculations to understand the ignition characteristics such as ignition limit and LOC (limiting oxygen concentration). The conclusions may be summarized as follows:

(1)
The LOC in microgravity is much lower than in normal gravity when the supplied energy is low (<14 J/cm), while the value at higher supplied energies in microgravity becomes closer to the

Acknowledgments

This research is partially supported by JAXA as the candidate experiments for the second phase utilization of JEM/ISS entitled “Quantitative Description of Gravity Impact on Solid Material Flammability as a base of Fire Safety in Space” and is also supported by JAXA Research Working Group to promote space utilization.

References (15)

S. McAllister et al.
Proc. Combust. Inst.
(2009)
S. McAllister et al.
Combust. Flame
(2010)
J. Takahashi et al.
Proc. Combust. Inst.
(2005)
Y. Nakamura et al.
Combust. Flame
(2000)
S.L. Olson et al.
Combust. Flame
(2001)
Y. Nakamura et al.
Proc. Combust. Inst.
(2005)
O. Fujita et al.
Proc. Combust. Inst.
(2011)

There are more references available in the full text version of this article.

Cited by (45)

Experimental study on fire spread behavior of single 110 kV cable under different layout conditions
2023, Fire Safety Journal
High-voltage cable fire accidents can cause large-scale power outages. In particular, the 110 kV large-section (diameter 10 cm) cable has complex cable structures. Therefore, based on the actual cable laying scenarios, three cable layout conditions are set, including the horizontal cable above horizontal floor structure (HCAHFS), the inclined cable above horizontal floor structure (ICAHFS), and the inclined cable above inclined floor structure (ICAIFS). The combustion behaviors and flame spread characteristics of 110 kV large-section cable are experimentally studied. It is found that the flame cannot spread on the surface of the flame-retardant outer sheath of the cable. However, when the cable-floor distance (l_cf) is between 0.75 d and 2 d (d is the cable diameter) in HCAHFS, i.e. there is a floor under the cable that can catch cable melt, and the flame spread characteristics is different. After the cable is ignited, the cable fire can be divided into two parts, outer sheath flame and melt flame, and the cable fire can spread under the joint action of two flames. When l_cf ≤ 0.75 d, the space under the cable is narrow. With the accumulation of melt on the floor, air entrainment is limited during cable combustion, resulting in the cable flame unsustainable. When l_cf ≥ 2 d, the thermal radiation and convection from the melt and outer sheath flame to the cable preheat zone are weakened, resulting in cable flame extinction as well. In ICAHFS, the cable flame spread rate is first increased and then decreased with the increasing l_cf at the fixed inclination angle; In ICAIFS, the upward spread rate of cable flame is faster than the downward spread rate at the fixed inclined angle. Moreover, the cable flame spread rate (FSR) in HCAHFS is the slowest compared to that in ICAIFS with different inclined angles. The preheating length for ICAIFS increases under both upper and lower ignition modes, while it is obviously at upper ignition, due to the melt flow. Finally, a correlation of FSR was proposed for various ignition positions and inclined angles. This study provides a reference for fire protection design and numerical simulation parameter setting of 110 kV cable fire.
Effects of applied AC electric fields on flame spread over inclined polyethylene-insulated electrical wire with Cu-core
2023, Fire Safety Journal
Effects of applied electric fields on downward and upward flame spreads (DFS and UFS) over polyethylene (PE) insulated electrical wires with Cu-core are investigated by varying the applied voltage (V_AC) and frequency (f_AC) at various inclination angles (θ). With AC electric fields, FSRs are influenced appreciably by complex dynamic behaviors of molten-PE and flame. For the DFSs, the FSR with θ exhibits increasing, decreasing, and increasing trends. For the UFSs, the FSR with θ shows an increasing tendency, along with dependencies on V_AC and f_AC. Further analyses are conducted at θ = ±70°. For the DFSs, the globular molten-PE size, the distance between flame front and globular molten PE, and electrospray influence FSR. For the UFSs, downward flow of molten-PE in some cases increases FSR. While the strong downward flow causes a flame detachment from the main body of the flame and/or a merging phenomenon between molten-PE drops, reducing FSR. A serious of bulged flames due to electrospray increases FSR. These FSR behaviors for DFS and UFS are well characterized with their related physical parameters. Flame extinction occurs only for DFSs with NiCr- and Cu-core. The critical extinction frequency for DFSs is characterized with log (f_AC,ext) = a $\times$ V_AC + b.
Identifying two ignition modes of polymer insulated wires with continuous excess current in microgravity
2023, Fire Safety Journal
Overload ignition of electric wires in microgravity has been investigated for space fire safety. Microgravity experiments from drop towers and parabolic flight have been conducted to observe the overall ignition phenomena. Two-dimensional numerical simulations were performed employing finite-rate chemistries in both condensed and gas phases. As the applied current changes, two distinct ignition modes were found in both experiments and simulations, one away from the wire and the other near the wire surface, named “spontaneous ignition” and “wire assisted ignition”, respectively. After validating ignition delay times by experiments, the simulation results were used to discuss the transition of two ignition modes. With a lower current, the spontaneous exothermic reaction at the pyrolysis gas front led to the far-distance ignition. While with a higher current, the local heat loss suppressed spontaneous ignition; meanwhile, the excessive heat from the exposed core wire caused the surface ignition. In addition, the ignition energy showed a non-monotonical increase with the applied current, where the minimum ignition energy was consumed with 5.5 A current supply implying the high fire risk of the “spontaneous ignition” regime. The control mechanisms of the ignition energy change in different stages were discussed.
Ignition of silicone rubber sheaths by series arcs at different currents and durations
2023, Fire Safety Journal
Understanding the igntion of combustibles by electric arc is important for the electric fire safety in energy systems. In this work, the ignition of a polymer sheath by series arcs with different supplied currents and arc durations was studied by a unique facility. Results show that the ignition process by different arcs can be categorized as one of three modes, including without ignition, with one-stage flaming ignition and with two-stage flaming ignition. Meanwhile, the ignition probability distribution versus current and duration was divided into three zones with different hazards levels (zone Ⅲ>Ⅱ>Ⅰ). The function of igntion probability with arc energy was established, obtaining the upper and lower limits of arc energy in zone Ⅱ, from 630 to 1515 J. Further, by comparing the test conditions where arc energy casued same ignition probability, it was found that the higher the supply current a lower arc energy was needed for ignition. Based on a quantitative analysis of the ignition time heated by sustained arcs and the arc current, the minimum ignition arc current (about 3 A) was in good agreement with experiment as predicted.
Fundamentals of window-ejected fire plumes from under-ventilated compartment fires: Recent progresses and perspectives
2023, Progress in Energy and Combustion Science
This paper intends to provide a comprehensive state-of-art review of recent progresses and to formulate perspectives on window-ejected fire plumes, originating from under-ventilated compartment fires (known as ‘Regime I’ fires). Various external boundary conditions are considered, as they contribute to the fire and plume dynamics, and as such affect decisions on fire prevention and firefighting. Hence this is an important fire combustion topic of both fundamental and practical significance. After discussing the general fundamentals, the paper focuses particularly on recent progresses on quantifying the ejected fire plume behavior: constrained by the presence of walls; at sub-atmospheric pressure (for fires at high altitudes) and under complex flow conditions caused by wind. Experiments, theoretical scaling analysis and basic models are reviewed. The key points cover systematically: the compartment fire evolution (and hence criteria for flame ejection through the window); flame interaction and merging behavior from two windows; air entrainment mechanisms and characteristic parameters (flame structure/dimensions, temperature profile and heat flux) of window-ejected fire plumes. Meanwhile, the limitations of present research and future challenges are also discussed.
Effect of pyrolysis kinetic parameters on the overload ignition of polymer insulated wires in microgravity
2023, Proceedings of the Combustion Institute
The overload ignition of polymer insulated wires in microgravity was investigated as basic information for space fire safety. This paper focuses on the effect of pyrolysis kinetic parameters of the insulation material on the ignition characteristics. A two-dimensional numerical model calculation under varied pyrolysis model was carried out to simulate the overload ignition, and microgravity experiments have been conducted for the model validation. With the help of thermogravimetric analysis, the original pyrolysis model of the polyethylene insulation used for wire ignition experiments was obtained. Three groups of varied pyrolysis models were proposed by changing the activation energy (E), pre-exponential factor (A) from the original model. Numerical simulations were conducted with the varied pyrolysis models. The simulated results employing the original pyrolysis model showed a good agreement with the microgravity experiments for ignition delay time with continuous current and the minimum ignition energy (MIE) with short-term current, respectively. From the simulation results with continuous overloading, the independent variation of A and E in the pyrolysis model has a limited effect on the ignition delay time but a significant effect on ignition modes in microgravity. Such effects are mainly reflected by changing the pyrolysis temperature during overloading, which determines the ease of spontaneous ignition. With a constant pyrolysis temperature, the change of mass loss rate in the pyrolysis model may affect the transition point from the “spontaneous ignition” to the “wire assisted ignition” when the current increases. Finally, for short-term current supply in microgravity, the single change of A or E showed higher MIE than the original pyrolysis model.

View all citing articles on Scopus

View full text

Ignition limits of short-term overloaded electric wires in microgravity

Abstract

Introduction

Section snippets

Experiments

Limiting oxygen concentration (LOC) by experiments

Temperature distributions with and without ignition

Conclusions

Acknowledgments

Proc. Combust. Inst.

Combust. Flame

Proc. Combust. Inst.

Combust. Flame

Combust. Flame

Proc. Combust. Inst.

Proc. Combust. Inst.