A pre-print of this article can be found here: Pre-print PDF
This is a pre-print of an article published in Fire Technology. The final authenticated version is available online at: DOI: 10.1007/s10694-018-0712-4
Dripping of polymer insulations in wire fire has a potential risk of igniting nearby objects and expanding the size of fire, but has not been well studied so far. In this experimental study, dripping behaviors during the flame spread over horizontal and vertical polyethylene (PE) insulated wires were investigated without external airflow. Two different wire dimensions – core/wire diameter of 3.5/8.0 and 5.5/9.0 mm – and three different PE insulations were tested. To identify effects of the core, wires with solid copper (Cu) core, hollow stainless steel (SS) core, and without core were tested, and both core and insulation temperatures were also measured during the flame spread. Experimental results showed that the high-conductance copper core acted as a heat source downstream to increase the flame-spread rate. However, in the upstream burning zone, the copper core also acted as a heat sink to cool the molten insulation and reduce its mobility. Thus, the copper core extended the residence time of molten insulation inside the flame to facilitate the burning while reducing the dripping. Moreover, for the downward flame spread, the heating by the dripping flow of hot molten insulation dominated over the heating by the core. The downward dripping flow is driven by gravity while limited by the viscous and surface tension forces. Therefore, the limited dripping flow along the cooler copper core reduced the downward flame spread. The trend of results was also found to be insensitive to the type of PE insulation. This is the first time that within a single flame, the simultaneous dual effect of the heat source and heat sink for the wire core was observed, and the influence of dripping on the flame spread over the wire was discovered.
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.
Selecting fabrics based on their fire resistance is important for professions with substantial fire risk such as firefighters, race car drivers, and astronauts suits. Generally, fire resistant materials are tested under standard atmospheric conditions. However, their flammability properties can change when the ambient conditions deviate from standard atmospheric conditions. Particularly in high altitude locations, aircraft, and spacecraft, the pressure and oxygen concentrations are different than in a standard atmosphere. Also, the presence of external radiation (i.e. overheating component or nearby fire) can reduced the fire resistance of a material. In this work, an experimental study was conducted to analyze the influence of environmental variables such as oxygen concentration, ambient pressure, and external radiant heat flux on the flame spread limits of two different fire resistant fabrics: Nomex HT90-40 and a blend made of Cotton/Nylon/Nomex. Ambient pressure was varied between 40 and 100 kPa and ambient oxygen concentrations were decreased until the Limiting Oxygen Concentration (LOC), limiting conditions which would permit flame propagation, were found. Experiments were conducted using no external radiant flux or a radiant flux of 5 kW/m2 to examine the influence of the presence of a nearby heat source. Among the results, it was found that as ambient pressure is reduced the oxygen concentration required for the flame to propagate must be increased. The external radiant heat flux acts as an additional source of heat and allows propagation of the flame at lower oxygen concentrations. An analysis of the propagation limits in terms of the partial pressure of oxygen suggest that the LOC of a material is not only determined by heat transfer mechanisms but also by chemical kinetic mechanisms. The information provided in this work helps characterize increased flammability risk of materials when in environments different from the standard atmospheric conditions at which they are typically tested.
The spotting ignition of combustible material by hot metal particles is an important pathway by which wild-land and urban spot fires are started. Upon impact with a fuel, such as dry grass, duff, or saw dust, these particles can initiate spot fires by direct flaming or smoldering can transition to flaming. In spite of the relative frequency that fires are initiated by hot metal particles, there is little work published that addresses the ignition capabilities of hot metal particles landing on natural fuels, especially regarding smoldering ignition. This work is an experimental and analytical study of how the flaming and smoldering propensities of powdered natural fuel beds in contact with hot metal particles are affected by differences in the particle characteristics, particularly the effect of particle melting, which adds energy to the particle. In the experiments, stainless steel and aluminum particles ranging in size from 1.6 to 8 mm in diameter are heated to various temperatures between 500 and 1100 °C and dropped onto a fuel bed composed of a powder grass blend. It is observed that the ignition boundary both for flaming and smoldering follows a hyperbolic relationship between particle size and temperature, with smaller particles requiring higher temperatures to ignite the fuel. For both metal particles smoldering ignition occurs at significantly lower temperatures than flaming ignition. A simplified numerical model is developed to help understand smoldering ignition by a metal particle and to examine how the melting influences the ignition process. Good qualitative agreement is obtained between the model predictions and the experiments suggesting that the model provides a first step toward the theoretical modeling of this complex problem.
Wildland and Wildland Urban Interface (WUI) fires are an important problem in many areas of the world and may have major consequences in terms of safety, air quality, and damage to buildings, infrastructure, and the ecosystem. It is expected that with climate changes the wildland fire and WUI fire problem will only intensify. The spot fire ignition of a wildland fire by hot (solid, molten or burning) metal fragments/sparks and firebrands (flaming or glowing embers) is an important fire ignition pathway by which wildfires, WUI fires, and fires in industrial settings are started and may propagate. There are numerous cases reported of wildfires started by hot metal particles from clashing power-lines, or generated by machines, grinding and welding. Once the wildfire or structural fire has been ignited and grows, it can spread rapidly through ember spotting, where pieces of burning material (e.g. branches, bark, building materials, etc.) are lofted by the plume of the fire and then transported forward by the wind landing where they can start spot fires downwind. The spot fire problem can be separated in several individual processes: the generation of the particles (metal or firebrand) and their thermochemical state; their flight by plume lofting and wind drag and the particle thermo-chemical change during the flight; the onset of ignition (smoldering or flaming) of the fuel after the particle lands on the fuel; and finally, the sustained ignition and burning of the combustible material. Here an attempt has been made to summarize the state of the art of the wildfire spotting problem by describing the distinct individual processes involved in the problem and by discussing their know-how status. Emphasis is given to those areas that the author is more familiar with, due to his work on the subject. By characterizing these distinct individual processes, it is possible to attain the required information to develop predictive, physics-base wildfire spotting models. Such spotting models, together with topographical maps and wind models, could be added to existing flame spread models to improve the predictive capabilities of landscape-scale wildland fire spread models. These enhanced wildland fire spread models would provide land managers and government agencies with better tools to prescribe preventive measures and fuels treatments before a fire, and allocate suppression resources and issue evacuation orders during a fire.