Bio: Aki Hosogai is an academic researcher from Japan Aerospace Exploration Agency. The author has contributed to research in topics: Limiting oxygen concentration & Scale model. The author has an hindex of 3, co-authored 3 publications receiving 24 citations.
••01 Jan 2019
TL;DR: In this article, a scale analysis of the flammability limits of thin flame retardant materials in an opposed flow under microgravity condition have been discussed by scale analysis, and the results were used to obtain proper preexponential factor, A, and activation energy, E, for blow-off phenomena.
Abstract: The flammability limits of thin flame retardant materials in an opposed flow under microgravity condition have been discussed by scale analysis The predicted trends of flammability were verified by parabolic flight experiments As the samples, meta- and para-aramid fabrics (NOMEX and Kevlar), polyimide film (Kapton) and polycarbonate (PC) were investigated In order to predict the limiting curve accurately, blow-off tests in high forced flow were conducted and the results were used to obtain proper pre-exponential factor, A, and activation energy, E, for blow-off phenomena The blow-off tests revealed that the material with high pyrolysis temperature, such as Kevlar and Kapton, had its minimum limiting oxygen concentration (MLOC) at higher opposed velocity region For such materials, it was predicted that the limiting oxygen concentration in microgravity environments was higher than that in normal gravity, and the prediction was consistent with the result of parabolic flight experiment For NOMEX and PC, the limiting oxygen concentration increased monotonically with increase of opposed flow velocity The developed model predicted that such materials could have their MLOC in microgravity environments Actually, the MLOCs of NOMEX and PC were observed under microgravity condition and they were 2% and 05% lower than their limiting oxygen concentrations in normal gravity (LOC1g), respectively The developed model with blow-off test data could predict the difference between the MLOC and LOC1g quantitatively, and it was found that the model was a good tool to discuss the flammability of the flame retardant materials in microgravity environments
TL;DR: In this article, the authors studied the near-limiting behavior of various thicknesses of thermoplastic materials under a candle-like burning configuration; followed by an ISO 4589-2 protocol.
Abstract: We studied the near-limiting behavior of various thicknesses of thermoplastic materials under a candle-like burning configuration; followed by an ISO 4589-2 protocol. The motivation for this work is to understand the sensitivity of the sample thickness on the limiting oxygen concentration in the range from 0.5 mm to 10 mm. In the first place, the effect of heating time on successive ignition was discussed. Through a simple analysis, it was suggested that a 30 s heating time, regulated in ISO 4589-2, might be insufficient to achieve a successful ignition when the specimen becomes thicker. Second, the effect of the thickness of the test specimen (PMMA, ABS) on the limiting oxygen concentration was examined. Flames formed over thicker PMMA (>4.0 mm thickness in this study) at near-limiting condition displayed a flickering motion, which then suddenly extinguished when the critical condition was achieved due to temporal acceleration of the surrounding flow. While the flame behavior with a thinner sample (<4.0 mm thickness in this study) at the limiting condition was found to be stationary, a gentle extinction was experienced as approached to the limit. This fact suggests that the key to leading extinction is different between thicker and thinner sample. Third, the temperature distribution over the 4.0 mm PMMA at the near-limiting condition was measured and a strategy to model/predict the limiting behavior is then proposed.
TL;DR: In this paper, a group of experiments is conducted to measure the flammability limit of polyethylene (PE) insulated wires under varying oxygen concentration and external radiation, and the results provide valuable information about the fire risk of electrical wires under variable oxygen concentration.
Abstract: 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.
TL;DR: In this article, the authors revisited the problem of opposed fire spread under limited and excessive oxygen supply and reviewed various near-limit fire phenomena, as recently observed in flaming, smoldering, and glowing spread under various environment and fuel configurations.
Abstract: Creeping fire spread under opposed airflow is a classic fundamental fire research problem involving heat transfer, fluid dynamics, chemical kinetics, and is strongly dependent on environmental factors. Persistent research over the last 50 years has established a solid framework for different fire-spread processes, but new fire phenomena and recent developments continue to challenge our current understanding and inspire future research areas. In this review, we revisit the problem of opposed fire spread under limited and excessive oxygen supply. Various near-limit fire phenomena, as recently observed in flaming, smoldering, and glowing spread under various environment and fuel configurations, are reviewed in detail. Particularly, aspects of apparent importance, such as transition phenomena and heterogenous chemistry, in near-limit fire spread are highlighted, and valuable problems for future research are suggested.
01 Jan 2019
TL;DR: In this article, the authors identify the transition from opposed flame spread to fuel regression under varying conditions, including sample size, opposed flow velocity, pressure, oxygen concentration, external radiation, and gravity level.
Abstract: The spread of flames over the surface of solid combustible material in an opposed flow is different from the mass burning (or fuel regression) in a pool fire. However, the progress of a flame front over a solid fuel includes both flame spread and fuel regression, but the difference between these two processes has not been well clarified. In this work, experiments using cylindrical PMMA samples were conducted in normal gravity and in microgravity. We aim to identify the transition from opposed flame spread to fuel regression under varying conditions, including sample size, opposed flow velocity, pressure, oxygen concentration, external radiation, and gravity level. For a thick rod in normal gravity, as the opposed flow increases to 50–100 cm/s, the flame can no longer spread over the fuel surface but stay in the recirculation zone downstream of the cylinder end surface, like a pool fire flame. The flame spread first transitions to fuel regression at a critical leading-edge regression angle of α ≈ 45°, and then, flame blow-off occurs. Under large opposed flow velocity, a stable flat blue flame is formed floating above the rod end surface, because of vortex shedding. In microgravity at a low opposed flow (
TL;DR: Photographic evidence identifies a flame-shedding process, most likely associated with continual sequential ignition of fuel vapor within a von Karman vortex street generated behind the falling burning drip, which is found to be a blue chain of flame.
Abstract: Dripping of molten fuels is a widely observed fire phenomenon, and, by igniting other fuels, it can promote fire spread and increase fire hazards. In this work, dripping phenomena from fires of horizontally oriented wires, coated with polyethylene (PE), are investigated in the laboratory. It is found that as long as a flame is attached to the drip, thin tissue paper can be ignited by a single drip. Below a minimum diameter (Dmin = 0.63 mm), the drip floats up. Above a critical diameter (Dcrt = 2.3 mm), a flame can remain attached to the drip and ignite tissue paper as it falls through a distance of at least 2.6 m, thereby posing a significant fire hazard. A falling burning drip appears to the eye to be a blue chain of flame as a result of persistence of vision. Photographic evidence identifies a flame-shedding process, most likely associated with continual sequential ignition of fuel vapor within a von Karman vortex street generated behind the falling burning drip. The frequency of flame shedding agrees with both the frequency of modeled vortex shedding and the frequency of the unexpected sound that is heard during the process. This is the first time that combustion characteristics of dripping fire phenomena have been studied in detail, and this helps to better evaluate the risk and hazards of wire and facade fires.
TL;DR: In this paper, flame-spread experiments on extruded poly(methyl)methacrylate (PMMA) rods with 10mm diameter were conducted in the SJ-10 Satellite.
Abstract: The enriched oxygen ambient may be applied to China’s next generation space station. To understand the fire behaviors under oxygen-enriched microgravity environment, flame-spread experiments on extruded poly(methyl)methacrylate (PMMA) rods with 10-mm diameter were conducted in the SJ-10 Satellite. The opposed flame-spread behaviors were studied at the oxygen-enriched ambient (33.5% and 49.4%) under low flow velocities in the range of 0 to 12 cm/s. After the ignition in the middle of the sample, an opposed flame spread was achieved, rather than the forward flame spread. The flame-spread rate increases with the opposed flow velocity, due to the decreased flame width and the enhanced flame heat flux. Moreover, a blue flame sheet with a frequent burst of bubbles is found throughout the opposed-flow spread process, showing a near extinction behavior. For the oxygen concentration above 25%, normal-gravity experiments suggest that whether PMMA is cast or extruded should have a negligible effect on the opposed flame spread in microgravity. Compared to normal gravity, the microgravity flame spread rate in the oxygen-enriched atmosphere is slower which is the order of 0.1 mm/s, only one-tenth to one-fifth of that in normal gravity at the same nominal opposed flow velocity, and the acceleration of flame spread in microgravity by increasing oxygen concentration is also much smaller. This result suggests that (1) if the environmental gas flow is small, the fire hazard increased by raising oxygen level in microgravity space cabin can be much smaller than that on Earth; and (2) the fire risk of oxygen-enriched microgravity environment might be overestimated when a ground-based test method is employed to evaluate the burning characteristics of solid material.