Koki Kizawa

Bio: Koki Kizawa is an academic researcher from Hokkaido University. The author has contributed to research in topics: Flame spread & Limiting oxygen concentration. The author has an hindex of 2, co-authored 2 publications receiving 45 citations.

Fire safety in space - Investigating flame spread interaction over wires

Abstract: A new rig for microgravity experiments was used for the study flame spread of parallel polyethylene-coated wires in concurrent and opposed airflow. The parabolic flight experiments were conducted at small length- and time scales, i.e. typically over 10 cm long samples for up to 20 s. For the first time, the influence of neighboring spread on the mass burning rate was assessed in microgravity. The observations are contrasted with the influence characterized in normal gravity. The experimental results are expected to deliver meaningful guidelines for future, planned experiments at a larger scale. Arising from the current results, the issue of the potential interaction among spreading flames also needs to be carefully investigated as this interaction plays a major role in realistic fire scenarios, and therefore on the design of the strategies that would allow the control of such a fire. Once buoyancy has been removed, the characteristic length and time scales of the different modes of heat and mass transfer are modified. For this reason, interaction among spreading flames may be revealed in microgravity, while it would not at normal gravity, or vice versa. Furthermore, the interaction may lead to an enhanced spread rate when mutual preheating dominates or, conversely, a reduced spread rate when oxidizer flow vitiation is predominant. In more general terms, the current study supports both the SAFFIRE and the FLARE projects, which are large projects with international scientific teams. First, material samples will be tested in a series of flight experiments (SAFFIRE 1-3) conducted in Cygnus vehicles after they have undocked from the ISS. These experiments will allow the study of ignition and possible flame spread in real spacecraft conditions, i.e. over real length scale samples within real time scales. Second, concomitant research conducted within the FLARE project is dedicated to the assessment of new standard tests for materials that a spacecraft can be composed of. Finally, these tests aim to define the ambient conditions that will mitigate and potentially prohibit the flame spread in microgravity over the material studied.

Experimental Study on Near-Limiting Burning Behavior of Thermoplastic Materials with Various Thicknesses Under Candle-Like Burning Configuration

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.

A Review of Fundamental Combustion Phenomena in Wire Fires

Abstract: Electrical wires and cables have been identified as a potential source of fire in residential buildings, nuclear power plants, aircraft, and spacecraft. This work reviews the recent understandings of the fundamental combustion processes in wire fire over the last three decades. Based on experimental studies using ideal laboratory wires, physical-based theories are proposed to describe the unique wire fire phenomena. The review emphasizes the complex role of the metallic core in the ignition, flame spread, burning, and extinction of wire fire. Moreover, the influence of wire configurations and environmental conditions, such as pressure, oxygen level, and gravity, on wire-fire behaviors are discussed in detail. Finally, the challenges and problems in both the fundamental research, using laboratory wires and numerical simulations, and the applied research, using commercial cables and empirical function approaches, are thoroughly discussed to guide future wire fire research and the design of fire-safe wire and cables.

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

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.

Transition from opposed flame spread to fuel regression and blow off: Effect of flow, atmosphere, and microgravity

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 (

Buoyancy Effects on Concurrent Flame Spread Over Thick PMMA

Abstract: The flammability of combustible materials in a spacecraft is important for fire safety applications because the conditions in spacecraft environments differ from those on earth. Experimental testing in space is difficult and expensive. However, reducing buoyancy by decreasing ambient pressure is a possible approach to simulate on-earth the burning behavior inside spacecraft environments. The objective of this work is to determine that possibility by studying the effect of pressure on concurrent flame spread, and by comparison with microgravity data, observe up to what point low-pressure can be used to replicate flame spread characteristics observed in microgravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs. Experiments in normal gravity were conducted over pressures ranging between 100 and 40 kPa and a forced flow velocity of 200 mm/s. Microgravity experiments were conducted during NASA’s Spacecraft Fire Experiment (Saffire II), on board the Cygnus spacecraft at 100 kPa with an air flow velocity of 200 mm/s. Results show that reductions of pressure slow down the flame spread over the PMMA surface approaching that in microgravity. The data is correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid, and the primary mechanism controlling the spread of the flame. The extrapolation of the correlation to low pressures predicts well the flame spread rate obtained in microgravity in the Saffire II experiments. Similar results were obtained by the authors with similar experiments with a thin composite cotton/fiberglass fabric (published elsewhere). Both results suggest that reduced pressure can be used to approximately replicate flame behavior of untested gravity conditions for the burning of thick and thin solids. This work could provide guidance for potential ground-based testing for fire safety design in spacecraft and space habitats.

Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity

Abstract: Concurrent flame spread over electric wire insulation was studied experimentally in microgravity conditions during parabolic flights. Polyethylene insulated Nickel-Chrome wires and Copper wires were examined for external flow velocities ranging from 50 mm/s to 200 mm/s. The experimental results showed that steady state flame spread over wire insulation in microgravity could be achieved, even for concurrent flow. A theoretical analysis on the balance of heat supply from the flame to the unburned region, radiation heat loss from the surface to the ambient and required energy to sustain the flame propagation was carried out to explain the presence of steady spread over insulated wire under concurrent flow. Based on the theory, the change in heat input (defined by the balance between heat supply from flame and radiation heat loss) was drawn as a function of the flame spread rate. The curve intersected the linear line of the required energy to sustain the flame. This balance point evidences the existence of steady propagation in concurrent flow. Moreover, the estimated steady spread rate (1.2 mm/s) was consistent with the experimental result by considering the ratio of the actual flame length to the theoretical to be 0.5. Further experimental results showed that the concurrent flame spread rate increased with the external flow velocity. In addition, the steady spread rate was found to be faster for Copper wires than for Nickel-Chrome wires. The experimental results for upward spreading (concurrent spreading) in normal gravity were compared with the microgravity results. In normal gravity, the flame did not reach a steady state within the investigated parameter range. This is due to the fact that the fairly large flame spread rate prevented the aforementioned heat balance to be reached, which meant that such a spread rate could not be attained within the length of the tested sample.