Takashi Kashiwagi

Experimental observations of spot radiative ignition and subsequent three-dimensional flame spread over thin cellulose fuels

Spontaneous radiative ignition and transition to flame spread over thin cellulose fuel samples was studied aboard the USMP-3 STS-75 Space Shuttle mission, and in three test series in the 10 second Japan Microgravity Center (JAMIC). A focused beam from a tungsten/halogen lamp was used to ignite the center of the fuel sample while an external air flow was varied from 0 to 10 cm/s. Non-piloted radiative ignition of the paper was found to occur more easily in microgravity than in normal gravity. Ignition of the sample was achieved under all conditions studied (shuttle cabin air, 21%-50% O2 in JAMIC), with transition to flame spread occurring for all but the lowest oxygen and flow conditions. While radiative ignition in a quiescent atmosphere was achieved, the flame quickly extinguished in air. The ignition delay time was proportional to the gas-phase mixing time, which is estimated using the inverse flow rate. The ignition delay was a much stronger function of flow at lower oxygen concentrations. After ignition, the flame initially spread only upstream, in a fan-shaped pattern. The fan angle increased with increasing external flow and oxygen concentration from zero angle (tunneling flame spread) at the limiting 0.5 cm/s external air flow, to 90 degrees (semicircular flame spread) for external flows at and above 5 cm/s, and higher oxygen concentrations. The fan angle was shown to be directly related to the limiting air flow velocity. Despite the convective heating from the upstream flame, the downstream flame was inhibited due to the oxygen shadow of the upstream flame for the air flow conditions studied. Downstream flame spread rates in air, measured after upstream flame spread was complete and extinguished, were slower than upstream flame spread rates at the same flow. The quench regime for the transition to flame spread was skewed toward the downstream, due to the augmenting role of diffusion for opposed flow flame spread, versus the canceling effect of diffusion at very low cocurrent flows.

Effects of slow wind on localied radiative ignition and transition to flame spread in microgravity

An experimental and numrical investigation of ignition and the subsequent transition to flame spread over a thermally thin cellulosic sample is described. The experiments were conducted using a lamp as an external radiant source in a 50% oxygen atmosphere at three diffeirent wind velocities of 0.2, and 5 cm/s in a 10 s drop tower. The results show that there are no significants effects of the slow wind on the ignition-delay time. Photographic sequences of both the experiments and the calculations show that the wind increases the flame propagation speed in the upwind direction. while decreasing it in the downstream direction. The downstream clame fails the transition to flame spread and becomes a tail of the upstream flame. The downstream char front propagates much slower than that for the upstream direction. Three-dimensional, time-dependent numerical solutions to the Navier-Stokes equations are used to simulate the experiments. Three global degradation reactions describe the pyrolysis of the sample paper, and one gasphase reaction describes the combustion of the fuel gases. The model results reflect the qualitative features of the experiments and also are in reasonable quantitative agreement, give the uncertainty of the gasphase reaction mechanism.

Effects of ignition and wind on the transition to flame spread in a microgravity environment

Abstract A two-dimensional, time-dependent model is developed describing ignition and the subsequent transition to flame spread over a thermally thin cellulosic sheet heated by external radiation in a microgravity environment. The effects of a slow external wind (0–5 cm/s), and of the flux distribution of the external radiation on the transition are studied mainly in an atmosphere of 30% oxygen concentration. The ignition is initiated along the width of a sample strip, giving rise initially to two flame fronts spreading in opposite directions. The calculated results are compared with data obtained in the 2.2-s drop tower. Both experimental and calculated results show that with a slow, imposed wind, the upstream flame front (opposed mode) is stronger and slightly faster than the quiescent counterpart due to a greater supply of oxygen. However, the downstream flame front (concurrent mode) tends to die during the transition period. For all calculated cases studied in this work using the selected kinetic constants for the global one-step gas phase reaction, the downstream flame front dies out in oxygen concentrations up to 50% and wind velocity up to 5 cm/s. This is caused by the “oxygen shadow” cast by the upstream flame. The ignition delay time depends mainly on the peak flux of external radiation, whereas the transition time to steady state flame spread depends mainly on the broadness of the flux distribution. The broader the radiative flux distribution, the greater the transient flame spread rate due to the preheating of the sample ahead of the flame front by the external radiation and thus the greater the delay to steady state flame spread.

High Throughput Methods for Polymer Nanocomposites Research: Extrusion, NMR Characterization and Flammability Property Screening

A large number of parameters influence polymer-nanocomposite performance and developing a detailed understanding of these materials involves investigation of a large volume of the associated multi-dimensional property space. This multi-dimensional parameter space for polymer-nanocomposites consists of the obvious list of different material types under consideration, such as “polymer” and “nano-additive,” but also includes interphase surface chemistry, and processing conditions. This article presents combinatorial library design and high-throughput screening methods for polymer nanocomposites intended as flame-resistant materials. Here, we present the results of using a twin-screwn extruder to create composition-gradient library strips of polymer nanocomposites that are screened with a solid-state NMR method to rapidly evaluate the optimal processing conditions for achieving nanocomposite dispersion. In addition, we present a comparison of a new rapid Cone calorimetry method to conventional Cone calorimetry and to the gradient heat-flux flame spread method.

FINGER-LIKE SMOLDERING OVER THIN CELLULOSIC SHEETS IN MICROGRAVITY

Microgravity smolder spread over a thin cellulosic fuel was studied with the Radiative Ignition and Transition to Spread Investigation (RITSI) apparatus in the Glovebox Facility on the STS-75 USMP-3 space shuttle mission. Radiative smoldering ignition was initiated by a focused beam from a tungsten/halogen lamp at the center of the smolder-promoted filter paper. The external airflow velocity was varied from 0.5 cm/s to 6.5 cm/s. The ignition and subsequent smolder spread events were recorded by a video camera, a 35-mm camera, and six thermocouples (two in the gas phase and four in the sample). Nonpiloted smoldering ignition of the paper in microgravity by external thermal radiation was demonstrated for the first time. Unlike the uniform normal gravity smolder front, a complex, unexpected finger-shaped char growth pattern was observed in microgravity. The preferred direction of smolder finger propagation was upstream into the fresh oxidizer. Downstream smolder was less viable and slower. Increasing external flow velocity increased the number of localized smoldering fronts, the number of the char fingers they left behind, and the frequency of bifurcations from the fingers. An analytical “oxygen shadow” model indicated that each localized smolder front cast an oxygen shadow that depleted the ambient oxygen in an egg-shaped region around itself. These oxygen shadows are a plausible explanation of the fingering smolder patterns observed in the experiments.

Flame Spread Along Free Edges of Thermally Thin Samples in Microgravity

The effects of imposed flow velocity on flame spread along open edges of a thermally thin cellulosic sample in microgravity were studied experimentally and theoretically. In this study, the sample was ignited locally at the middle of the 4 cm wide sample, and subsequent flame spread reached both open edges of the sample along the direction of the flow. The following flame behaviors were observed in the experiments and predicted by the numerical calculation, in order of increased imposed flow velocity: (1) ignition but subsequent flame spread was not attained, (2) flame spread upstream (opposed mode) without any downstream flame, and (3) the upstream flame and two separate downstream flames traveled along the two open edges (concurrent mode). Generally, the upstream and downstream edge flame spread rates were faster than the central flame spread rate for an imposed flow velocity of up to 5 cm/s. This was due to greater oxygen supply from the outer free stream to the edge flames and more efficient heat transfer from the edge flames to the sample surface than the central flames. For the upstream edge flame, flame spread rate was nearly independent of, or decreased gradually with, the imposed flow velocity. The spread rate of the downstream edge, however, increased significantly with the imposed flow velocity.

Combustion Behaviour Over ETFE Insulated Wire in Slow External Flow under Microgravity

Opposed flame spreading over ETFE (ethylene-tetrafluoroethylene co-polymer) insulated wire, which is used for actual wire in space, in low air flow velocity has been investigated under microgravity environment. The experiments were performed with Japan Microgravity Center (JAMIC) 10 s dropshaft and NASAs KC 135 parabolic flight. Experiments were performed with changing O2 concentration, 35 and 40%, external air flow velocity, 0 (quiescent) -22 cm/s and dilution gas, N2, CO2, He. The sample of 0.32 mm inner core diameter with 0.15 mm insulation thickness was used. The results showed that flame spread rate over wire insulation was strongly affected by air flow velocity and the dilution gases. Flame spread with N2 dilution had maximum value in a low flow velocity region. However, CO2 dilution resulted in monotonic increase in flame spread rate with decrease in air flow velocity and the maximum flame spread at Ve=0 cm/s (quiescent). The different dependency of the spread rate on flow velocity with different dilution gas was explained by the reabsorption effect of CO2 gas ahead of flame front. Reabsorption of radiation heat with CO2 gas recovers the radiation heat from the flame and prevent the flame temperature decrease even in very low flow velocity region, which is known as radiation control region for optically thin gas.

Three-Dimensional Kinetic Model for the Swelling of Intumescent Materials (NISTIR 5499)

This paper presents the first measurements of the burning rate of premixed flames inhibited by three fluorinated hydrocarbons who’s chemistry is similar to agents which may he used as replacements for CF3Br. Measurements were made of the reduction in the burning rate of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner. The burning rate was determined with the total area method from Schlieren images of the flame. The inhibitors were tested over a range of concentrations and fuel-air equivalence ratios. The measured burning rate reductions are compared with those predicted by numerical solution of the species and energy conservation equations employing a detailed chemical kinetic mechanism recently developed at the National Institute of Standards and Technology (NIST). This paper presents initial efforts at testing and validation of the mechanism using burning rate data. The mode of inhibition of these chemicals is inferred through interpretation of the numerical results.