Experimental comparison of opposed and concurrent flame spread in a forced convective microgravity environment
TL;DR: In this article, a 5.18-s drop tower with a thin cellulose fuel was used to investigate flame spread in both concurrent and opposed flow in a spacecraft, with a focus on pressure/oxygen combinations that result in earth-equivalent oxygen partial pressures (normoxic conditions).
Abstract: Flame spread experiments in both concurrent and opposed flow have been carried out in a 5.18-s drop tower with a thin cellulose fuel. Flame spread rate and flame length have been measured over a range of 0–30 cm/s forced flow (in both directions), 3.6–14.7 psia, and oxygen mole fractions 0.24–0.85 in nitrogen. Results are presented for each of the three variables independently to elucidate their individual effects, with special emphasis on pressure/oxygen combinations that result in earth-equivalent oxygen partial pressures (normoxic conditions). Correlations using all three variables combined into a single parameter to predict flame spread rate are presented. The correlations are used to demonstrate that opposed flow flames in typical spacecraft ventilation flows (5–20 cm/s) spread faster than concurrent flow flames under otherwise similar conditions (pressure, oxygen concentration) in nearly all spacecraft atmospheres. This indicates that in the event of an actual fire aboard a spacecraft, the fire is likely to grow most quickly in the opposed mode as the upstream flame spreads faster and the downstream flame is inhibited by the vitiated atmosphere produced by the upstream flame. Additionally, an interesting phenomenon was observed at intermediate values of concurrent forced flow velocity where flow/flame interactions produced a recirculation downstream of the flame, which allowed an opposed flow leading edge to form there.
••01 Jan 2015
TL;DR: In this paper, the authors introduce fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed.
Abstract: This paper introduces fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat. Two types of existing standards include material evaluation by pass/fail criteria corresponding to Test 1 of NASA STD 6001B and evaluation by a flammability index such as maximum oxygen concentration (MOC) corresponding to the improved Test 1. The advantage of the latter is the wide applicability of the MOC index to different atmospheres in spacecraft. Additionally, the limiting oxygen index (LOI) method is introduced as a potential alternative index for the evaluation using the improved Test 1 method. When criteria based on an index such as MOC or LOI are applied for material screening, the discrepancy of the index to the actual flammability limit in microgravity such as minimum limiting oxygen concentration (MLOC) is essential information for guaranteeing fire safety in space because material flammability can be higher in microgravity. In this paper, the existing research on the effects of significant parameters on material flammability in microgravity are introduced, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed. Finally, on-going efforts to develop estimation methods of material flammability in microgravity according to normal gravity tests are summarized.
Glenn Research Center1, Case Western Reserve University2, University of California, Berkeley3, University of Maryland, College Park4, University of Paris5, University of Bremen6, Moscow State University7, Hokkaido University8, European Space Research and Technology Centre9, University of Edinburgh10
TL;DR: In this paper, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth.
Abstract: For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called “Saffire,” studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed.
TL;DR: In this article, a series of concurrent-flow rod flammability tests were conducted in microgravity aboard the International Space Station (ISS), where a small flow duct was used to create 0 to 55 cm/s flows past three sizes of clear and black PMMA rods.
Abstract: For the first time, a series of concurrent-flow rod flammability tests were conducted in microgravity aboard the International Space Station. A small flow duct was used to create 0 to 55 cm/s flows past three sizes of clear and black PMMA rods. The ambient oxygen concentration in the Microgravity Science Glovebox was varied from 13.6% to 22.2%. Oxygen, carbon dioxide, and carbon monoxide gas sensors provide initial and final readings for each test and indicate that the flames are globally stoichiometric at higher oxygen concentrations, but become more globally fuel rich as the minimum oxygen concentration is approached due to excess pyrolyzate leakage out of the open tail of the hemispherical flames. Quenching extinction occurs at very low forced flows, where the flame shrinks to a hemispherical blue flame and oscillates with increasing amplitude just before going out. Blowoff extinction is initiated by the formation of a hole in the flame sheet in the stagnation region of the flame. A critical Damkohler number formulation is applied across the flammability boundary, and the critical flame temperatures are derived. These critical flame temperatures are then used in a Nusselt number correlation to estimate the convective heat flux to the stagnation region of the rod. A model of surface energy balance is formulated that uses the critical flame temperature and convective heat flux to derive the mass burning rate along the boundary. The rod regression rates calculated from this model compare favorably with the experimental measurements. The surface energy balance reveals that along the blowoff branch, heat losses are negligible whereas in the quenching region, surface radiative loss dominates. At the bottom of the flammability map, the transition from blowoff to quenching occurs when the convective flows become the same order of magnitude as diffusive flows, shifting the critical Damkohler number from residence time limitations to diffusive time limitations.
••01 Jan 2015
TL;DR: In this paper, the normal gravity (1 g) and microgravity (lg) flame spread limits (LOC) of ETFE insulated copper wires exposed to an external radiant flux were studied.
Abstract: The present work studied the normal gravity (1 g) and microgravity (lg) flame spread limits (LOC) of ETFE insulated copper wires exposed to an external radiant flux. Experiments with sample wires of a 0.50 mm copper core and 0.30 mm ETFE insulation thickness were conducted in oxygen concentrations ranging from 20% to 32% and external radiant fluxes from 0 to 25 kW/m. Microgravity experiments conducted in parabolic flights showed that lg reduced the Limiting Oxygen Index of the material. The addition of an external radiant flux further extends the Limiting Oxygen Concentration (LOC) for flame spread over ETFE insulated wires. Microgravity reduced heat losses and allowed the flame to propagate in lower oxygen concentrations. The addition of an external radiant flux further compensates for lower flame temperatures in reduced oxygen concentrations and further extends the LOC of the material. Limiting Oxygen Index (LOI) results obtained with ETFE were also compared to available results with PE and show that lg conditions have a larger impact in ETFE than PE. The results of this work are relevant given that the flammability of materials is routinely tested without considering the effects of environmental variables and according to the results presented in here may not be indicative of the absolute flammability limits.
TL;DR: In this paper, the effects of sample width and altitude on the burning characteristics of wood were explored at two different altitudes with varying widths (W) of thin wood sheets.
Abstract: To explore the effects of altitude and sample width on the burning characteristics of wood, a series of experiments are carried out at two different altitudes with varying widths (W) of thin wood sheets. Flame size and flame spread rate are measured over a range of sample widths from 2 cm to 12 cm. At both altitudes, the width effects on both the dimensionless flame height (Hf/W) and the spread rate are analyzed. The dimensionless flame heights at both altitudes show negative power law relationships with the sample width, and the decline at a low altitude (50 m, Hefei) is much smaller than that at a high altitude (3658 m, Lhasa). The spread rate curves at both altitudes contain a turning point: Before the point, the spread rate decreases with sample width, and after it, the rate increases. Furthermore, heat transfer theory was applied to explain the altitude and sample width effects on the flame spread rate, and the experimental results agree well with the theoretical analysis.
01 Jan 1995
TL;DR: In this article, G. Cox the sold phase, C. Fernandez-Pello bouyant fire plumes, E.E. Zukoski turbulent diffusion flames, J.B. Moss the growth of fire, P.H. Thomas compartment fire modelling, G Cox fire chemistry, R.F. Simmons.
Abstract: Some fundamentals, G. Cox the sold phase, C. Fernandez-Pello bouyant fire plumes, E.E. Zukoski turbulent diffusion flames, J.B. Moss the growth of fire, P.H. Thomas compartment fire modelling, G. Cox fire chemistry, R.F. Simmons.
TL;DR: In this article, a flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process: near-quenching region, very low characteristic relative velocities, a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed.
Abstract: Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be described by a thermal theory, is controlled by gas-phase conduction.
TL;DR: In this article, a numerical model was developed to examine steady, laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flow, incorporating an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point, and a parabolic treatment of downstream flame, which has a higher flow Reynolds number.
Abstract: A numerical model is developed to examine steady, laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flow. The model incorporates an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point, and a parabolic treatment of the downstream flame, which has a higher flow Reynolds number. This provides a more precise fluid-mechanical description of the flame than using parabolic equations throughout, and is the first time such an approach has been used in concurrent flame spread modeling. The parabolic and elliptic regions are coupled smoothly by matching boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Calculations (with the flame spread rate being an eigenvalue) are performed for forced flow without gravitational influences in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Steady spread with constant flame and...
TL;DR: In this article, 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.
Abstract: Non-piloted 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% O 2 in JAMIC), with transition to flame spread occurring for all but the lowest oxygen and flow conditions. Although 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 by 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. A surface energy balance reveals that the heat feedback rate from the upstream flame to the surface decreases with decreasing oxygen mass transport via either imposed flow velocity or ambient oxygen concentration. Quenching extinction occurs when the heat feedback rate from the flame is no longer sufficient to offset the ongoing surface radiative heat losses. 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, because of the augmenting role of diffusion for opposed flow flame spread, versus the canceling effect of diffusion at very low cocurrent flows.
TL;DR: In this article, the results from experiments on flame spread over a thin cellulosic fuel in a quiescent, microgravity environment of a 50/50 volumetric mixture of oxygen and nitrogen (oxygen mass fraction 0.53) at three different pressures-101, 152, and 203 kPa (1, 1.5, and 2.0 atm).
Abstract: Results from recently conducted experiments on flame spread over a thin cellulosic fuel in a quiescent, microgravity environment of a 50/50 volumetric mixture of oxygen and nitrogen (oxygen mass fraction 0.53) at three different pressures-101, 152, and 203 kPa (1, 1.5, and 2.0 atm)-are analyzed. The results are compared with established theoretical results and two different computational flame spread models : one that includes gas-phase radiation, and one that does not. The spread rate behavior from experiment, i.e., an increase of spread rate with pressure, is consistent with the theoretical model that includes gas-phase radiation, and side-view photographs of the flames compare favorably with two-dimensional temperature contours produced computationally from the same model. In contrast, neither the dependence of spread rate on pressure nor the flame shape can be predicted with favorable comparison to experiment if radiation is neglected.
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