Effects of radiative and diffusive transport processes on premixed flames near flammability limits (original) (raw)
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Flame spread over thin fuels in actual and simulated microgravity conditions
Combustion and Flame, 2009
Most previous research on flame spread over solid surfaces has involved flames in open areas. In this study, the flame spreads in a narrow gap, as occurs in fires behind walls or inside electronic equipment. This geometry leads to interesting flame behaviors not typically seen in open flame spread, and also reproduces some of the conditions experienced by microgravity flames. Two sets of experiments are described, one involving flame spread in a Narrow Channel Apparatus (NCA) in normal gravity, and the others taking place in actual microgravity. Three primary variables are considered: flow velocity, oxygen concentration, and gap size (or effect of heat loss). When the oxidizer flow is reduced at either gravity level, the initially uniform flame front becomes corrugated and breaks into separate flamelets. This breakup behavior allows the flame to keep propagating below standard extinction limits by increasing the oxidizer transport to the flame, but has not been observed in other microgravity experiments due to the narrow samples employed. Breakup cannot be studied in typical (i.e., "open") normal gravity test facilities due to buoyancy-induced opposed flow velocities that are larger than the forced velocities in the flamelet regime. Flammability maps are constructed that delineate the uniform regime, the flamelet regime, and extinction limits for thin cellulose samples. Good agreement is found between flame and flamelet spread rate and flamelet size between the two facilities. Supporting calculations using FLUENT suggest that for small gaps buoyancy is suppressed and exerts a negligible influence on the flow pattern for inlet velocities 5 cm/s. The experiments show that in normal gravity the flamelets are a fire hazard since they can persist in small gaps where they are hard to detect. The results also indicate that the NCA quantitatively captures the essential features of the microgravity tests for thin fuels in opposed flow.
42nd International Conference on Environmental Systems, 2012
NASA's current flammability testing method for non-metallic solids is NASA-STD-(I)-6001A Test 1. Materials that allow for an upward flame propagation of six inches or more fail the flammability test. The flames in the Earth-based Test 1 are dominated by the upward-flowing buoyant gases, and this is not representative of actual flame behavior in microgravity, where there are no buoyant effects on flames. Scientists at NASA have shown that by spatially confining a horizontally spreading flame in the vertical direction, buoyant forces can be minimized in an Earth-based flame spread test. Results from flammability tests conducted in San Diego State University's Narrow Channel Apparatus (SDSU NCA) closely match results from NASA's Narrow Channel Apparatus at 1 atmosphere of pressure and 21% oxygen. The advantage of the SDSU NCA is that it not only minimizes buoyant effects, but it also allows flammability tests to be performed at normoxic equivalent atmospheres that more closely match future spacecraft cabin atmospheres. Normoxic conditions are achieved in the SDSU NCA by varying the total pressure, opposed flow oxidizer velocity, and oxygen concentration in the test channel.
Enclosure effects on flame spread over solid fuels in microgravity‡
Combustion and Flame, 2002
Enclosure effects on the transition from a localized ignition to subsequent flame growth over a thermally thin solid fuel in microgravity are numerically investigated by solving the low Mach number time-dependent Navier-Stokes equations. The numerical model solves the two and three dimensional, time-dependent, convective/diffusive mass, and heat transport equations with a one-step global oxidation reaction in the gas phase coupled to a three-step global pyrolysis/oxidative reaction system in the solid phase. Cellulosic paper is used as the solid fuel and is placed in a slow imposed flow parallel to the surface. Ignition is initiated across the width of the sample or at a small circular area by an external thermal radiation source. Two cases are examined; an open configuration (i.e., without any enclosure) and the case with the test chamber used in our previous microgravity experiments. Numerical results show that the upstream centerline flame spread rate for the case with the enclosure is faster than that for the case without any enclosure. This is due to the confinement of the flow field and also thermal expansion initiated by heat and mass addition in the chamber. The confinement accelerates the flow in the chamber, which enhances oxygen transport into the flame. In the three-dimensional configuration with the spot ignition, the flame growth in the direction perpendicular to the flow is significantly enhanced by the confinement effects. The effect of the enclosure is most significant at the slowest flow condition investigated and the effect becomes less important with an increase in imposed flow velocity. The total heat release rate from the flame during a flame growth period increases significantly with the confinement and the enclosure effects should be accounted to avoid underestimating fire hazard in a spacecraft.
Localized Ignition And Subsequent Flame Spread Over Solid Fuels In Microgravity
Localized ignition is initiated by an external radiant source at the middle of a thin solid sheet under external slow flow, simulating fire initiation in a spacecraft with a slow ventilation flow. Ignition behavior, subsequent transition simultaneously to upstream and downstream flame spread, and flame growth behavior are studied theoretically and experimentally. There are two transition stages in this study; one is the first transition from the onset of the ignition to form an initial anchored flame close to the sample surface, near the ignited area. The second transition is the flame growth stage from the anchored flame to a steady fire spread state (i.e. no change in flame size or in heat release rate) or a quasi-steady state, if either exists. Observations of experimental spot ignition characteristics and of the second transition over a thermally thin paper were made to determine the effects of external flow velocity. Both transitions have been studied theoretically to determine...
Emulation of condensed fuel flames with gases in microgravity
Combustion and Flame, 2015
A gaseous fuel burner has been designed to emulate the burning behavior of liquids and solids. The burner is hypothesized to represent a liquid or solid fuel through four key properties: heat of combustion, heat of gasification, vaporization temperature, and laminar smoke point. Previous work supports this concept, and it has been demonstrated for four real fuels. The technique is applied to flames during 5 s of microgravity. Tests were conducted with a burner of 25 mm diameter, two gaseous fuels, and a range of flow rates, oxygen concentrations, and pressures. The microgravity tests reveal a condition appearing to approach a steady state but sometimes with apparent local extinction. The flame typically retains a hemispherical shape, with some indication of slowing growth, and nearly asymptotic steady flame heat flux. A one-dimensional steady-state theory reasonably correlates the data for flame heat flux and flame length. The burning rate per unit area is found to be inversely dependent on diameter and a function of the ratio of the ambient oxygen mass fraction to the heat of gasification. The flame length to diameter ratio depends on two dimensionless parameters: Spalding B number and the ratio of the heat of combustion per unit mass of ambient oxygen to the heat of combustion of the fuel mixture stream.
Characteristics of Gaseous Diffusion Flames with High Temperature Combustion Air in Microgravity
41st Aerospace Sciences Meeting and Exhibit, 2003
The characteristics of gaseous diffusion flames have been obtained using high temperature combustion air under microgravity conditions. The time resolved flame images under free fall microgravity conditions were obtained from the video images obtained. The tests results reported here were conducted using propane as the fuel and about 1000 o C combustion air. The burner included a 0.686 mm diameter central fuel jet injected into the surrounding high temperature combustion air. The fuel jet exit Reynolds number was 63. Several measurements were taken at different air preheats and fuel jet exit Reynolds number. The resulting hybrid color flame was found to be blue at the base of the flame followed by a yellow color flame. The length and width of flame during the entire free fall conditions has been examined. Also the relative flame length and width for blue and yellow portion of the flame has been examined under microgravity conditions. The results show that the flame length decreases and width increases with high air preheats in microgravity condition. In microgravity conditions the flame length is larger with normal temperature combustion air than high temperature air.
Proceedings of the Combustion Institute, 2009
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. Published by Elsevier Inc. on behalf of The Combustion Institute.
Symposium (International) on Combustion, 1998
Premixed gas flames in mixtures of CH 4 , O 2 , N 2 and CO 2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively-induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically-thin radiation models, particularly when CO 2 , a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H 2 O and CO 2 , most of the radiation from product H 2 O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO 2 is broader at flame temperatures than ambient temperature, thus some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO 2 . Via both mechanisms some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison to experiment demonstrates the importance of reabsorption in CO 2 -diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emissionabsorption spectra of the reactant and product gases and their temperature dependence, and cannot be predicted using gray-gas or optically-thin model parameters. Applications to practical flames at high pressure, in large combustion chambers and with exhaust-gas or flue-gas recirculation are discussed.