Flame spread along free edges of thermally thin samples in microgravity (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
Opposed flow flammability experiments are conducted for the first time using a planar Couette Flow Apparatus (CFA). The CFA produces a linear velocity profile above the fuel to mimic in normal gravity the boundary layer encountered by a flame in an actual microgravity fire. Similar apparatuses (e.g., the Narrow Channel Apparatus) have been used to test materials using a parabolic velocity profile, but the purpose of this research is to determine whether or not using a linear velocity profile above the fuel surface will produce different results in the flame spread rate or flame appearance. The apparatus is a 1.1 meter long channel, 8 centimeters wide, and has a height adjustable 0 to 4 cm. The channel consists of a fixed bottom plate, two glass side walls, a moving belt at the top, and a fan at the outlet to help pull air in the inlet to initiate the flow. Images and videos were recorded through windows in the channel to show flame behavior. Air flow in the channel was characterized using hot wire anemometers, and flame speed was determined using image tracking software. The belt velocity, gap height, and velocity gradient were varied to study their effect on flame spread. Comparisons are made to flame spread rate in the Narrow Channel Apparatus with the same average velocity. The results for thin cellulosic fuel indicate that the flame spread rate is affected by gap height and may also be affected by velocity gradient.
Proceedings of the Combustion Institute, 2019
Concurrent-flow flame spread over a thin charring material was studied in an unmanned spacecraft. Two sample sizes were used: 5 cm by 29 cm and 41 cm by 94 cm, the largest ever samples burned in controlled experiments in microgravity. A low-speed ambient airflow of 20 cm/s was used. The samples were ignited from their upstream ends and were allowed to burn for several minutes. Video recorded during the burning process and readings from thermocouples on the sample surfaces were examined. The results showed that flames reached a quasi-steady state with nearly constant flame length for both sample sizes. However, the pyrolysis length exhibited an initial overshoot before reaching steady state for the large sample. This phenomenon has not been reported or observed until now. While steady state pyrolysis lengths were similar, the 5 cm wide sample had a slightly larger spread rate (by ∼13%) and a shorter burnout time (pyrolysis length over spread rate) compared to the 41 cm wide sample. A previously developed three-dimensional transient model was used to conduct the numerical study. Detailed profiles of the gas and solid phases, including flow patterns, species concentrations, temperature, solid burning rate, and heat flux distributions are examined. The modeling results reveal that flow reduction in a growing boundary layer along the sample surface accounts for the overshoot of the flame length observed for the large sample. Compared to a wide sample, a narrow sample has more effective oxygen transport across the width of the sample. This results in a stronger flame, a shorter flame standoff distance from the sample surface, and larger heat feedback to the sample. This accounts for the shorter burnout time for the narrow sample compared to the large sample.
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...
Flame spread: Effects of microgravity and scale
Combustion and Flame, 2019
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 cottonfiberglass 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.
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.
Effect of wind velocity on flame spread in microgravity
Proceedings of the …, 2002
his report is a preprint of an article submitted to journal for publication. Because of changes that l ay be made before formal p ublication, th is. reprint is made avail able with th e unde rstanding lat it will not be cited or reprod uced without the ermission of the author.
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.