Browsing by Author "Olson, Sandra"
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Item Analysis of Saffire II two-sided concurrent flame spread over a thick PMMA slab(48th International Conference on Environmental Systems, 2018-07-08) Olson, Sandra; Urban, David; Ruff, Gary; Ferkul, Paul; Toth, Balazs; Eigenbrod, Christian; Meyer, FlorianThis paper reports the results of the microgravity flame spread over a 10 mm flat slab sample and a 4-10 mm structured sample. The samples were 50 mm wide and 290 mm long. They were ignited for 30 seconds in a constant air flow rate of 200 mm/s at one atmosphere pressure. The flame over the structured sample grew faster and was much more luminous than the flat sample. Interestingly, it was also quite non-symmetric on the two sides of the fuel card based on the radiometer readings. This non-symmetry affected the flow in the duct, implying the flame significantly affected the local flow field even in microgravity. The total radiometric output increased for the entire test time, indicating that the overall flame strength was increasing. Both flames over the samples grew to be 80-100 mm long, with soot streaming out the tip of the flames. Compared to normal gravity tests, the microgravity pyrolysis front spread rates were 18 to 24 times slower, but the fuel burnout rates were only 1.6 to 3 times slower. This resulted in a significant difference in overall flame size, with the microgravity flames being much smaller. Vapor jetting of bubbles of MMA monomer rupturing at the surface caused violent perturbations in the flame that got progressively worse as the test progressed. Both tests set off the vehicle smoke detector at levels well above the background reading of the sensor. The highest smoke detector reading occurred after the flow was turned on at the start of the flat sample test, as the residual smoke in the flow duct was flushed out. This may be due to soot agglomeration and/or the generation of a cloud of condensed fuel vapor.Item Fire Safety Implications of Preliminary Results from Saffire IV and V Experiments on Large Scale Spacecraft Fires(50th International Conference on Environmental Systems, 7/12/2021) Urban, David; Ruff, Gary; Ferkul, Paul; Easton, John; Owens, Jay; Olson, Sandra; Meyer, Marit; Fortenberry, Claire; Brooker, John; Graf, John; Casteel, Michael; Jomaas, Grunde; Toth, Balazs; Eigenbrod, Christian; T'Ien, James; Liao, Ya-Ting; Fernandez-Pello, Carlos; Meyer, Florian; Legros, Guillaume; Guibaud, Augustin; Smirnov, Nikolay; Fujita, OsamuThe spread and growth of flames over large solid fuel samples and their effect on the pressurized spacecraft were studied inside Cygnus spacecraft while in orbit after departing the International Space Station. These experiments were developed by NASA�s Advanced Exploration Systems Division in the Human Exploration and Operations Mission Directorate. The ignited materials consisted of poly-methyl methacrylate (PMMA), cotton fabric and a cotton/fiberglass fabric blend. The samples were all 40 cm wide and with various lengths ranging from 18 cm for the PMMA samples to 50 cm for the fabrics. The overall results from these tests and their impact on the spacecraft are presented with emphasis on the fire safety implications of the results. The experiments included, a post-fire cleanup system, vehicle internal volume measurements, and transport of acid gases (HCl and HF). Measurements included video images, flame spread rate, flame temperatures and radiant heat output; energy release through oxygen calorimetry; distributed measurements of CO2 concentration and temperature at six locations in the spacecraft; CO2, CO, O2, HF and HCl concentrations; vehicle pressurized volume; and aerosol concentrations. Details of the flame growth and spread are discussed in other papers as are details of the post-fire cleanup system performance. The fire events had a measurable impact on the vehicle pressure, temperature, and carbon dioxide concentration. However, despite having heat release rates up to 10 kW, the average vehicle conditions did not rise to unacceptable levels. The combined results of the experiments provide significant new understanding of the impact of sample and flow duct height on flame spread and growth in addition to an improved perspective of the impact of a fire event on a spacecraft.Item Flammability Limits from BASS-II Testing in Microgravity Compared to Normal Gravity Limits(49th International Conference on Environmental Systems, 2019-07-07) Olson, Sandra; Ferkul, Paul; Fernandez-Pello, Carlos; Miller, Fletcher; Wichman, Indrek; T'Ien, JamesNormal gravity flammability limits are often reported as downward spread limits (Limiting Oxygen Index), or upward spread limits (NASA Test 1, UL 94V). In microgravity, the comparable limits are opposed flow and concurrent flow. During BASS-II experiments in the Microgravity Science Glovebox on the ISS, PMMA (polymethylmethacrylate) rods, sheets, and slabs, and cotton-fiberglass fabric sheets were burned in microgravity with a variety of oxygen concentrations and at different flow speeds in both opposed and concurrent flow directions. Flame extinctions occurred under conditions indicating that materials can burn at a lower oxygen concentration in microgravity compared to normal gravity. Flames at very low velocity shrink to small spherical sections at the ends of rods, or to small nearly circular disks (a.k.a. flamelets) above flat samples. These spherical/circular flames sometimes oscillate prior to extinction. At high velocity, flames quickly grow but the stabilization zone thins and then fails as the flow time becomes too short for the reactions to occur in that hot flame stabilization zone. The flame blows off, often after a few oscillations. At the other extreme, the flames always quenched when the fan was turned off. This shows that the first line of fire defense on the ISS, to turn off all ventilation systems when a fire is detected, is an excellent mitigation strategy for the ISS. Previous quiescent testing at elevated oxygen concentrations, however, suggests that ventilation may not be necessary for a flame to survive under conditions that may be present in high oxygen low pressure atmospheres in spacecraft or in partial gravity environments, so this mitigation strategy may not be applicable in future exploration missions. The role of heat losses, sample dimension, and near-limit phenomena are discussed. A normal gravity test method is suggested that can account for partial gravity flammability, where more research is needed.Item Limiting Oxygen Concentration of Burning PMMA Cylinders under External Radiant Heating and Subatmospheric Pressures(51st International Conference on Environmental Systems, 7/10/2022) Scudiere, Charles; Liveretou, Christina; Fernandez-Pello, Carlos; Gollner, Michael; Olson, Sandra; Ferkul, PaulUnderstanding flame spread over combustible solids under different environmental conditions is important for fire safety in spacecraft applications due to the criticality associated with such events. Environmental variables such as oxygen concentration, ambient pressure, external radiant heating, or gravity may change the flammability and fire dynamics of materials. The overall objective of this work is to study the effect of an external radiant flux on the opposed flame spread rate and the limiting oxygen concentration (LOC) for flames spreading over the surface of cylindrical samples of polymethyl methacrylate (PMMA) in Space Exploration Atmospheres (SEA). In the work presented here, experiments under normal gravity and subatmospheric pressure are conducted using a variable heat flux with peak values ranging from 0 kW/m2 to 1.9 kW/m2. A forced flow of air with a velocity of 10 cm/s is used to mimic the air flow velocity generated by the HVAC system inside a spacecraft. Flame spread rates and limiting conditions for flame spread were measured from video processing of the experiments at different environmental conditions and external radiant heat flux. Results show that the limiting oxygen concentrations depend on the amount of radiant heating received by the PMMA sample, decreasing as the radiant flux is increased. The data presented in this work provides a baseline for comparison with future microgravity experiments to be performed by NASA as part of the SoFIE/MIST project aboard the International Space Station. It is expected that the results will provide insight for what is to be expected in different conditions relevant for fire safety in future space facilities. The work presented here was supported by NASA Grants NNX10AE01G and NNX13AL10A.Item Limiting Oxygen Concentrations of Burning PMMA Cylinders under External Radiant Heating and Subatmospheric Pressure(2023 International Conference on Environmental Systems, 2023-07-16) Liveretou, Christina; Scudiere, Charles; Rivera, Jose; Fernandez-Pello, Carlos; Gollner, Michael; Olson, Sandra; Ferkul, PaulThis work considers the effect of ambient pressure and an external radiant flux on the limiting oxygen concentration (LOC) for flames spreading downward over the surface of cylindrical samples of black polymethyl methacrylate (PMMA). The objective is to determine the effect of an external radiant flux on the LOC of combustible solid materials in environments expected in future spacecraft cabins. The experimental apparatus and testing methodology is a combination of the LOI and LIFT test apparatuses. The radiant heating ranges from 0 to 3.3 kW/m^2 and the ambient pressures tested from 100 kPa to 40 kPa. An upward forced flow of a mixture of oxygen and nitrogen with a velocity of 10 cm/s is used to determine the LOC of the PMMA as a function of ambient pressure and external heat flux. Results show that increasing the ambient pressure or external radiant flux increases the flame spread rate and decreases the LOC of the PMMA. Correlating the LOC data in terms of the partial pressure of oxygen and the ambient pressure shows that the LOC occurs at an approximately constant oxygen mole fraction that depends weakly on the ambient pressure and radiant flux. The combustion mechanisms leading to this result are discussed based on simple equations and phenomenological arguments. The data from this work will be compared with data from experiments to be conducted in the International Space Station (ISS) under the SoFIE-MIST project, to provide further understanding of the effect spacecraft environments on the LOC of materials. The results will give further insight into the flammability of materials, particularly at subatmospheric ambient pressures, such as in spacecraft, aircraft, and high-altitude locations.Item Mixed Convection Blowoff Limits as a Function of Oxygen Concentration and Upward Forced Stretch rate for Burning PMMA Rods of Various Sizes(47th International Conference on Environmental Systems, 2017-07-16) Marcum, Jeremy; Olson, Sandra; Ferkul, PaulNormal gravity flame blowoff limits in an axisymmetric pmma rod geometry in upward axial stagnation flow are compared with microgravity Burning and Suppression of Solids – II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but with the addition of normal gravity buoyant flow. Cast polymethylmethacrylate (pmma) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 14 to 18% by volume. The forced flow velocity where blowoff occurred was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch is included and the flow is corrected by considering the blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration directly proportional to flow speed. We describe a new normal gravity ‘upward flame spread test’ method which extrapolates the linear blowoff boundary to the zero stretch limit in order to resolve microgravity flammability limits—something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain good estimates of material flammability in low gravity.Item Opposed flame spreading along a structured PMMA sample in exploration atmosphere under microgravity(50th International Conference on Environmental Systems, 7/12/2021) Eigenbrod, Christian; Meyer, Florian; Jomaas, Grunde; Olson, Sandra; Ferkul, Paul; Urban, David; Ruff, Gary A.; Toth, BalazsAs part of SAFFIRE V experiments on the ISS supply spacecraft CYGNUS, flame propagation along different surface structures on a PMMA sample was investigated in opposed flow. The sample was 200 x 400 mm (length x width) with a thickness of 10 mm and contained ribs of different widths (1-10 mm), each 3 mm high, on both sides, arranged in the flow direction. The total thickness of the specimen was thus 16 mm for the ribs. For the first 360 s after ignition with a Kanthal wire, the flow velocity was 20 cm/s in the opposed direction. Then the flow velocity was reduced to 5 cm/s for a duration of 300 s before it was switched off to terminate the experiment. The experimental pressure was 761 hPa and the oxygen concentration was 26.9 vol %. These conditions correspond to atmospheric conditions envisioned for future exploration missions. It was found that the flames ignited downstream propagated forward along the rib edges surprisingly fast with up to 0.88 mm/s (1 mm rib width). Also, it was observed that the wider the rib, the slower the propagation. The widest rib (10 mm) already showed two largely independent edge flames. Unintentionally, the experiment showed how sensitive the flames are to small perturbations of the flow field, as remnants of a preceding and upstream experiment disturbed the incoming flow stratification partially.Item Results of Large-Scale Spacecraft Flammability Tests(47th International Conference on Environmental Systems, 2017-07-16) Ferkul, Paul; Olson, Sandra; Urban, David; Ruff, Gary; Easton, John; T'Ien, James; Liao, Ya-Ting; Fernandez-Pello, A. Carlos; Torero, Jose; Eigenbrod, Christian; Legros, Guillaume; Smirnov, Nickolay; Fujita, Osamu; Rouvreau, Sebastien; Toth, Balazs; Jomaas, GrundeThe preliminary results for two flights of the Spacecraft Fire Experiment (Saffire), conducted on an orbiting spacecraft, are presented. These experiments directly address the risks associated with our understanding of spacecraft fire behavior at practical length scales and geometries. The result of this lack of experimental data has forced spacecraft designers to base their designs and safety precautions on 1-g understanding of flame spread, fire detection, and suppression. However, low-gravity combustion research has demonstrated substantial differences in flame behavior in low-gravity. Over the past several years, NASA and an international team of investigators have worked to address open issues in spacecraft fire safety. NASA’s Spacecraft Fire Safety Demonstration Project was developed with a goal to conduct a series of large-scale experiments in true confined spacecraft environments that represent practical spacecraft fires. The first two flights are complete and examined spread over a large thin sheet of flammable fuel (cotton/fiberglass 41 x 94 cm) and over 9 samples (5 x 30 cm) of various materials (silicone (4), PMMA (2), cotton/fiberglass (2) and Nomex®) that addressed the conditions of NASA STD 6001 Test 1 (material flammability). These experiments were performed on two separate unmanned ISS re-supply spacecraft after they had delivered their cargo and had begun their return journeys to Earth (destructive reentry). Preliminary flame spread rates and flammability assessments are presented for the conditions studied with comparison to prior data. A computer modeling effort is underway to complement the experimental effort. In addition, conceptual development has begun for three more flights that will include fire detection and suppression objectives to the program.Item The Effect of Gravity on Flame Spread over PMMA Cylinders in Opposed Flow with Variable Oxygen Concentration.(46th International Conference on Environmental Systems, 2016-07-10) Link, Shmuel; Huang, Xinyan; Olson, Sandra; Ferkul, Paul; Fernandez-Pello, CarlosFire safety is an important concern for space travel, particularly with the operation of proposed space missions such as those of NASA’s Constellation Program. Currently there is not sufficient knowledge regarding the fire behavior of materials in environments similar to those expected in those future spacecraft (micro-gravity, low velocity flow, elevated oxygen, and reduced pressures. Flammability of solid materials is typically characterized by ignitability, flame spread rate, heat release rate, and toxicity. Although, the most effective fire safety strategy is to prevent ignition altogether, if ignition occurs the fire has to spread to present a risk. The spread of flames over cylindrical samples of cast PMMA of different diameters was investigated to better understand the effects of gravity and oxygen on the mechanisms of flame spread. The 1g experiment consisted of determining the opposed flow spread rate for 3 sample diameters (6.3, 9.5, and 12.7 mm) in gas flows with oxygen concentrations from 21% to 15% and a velocity of 25 cm/s. The micro-gravity flame-spread experiments were conducted as a part of the Burning and Suppression of Solids - II (BASS-II) campaign of micro-gravity combustion experiments conducted aboard the ISS. Opposed flame spread rates were measured for the above samples under flow velocities from 0.5 to 5 cm/s. and oxygen concentrations from 21% to 15%. It was found that for all comparable oxygen concentrations and diameters flame spread in μg was faster than in 1g conditions. A numerical modeling of flame-spread over cast PMMA rods as a function of ambient oxygen concentration and gravity was conducted using FDS. The modeling predicts the observed experimental trends qualitatively but not quantitatively. This is most likely due to the approximate description of the solid phase pyrolysis and gas phase chemistry. The work was supported by NASA Grants NNX10AE01GItem Upward Flame Spread over a Thin Composite Fabric: the Effect of Pressure and Microgravity(48th International Conference on Environmental Systems, 2018-07-08) Thomsen, Maria; Fernandez-Pello, Carlos; Urban, David; Ruff, Gary; Olson, SandraThe influence of the environment on the flammability of combustible solid materials is of utmost importance for applications where fire safety must be ensured. Such is the case of spacecrafts vehicles where a fire could be catastrophic, and the environmental conditions can greatly differ from what is encountered on earth. Moreover, experimental testing under these conditions can be difficult and expensive, with limitations in sample size and duration. Reducing buoyancy by decreasing ambient pressure is a possible approach to simulate a spacecraft environment, thus facilitating testing. The objective of this work is to obtain information on the effect of pressure on the flammability of a thin material, and by comparison with microgravity data, determine up to what point reducing pressure can be used to simulate reduced gravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread rates and flame appearance of a burning thin fabric sample made of 75% cotton and 25% fiberglass. Experiments in normal gravity were conducted using pressures ranging between 100 and 30 kPa and a forced flow velocity of 20 cm/s. Microgravity experiments were conducted during NASA's Spacecraft Fire Experiment (Saffire), on board of the Orbital Corporation Cygnus spacecraft at 100 kPa and an air flow velocity of 20 cm/s. Results show that reductions in pressure slow down the flame spread over the fabric. As pressure is reduced, flame intensity is also reduced, until eventually a very weak blue/purple flame was observed for the lower pressure tested. Comparison with the flame spread rates in microgravity show that similar flame spread rates are obtained at around 30 kPa. The results of this work provide information about the similitudes of the flame spread process in low pressure and microgravity environments, providing guidance for potential ground-based testing for fire safety design in space exploration.