Browsing by Author "Thomsen, Maria"
Now showing 1 - 8 of 8
- Results Per Page
- Sort Options
Item Concurrent Upward Flame Spread over a Fire Resistant Fabric (Nomex) under External Heating(47th International Conference on Environmental Systems, 2017-07-16) Thomsen, Maria; Huang, Xinyan; Alonso, Alain; Fernandez-Pello, Carlos; Urban, David; Ruff, GaryFire resistant materials are used in multiple applications (clothing, curtains, tents, etc.) were protection from a potential fire is needed. Particularly relevant for this work is the application for astronaut space suits since a spacecraft environment may be different than atmospheric ones. Furthermore, their fire resistant capacity are often tested under very specific conditions that might not represent the real fire situations. For example, when a material is exposed to a near fire or different environmental conditions like reduced pressure, enriched oxygen concentration and micro-gravity, its flammability and fire behaviors can be altered. In this work, an experimental study was performed to investigate the effect of ambient pressure and oxygen concentration on the upward flame spread over a typical fire resistant fabric (Nomex HT90-40) exposed to two different external heat sources. One is the radiation from infrared lamps and the other is the flame from a burning polymethyl methacrylate (PMMA) sheet placed below the fabric. The limiting oxygen concentration (LOC) was first quantified under different external heating, and then the upward flame-spread rate above LOC was measured. Experiments show that the flame from nearby burning object not only can ignite the fire resistant fabric, but also extend the LOC of the material to lower oxygen concentrations. Moreover, the heating from the attached flame is different from an external radiant flux. The results of this work also provide important information about the fire interactions of different materials, and guide the future fire safety design in space exploration.Item Determining the Cause of Reduced Concurrent Flame Spread over Thin Solid Fuels in Low Pressure and Low Gravity(50th International Conference on Environmental Systems, 7/12/2021) Thomsen, Maria; Fereres, Sonia; Carmignani, Luca; Fernandez-Pello, Carlos; Ruff, Gary A.; Urban, David L.The spread of flames over the surface of solid combustible materials has been widely investigated and is known to be affected by environmental conditions. Variables such as flow condition, oxygen concentration, ambient pressure, and partial or microgravity, may change the material flammability and influence the fire dynamics. This is a critical fire safety issue for space exploration vehicles and future habitat atmospheres which will very likely have reduced pressure and enriched oxygen concentration environments, different than those currently used on the International Space Station. However, testing experimentally the materials to be used and qualified for space exploration under these conditions is a cumbersome and expensive task. The objective of this work is to provide a better understanding through numerical modeling of the dominant physico-chemical processes on the concurrent flame spread over thin fabrics under reduced ambient pressure (and in turn, buoyancy) under variable gravitational conditions. Numerical modeling is performed using the Fire Dynamics Simulator (FDS6) code with a single-step Arrhenius reaction rate for the solid phase decomposition. Different models are tested for the gas phase combustion kinetics. The model results are validated with experimental results obtained at similar reduced ambient pressure and flow conditions at 1 g. It is shown that as ambient pressure is reduced the flame spread rate over a thin fabric is also reduced, both experimentally and numerically. Numerical results are compared to an analytical approach previously developed to explain the experimental trends. Further interpretation of the model results provides information regarding the physics of the process and how they are affected by the lower pressure environments. The results of this work provide guidance for potential on-earth testing for fire safety design in spacecraft and space habitats.Item The Effect of Buoyancy on Upward-Concurrent Flame Spread over Thin Paper(49th International Conference on Environmental Systems, 2019-07-07) Thomsen, Maria; Fernandez-Pello, Carlos; Urban, David; Ruff, GaryUnderstanding material flammability inside a spacecraft is important because the conditions in spacecraft environments can greatly differ from those on earth. Because in a gravity field there is a flame-induced buoyancy, it is very difficult to reproduce on Earth the environmental conditions of a spacecraft, thus making fire testing harder. To overcome this problem, alternative approaches that reduce buoyancy are required. One possibility to reduce buoyancy effects relies in using reduced ambient pressure. The objective of this work is to study the effect of pressure, and consequently buoyancy, on upward/concurrent flame spread over a thin combustible solid, and by comparison with partial gravity data, observe up to what point low-pressure can be used to replicate flame characteristics observed in different gravity levels. Experiments in normal gravity were conducted over pressures ranging between 100 and 30 kPa and a forced flow velocity of 10 cm/s. Results show that reductions of pressure slow down the flame spread over the material surface. As pressure is reduced, flame intensity is also reduced. Comparison with partial gravity data shows that as the pressure is reduced, the normal gravity flame spread rate approaches that observed at different gravity levels. The data presented is correlated in terms of a mixed convection non-dimensional number that describes the convective heat transferred from the flame to the solid, and that also describes the primary mechanism controlling the spread of the flame. The correlation provides information about the similitudes of the flame spread process in variable pressure, flow velocity and gravity environments, providing guidance for potential ground-based testing for fire safety design in spacecraft.Item The effect of reduced pressure on the characteristics of spreading flames(50th International Conference on Environmental Systems, 7/12/2021) Carmignani, Luca; Thomsen, Maria; Fereres, Sonia; Gollner, Michael; Fernandez-Pello, Carlos; Urban, David; Ruff, GaryFlame spread over solid fuels is a canonical problem in fire science, due to its direct implications on material flammability and importance in fire development. In a microgravity environment, such as onboard a spacecraft, flames can behave very differently than on Earth. This is concerning for spaceflight life safety, especially in higher-oxygen environments. Due to the difficulties associated with microgravity testing, low-pressure environments have been proposed as an alternative to approximately replicate reduced gravity conditions because of the reduction in buoyancy. However, the roles played by gravity and pressure on flame length, standoff distance, and flame spread rate vary with the burning configuration. In concurrent flame spread, the buoyant flow enhances the spread rate by bringing the flame closer to the fuel surface and increasing the heating of the solid fuel. In opposed flame spread, the sample is preheated by the flame ahead of the flame leading edge, which is strongly affected by the surrounding flow field. In this work, we consider flames spreading over thin cotton samples in both downward (opposed) and upward (concurrent) configurations to investigate the effect of pressure (30-100 kPa) on flame characteristics, such as spread rate and standoff distance. A small forced flow is induced upward so that the flames are exposed to a mixed (forced and free) flow. By reducing pressure, flames become less bright, their standoff distance increases, and their spread rates decrease in analogy with low-gravity flames. These results could in help understanding the differences between flames at low pressure and low gravity environments for these similar, yet very different, spreading configurations. They could also provide further information about potential Earth testing of the flammability of materials in spacecraft environments.Item Modeling the Effect of Buoyancy and External Heating on the Flame Spread in Fire Resistant Fabrics(48th International Conference on Environmental Systems, 2018-07-08) Thomsen, Maria; Fereres, Sonia; Alonso IpiƱa, Alain; Fernandez-Pello, Carlos; Urban, David; Ruff, GarySpacesuits are fabricated with Nomex, Kevlar and other fire resistant fabrics. The flammability behavior of these materials has been widely studied experimentally, mostly under standard sea level atmospheric conditions. However, future human space exploration vehicles and habitat environments will very likely have different environments, i.e. reduced pressure and enriched oxygen concentration. Experiments under these conditions, particularly in microgravity, can become a difficult and expensive task. Numerical investigations of the flammability of high performing fibers/fabrics may be a viable alternative to experiments. Here we present a numerical model formulated to understand the effect of environmental conditions on the flame propagation characteristics of thin fire-resistant material such as Nomex. Moreover, the effect of external radiant heating on material flammability is also studied. Thermogravimetric analysis (TGA) experiments were performed with Nomex to estimate the kinetic parameters, which were then used to model the thermal decomposition of the fabric sample using a Computational Fluid Dynamics (CFD) code, Fire Dynamics Simulator (FDS6). Two-dimensional simulations are performed using finite-rate single-step combustion kinetics in the gas phase and an Arrhenius reaction mechanism with multiple steps for the solid phase decomposition. The model results are then compared to previous experimental results at high oxygen concentrations and/or reduced pressure conditions. It is shown that with the appropriate kinetic parameters the model is able to capture the main physical aspects of the flame spread of a thin solid fuel and it provides a basis for future modeling of fire resistant fabrics for space exploration.Item Opposed-flow spreading flames: Effect of sub-atmospheric pressure on spread and burning rates(51st International Conference on Environmental Systems, 7/10/2022) Carmignani, Luca; Garg, Priya; Thomsen, Maria; Gollner, Michael; Fernandez-Pello, Carlos; Urban, David; Ruff, GaryFlame spread over solid fuels is a canonical problem in fire science, due to its direct implications on material flammability and importance in fire development. Flames in a microgravity environment can behave very differently than on Earth, posing additional risks for spaceflight life safety. Due to the difficulties associated with microgravity testing, sub-atmospheric pressures in ground-based experiments have been proposed to approximately replicate the burning behavior of solid fuels in reduced gravity conditions because of similar effects on heat and mass transfer mechanisms from the flame to the solid. However, the roles played by gravity and pressure vary with the flame spread configuration. In opposed flame spread, the solid fuel is heated by the flame ahead of its leading edge, and this process is strongly affected by the ambient conditions. In this work, we consider flames spreading over acrylic samples exposed to a forced flow of 20 cm/s, and pressures between 30 and 100 kPa. Flame characteristics such as spread rate, standoff distance, and length are obtained from the video analysis of the experiments and compared at different pressures. Mass burning rates are calculated from the samples weight measured before and after the experiments. Additionally, gas emissions measured during the experiments are used to estimate the heat release rate of the spreading flames. Results show a decreasing non-monotonic behavior of flame length, spread rate, and mass burning rate with reducing pressure. The comparison of the heat release rate obtained from the measured emissions and the estimated mass burning rate, suggests that chemical kinetics is not driving the decrease in flame spread rate observed at low pressures. These results could provide more information to guide future Earth-based flammability testing of materials for spacecraft applications. This research was supported by NASA Grant NNX12AN67A.Item 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.Item Upward Flame Spread over a Thin Fabric in Normoxic Atmospheres(2023 International Conference on Environmental Systems, 2023-07-16) Thomsen, Maria; Carmignani, Luca; Garg, Priya; Fernandez-Pello, Carlos; Gollner, Michael; Urban, David; Ruff, GaryThe influence of environmental conditions on the flammability of combustible solids is of importance to spacecraft fire safety. In a manned spacecraft the environment is maintained at a normoxic condition, which is the combination of ambient pressure and oxygen concentration that results in a partial pressure of oxygen equal to that of normal atmosphere at sea level. Future spacecraft will have atmospheres with reduced pressures and increased oxygen concentrations at normoxic conditions (Space Exploration Atmospheres - SEA), designed to reduce preparation time for extravehicular activities. This paper presents experimental results on upward spread of flames over a flat thin cotton fabric under normoxic conditions. Experiments are conducted with ambient pressures ranging between 100 and 60 kPa and oxygen concentrations between 21% and 35% by volume. Additional experiments are carried out with a fixed oxygen concentration of 21% (ambient air) and a pressure ranging between 100 and 60 kPa. Results show that moving to normoxic environments with reduced pressure and increased oxygen concentration increases the flammability of the fabric and the flame spread rate. Low pressures with 21% oxygen concentration, on the other hand, led to a decrease in flame spread rate. Normoxic conditions also showed a greater increase in distance between the outer flame and the fuel bed at lower pressures. During the experiments, O2, CO, and CO2 were measured in the combustion products, and were used to calculate the heat release rate at different pressures and in normoxic conditions. The results show that as the cabin environment transitions to normoxic conditions with higher O2 concentrations, the heat released by the flame also increases. The data presented here provides information about the flammability of spacecraft materials in future SEA in support of the NASA research, yielding insight for future designs when considering fire safety in spacecrafts.