Browsing by Author "Urban, David L."
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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 Experimental Results on the Effect of Surface Structures on the Flame Propagation Velocity of PMMA in Microgravity(47th International Conference on Environmental Systems, 2017-07-16) Eigenbrod, Christian; Hauschildt, Jakob; Meyer, Florian; Urban, David L.; Ruff, Gary A.; Olson, Sandra L.; Ferkul, Paul; Jomaas, Grunde; Toth, BalazsMaterials foreseen for the design of manned spacecraft must pass the NASA-STD 6001B Test 1 regarding its fire hazard. During this qualification test in 1g conditions, a flat sample with fire protected edges is placed vertically in a quiescent environment, and ignited at its lower end. To pass the test, it must extinguish within 150 mm propagation length. Even though PMMA does not pass this test, it is extensively used for scientific investigations because of its repeatability and use in previous studies. Systematic ground tests of generic geometries have revealed that almost any realistic machined geometry like sharp or rounded edges, fins or grooves may lead to a rise in flame propagation velocity up to a factor of four related to the flat standard sample. For the first time, the flamed spread over a structured, thick PMMA sample of 290 x 50 mm was examined in microgravity (3x10-5g0) under concurrent flow of 0.20 m/s onboard Orbital ATK’s re-supply spacecraft Cygnus. The results were compared to the behavior of a similarly-sized flat sample. Just as in 1g, it was found that vertical structures promote faster flame spread compared to a flat sample but to a lesser degree than what is observed in 1g. While the structured sample burned 70% faster than the flat sample in 1g, this difference was reduced to only 32% in microgravity. Both samples burned drastically slower in microgravity: 23 times slower for the structured sample and 18 times slower for the flat sample. In 1g the pyrolysis front rapidly spreads along the surface and takes advantage of improver in depth heat transfer afforded by edges but, in microgravity, the burning mostly confined to the leading edge which has the best supply of oxygen. Finally, the microgravity flames produced more smoke and exhibited a larger preheat area.Item Modeling and Analysis of Realistic Fire Scenarios in Spacecraft(45th International Conference on Environmental Systems, 2015-07-12) Brooker, John E.; Dietrich, Daniel L.; Gokoglu, Suleyman A.; Urban, David L.; Ruff, Gary A.An accidental fire inside a spacecraft is an unlikely, but very real emergency situation that can easily have dire consequences. While much has been learned over the past 25+ years of dedicated research on flame behavior in microgravity, a quantitative understanding of the initiation, spread, detection and extinguishment of a realistic fire aboard a spacecraft is lacking. Virtually all combustion experiments in microgravity have been small-scale, by necessity (hardware limitations in ground-based facilities and safety concerns in space-based facilities). Large-scale, realistic fire experiments are unlikely for the foreseeable future (unlike in terrestrial situations). Therefore, NASA will have to rely on scale modeling, extrapolation of small-scale experiments and detailed numerical modeling to provide the data necessary for vehicle and safety system design. This paper presents the results of parallel efforts to better model the initiation, spread, detection and extinguishment of fires aboard spacecraft. The first is a detailed numerical model using the freely available Fire Dynamics Simulator (FDS). FDS is a Computational Fluid Dynamics (CFD) code that numerically solves a large-eddy simulation form of the Navier-Stokes equations. FDS provides a detailed treatment of the smoke and energy trans- port from a fire. The simulations provide a wealth of information, but are computationally intensive and not suitable for parametric studies where the detailed treatment of the mass and energy transport are unnecessary. Application of this model to define flow conditions for the Spacecraft Fire Experiment (Saffire) in Orbital’s Cygnus vehicle is presented in the paper. The second path extends a model previously documented at ICES meetings that at- tempted to predict maximum survivable fires aboard spacecraft. This one-dimensional model simplifies the heat and mass transfer as well as toxic species production from a fire. These simplifications result in a code that is faster and more suitable for parametric studies. The paper discusses a case study applying this model to assist hatch design for the Multi-Purpose Crew Vehicle (MPCV) and shows that the Cabin Pressure Equalization (CPE) valve could prevent cabin over-pressurization (enabling hatch opening by the crew) for reasonable fire scenarios. The overall goal of this work is to develop these two models as complementary tools. The FDS model will enable extrapolating Saffire experiment results to other vehicles and will help in verification of future vehicle safety plans. The one-dimensional model is helpful for rapid parametric studies of design options.Item Spacecraft Fire Experiment (Saffire) Development Status(44th International Conference on Environmental Systems, 2014-07-13) Ruff, Gary A.; Urban, David L.; Fernandez-Pello, A. Carlos; T'ien, James S.; Torero, Jose L.; Legros, Guillaume; Eigenbrod, Christian; Smirnov, Nickolay; Fujita, Osamu; Cowlard, Adam J.; Rouvreau, Sebastien; Minster, Olivier; Toth, Balazs; Jomaas, GrundeThe status is presented of a spacecraft fire safety research project that is under development to reduce the uncertainty and risk in the design of spacecraft fire safety systems for exploration missions. The Spacecraft Fire Safety Demonstration Project is developing three Spacecraft Fire Experiments (Saffire-I, -II, and -III) to conduct a series of material flammability tests at a length scale that is realistic for a serious spacecraft fire in low-gravity. The objectives of these experiments are to (1) determine how rapidly a large scale fire grows in low-gravity and (2) investigate the low-g flammability limits compared to those obtained in NASA’s normal gravity material flammability screening test. The experiments will be conducted in Orbital Science Corporation’s Cygnus vehicle after it has deberthed from the International Space Station. Although the experiment will need to meet rigorous safety requirements to ensure the carrier vehicle does not sustain damage, the absence of a crew removes the need for strict containment of combustion products. The tests will be fully automated with the data downlinked at the conclusion of the test before the Cygnus vehicle reenters the atmosphere. A computer modeling effort will complement the experimental effort. An international topical team is collaborating with the NASA team in the definition of experiment requirements and performing supporting analysis, experimentation and technology development. The status of the overall experiment are summarized in this paper along with a brief look at future experiments that could further enhance NASA’s approach to spacecraft fire safety.