Browsing by Author "Carmignani, Luca"
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Item Boundary Layer Effect on Opposed-Flow Flame Spread in the Microgravity Regime(46th International Conference on Environmental Systems, 2016-07-10) Bhattacharjee, Subrata; Simsek, Aslihan; Carmignani, LucaOpposed flow flame spread over thin solid fuels can be divided into three different regimes based on the strength of the opposing flow velocity. In the thermal regime, the spread rate is independent of flow velocity. As the flow velocity is increased indefinitely, the kinetic regime is reached where the spread rate decreases with an increasing flow velocity, leading to blow off extinction. On the other hand, as the flow velocity is reduced indefinitely, which is possible only in a microgravity environment due to the lack of buoyancy induced flow, the radiative regime is reached where the spread rate decreases with a decrease in flow velocity, leading to radiative quenching of the flame. In this work, the role played by boundary layer in the radiative regime is studied both experimentally and computationally. The experiments were conducted with thin sheets of PMMA ignited in an opposed-flow configuration in a flow tunnel in the International Space Station. Fuel thickness, sample width, flow velocity, and the oxygen level were varied in these experiments. The results show that the flame size changes significantly as the flame spread across a developing boundary layer as predicted by the computational model. However, over the limited range of boundary layer development length, the experiment did not show a rise in spread rate with ta thinnin boudary layer as expected from the computational results.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 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 Flame spread over acrylic cylinders in microgravity: effect of surface radiation on flame spread and extinction(48th International Conference on Environmental Systems, 2018-07-08) Carmignani, Luca; Sato, Shun; Bhattacharjee, SubrataDuring the 1990s several combustion experiments involving cellulose and PMMA as burning fuels were conducted on the Space Shuttle varying the oxygen concentration up to 50% in two pressure levels of 1 and 2 atm (Solid Surface Combustion Experiments, SSCE). These pioneering experiments were among the first attempts to explore flame spread in a quiescent microgravity environment. Although a number of papers have been published on the flame spread rate over thin and thick fuels, digitizing the videos, previously stored in VHS media, and application of recently developed image analysis tools have allowed us to re-analyze those videos for further understanding of these unique experimental results. Specifically, this work explores the effect of surface radiation on flame spread and extinction, starting from a qualitative analysis of the experiments. The comparison with samples from the more recent BASS (Burning And Suppression of Solid fuels) investigation suggest that radiative effects for flat and cylindrical fuels can be quite different, and are affected by the oxygen concentration. A non-dimensional surface radiation number is proposed that captures the geometric effect on radiation.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 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.