Browsing by Author "Gollner, Michael"
Now showing 1 - 5 of 5
- Results Per Page
- Sort Options
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 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 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.