NOTE: End date changed to 10/31/2021 per NSSC information (Ed., 9/9/20)

NOTE: Extended to 10/31/2020 per PI and S. Olson/GRC (Ed., 3/23/16)

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. The flammability, or fire hazard, of materials is normally assessed by the material's ease of ignition, rate of flame spread, heat released and extinction. In this research project, an experimental investigation is conducted to study the flammability of thermoplastic materials under SEA is being conducted.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

Experiments

The experimental apparatus used in the normal gravity experiments has been described in previous progress reports except for the inclusion of radiant heaters, and thus it will be described here only briefly. It consists of a laboratory scale combustion tunnel that is inserted in a 105 L pressure chamber. The chamber is provided with a flow system that provides constant supply, and exhaust, of gases to avoid vitiation problems. Compressed house air, or nitrogen and oxygen, are supplied through critical nozzles (O’Keefe Controls) to the bottom of the combustion tunnel while constantly evacuating to maintain constant the pressure inside the chamber. The chamber pressure is controlled with a high-capacity vacuum generator (Vaccon JS-300) and a mechanical vacuum regulator. The chamber pressure is monitored constantly with an electronic pressure transducer (Omega Engineering, Inc. PX303-015A5V). Tests can be conducted at sub-atmospheric pressures ranging between 100 kPA and 30 kPa ± 2 kPa, and varied oxygen concentrations. The forced flow velocity can be varied from 300 mm/s.

The flow duct was built following a similar setup used to study flame spread over solid fuels together with the details of the heater attachment that will be used in the Solid Fuel Ignition and Extinction / Material Ignition and Suppression Test (SoFIE/MIST) microgravity experiments that are planned to be conducted on board the International Space Station (ISS). The laboratory scale combustion tunnel has a cross sectional area of 125 mm by 125 mm, and is 600 mm in length. The first 350 mm of the duct serves as a flow straightener, where inlet gases pass sequentially through a 20 mm layer of 5 mm borosilicate beads, a stainless-steel mesh and 63 mm thick aluminum honeycomb with 3 mm cells before entering the test section. The last 250 mm portion of the duct is used as the test section. The side walls of the top portion of the test section are made of Pyrex glass to withstand the heat from the heater array and the burning samples, while allowing the visualization of the burning samples.

The samples used in the experiments are cylindrical rods made of cast black polymethyl methacrylate (PMMA). The rods are placed vertically in the center of the test section, supported on a metal cylinder of the same diameter to prevent flow disturbances. The black PMMA was selected to maximize the absorptance of the thermal radiation at the rods' surface. The PMMA rods are 90 mm long with diameters of 6.4 mm, 9.5 mm, and 12.7 mm. The sample material and geometry were selected following previous experiments conducted in the ISS under the BASS-II project and future experiments that will be done as part of the SoFIE/MIST project. Also, PMMA is potentially going to be used in some spacecraft components, such as windows, in future space vehicles. The samples are positioned in the center of the test section using two actuators (Firgelli Automations FA-150-S-12-6) situated on opposite sides of the flow straightener section of the duct and powered with a power supply (MASTECH HY1803D). The actuators are connected by a central shaft that holds and positions the sample holder. Several K-type thermocouples are embedded on the surface of the PMMA samples to record surface temperature from the beginning of the test until the moment the flame arrives to the location of the bead.

Four flat ceramic heaters (Bach Resistor Ceramics GmbH) are used as the source of external thermal radiation. These heaters were selected to match the heaters that will be used in the MIST/SoFIE experiments. The selection of the heaters for the MIST/SoFIE experiments was made based on concerns that other heaters available, like halogen lamps, may shatter during the launch of the rocket where the MIST apparatus would be transported to the ISS. Each ceramic heater has a length of 75 mm, a width of 14.7 mm, and a heated section of 50 mm. The four heaters are mounted on fixed rails positioned on each one of the corners of the experimental section surrounding the sample. Heaters are equi-spaced with a distance of 29.85 mm from the heater surface to the center of the duct, where the cylindrical sample is positioned. Parabolic reflectors made of electropolished stainless steel are positioned behind the heaters with the polished side facing the sample. The reflectors are 70 mm in length by 51 mm in width and they are positioned 18.5 mm from the back of the heater. The heater and reflector are positioned 83 mm and 93.2 mm, respectively, from the top opening of the duct, aligned with the position of the sample. The four heaters are used to provide a variable heat flux in the direction of flame spread (downward). The distribution of heat flux over the solid surface provided by the heaters was characterized under different power settings using a Schmidt-Boelter radiometer from Medtherm Corporation. The characterization of the heaters was performed considering the heaters assembled in the tunnel with the array of reflectors. Distribution in the direction of flame spread is monitored for five different power settings ranging between 30 V and 70 V, with peak heat fluxes ranging from 0.9 to 9.3 kW/m2 respectively. Higher voltage settings are not considered because at those conditions the peak heat flux over the sample would exceed the critical heat flux for ignition of PMMA.

Experimental results

The opposed flow flame spread over the surface of the PMMA rods was investigated under the influence of different levels of radiation. The primary data collected during the experiments was the flame spread rate over the PMMA surface, and the surface temperature at the thermocouples’ location. The rate of flame spread was determined by tracking the position of the flame leading edge in the recorded videos on a frame by frame basis with an internally developed Python-based program called Flame Tracker. Once ignition was obtained and the sample loaded into the test section, the flame was observed as it propagated downward over the sample. As the radiant heat flux is increased, the flame spread rate speeds up and the flame burns stronger and brighter. As the PMMA rods burn, a conical shape at the tip of the sample is formed and no dripping of molten plastic is observed. During the burning process, significant vapor jetting of bubbles of PMMA bursting at the surface are also observed, causing distortions in the flame as the test progresses.

The measurements show that as the heat flux is increased the flame spreads more rapidly due to the preheating of the PMMA before the arrival of the flame. Furthermore, the flame has an initial acceleration period (soon after ignition) and a deceleration period (closer to the end of the spread) following the profile of the applied heat flux. From the flame spread data, it is possible to see how the flame spread rate is influenced by the radiant heat flux, resulting in faster flame spread rates for the higher heat fluxes. In addition, the preheating time along the fuel sample at a given location is different for different heat fluxes because of the different flame spread rates.

Data correlation

The behavior of the flame spread over a solid fuel is a process that involves multiple physicochemical processes that affect the gas (transport, mixing, chemical kinetics) and solid (heat transfer, thermal decomposition, gasification) phases. As the flame spreads over the solid surface, enough heat has to be transferred from the flame to the unburnt solid ahead of the flame to raise its temperature to the pyrolysis temperature and pyrolyze it. The pyrolyzed fuel is then convected and diffused away from the surface where it mixes with the oxidizer, forming a flammable mixture that is later ignited by the flame in a continuous process. The spread of the flame is therefore determined by the heat transferred from the flame to the solid, the rate at which the fuel is pyrolyzed and the ability of the flame to ignite the flammable mixture. In the presence of an external source of radiant heating, the external radiant flux contributes in two primary ways to the enhancement of the flame spread rate: on the one hand, the added radiant heat increases the temperature of the unburnt solid in the region ahead of the flame; on the other, the external radiant heat flux also increases the fuel gasification rate in the burning region because of the thermal energy applied to the surface in addition to that provided by the flame.

Despite the complexities involved in the flame spread process, several theoretical models have been developed to simulate and provide a better understanding of the phenomenological process and the effect of different parameters. Most of the different theoretical models for downward flame spread proposed in the literature predict a flame spread rate that is proportional to the external radiant flux and inversely proportional to the difference between the pyrolysis temperature of the solid and the surface temperature of the solid at the time of flame front arrival to a specific location. Consequently, the flame spread rate should correlate with either the solid surface temperature at the time of flame arrival, or with the energy received by the solid at the time of flame arrival. This is described in the following two sections.

Data correlation based on the surface temperature at the moment of flame arrival

Looking at the theoretically derived expression of the flame spread rate, it is seen that the applied external radiant heat flux affects the spread rate in two ways: directly, and indirectly (through variations in the surface temperature beyond the region heated by the flame). With no external radiant flux, the temperature of the surface that the flame experiences can be assumed to be the sample’s initial temperature T_o. However, in the presence of an external source of radiant heating, the surface temperature will increase as a function of the magnitude of the heat flux applied and the time that the solid is exposed to the heat flux. For downward/opposed flame spread, the flame does not heat the portion of the solid far ahead of the flame front as it spreads; thus, approximately it can be assumed that T_s is the surface temperature at the time of flame arrival to a particular location t_arr. Accordingly, in an opposed flow configuration under an external heating like the one studied in the present work, the spread process is strongly influenced by the temperature of the fuel, which in turn is changing in time as a result of the applied non-uniform external heating. Therefore, as the flame spreads over the solid, the leading edge of the flame encounters a surface temperature that is changing as a function of time and space, influencing in turn the rate at which the flame spreads.

Following this concept, the measured flame spread velocity was correlated in terms of a nondimensional temperature at the time of flame arrival. It was found that the flame spread rate measured correlates fairly well with the surface temperature, with larger spread rates following larger surface temperatures. The good agreement of the correlation is a statement of the importance of the surface temperature, and in turn the external radiant heat flux, in determining the rate of spread of the flame.

Data correlation based on the energy applied to the surface prior to flame arrival

Although useful and widely used in the literature, thermocouples present some limitations when used to measure the surface temperature of the solid (i.e., they need to be carefully attached to the surface to minimize inconsistencies, they are susceptible to heating from the radiant flux and signal noise, they are labor intensive, among others). Furthermore, thermocouple measurements will not be available during the flight experiments performed as part of the SoFIE/MIST experiments. Thus, additional methods to determine the surface temperature, and to correlate the flame spread rate at different environmental conditions, need to be explored.

For downward flame spread over a surface exposed to external radiant heating, the applied external heat flux determines the amount of energy that the sample receives as preheating while the flame is spreading. Consequently, the surface temperature that the flame experiences at the moment of arrival to a given location in the sample is also determined by the energy received by the sample at that location and time. Given the complexities associated with temperature measurements, and the limited accessibility for those type of measurements in future spacecraft experiments, an alternative approach is pursued here with the purpose of correlating the flame spread rate with the energy levels applied to the surface. The reasoning behind this approach relies on the idea that the surface temperature is determined by the balance between the energy applied at the surface and the energy removed from it due to heat losses. For a flame spreading in a downward configuration away from extinction conditions, the energy applied to the surface is larger than the energy losses, and consequently the spread process develops in a continuous fashion.

In the presence of an external radiant heat source, the total energy applied to the surface becomes dominant over the energy losses and thus it is expected that the flame spread rate could be directly correlated to this parameter, as it was previously done with the surface temperature. The energy values are obtained from the relation between the external heat flux applied to the surface and the time the sample is exposed to the applied heat flux (tarr). Plots of the surface temperature and energy received by the solid at the time of flame arrival and energy are then compared; as expected, both the spatial and temporal profiles of the energy applied to the surface distributions follow similar distributions as those of the heater calibration, sample surface temperature at the time of flame arrival, and the flame spread rate. The similarities between these distributions are a clear demonstration of how intrinsically related the spread rate is to the surface temperature and to the energy applied to the surface.

The measured flame spread velocity was plotted in a nondimensional form as a function of a nondimensional energy received by the sample at the location and time of flame arrival. Flame spread rate and energy are nondimensionalized using the average flame spread rate obtained under no external heat flux, V_(o,exp), and the energy required to raise the surface temperature to the pyrolysis temperature, respectively. The energy applied to the surface is determined as a function of the time it takes the flame to reach a specific location (t_arr) and the external heat flux applied to the surface. It was found that the measured flame spread rate correlates well with the energy, with larger spread rates following larger energies.

These results provide an alternative method to correlate the flame spread rate under variable radiant heat fluxes, based only on the energy applied to the surface. The use of the correlations presented here removes the need to measure the surface temperature, and thus facilitates the analysis of data in which the means to constantly measure surface temperature are not available. A paper reporting these results has been accepted for publication in Fire Technology.

NOTE: End date changed to 10/31/2021 per NSSC information (Ed., 9/9/20)

NOTE: Extended to 10/31/2020 per PI and S. Olson/GRC (Ed., 3/23/16)

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. The flammability, or fire hazard, of materials is normally assessed by the material's ease of ignition, rate of flame spread, heat released and extinction. In this research project, an experimental investigation is conducted to study the flammability of thermoplastic materials under SEA is being conducted.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

The opposed flame spread rate, flame appearance, and surface regression rate over polymethylmethacrylate (PMMA) cylinders has been studied under different normoxic conditions potentially applicable in future space exploration atmospheres (SEA). The environmental conditions were obtained by simultaneously reducing ambient pressure and increasing oxygen concentration. The flame spread rate is found to increase linearly with normoxic environments with high oxygen concentration, and with the mixed convection flow velocity that the flame is exposed to. It is also found that the flame spread rate depends on the rod diameter, with a transition to a stronger dependence as the normoxic conditions are changed to lower pressures and higher oxygen concentrations. This last result not only seems to contradict the predictions of analytical models of flame spread but also a phenomenological interpretation of the flame spread process. Similarly, the observation that the surface regression rate decreases as the normoxic conditions are changed to lower pressure and oxygen concentration appears to contradict phenomenological arguments of the process, indicating that cautions should be taken when applying simplified analyses of flame spread and surface regression to spacecraft environments. The modifications implemented in the normal gravity MIST apparatus to incorporate the radiant heaters will permit testing at different radiant fluxes to observe the effect of an external radiant flux on the flammability of materials in SEA. This testing will be initiated once the accurate operation of the modified apparatus is verified.

NOTE: Extended to 10/31/2020 per PI and S. Olson/GRC (Ed., 3/23/16)

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. The flammability, or fire hazard, of materials is normally assessed by the material's ease of ignition, rate of flame spread, heat released and extinction. In this research project, an experimental investigation is conducted to study the flammability of thermoplastic materials under SEA is being conducted.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

Results show that in normal gravity, as the opposed flow increases to 50~100 cm/s, the flame can no longer spread over the fuel surface, but becomes a mass burning process, staying in the recirculation zone downstream of the cylinder top surface. Thus, the observed “flame-spread” is actually a fuel-regression like a pool fire flame or a candle flame. The fuel regression only occurs for the thick rods, but not for the thin rods which do not have a downstream recirculation zone. Such transition can also be indicated by a critical leading-edge regression angle of ˜45o. Increasing the opposed flow velocity, the shape of cone becomes flatter until flame blow-off occurs. In the present experiments, before blow-off, a flat blue flame is formed floating above the rod top s urface, like a classical counterflow diffusion flame. Such blue flame is stable and is a result of vortex shedding.

As the ambient pressure or O2 concentration decreases, the flame spread first transitions to the fuel regression regime, and then, extinction occurs (two-step extinction process). On the other hand, external radiation in the perpendicular to the flame-spread direction may prevent the transition to fuel regression if the radiant flux is elevated enough. In microgravity, the fuel regression angle is found to be much smaller (<5o) in the low opposed flow (<10 cm/s) and the cone-shape fuel may not be formed after the flame spread, which is different from normal gravity. This work not only provides a better understanding of flame-spread and blow-off phenomena under various environmental conditions, but also clarifies the differences between the flame-spread and fuel-regression on solid fuels.

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

A paper has been submitted to Scientific Reports: S.L. Link, X. Huang, C. Fernandez-Pello, S. Olson, and P. Ferkul. "The effect of Gravity on Flame Spread over PMMA Cylinders."

NOTE: Extended to 10/31/2015 per NSSC information (Ed., 3/24/15)

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

In the case of mass flux at ignition, there is no discernible trend in the data as a function of external radiant heat flux. Nor should a trend in mass flux be expected as the gas-phase chemistry at the pilot is independent of the solid heating as long as a flammable mixture is present. Thus across the range of radiant heat fluxes tested, an observed mass flux of between 0.5 and 0.9 g m-2 s-1 corresponds very well to the theoretical range from De Ris’s seminal model of 0.7614 and 0.9021 g m-2 s-1.

Concurrently, the opposed flow flame spread experiments conducted aboard the International Space Station (ISS) with clear and black PMMA cylindrical rods as a part of the BASS-II campaign have helped elucidate the effects that oxygen concentration and opposed flow velocity have on flame spread over rod-like samples of thermoplastics in a micro-gravity environment. In particular, it is seen that increasing oxygen concentration or opposed flow velocity acts to increase the flame-spread rate for all three rod diameters tested. Yet, decreasing rod diameter increases flame-spread rate, an effect that can be at least in part be attributed to the curvature of the sample surface, and resultant effects of geometry on both the gas-phase boundary layer and on the solid phase conduction.

To extend the results of the BASS-II experiments, and in preparation for the SoPHIE campaign of ISS bound combustion experiment, the effects of oxygen concentration, external radiant heating, and sample diameter on flame-spread rate over cast black and clear PMMA rods has been also investigated. It was found that flame spread rate decreased with decreasing oxygen concentration or eternal radiant heat flux, but increased with decreasing sample diameter. It was also found that with the use of external radiant heating, the effective Limiting Oxygen Index (LOI), or oxygen concentration at which sustained flame-spread was possible, could be reduced. Yet, only the thickest samples were able to sustain flames near or below the LOI of PMMA with the aid of the external radiant heating. It is hypothesized that this is a joint effect caused by the thermal mass of the thicker samples and the fact that the smaller, less radiant flames at lower oxygen concentrations can stand in the downstream shadow of the thicker samples, where for the thinner samples, this does not seem to be an option for the flame.

In comparing the BASS-II micro-gravity results to those obtained in 1g, it is clear that flame spread in micro-gravity is faster if one accounts for the fact that the flow velocities tested in both cases are near the lower bound of what is feasible or relevant. Similar trends in flame-spread rate with sample diameter, oxygen concentration, and flow velocity (beyond the natural convection break-point) were observed, but for the tested conditions, flame-spread in µg is categorically faster than in 1g.

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station (ISS). It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

In the original MIST design, a small-scale cylindrical flow duct with fuel samples attached to its inside wall was heated by a cylindrical infra-red heater located along the central axis of the cylinder. However, as the project evolved it was decided by NASA that it would be better to produce an experimental design that could accommodate other experiments and experimenters with different concepts or sample geometries. Based on those instructions and input regarding the requirements of other researchers that may share the hardware in an ISS/CIR experiment, a cylindrical design placing the sample at the center of an optically transparent tube with heaters equally spaced along the exterior of the cylinder wall was developed. A hot wire igniter in the downstream boundary layer of the fuel sample initiates piloted ignition. Environment variables that can be studied via this experimental apparatus include: external radiant heat flux, flow oxygen concentration, flow velocity, ambient pressure, and gravity level (if flown in the ISS/CIR). This new geometry experimental design maintains good consistency with Dr. Tien’s and Dr. Olson’s project approaches and experimental objectives. A further goal of the project has been to develop a combined solid/gas phase numerical model based on the MIST test methodology to predict the flammability behavior of practical materials in spacecraft. Additionally, with the current flight of the Burning and Suppression of Solids – II (BASS – II) project aboard the ISS, work being done to accommodate the ground-based experiments associated with that project in the MIST experimental apparatus.

MIST has developed an usable experimental apparatus for evaluation of an optimum configuration of sample, heater, and igniter placement to fall within the power supply and heat dissipation requirements of the NASA ISS/CIR. Using this experimental apparatus, time to ignition, and mass loss at ignition data have been collected as a means of establishing a baseline for eventual comparison with space flight data. In a parallel effort a 2-D axisymmetric numerical model of the piloted ignition of a sample has been developed and prediction of ignition delay is being obtained as a function of several ambient variables. In parallel, work is undergoing to employ this computational model to predict and extend the results of the BASS – II standard and microgravity experiments.

For the ground-based (1g) MIST tests, fame spread rates were found for four diameters (0.3175, 0.635, 0.9525, 1.27 cm) of clear and black cast Poly(methyl-methacrylate) (PMMA) rods under four different external radiant heat fluxes (0, 2, 5, 10 kW/m2). It was found that there is a clear decreasing trend in flame-spread rate with increasing PMMA rod diameter. Similarly, for both clear and black cast PMMA rods and at both 19 and 21% oxygen by volume increasing the external radiant flux acts to increase the flame spread-rate.

Comparing flame-spread rates from 19 and 21% oxygen concentration (1g) experiments for both clear and black cast PMMA rods, the flame-spread rates are almost a magnitude higher for the 21% oxygen environment than for the 19% oxygen environment. It seems though, that the effect of oxygen concentration between the 19% and 21% environments is magnified by the clear cast PMMA rods compared to the results in the black cast PMMA rods.

As a part of the BASS-II campaign, tests were recently carried out in the Microgravity Science Glovebox (MSG) aboard the International Space Station (ISS). Three different diameters (0.635, 0.9525, 1.27 cm) of clear and black cast PMMA rods were tested. Oxygen concentration was varied between 16% and 21% by volume, and flow velocities were varied between 0.4 and 8 cm/sec. Tests were conducted at fixed oxygen concentration, and flow velocity was varied downward towards extinction. Across all three diameters, there seems to be a correlation between decreasing oxygen concentration and decreasing flame spread rate, as well as a trend in decreasing flame-spread rate with increasing diameter as was seen in the 1g experimental data. Additionally, there seems to be a mild trend of increasing flame-spread rate with increasing opposed flow velocity.

For all three diameters for which tests were conducted in both µg and 1g the flame spread rates are higher in the case of the microgravity tests. Additionally, as previously discussed, we see decreased flame-spread rates as the diameter of the rods increases.

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station. It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.

Current experimental investigations surround the the piloted ignition of cylindrical samples of Poly(methyl-methacrylate) (PMMA) under an externally applied radiant heat flux. Clear and black PMMA rods are tested in a longitudinal orientation in a natural convection air flow. This work expands upon previous investigations of ignition phenomena which were concerned with the piloted ignition characteristics of flat samples of PMMA or similar thermoplastics. The primary novel aspect of this experimental investigation is the cylindrical symmetry of the samples and the associated curvature of the fluid, thermal, and species boundary layers which act to change the thermo-chemical characteristics of the sample heating and ignition. Thus far, there seem to be (1) slightly longer ignition delay times when compared to similar conditions for flat samples, and (2) similar fuel mass flux at ignition. Both of these phenomena can be attributed at least in part to the symmetrical boundary condition at the center of the cylindrical samples, and the thinner boundary layer present in the gas phase -- due to the curvature of the sample surface. It is proposed that the thinner boundary layer may result in greater convective cooling of the sample, thus increasing the time to ignition, without significantly affecting the critical mass flux necessary for ignition.

Modeling has been carried out in conjunction with the above experimental work to simulate the MIST experiments. The 2-D antisymmetric computational model mimics the geometry of the current experiment using a long antisymmetric fuel rod in a duct of similar dimensions to that in the current iteration of the experiment. Fire Dynamics Simulator (FDS) version 6.0.1, developed by NIST, in its Direct Numerical Simulation (DNS) mode was employed. The model simultaneously considers the processes in the solid and in the gas phases. The solid phase decomposition is modeled with a single step global Arrhenius reaction rate. Oxidative pyrolysis is not considered and the in-depth formed pyrolyzate is assumed to flow unrestricted through the solid combustible. Additionally, due to limitations in FDS, conduction in the solid is strictly radial. Gas phase kinetics are modeled with a single-step second order Arrhenius reaction rate. Although the current model simulates the current set of experiments quite well, it may be necessary to validate the predictive capabilities of the model and quantify how well the current values for physical parameters are able to handle environmental changes and then implement any necessary modification as necessary.

Thus far, the modeling results seem to indicate that buoyancy plays an important role via the Rayleigh-Taylor instability at normal gravity, particularly once the flame is established over the sample surface. However, the Rayleigh-Taylor instability disappears for zero gravity conditions and the flame shows a distinctly round, more uniform shape in that case. Furthermore, ignition occurs significantly earlier in 0-g than in 1-g conditions.

The computational model is moving towards becoming useful predictive tools for the study of ignition and flame spread phenomena. Eventually, the goal will be to have validated the model using both 1-g and 0-g experimental data and then be able to extend the model to the predictive realm with better confidence as to its output.

Both the experimental and computational results will hopefully assist in the future development of better fire safety tests for employ in manned space capsules.

Central Objectives: The next generation of NASA spacecraft will use lower pressures and increased oxygen concentrations than current designs. Although there is insufficient knowledge about fire behavior of materials in these Space Exploration Atmospheres (SEA), recent research suggests that material ignitability is increased in SEA. These results, imply that next generation spacecraft present a greater fire safety hazard than current designs. Although the most effective fire safety strategy is to prevent ignition, if ignition does occur, it is critical to rapidly extinguish the fire. Therefore, an experimental investigation into ease of ignition and fire suppression characteristics of materials under SEA is proposed.

Methods/techniques: The proposed experimental apparatus is simple and easily incorporated in the Combustion Integrated Rack on the International Space Station. It consists of a small-scale cylindrical flow duct that has fuel samples attached to its inside wall. A cylindrical radiant heater is located at the center of the duct. An oxidizer gas flows at down the length of the tube. Electrical igniters are placed downstream of each fuel sample. Time to ignition and the time between introduction of suppression agent and extinction are measured as a function of environmental conditions. The normal gravity and microgravity experimental programs will be supported by a comprehensive model of piloted ignition and extinction of materials in SEA. Microgravity experiments will be used to validate the model, which in turn will be used to predict the fire behavior of different combustible materials in SEA.

Perceived Significance: Currently, there exists no testing methodology specifically designed to determine the fire hazards of materials under those conditions. The proposed work will fill that void and provide additional information about the effect of SEA on the flammability and suppression of materials. It will also provide guidance to interpret and extend current NASA testing procedures to SEA.