This article examines a transient, one-dimensional model of surface regression during opposed flow flame spread over finite thickness, non-charring thermoplastics. Gas to solid conductive transfer from the flame drives sample heating. The solid degrades into volatile fuel molecules in two stages: (1) the pre-vaporization heat-up stage and (2) the vaporization/regression stage. The influence of the external (flame) heat flux on heat-up and regression is examined. Regression stalls eventually because the sample bottom temperature, which is fixed, is lower than the sample top vaporization temperature. The moving boundary value problem is described by the energy equation subject to a variable top surface energetic boundary condition. The energy flux from the external source (flame) is taken as constant. The regressing solid is transformed to a fixed-domain coordinate system in which it is coupled, nonlinearly, to the energetic boundary condition. This nonlinear system is solved numerically. Term-by-term evaluation of the numerical solution allows development of analytical energy balance formulas in the short and long time intervals of the regression stage. A physics based Peclet number analysis describes features of time dependent surface regression.

The SoFIE-GEL (Solid Fuel Ignition and Extinction-Growth and Extinction Limit) microgravity combustion experiment was recently conducted aboard the International Space Station (ISS). In these tests, diffusion flame extinction limits of 4-cm diameter PMMA (polymethacrylate) spheres were determined as a function of forced flows (0.2 to 80 cm/s), oxygen concentrations (13 % to 34 %) and pressure (0.15-1.1 atm). In particular, the effect of sample heat-up is investigated. The sample surface layer is preheated by the ignitor and the diffusion flame itself before the extinction procedure (e.g. flow velocity decrease or increase or oxygen decrease) is implemented. The degree of sample heat-up is inversely related to the gas flame heat loss to the solid which is quantified by embedded thermocouples inside the sample sphere near the flow forward stagnation point. This region controls the flame extinction process. In this paper, the extinction boundary in one atmosphere is presented in a map using oxygen percent and flow velocity (and stagnation flow strain rate) as coordinates. The oxygen-velocity (strain rate) boundary is a truncated U-shaped band when plotted with a log x axis. The data at high flow velocity (80 cm/s) is truncated because of the limitation of the test facility. The boundary contains data with different degrees of sample preheating inside the band. More preheat widens the flammable domain. Two modes of extinction are distinguished: quenching at low flow speeds or small stagnation flow strain rates, stagnation and wake blowoffs at high flow velocities or high flow strain rates. Merging between the two branches occurs at a flow strain rate of 7.5 s(-1). The minimum oxygen percentage is slightly <14 %. This long duration, remotely controlled microgravity experiment enables us to study the effect of purely forced flow on the burning of bulk solids, especially in the low velocity regime, not easily achievable in normal gravity. In total, 7 hours of microgravity PMMA burning time is recorded. Several examples from recorded test runs are shown to illustrate the special features of the experiment.

Opposed flame spread is a classic fundamental fire research problem that is controlled by the simultaneous interactions of heat transfer, chemical kinetics, and fluid dynamics mechanisms. Although the problem has been widely studied before, there is still a need for more fundamental research particularly at limiting conditions, such as near extinction in non-standard atmospheres such as those expected in future spacecraft and habitats. The determination of the extinction of a material is often based on the oxygen volume fraction below which the material does not burn. and is referred to as the limiting oxygen index, or concentration (LOI or LOC). This extinction limit depends on the fuel properties and environmental conditions. This work presents a simplified analytical model of the oxygen volume fraction (LOC) leading to the extinction of opposed flame spread over thin combustible materials. The analysis predicts two oxygen volume fraction boundaries for extinction: a “heat transfer” boundary and a “chemical kinetics” boundary. The former is given by the conditions needed to heat the solid to its pyrolysis temperature, and the latter by the conditions needed for the gas phase reaction to be maintained. The combination of the two boundaries provides an overall boundary for the oxygen volume fraction for extinction as a function of material properties, gas flow velocity, ambient pressure, gravity level, and external heating. The results are compared with available experimental data for the LOC of thin paper sheets under different environmental variables to provide relevant information about the material flammability at near extinction conditions, particularly in spacecraft environments.

Six Saffire experiments with SIBAL fabric (75 % cotton, 25 % fiberglass) were carried out in orbit aboard different Cygnus vehicles after unberthing from the ISS. In five tests the ambient atmosphere was ISS cabin air, which contained approximately 21 % O2 at ambient pressure. The sixth test was an exploration atmosphere (31 % O2 at 54 kPa). Relative humidity was measured but not controlled. In each experiment, after the concurrent flame passed over the fabric, glowing smolder spots were observed to spread over the hot reactive char. Characteristics of this smolder wave have been measured as a function of forced flow and relative humidity level. The spots showed a slight preference to spread concurrently in air but had no preference in lateral spread. An increase in the flow modestly increased the number of spots while an increase in ambient air relative humidity diminished the length of the smolder wave and linearly decreased the number of spots due to the endothermic heterogenous surface reaction C(s) + H2O > CO + H2. The smolder wave was found to have a lognormal distribution of glowing spots relative to distance from the onset of smolder. The smolder wave spread at the same speed as the flame after an ignition transient since the spots were generated just beyond the flame base when the hot reactive char was exposed to the fresh oxidizer flow. The smolder wave was made up of growth and extinction phases. The initial population growth occurs at an intrinsic growth rate, and after peaking, the population exhibits an exponential decay in the number of spots as extinctions become dominant. The populations follow a logistic population model with carrying capacity (K) and intrinsic growth rate (ro) parameters taken from the data and assuming a starting population of unity.

The limiting oxygen volume fraction (LOC) for flame spread is a parameter used to determine flammability and fire hazard. The LOC depends on the solid material, geometry, and environmental conditions. It is important to understand the relationship between these conditions and the LOC, particularly for environments different than Earth atmospheres. Future spacecraft will have sub-atmospheric cabin pressures designed to reduce preparation time for extravehicular activities. This work attempts to provide further information on this aspect of solid fuel flammability by conducting experiments to determine the effect of ambient pressure and external radiant flux for flames spreading downward over cylindrical samples of black polymethyl methacrylate (PMMA). The experimental methodology is based on test methods ASTM 2863 (LOI) and ASTM E-1321 (LIFT), that are used to determine some aspects of material flammability, specifically extinction and flame spread under external radiant heating. Experiments are conducted in a pressure chamber at pressures from 40 to 100 kPa with radiant heating from 0 to 3.3 kW/m2. 2 . Results show that decreasing either the ambient pressure or the external radiant flux increases the LOC of the PMMA. The results are correlated in terms of the ratio of the partial pressure of oxygen, radiant flux, and ambient pressure with an equation that includes the LOC at atmospheric pressure. It is found that the heat transfer and chemical kinetic mechanisms that control the flame spread are primarily dependent on the oxygen molar fraction, explaining the experimental observations. The data from this work will be compared with experiments to be conducted in the International Space Station (ISS) under the SoFIE-MIST project to provide further understanding of the effect spacecraft environments have on the LOC of materials. The results will give further insight into the flammability of materials, particularly at sub-atmospheric ambient pressures found in spacecraft, aircraft, and high-altitude locations.

Testing was performed in a partial gravity centrifuge drop vehicle in the Zero Gravity Research Facility with cast PMMA rods in concurrent axial buoyant stagnation flow to determine flammability limits as a function of partial gravity level. The partial gravity levels studied varied from 0.04 g to a simulated 1g for ambient oxygen concentrations from 13.2% to 15.2% O2 by volume at an average 57.6 kPa (8.4 PSIA) ambient pressure, which is very close to the anticipated Lunar habitat pressure. The PMMA flammability boundary as a function of oxygen concentration and gravity level has been determined at five gravity levels. Coriolis effects appear to be minimal in the stagnation region where the flame is stabilized while the tips of the flame do show some bending at the higher gravity levels. Lunar gravity levels are near the minimum in the oxygen - gravity level flammability boundary. This suggests that fire is a significant safety risk for future exploration missions to the Moon since materials are screened in normal gravity to evaluate their safe use in space. If normal gravity screening is not conservative, a material derating method will need to be applied to ensure the material is not flammable on the Moon. Since the blowoff boundary appears to be linear with forced flow velocity, it may be possible to conduct elevated forced flow blowoff testing that could then be extrapolated down to effective Lunar gravity levels to provide an oxygen delta between 1g and Lunar flammability limits to derate the material.

The response of a diffusion flame around a PMMA solid sphere to a sudden transition to zero-gravity is investigated both experimentally and numerically. The initial flame is established in normal gravity with a low speed forced flow in a 17% oxygen by volume atmosphere. An abrupt (step change) normal-to-zero gravity transition occurs when the test package is released in the drop tower. The dynamic flame response is recorded by video and modeled numerically. By the end of the 5.18-s drop, the flame tip retreats, and the flame base may either remain stabilized near the forward stagnation region or extinguish depending on several parameters. The two parameters investigated are: forced flow velocity and the degree of preheating in the surface layer of the solid sample. The amount of preheating or equivalently the conductive flame heat loss rate to the solid interior is varied by controlling the duration of the normal gravity burning before releasing to microgravity. The heat loss is quantified by using embedded thermocouples to measure the solid subsurface temperature gradient near the stagnation point. The detailed numerical model reveals details of the flow field and flame structure including oscillatory extinction and quantifies the various transient gas-solid surface energy balance terms.

Long duration microgravity experiments aboard the International Space Station determine diffusion flame extinction limits of PMMA spheres. Upon ignition from an electrically-heated coil, the 4-cm-diameter samples are exposed to forced flows ranging from 0.2 to 80 cm/s and an oxygen ranging from 13 to 28 % in one atmosphere total pressure. Extinction is reached as oxygen concentration gradually decreases by natural depletion. Five extinction tests are presented at different flow velocities and oxygen concentrations. Quenching at low velocity is observed with the flame tip shrinking upstream and blowoff is observed when a hole forms in the flame at the forward flow stagnation point. However, these processes are not quasistatic. The quenching motion involves periodic flame tip pulsating toward downstream and shrinking upstream with a continually decreasing flame size at the end of each cycle. In blowoff, the flame base pulsates between the flame hole and the downstream location, although with far fewer cycles compared to quenching. The pulsations appear to be the result of a premixed flame front spreading into a combustible mixture. Two specific cases are discussed in more details. In the first, at very low flow velocity ( 0.4 cm/s) and elevated oxygen, self-sustained flame tip cyclic pulsations are observed for a lengthy period ( 15 min). In the second case with a higher flow velocity (50 cm/s), the diffusion flame is stabilized at the shoulder of the spherical sample after local stagnation point blowoff. With steady decrease in ambient oxygen due to depletion, spinning flamelets are formed. The long-duration microgravity environment makes it possible to observe these interesting and detailed extinction processes.

Concurrent flame spread data for thermally-thin charring solid fuels are presented from Saffire and BASS experiments performed in habitable spacecraft for three duct sizes, five sample sizes, two materials, and two atmospheres. The flame spread rates and flame lengths were strongly affected by duct size even for the relatively large ducts (> 30 cm tall). A transient excess pyrolysis length (i.e., flame length overshoot) was observed for the cotton fabric that burned away, which indicates that the transient excess pyrolysis length phenomenon is caused by more than just the flame moving into the developing boundary layer thickness as was the case with the SIBAL sample. A burnout time, defined as the pyrolysis length divided by the flame spread rate, normalized the pyrolysis length histories into a single curve with a steady burnout time of 22 s for the SIBAL fabric. The transient excess pyrolysis length is hypothesized to be a post-ignition flame growth transient for the essentially two-dimensional flames where the burnout time becomes very long until the preheat and pyrolysis lengths develop. The three-dimensional flames over narrow samples have lateral thermal expansion and lateral oxygen diffusion which allows them to transition to a steady state length without the transient excess pyrolysis length. Surface temperature profiles, nondimensionalized by the pyrolysis length, indicate that the temperature profiles exhibit the same shape across the pyrolysis zone. A surface energy balance calculation in the preheat region revealed that the heat flux increased rapidly at the pyrolysis front to near the critical heat flux for ignition. An estimate of the acceleration of the inviscid core flow in the duct due to thermal expansion and developing boundary layers on the duct walls and the SIBAL sample surface seems to explain the observed spread rate trends across three duct sizes and multiple sample sizes. (c) 2022 Published by Elsevier Inc. on behalf of The Combustion Institute.

Fire is a very serious threat in any confined volume but particularly so when a crew is confined in a spacecraft far from Earth, where there is little or no possibility of receiving aid or being rescued. Therefore, every measure must be taken to prevent a fire from happening. Even with material controls and other fire prevention measures, fire detection, suppression, and clean-up systems must be provided on spacecraft to allow the crew to respond to a fire. Unlike flames in normal gravity, fires in low gravity are not supported by the strong pumping of oxidizer and fuel into the flame zone and the subsequent pumping of the reactants out of the reaction zone. The lack of buoyancy allows a flame to propagate at lower air velocities than on Earth that changes the transport of oxygen into the flame, heat transfer to the fuel surface, and transport of heat out of the reaction zone. Under these conditions, the flame spread can occur at conditions and with characteristics that are not duplicated readily in normal gravity because of the large buoyant flows generated by the flame. Differences in flame characteristics also impact requirements for fire detection and suppression of spacecraft fires. Of course, any fire response protocol must be compatible with the other spacecraft systems such as the Environmental Control and Life Support System (ECLSS).

Although thermoplastics possess many desirable traits as solidified materials, they are combustible and burn readily. Post burning samples of polymethylmethacrylate (PMMA) are the focus of this study. During flame spread, PMMA forms a bubble containing molten surface layer that influences the spread rate and the surface mass efflux. This study examines the formation and distribution of bubbles inside a PMMA sample that has previously been subjected to flame spread and then re-hardened into its solid state. Experiments discussed herein were conducted in a Narrow Channel Apparatus (NCA), which reduces the influences of gravitation (buoyancy) on flame spread. Bubble sizes and counts were determined using digital image analysis (DIA). The burnt samples were analyzed by dividing sample images into eight equal area sections. Frequency distributions of bubble size (area) were compared. Distribution functions were fitted against the empirical probability density function (PDF) for the bubble size distribution. The log-normal distribution predicts the bubble size distribution for all segments. The bubble distribution function can be used to describe physical processes inside the polymeric material as it undergoes thermal transformation by the spreading flame.

Surface regression was studied during opposed flow flame spread over thermally thick polymethyl-methacrylate (2.54 cm) under variable forced flows in a horizontal Narrow Channel Apparatus (NCA). The sample nominal thickness was 2.54 cm and opposed flow velocities varied from 8 -45 cm/s. The post-burn samples were examined for the regressed surface shape. A formula was derived whose primary input is the re-gression profile: either the spread rate or the mass flux can be calculated from other inputs. The experimental video record gives the flame spread rate. The measured surface profile, along with the flame spread rate, gives the mass flux from the surface. The regression depth increases with increased mass average flow velocity. For mass average flow velocities below 12 cm/s, when the surface barely regresses (2 mm over an & AP; 12 cm length), the average power law exponent for mass loss rate approximates to-1/2. For mass average flow velocities greater than 25 cm/s, the average power law dependency approximates to-1/4. The transition from the-1/2 to-1/4 power occurs over a narrow range of opposed flow stretch rates and mass mean flow velocities. For the higher opposed flow velocities the regression depth is a prominent feature of the burned sample. Based on the hypothesis of partitioning of physical influences, the contributions to flame spread of the flow field, finite rate chemistry and surface regression are evaluated from the experimental data. A semi-empirical flame spread formula is developed & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Ignition limits and ignition delay times of thermally-thick polymethyl methacrylate (PMMA) spheres (4-cm diameter) and cylinders (5.08-cm diameter) are investigated by placing an electrically-heated wire coil beneath the sample in a slow forced flow. The wire serves as both a heater for the solid sample and a gas-phase hot spot to ignite the oxidizer-fuel vapor mixture. In the experiments, the wire's electrical current and power-on time, its distance from the sample surface, the forced flow velocity, oxygen percentage, and pressure are systematically varied. Ignition limits as a function of the oxygen percentage, pressure, and igniter current are determined. Two ignition delay times are determined: igniter-assisted ignition and self-sustained ignition. These critical times are also functions of the degree of heat-up of the sample interior. Solid temperature profiles are measured using embedded thermocouples to determine the sub-surface temperature gradients at the moment of ignition. The heat transfer modes from the igniter to the solid sample are numerically investigated during the pre-pyrolysis heat-up period. At the forward stagnation-point region where ignition occurs, radiation is greater than convection and accounts for approximately 60% of the total heat input. This is consistent with the experimental findings on ignition delay times and heat flux measurements.

There are multiple situations in which fires may occur at environmental conditions that are different than standard atmospheric conditions. Changes in ambient pressure, oxygen concentration, flow velocity, the presence of an external heat source or gravity may change the flammability and fire dynamics of materials. The objective of this work is to study the effect of external radiant heating on downward flame spread over cylindrical samples of polymethyl methacrylate (PMMA). In this work, experiments under normal gravity and atmospheric ambient conditions are conducted using a variable heat flux with peak values up to 13.2 kW/m(2). A forced flow of air with a mass-mean velocity of 10 cm/s is used during the experiments. Flame spread rates were measured from video processing of the experiments at different conditions. Results show that the flame spread rate measured depends strongly on the amount of radiant heating provided. An analysis is presented to correlate the flame spread rate with the energy applied to the surface of the sample and the surface temperature. The results provide 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.

Better understanding the burning behavior of solid fuels is necessary in order to be able to estimate more accurately the fire risk that they might represent. The burning rate is an essential component when assessing the growth of a fire, since it determines the heat released during the fuel burning. The objective of the present work is to study the effect of ambient pressure and oxygen concentration on the burning of cylindrical samples of polymethyl-methacrylate (PMMA) in opposed flow flame spread conditions. Experiments are conducted using pressures ranging between 100 and 30 kPa and oxygen concentrations between 19% and 23%, with a forced flow velocity of 100 mm/s. The experimental conditions reflect environments potentially encountered in future spacecraft cabins. The data collected show that during downward opposed flame spread the surface regression rate, and consequently the mass burning rate, decreases with decreasing ambient pressure and increases with decreasing oxygen concentration. The data presented is correlated with a simplified model to estimate the surface regression rate based on the mixed heat convection coefficient and the B number. Model predictions are made based on fuel properties as well as the relevant environmental conditions. The predictions of the model agree well capturing the effect of ambient pressure and oxygen concentration. The correlations provide information about the variations in the burning rate in environments with variable pressure and oxygen concentration, providing guidance for potential for fire safety testing and design in different ambient conditions.

The Burning and Suppression of Solids (BASS) experiment hardware is a small flow duct that provides containment for small scale burning of solid samples within the Microgravity Science Glovebox (MSG) aboard the International Space Station (ISS). A video camera with a data overlay and a 35-mm still camera record the combustion events. The controls (ignition, fan speed, etc.) are operated by an astronaut while the principal investigator team monitors the experiment from the ground and communicates directly with the astronaut. For the first time on ISS, BASS–II utilized MSG working volume dilution with gaseous N2. We developed a perfectly stirred reactor model to determine the N2 flow time and flow rate to obtain the desired reduced O2 concentration in the working volume for each test. We calibrated the model with the Compound Specific Analyzer-Combustion Products (CSA-CP) O2 readings offset using the Major Constituents Analyzer reading of the ISS ambient atmosphere data for that day. This worked out extremely well for operations, and added a new vital variable, ambient O2 level, to our test matrices. The main variables tested in BASS–II were ambient O2 concentration, ventilation flow velocity, and fuel type, thickness, and geometry. BASS–II also utilized the onboard CSA-CP for O2 and CO readings, and the Carbon Dioxide Monitor for CO2 readings before and after each test. Readings from these sensors allow us to evaluate the completeness of the combustion. The O2 and CO2 readings before and after each test were analyzed and compared very well to stoichiometric ratios for a one-step gas-phase reaction. The CO versus CO2 followed a linear trend for some datasets, but not for all the different geometries of fuel and flow tested. We calculated the heat release rates during each test from the O2 consumption and burn times, using the constant 13.1 kJ of heat released per gram of O2 consumed. The results showed that most of the tests had heat release rates well below 100 W. Lastly, the global equivalence ratio for the tests is estimated to be fuel rich, 1.3 on average using mass loss and O2 consumption data.

Quantitative image analysis of infrared (IR) measurements are compared to visible color images for simultaneous upstream and downstream flame spread over thermally thin cellulose samples in low speed forced flow in microgravity. These results are a unique set of data that provide a wealth of information to guide future modeling effort s. Test conditions at ambient pressure were conducted in 21-50% O2 by volume (N2 balance) and forced flow velocities from 2 cm/s to 20 cm/s. The tests conditions encompass strictly upstream flame spread to simultaneous upstream and downstream flame spread. Steady upstream flame spread is observed while the downstream flame only begins to spread at the highest flow velocities due to the oxygen shadow cast by the upstream flame (oxygen depleted region downstream of the flame). Blackbody surface temperature gradients for the upstream flame are an order of magnitude larger than the downstream gradients. Upstream preheat lengths decrease with flow while downstream preheat lengths increase. Downstream pyrolysis lengths reach steady state for most cases and increase with convective heating while upstream pyrolysis lengths increase with time. Surface thermocouple (TC) histories were non-dimensionalized to obtain a characteristic surface temperature profile using new scaling analyses. Applying the ellipticity of the leading edge to the scaling reveals the local flow velocity at the leading edge is on the order of diffusive velocities and the flame thermal expansion acts as a significant barrier to divert the flow. A surface energy balance reveals that the peak heat flux for both upstream and downstream flames increases with oxygen concentration and forced flow velocity. The upstream flame heat flux is equally divided between fuel heat up and surface radiation. The downstream heat flux goes almost exclusively to surface radiation. Comparisons of these results to previous applicable research are provided throughout the text and generally agree well. (c) 2021 Published by Elsevier Inc. on behalf of The Combustion Institute.