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Ambient pressure and gravity are important parameters in buoyant flow that governs upward flame spread process. Based on the concept of pressure modelling, this experimental study investigates extinction and upward flame spread process of a thermally-thin solid fuel in different pressure and oxygen conditions. Experiments are performed in a combustion chamber in air at different pressures (ranging from 10 kPa to 100 kPa) and different oxygen molar fraction environment (9–21 %). As pressure increases, different burning behaviors are observed: no ignition, partial flame spread, steady flame spread, and accelerating flame spread. Similar trend is observed as the ambient oxygen molar fraction increases. In partial pressure conditions (e.g., 25–50 kPa), flames exhibit characteristics that are typically observed in micro- and partial gravity environments: blue and dim. Flame spread rate and sample burnt length are deduced and compared between different pressure and oxygen levels. Overall, the burning intensity and the flame spread rate decrease with the decrease in ambient pressure and oxygen. The decrease in flame spread rate at reduced pressure is attributed to increase in flame standoff distance and decrease in convective heat transfer to the solid, whereas the decrease in flame spread rate in reduced oxygen molar fraction environment is attributed to decrease in flame temperature. Lastly, current and previous studies performed at different ambient environments are correlated using the concept of flame standoff distance (δ f) , which is estimated using the theoretical viscous boundary layer thickness (δ v). It was found that approximating δ f ∼ δ v for forced flow and δ f ∼ 1 / 3 δ v for natural flow can predict the flame spread rate reasonably well for data obtained in micro-, partial, and normal gravities, for a wide range of environmental conditions away from extinction limits. [ABSTRACT FROM AUTHOR]

Concurrent flow flame spread experiments are conducted over thermally thin solid fuels in microgravity aboard the International Space Station (ISS) under varying levels of confinement. Samples of cotton fiberglass blended textile fabric are burned in air flows in a small flow duct. Baffles are placed parallel to the sample sheet, one on each side symmetrically. The distance between the baffles is varied to change the confinement of the burning event. Three different materials of baffles are used to alter the radiative boundary conditions of the space that the flame resides: transparent polycarbonate, black anodized aluminum, and polished aluminum. In all tests, samples are ignited at the upstream leading edge and allowed to burn to completion. The results show that at low flow speeds (<17 cm/s), the flame reaches a steady state for all tested baffle types and baffle distances. The spread rates and flame lengths at the steady state increase first and then decrease when the baffle distance decreases, resulting in an optimal baffle distance for flame spread. Furthermore, there exists a limiting baffle distance below which the flame fails to spread. It is concluded that the confinement imposed by the baffles accelerates the flow during the combustion thermal expansion and the baffles reflect flame radiation back to the sample surface, both of which intensifying the burning. However, the confinement also limits the oxygen supply and introduces conductive heat loss away from the flame. At the same baffle distance and imposed flow speed, flame length and spread rate are largest for polished aluminum baffles, and lowest for transparent polycarbonate baffles. The differences are most prominent at intermediate tested baffle distances. While the radiative heat feedback from the baffles is expected to increase when the baffle distance decreases, flame length and flame spread rate are similar for all baffle types at small baffle distances as the combustion is limited by the reduced oxygen supply. [ABSTRACT FROM AUTHOR]

Confinement has been shown to play an important role on burning behavior of solid materials in microgravity. Previous studies using small flow ducts (duct height < 7.6 cm) concluded that the flame spread rate is linear to the inverse of the duct height. The underlying physics for this correlation is the combustion thermal expansion that leads to different flow acceleration in different flow duct cross-section areas. In recent NASA microgravity fire experiments, Saffire, wide solid fuel samples were burned in two large flow ducts. In these experiments, the duct heights (30 cm and 50 cm) were significantly larger than the flame standoff distance (∼1 cm) and the effect of thermal expansion was expected to be minimal. However, the experiment results showed that both flame spread rate and pyrolysis length increased more than 95 % when the duct height decreased by 40 %. To understand the underlying physics of the experimental results, two-dimensional transient numerical simulations are performed. The model configuration is based on the Saffire experiments. The model successfully predicts the transient flame development processes and the flame spread rates observed at both duct heights. In addition, a methodology is developed to deduce net heat flux distribution on the sample surface using thermocouple data obtained in the experiments. The deduced heat flux profiles in the experiments and in the numerical results agree qualitatively and quantitively. After the model is validated against the experimental data, a parametric study on the duct height is performed. When duct height decreases, flame spread rate increases. It is found that, at large duct heights (> 20 cm), the inverse of the flame spread rate has a linear dependency on the inverse of the duct height. Analytical analysis of cold flow (without combustion) demonstrates that this relationship is due to the different flow profiles on duct cross-section plane when duct height varies (i.e., a hydrodynamic effect). Effects of duct ceiling radiation properties are also considered. Radiation reflection from the duct ceiling increases the heat input on the sample surface, resulting in an increased flame spread rate. This effect is stronger at a smaller duct height. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical study is performed to investigate concurrent-flow flame spread over thin solid fuels in microgravity. The model considers the burning scenarios of a recently concluded ISS microgravity experiment, Confined Combustion. Cellulose based thin samples are burned in a small flow duct. The height of the flow duct and the radiation reflectance of the duct wall are varied. Flame development and steady spread flame characteristics are compared with the experimental results at various duct heights. The numerical results demonstrate that the confinement imposed by the duct walls accelerates the flow during the combustion thermal expansion, enhancing the conductive heat transfer to the solid samples. When the duct height is below a critical height, the flow confinement limits oxygen supply to the flame, and the duct wall acts as a conductive heat sink. As a result of the interplay of these effects, the flame spread rate and pyrolysis length first increase and then decrease as the duct height decreases. Eventually, the flame fails to spread at a quenching duct height. In addition, side-leading concave (two-teeth fork shaped) flames are observed below the critical duct height. This flame shape increases the flame surface area and facilitates oxygen transport to the combustion zone. When the duct wall reflectance varies, a higher reflectance yields a longer pyrolysis length and a faster spread rate. This is due to enhanced heat input to both the solid sample surface and the gaseous flame. This effect is most significant for medium duct heights. At large duct heights, the duct wall is far from the flame and the sample. At small duct heights, while flame spread rate increases with the wall reflectance, the pyrolysis and flame length remain similar as combustion is limited by oxygen supply. [ABSTRACT FROM AUTHOR]

Microgravity experiments are performed to study the effects of confinement on the burning behavior of polymeric solid materials. Flat, 100 × 22 × 1 mm PMMA samples are burned in concurrent air flow in a small flow duct aboard the International Space Station. Three different burning scenarios are examined, double-sided, single-sided, and parallel samples. In the first two scenarios, single samples are burned on both sides and on one side, respectively. Flat baffles are placed parallel to the sample to confine the available space for combustion. The distance between the baffle and the sample (H) is varied in different tests. In each test, imposed flow is reduced in steps and steady flame spread is achieved at each flow speed until the flame quenches. The results show that at the same confined condition, steady state flame length and spread rate are proportional to flow speed over the range tested. When confinement increases (or H decreases), the flame spread rate and flame length increase first and then decrease. In addition, the quenching flow speed decreases and then increases with decreasing H. These results suggest that the confinement can increase or decrease solid fuel flammability depending on conditions. In the third burning scenario, two PMMA samples are placed parallel to each other separated by a distance H. Twin flames are observed and combustion is confined between the two samples. Among the three tested burning scenarios, twin flames have the largest flame length and burning rate at the same confinement level (H). This is because the thermal interaction between the twin flames enhances the heat feedback to the solid fuel and reduces the relative heat loss to the surrounding flow duct. Comparing single- and double-sided flames with the same baffle-sample distance, the spread rate of a single-sided flame is slightly less than half of that of a double-sided flame. This is due to the halved pyrolysis area exposed to the flame and heat loss on the back side of the sample. Optimal transport of oxygen to the flames also plays a role. [ABSTRACT FROM AUTHOR]

Microgravity experiments are performed to study concurrent-flow flame spread over an array of thin cellulose-based fuel samples, using NASA Glenn Research Center's 5.18 s drop tower. Sample segments are distributed uniformly, separated by air gaps, on a sample holder. The exposed width of each sample segment is 5 cm. Two segment lengths, 0.5 cm and 1 cm, are tested. The gap sizes are varied in different tests, ranging from 0.5 to 5 cm. In all tests, a low-speed air flow (30 cm/s) is imposed and the upstream-most fuel segment is ignited by an electrical ignition wire. Upon ignition, the flame spread is recorded by two video cameras from the front and side-view angles. Spread rates, flame lengths, and burning durations are extracted using a custom video processing code. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. A burning discrete fuel segment, before it is consumed, needs to ignite the subsequent segment in order to have flame propagation across the gap. During this process, larger gaps between samples reduce the effective fuel load, increasing the apparent flame spread rate. However, larger gaps also reduce the heat transfer between adjacent samples, decreasing the sample burning rate. As a result, as the gap size increases, the flame spread rate increases but the burning rate decreases. At the same gap size, the flame spread rate is higher for the shorter tested sample segments. When considering sample configurations of the same fuel ratio (fuel length over the summation of the fuel and gap lengths), the spread rates are similar. This trend remains until a critical gap size is reached and flame fails to propagate across the entire array of samples. The critical gap sizes are similar for the two tested sample segment lengths and are suspected to be determined by the flame length. [ABSTRACT FROM AUTHOR]

Abstract For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called "Saffire," studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed. [ABSTRACT FROM AUTHOR]

An unsteady two-dimensional numerical model was used to simulate concurrent flame spread over paper-like thin solid fuels of discrete configurations in microgravity (0 g with 20 cm/s) and in normal gravity (1 g). An array of ten 1 cm-long fuel segments was uniformly distributed in the flow direction (0 g) or in the vertical direction (1 g). A hot spot ignition source was applied at the upstream leading edge of the first fuel segment. The separation distance between the fuel segments was a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3 cm. Using this setup, the burning characteristics, spread rate of the flame base, and the solid burning rate were examined. The flame base spread rates in both 1 g and 0 g cases increase with the separation distance. This is due to the flame jumping across the gaps. For the solid burning rate, the dependency on the separation distance is different in 1 g and 0 g cases. At a flow velocity of 20 cm/s in 0 g, the flame reaches a limiting length and the flame length is approximately the same for all fuel configurations. As the separation distance increases, the heating length (the fuel area exposed to the flame) decreases, resulting in a decreasing total heat input and a decreasing solid burning rate. In 1 g, the flame is long and extends to the last fuel segment before the first fuel segment burns out. This suggests that the heating length is approximately the same in all simulated cases (∼total fuel length). However, the flame standoff distance decreases when the separation distance increases. This results in an increasing total heat input and an increasing solid burning rate. Terrestrial experiments were conducted to validate the 1 g model. The experimental results agreed reasonably with the model predictions of burning characteristics, burn durations, and flame spread rates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]