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Abstract Opposed-flow flame spread over solid materials has been investigated in the past few decades owing to its importance in fundamental understanding of fires. These studies provided insights on the behavior of opposed-flow flames in different environmental conditions (e.g., flow speed, oxygen concentration). However, the effect of confinement on opposed-flow flames remains under-explored. It is known that confinement plays a critical role in concurrent-flow flame spread in normal and microgravity conditions. Hence, for a complete understanding it becomes important to understand the effects of confinement for opposed-flow flames. In this study, microgravity experiments are conducted aboard the International Space Station (ISS) to investigate opposed-flow flame spread in different confined conditions. Two materials, cotton-fiberglass blended textile fabric (SIBAL) and 1 mm thick polymethyl methacrylate (PMMA) slab are burned between a pair of parallel flow baffles in a small flow duct. By varying the sample-baffle distance, various levels of confinement are achieved (H = 1–2 cm). Three types of baffles, transparent, black, and reflective, are used to create different radiative boundary conditions. The purely forced flow speed is also varied (between 2.6 and 10.5 cm/s) to investigate its interplay with the confinement level. For both sample materials, it is observed that the flame spread rate decreases when the confinement level increases (i.e., when H decreases). In addition, flame spread rate is shown to have a positive correlation with flow speed, up to an optimal value. The results also indicate that the optimal flow speed for flame spread can decrease in highly confined conditions. Surface radiation on the confinement boundary is shown to play a key role. For SIBAL fabric, stronger flames are observed when using black baffles compared to transparent. For PMMA, reflective baffles yield stronger flames compared to black baffles. When comparing the results to the concurrent-flow case, it is also noticed that opposed-flow flames spread slower and blow off at larger flow speeds but are not as sensitive to the flow speed. This work provides unique long-duration microgravity experimental data that can inform the design of future opposed-flow experiments in microgravity and the development of theory and numerical models. 

Firebrands are known to be able to ignite not only vegetation but also various structures found in wildland-urban interface (WUI) area. Especially, firebrands located close to each other on a combustible substrate increase the likelihood of ignition and the subsequent fire. To elucidate the ignition mechanism of firebrands, experiments are performed using a 3 by 3 square array of flaming firebrands deposited on a 6.35 mm thick birch plywood. The spacing of the firebrand is varied in each experiment, ranging from 10 to 30 mm. The deposited mass of firebrands lies between 13 and 15 g. Ambient wind is imposed parallel to the plywood surface to investigate its effect on the ignition and the subsequent flame spread over the fuel. Three different wind speeds 0, 0.5, and 0.75 m/s are tested. During the experiments, mass loss of the plywood and the deposited firebrands is recorded. Video cameras are used to monitor the burning process. An infrared camera is also used to monitor the temperature of the firebrands and the plywood. The experiment results indicate that the firebrands with the spacing greater than 20 mm are able to burn only the surface of the plywood until the firebrands burn out. When the spacing between firebrands is smaller than 20 mm, the plywood is ignited and continues to burn even after the firebrands are fully consumed. It is also observed that the flame is able to spread downstream at 10 mm spacing under ambient wind speed of 0.5 m/s. Results from this study demonstrate the significant influence of spacing between the firebrands on the ignition and the burning behavior of the substrate materials. 

Firebrand attack has been shown to be one of the key mechanisms of wildfire spread into Wildland-Urban Interface (WUI) communities. The ignition propensity of materials caused by firebrands depends on not only the attributes (e.g., shape, size, numbers) but also the distribution of firebrands after landing on the substrate materials. To help characterize this process, this study aims to first investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments were conducted using 9 wooden cubes, 19mm-long on each side. These samples were arranged in a 3 by 3 square pattern on suspension wires. The gap spacing (s) between the cube samples varies from 0 to 30 mm. Burning process was recorded using video cameras. Sample mass loss and temperatures were monitored during the flaming and smoldering processes. The results show that when s ≤ 10 mm, flames from individual samples merged. When the gap spacing reduces, the mass loss rate first increases but starts decreasing at s = 10 mm where flame merging occurs. The flame height has a similar non-monotonic dependency on the gap spacing and the maximum flame height occurs at s = 5 mm. Compared to the case with s = 10 mm, cases with a smaller gap spacing (s = 2.5 and 5 mm) have a larger flame height but a smaller sample mass loss rate. This indicates that a reduced air entrainment leads to an increase in the flame height despite of a decreased flame heat feedback to the solid samples. The heating rates of each sample were also calculated to investigate the local burning behaviors. The analysis showed a weaker flame heat feedback to the sample at the center for cases with under-ventilated combustion. Last, gaseous flame height was corelated to the solid burning rate. The correlation was also compared with previous empirical equations concerning liquid pool fires of different heat release rates. 

Abstract The objective of this work is to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. A three-dimensional transient computational fluid dynamics (CFD) combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady-state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has multiple effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned nonmonotonic trend of flame spread rate as duct height varies. Near the quenching duct height, the transient model reveals that the flame exhibits oscillation in length, flame temperature, and flame structure. This phenomenon is suspected to be due to thermodiffusive instability. 

A numerical study is pursued to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. This is done in support of upcoming microgravity experiments aboard the International Space Station. For the numerical study, a three-dimensional transient Computational Fluid Dynamics combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has competing effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned non-monotonic trend of flame spread rate as duct height varies. This work relates to upcoming microgravity experiments, in which flat thin samples will be burned in a low-speed concurrent flow using a small flow duct aboard the International Space Station. Two baffles will be installed parallel to the fuel sample (one on each side of the sample) to create an effective reduction in the height of the flow duct. The concept and setup of the experiments are presented in this work.