Olson, SL; Ruff, GA; Ferkul, PV; Owens, JC; Easton, J; Liao, YT; T'ien, JS; ... Urban, DL; + view all Olson, SL; Ruff, GA; Ferkul, PV; Owens, JC; Easton, J; Liao, YT; T'ien, JS; Toth, B; Jomaas, G; Fernandez-Pello, C; Legros, G; Guibaud, A; Fujita, O; Smirnov, N; Urban, DL; - view fewer (2023) The effect of duct size, sample size, and fuel composition on concurrent flame spread over large cellulose samples in microgravity. Combustion and Flame , 248 , Article 112559. 10.1016/j.combustflame.2022.112559.

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Abstract

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.

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