Concurrent flame growth, spread, and quenching over composite fabric samples in low speed purely forced flow in microgravity

Flame growth, spread, and quenching extinction over a thin composite cotton-fiberglass fabric blend (referred to as the SIBAL fabric) were studied in low-speed concurrent purely forced flows aboard the International Space Station. The tests were conducted in a small flow duct within the Microgravity Science Glovebox. The fuel samples measured 1.2 and 2.2 cm wide and 10 cm long. Ambient oxygen was varied from 21% down to 16% molar concentration and flow speed from 55 cm/s down to 1 cm/s. A slow purely forced flow resulted in a small flame, enabling us to observe the entire history of flame development including ignition, flame growth, steady spread (in some cases), and decay at the end of the sample. In addition, by decreasing flow velocity during some of the tests, low-speed flame quenching extinction limits were determined as a function of oxygen percentage. The quenching speeds were found to be between 1 and 5 cm/s with higher extinction speeds in lower oxygen atmospheres. The shape of the quenching boundary supports the prediction by earlier theoretical models. These long duration microgravity experiments provide a rare opportunity for solid fuel combustion since microgravity time in ground-based facilities is generally not sufficient. This is the first time that a low-speed quenching boundary in concurrent spread is mapped in a clean and unambiguous manner. A previously developed three-dimensional transient model is modified to compare with the experiment. The modification includes the use of two-step SIBAL fabric pyrolysis kinetics where the rate constants are determined using Thermo-Gravimetric Analysis data. The model yields good quantitative comparison on the quenching boundary, the flame transient development, and the steady flame spread rates. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.