Flame spread over thin circular PMMA rods

This article presents a series of opposed flow flame spread experiments, conducted using cast cylindrical PMMA (acrylic) rods, 80 mm long and of diameters 1 mm and 0.5 mm, in normal gravity and microgravity environments. The experiments are primarily conducted for molar oxygen levels of 21%, 23% and 40% at 1 atmosphere pressure and opposed flow speed ranging from 0 cm/s to 25 cm/s. Experiments are also conducted in normal gravity for oxygen levels 21% to 60% to study the effect of oxygen level. At near ambient oxygen levels, the flame shape in microgravity resembles a mushroom and there are fluctuations at the leading edge due to sporadic fuel jets emanating from bursting bubbles at the fuel surface. The flame spreads faster in microgravity, which is determined to be due to increased preheat length. Preheat length is measured for flame spreading over 1 mm diameter fuel rod under no flow condition using fine thermocouples and is found to be 0.98 cm in microgravity and 0.34 cm in normal gravity. It is found that for scaling analysis, including Stefan flow velocity in the definition of reference velocity results in a reasonable estimate of the preheat length of a flame spreading in quiescent microgravity environment. At high oxygen levels (> 35%) the flame becomes turbulent and spreads at nearly the same rate in normal gravity and microgravity environments and the flame spread rates are not affected by external flow speed.