Flame Spread Over Thin Circular PMMA Rods: Experiments

This article presents a series of opposed-flow flame spread experiments conducted using cast cylindrical PMMA (acrylic) rods of the smallest diameters (1 mm and 0.5 mm) studied so far in normal-gravity and microgravity environments. The experiments are primarily conducted in 1 atmosphere pressure, for near ambient oxygen levels of 21%, 23% and elevated oxygen level of 40%. The opposed flow speed ranges from 0 cm/s to 25 cm/s. Additional experiments are conducted in normal gravity over a wider range of oxygen levels from 21% to 60% to examine its effect. At near ambient oxygen levels, the laminar oblong flame in normal gravity transforms into a fluctuating mushroom-shapedfaster-spreading flame in microgravity. This unique shape of the microgravity flame is determined to be due to sporadic fuel jets emanating from bursting bubbles on the fuel surface. At elevated oxygen levels (>35%), the flame becomes turbulent even in normal gravity and spreads at nearly the same rate in both normal gravity and microgravity. The flame transiting from laminar to turbulent is found to be typical of thin fuel rods and reported here for the first time. The flame spread rate is unaffected by external flow speed. The flame spread rate increases more steeply with oxygen for oxygen levels > 35% compared with when oxygen levels are < 35%. The variation of flame height (h) on the other hand with heat release rate (Q) follows a linear trend, h similar to Q for oxygen levels < 35% and h similar to Q(0.5) for higher oxygen levels > 35%.