Abstract
A series of microgravity experiments using 50 m-
droptower for burning polymer sphere have been performed
to elucidate the role of the “abrupt” dynamic behaviors, such
as soot/bubble bursting, on determination of its burning rate
constant. With 23 successive runs at the same condition, we
could learn precisely the source of the variation of burning
behaviors and the strategy to define the burning character of
polymers in a precise manner. Specimen is a layered PMMA
(poly-methyl methacrylate) coated over a small ceramic ball supported by 14 m SiC fiber. Tiny amount
of ignitor-gel is employed to attain quasi-steady burning within the limited microgravity time (< 2.5 s). It
was found that there are two distinctive stages during the event; square of specimen diameter (d
2) nearly
stays at constant in early stage (Stage I), whereas it almost linearly decreases in time (Stage II) enabling us
to define the burning rate constant (K, mm2/s) for given conditions. Notably, soot bursting is found to be
mainly occurred in Stage I, while the bubble bursting is pronounced in Stage II. With 23 runs of
microgravity experiment, burning rate constant, K, is found to be varied randomly, revealing that the
randomness of bubble generation in molten specimen is the key and its control is crucial to obtain the
characteristic K value. Present work is firstly achieved with sufficient number of microgravity tests of
polymeric material for the specific condition and pointed out the importance to take it account the inherent
less-repeatable burning character of polymers; such that bubble-generating/growing and “abrupt”
bursting processes are essentially unavoidable. It is suggested that certain attempt to suppress such
dynamic behaviors is strongly demanded in order to extract “ideal” value of the burning character.
