Reduced Gravity Combustion of Thermoplastic Spheres.
Reduced Gravity Combustion of Thermoplastic Spheres.
(1657 K)
Yang, J. C.; Hamins, A.; Donnelly, M. K.
Combustion and Flame, Vol. 120, No. 1/2, 61-74, January
2000.
Keywords:
combustion; thermoplastics; fire safety; polymers;
polymethyl methacrylate; polypropylene; polystyrene;
reduced gravity; spheres
Abstract:
A series of low-gravity experiments were conducted to
investigate the combustion of supported thermoplastic
polymer spheres under varying ambient conditions. The
three types of thermoplastics investigated were
polymethyl methacrylate (PMMA), polypropylene (PP), and
polystyrene (PS). The low-gravity environment was
achieved by performing the experiments aboard the NASA
DC-9 and the KC-135 Reduced Gravity Aircraft. Spheres
with diameters ranging from 2 mm to 6.35 mm were tested
yielding Grashof numbers calculated to be less than 0.1.
The polymer sphere was supported using a 75mum-diameter
Al/Cr/Fe alloy wire. The total initial pressure varied
from 0.05 MPa to 0.15 MPa whereas the ambient oxygen
concentration varied from 19% to 30% (by volume). The
ignition system consisted of a pair of retractable
energized coils. Two CCD cameras recorded the burning
histories of the spheres. The video sequences revealed
a number of dynamic events including bubbling, and
sputtering as well as soot shell formation and break-up
during combustion of the spheres at reduced gravity.
The ejection of combusting material from the burning
spheres represents a fire hazard that must be considered
at reduced gravity. The ejection was found to be
sensitive to polymer type, but independent of oxygen
concentration and pressure. The average value of the
ejection frequency was found to be 3 Hz, 5 Hz, and 5 Hz
for PMMA, PS, and PP, respectively. The velocities of
the ejected material were estimated by tracking the
material in two consecutive video frames. For the PP
spheres, Va = 2.3 (+ 1.2) cm/s (with 60 events
observed). The ejected material appeared to decelerate
at an average rate of = 40 cm/s2, and traverse an
average distance of only 8 mm before burning to
completion. The Va for PS and PMMA was not determined
because the ejected material was never observed to exist
beyond the visible flame of the parent sphere. The
average mass burning rates were measured to increase
with initial sphere diameter and oxygen concentration,
whereas the initial pressure had little effect. The
three thermoplastic types exhibited different burning
characteristics. For the same initial conditions, the
mass burning rate of PP was slower than PMMA, whereas
the mass burning rate of PS was comparable to PMMA. The
transient diameter of the burning thermoplastic
exhibited two distinct periods: an initial period
(enduring approximately half of the total burn duration)
when the diameter remained approximately constant, and a
final period when the square of the diameter linearly
decreased with time. A simple homogeneous two-phase
model was developed to understand the changing diameter
of the burning sphere. Its value is based on a
competition between diameter reduction due to mass loss
from burning and sputtering, and diameter expansion due
to the processes of swelling (density decrease with
heating) and bubble growth. The model relies on
empirical parameters for input, such as the burning rate
and the duration of the initial and final burning
periods.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899