Combustion of a Polymer (PMMA) Sphere in Microgravity.
Combustion of a Polymer (PMMA) Sphere in Microgravity.
(3301 K)
Yang, J. C.; Hamins, A.; Donnelly, M. K.
NISTIR 6331; 44 p. May 1999.
Sponsor:
National Aeronautics and Space Administration, Lewis
Research Center, Cleveland, OH
Order number: PB99-144305
Keywords:
microgravity; combustion; polymethyl methacrylate; fire
safety; plastics; 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 thermoplastic investigated were
polymethylmethacrylate (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 75 mum diameter
A1/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 PAMA was
not determined because the ejected material was never
observed to exist beyond the visible flame of the parent
sphere. The average 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 burning rate of PP was slower than PMMA,
whereas the 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