Bursting Bubbles From Combustion of Thermoplastic Materials in Microgravity.
Bursting Bubbles From Combustion of Thermoplastic
Materials in Microgravity.
(365 K)
Butler, K. M.
NASA/CP-1999-208917;
Microgravity Combustion Workshop, Fifth (5th)
International. Proceedings. Sponsored by NASA Office
of Life and Microgravity Sciences and Applications and
the Microgravity Combustion Science Discipline Working
Group hosted by NASA Glenn Research Center and the
National Center for Microgravity Research on Fluids and
Combustion. NASA/CP-1999-208917. May 18-20, 1999,
Cleveland, OH, 93-96 pp, 1999.
Keywords:
microgravity; bubbles; bursting; combustion; combustion
models; thermoplastics
Abstract:
Many thermoplastic materials in common use for a wide
range of applications, including spacecraft, develop
bubbles internally as they bum due to chemical reactions
taking place within the bulk. These bubbles grow and
migrate until they burst at the surface, forceably
ejecting volatile gases and, occasionally, molten fuel.
In experiments in normal gravity, Kashiwagi and
Ohlemiller observed vapor jets extending a few
centimeters from the surface of a radiatively heated
polymethylmethacrylate (PMMA) sample, with some molten
material ejected into the gas phase. These physical
phenomena complicated the combustion process
considerably. In addition to the non-steady release of
volatiles, the depth of the surface layer affected by
oxygen was increased, attributed to the roughening of
the surface by bursting events. The ejection of burning
droplets in random directions presents a potential fire
hazard unique to microgravity. In microgravity
combustion experiments on nylon Velcro fasteners and on
polyethylene wire insulation, the presence of bursting
fuel vapor bubbles was associated with the ejection of
small particles of molten fuel as well as pulsations of
the flame. For the nylon fasteners, particle velocities
were higher than 30 cm/sec. The droplets burned
robustly until all fuel was consumed, demonstrating the
potential for the spread of fire in random directions
over an extended distance. The sequence of events for a
bursting bubble has been photographed by Newitt et al.
As the bubble reaches the fluid surface, the outer
surface forms a dome while the internal bubble pressure
maintains a depression at the inner interface. Liquid
drains from the dome until it breaks into a cloud of
droplets on the order of a few microns in size. The
bubble gases are released rapidly, generating vortices
in the quiescent surroundings and transporting the tiny
droplets. The depression left by the escaping gases
collapses into a central jet, which rises with a high
velocity and may break up, releasing one or more
relatively large drops (on the order of a millimeter in
these experiments). A better understanding of bubble
development and bursting processes, the effects of
bursting behavior on burning rate of the bulk material,
and the circumstances under which large droplets are
expelled, as well as their trajectories, sizes, and
burning rates, is sought through computer modeling
compared with experiment.
Building and Fire Research Laboratory
National Institute of Standards and Technology
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