Two-Sided Ignition of a Thin PMMA Sheet in Microgravity.
Two-Sided Ignition of a Thin PMMA Sheet in Microgravity.
(1290 K)
Nakamura, Y.; Kashiwagi, T.; Olson, S. L.; Nishizawa,
K.; Fujita, O.; Ito, K.
Combustion Institute, Symposium (International) on
Combustion, 30th. Proceedings. Volume 30. Part 2.
July 25-30, 2004, Chicago, IL, Combustion Institute,
Pittsburgh, PA, Chen, J. H.; Colket, M. D.; Barlow, R.
S.; Yetter, R. A., Editor(s)(s), 2319-2325 pp, 2005.
Keywords:
combustion; microgravity; polymethyl methacrylate;
ignition; experiments; lasers; flame spread;
irradiation; heat release rate
Abstract:
Numerical computations and a series of experiments were
conducted in microgravity to study the ignition
characteristics of a thin polymethylmethacrylate (PMMA)
sheet (thicknesses of 0.2 and 0.4 mm) using a CO2 laser
as an external radiant source. Two separate ignition
events were observed, including ignition over the
irradiated surface (frontside ignition), and ignition,
after some delay, over the backside surface (backside
ignition). The backside ignition was achieved in two
different modes. In the first mode, after the laser was
turned off, the flame shrank and stabilized closer to
the fuel surface. This allowed the flame to travel from
the frontside to the backside through the small, open
hole generated by the laser\'01s vaporization of PMMA.
In the second mode, backside ignition was achieved
during the laser irradiation. The numerical calculation
simulating this second process predicts fresh oxygen
supply flows from the backside gas phase to the
frontside gas phase through the open hole, which mixes
with accumulated hot MMA fuel vapor which is ignited as
a second flame in the frontside gas phase above the
hole. Then, the flame initiated from the second ignition
travels through the hole to ignite the accumulated
flammable mixture in the backside gas phase near the
hole, attaining backside ignition. The first backside
ignition mode was observed in
21% oxygen and the second backside ignition mode in 35%.
The duration of the laser irradiation appears to have
important effects on the onset of backside ignition. For
example, in 21% oxygen, the backside ignition was
attained after a 3 s laser duration but was not observed
after a 6 s laser duration (within the available test
time of 10 s). Longer laser duration might prevent
two-sided ignition in low oxygen concentrations.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899