Effects of Sample Orientation on Nonpiloted Ignition of Thin Poly(methyl methacrylate) Sheets by a Laser. Part 2. Experimental Results.
Effects of Sample Orientation on Nonpiloted Ignition of
Thin Poly(methyl methacrylate) Sheets by a Laser. Part
2. Experimental Results.
(1164 K)
Gotoda, H.; Manzello, S. L.; Saso, Y.; Kashiwagi, T.
Combustion and Flame, Vol. 145, No. 4, 820-835, June
2006.
Keywords:
ignition; polymethyl methacrylate; lasers; experiments;
buoyancy; absorption; carbon dioxide; ignition delay
Abstract:
The effect of the sample orientation angle on frontside
(irradiated surface) ignition and subsequent backside
(nonirradiated surface) flame appearance over thin
poly(methyl methacrylate) (PMMA) sheets having
thicknesses of 0.2 and 0.5 mm has been experimentally
investigated, using a CO2 laser as an external radiant
source in quiescent normal gravity. The sample
orientation angle was varied from (ceiling
configuration) to +90DG (floor configuration) at
intervals of 15DG under three different laser powers of
16.0, 17.3, and 26.1 W. The shortest frontside ignition
delay time was observed for the ceiling configuration
and frontside ignition delay time significantly varied
with increase in sample orientation angle at a laser
power of 16.0 W. As the laser power was increased,
frontside ignition was observed at all angles and its
delay time became less dependent on the sample
orientation angle. The appearance of a backside flame
was achieved after the formation of an open hole (due to
local consumption of the sample) by two different
processes: the onset of laser induced ignition over the
backside sample (backside ignition) and a flame
traveling from the frontside through an open hole to the
backside (backside flame). The former process was
observed for a limited number of cases only around the
vertical configurations The delay time for the
appearance of backside flame tended to be longer for
sample surfaces facing downward than for the sample
surface facing upward regardless of the laser power.
When the duration of laser irradiation was shortened
from 10 to 4 s, as soon as the laser was shut off, the
flame on the frontside immediately shrank, moved close
to the sample surface, and then traveled rapidly to the
backside. Therefore, the delay time of backside flame
appearance (about 6 s) became longer with longer
duration of laser irradiation after the onset of a
frontside flame. The size of the hole (about 4 mm
diameter) was large enough for the flame to travel
through it, even after 4 s of laser irradiation to
sample. These results indicate that the size of the hole
appears to be not a critical parameter for the
appearance of the backside flame.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899