Effects of Sample Orientation on Nonpiloted Ignition of Thin Poly(methyl methacrylate) Sheet by a Laser. Part 1. Theoretical Prediction.
Effects of Sample Orientation on Nonpiloted Ignition of
Thin Poly(methyl methacrylate) Sheet by a Laser. Part 1.
Nakamura, Y.; Kashiwagi, T.
Combustion and Flame, Vol. 141, No. 1/2, 149-169, April
poly(methyl methacrylate); ignition; gravity; lasers;
radiant source; equations; heat release rate;
temperature contour; oxygen
Nonpiloted ignition processes of a thin poly(methyl
methacrylate) (PMMA) sheet (0.2 mm thick) with a laser
beam as an external radiant source are investigated
using three-dimensional, time-dependent numerical
calculations. The effects of sample orientation angle on
ignition delay time in quiescent air in a normal-gravity
environment and of imposed velocity in a microgravity
environment are determined. The numerical model includes
heat and mass transport processes with global one-step
chemical reactions in both gas and solid phases. A
simple absorption model based on Beer's law is
introduced and bulk absorption coefficients are applied
to the solid PMMA and evolved methylmethacrylate (MMA).
The PMMA sample surface is kept normal to the incident
radiation at all sample orientation angles. In a zero
gravity environment, ignition delay time increases with
an increase in imposed flow velocity. In quiescent
normal gravity, ignition delay time has a strong
dependency on the sample orientation angle due to a
complex interaction between the buoyancy-induced flow
containing evolved MMA and the incident laser beam.
Without absorption of the incident radiation by the
evolved MMA, ignition is not achieved. The most
favorable ignition configuration is the ceiling
configuration (downward-facing horizontal sample
irradiated by upward laser beam). The formation of a
hole through the thin sample due to consumption has two
counteractive effects on the ignition process: one is a
reduction in the fuel supply rate, and the other is an
increase in the air supply from the side opposite to the
irradiated side by the buoyancy-induced flow through the