Numerical Study on Non-Piloted Ignition of Thin PMMA Sheet by a Laser - Effects of Sample Orientation
Y N. Nakamura, Takashi Kashiwagi
Non-piloted ignition processes of a thin poly(methylmethacrylate) (PMMA) sheet (0. 2 mm thick) by a laser beam as an external radiant source are investigated by the three-dimensional, time-dependent numerical calculation. The effects of the 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 methymethacrylate (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 of the buoyancy induced flow containing evolved MMA with the incident laser beam. Without the absorption of the incident radiation by evolved MMA, ignition is not achieved. The most favorable ignition configuration is the ceiling configuration (face down 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 reduction in fuel supply rate and the other is an increase in air supply from the back side to the irradiated side by the buoyancy induced flow through the hole.