Effects of sample orientation on nonpiloted ignition of thin poly(methyl methacrylate) sheet by a laser

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 hole.