Abstract
Ignition limits and ignition delay times of thermally-thick polymethyl methacrylate (PMMA) spheres (4-cm diameter) and cylinders (5.08-cm diameter) are investigated by placing an electrically-heated wire coil beneath the sample in a slow forced flow. The wire serves as both a heater for the solid sample and a gas-phase hot spot to ignite the oxidizer-fuel vapor mixture. In the experiments, the wire's electrical current and power-on time, its distance from the sample surface, the forced flow velocity, oxygen percentage, and pressure are systematically varied. Ignition limits as a function of the oxygen percentage, pressure, and igniter current are determined. Two ignition delay times are determined: igniter-assisted ignition and self-sustained ignition. These critical times are also functions of the degree of heat-up of the sample interior. Solid temperature profiles are measured using embedded thermocouples to determine the sub-surface temperature gradients at the moment of ignition. The heat transfer modes from the igniter to the solid sample are numerically investigated during the pre-pyrolysis heat-up period. At the forward stagnation-point region where ignition occurs, radiation is greater than convection and accounts for approximately 60% of the total heat input. This is consistent with the experimental findings on ignition delay times and heat flux measurements.
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