In the area of fire safety, an ignition phenomenon of devolatilizing solid fuel exposed to a heat source in a confined geometry is one of very important combustion subjects. However, many formidable difficulties arising from the various physical processes involved therein have hindered a more comprehensive investigation. In order to enhance the physical understanding of this problem, in this study, the experimental and numerical analyses have been performed to study the ignition of solid fuel (PMMA) in the rectangular enclosure, when it is suddenly exposed to a radiative source with high emissivity.
Experimentally, a high-speed camera has been used to record the flame initiation and propagation in the enclosure for various radiative heat source temperatures, while the ignition delay time as well as the gas phase temperature was measured. Numerical analysis has also been performed in a two-dimensional geometry by taking account of the effects of gas radiation due to fuel vapor.
The ignition delay time was found to decrease, as the radiative heat source temperature increased. It was found the numerical analysis well predicted the experimental ignition delays for low radiative heat source temperature, since the heat and mass transfer governed the physical process. But for high radiative heat source temperature, the numerical ignition delays under-predicted the experimental ones, for the thermal pyrolysis behavior of PMMA played an important role in the whole ignition phenomenon.
When the radiative heat source temperature was low, the thermally decomposed volatile gas from the PMMA wall migrated into the hot wall region to be finally ignited, assisted by natural convective flow. Consequently, the mixing of fuel vapor and oxidizer in the vicinity of hot wall controlled this ignition process so that it was termed as the mixing and transport controlled ignition process. But, when the hot wall temperature was high, the ignition process was thermally controlled by an infiltration of hot air originating from the hot wall into the fuel vapor region formed near the PMMA wall. Thereby, the ignition mechanism seemed to be changed from the mixing and transported controlled ignition to the thermally controlled ignition.
