Optical and radiative properties of clear PMMA samples exposed to a radiant heat flux

摘要

Radiative transfer through clear PMMA samples subjected to a constant radiant heat flux was studied. The aims of the present study were threefold. First, optical indices of clear PMMA were determined from reflectivity and transmissivity measurements. Clear PMMA exhibits a strong non-grey behavior, quite transparent in the visible and near infrared ranges of the spectrum, and almost opaque in some bands of the mid infrared. Absorption and refraction indices were obtained using an identification method applied to the transmissivity data of several samples with different thicknesses whenever possible (i.e. in ranges with sufficiently high transmission levels), whereas elsewhere, the Lorentz oscillators model coupled with a genetic algorithm was applied to the reflectivity data. Secondly, the absorption coefficient was determined at a very fine resolution of 4 cm~(-1). A nineteen-band model was proposed to represent the spectral variation of the absorption coefficient in the range of [590-18999 cm~(-1)]. Third, a reciprocal Monte Carlo simulation was performed to calculate the response of a 3-cm-thick PMMA sample to a 18 kW m~(-2) externally incident radiation, involving absorption and internal emission into the solid. Radiative transfer, pure or combined with conduction and convection, was considered to simulate the early stages of sample heating (before the glass transition of PMMA) from two different radiative sources, namely a cone calorimeter and a tungsten lamp. The influence of the type of the radiation source was clearly demonstrated in terms of temperature distribution inside the material and divergence of the radiative flux. All input data (optical and radiative properties) are available upon request to the corresponding author.