Absorption of thermal energy in PMMA by in-depth radiation

An experimental technique is developed to quantify the absorption of thermal energy in black PMMA (Polycast) by in-depth radiation in semi-transparent media. In-depth heating occurs when non-reflected incident heat flux enters the solid without first being absorbed at the exposed surface. Transient conduction due to temperature gradients occurs within the solid in response to this in-depth absorption. An analytical model is developed for predicting time to ignition for such in-depth heating situations. Using the measured absorption coefficient, κ, the analytical prediction for time to ignition is found to be in excellent agreement with data from experiments of Saito and Delichatsios.