Blind predictions of polymer pyrolysis and the dual threshold for ignition

In a previous recent paper (Vermesi et al., Comb. Flame, 2016), we investigated the pyrolysis of a polymer under transient irradiation. A model was validated by in-depth temperature measurements, and blind predictions of the mass loss rate were made. The predictions and measurements were then used to establish a dual threshold for ignition based on surface temperature and mass loss rate, which indicates the shortest possible time to ignition. There was a need to verify the validity of the blind predictions of mass loss rate and confirm the previous measurements with additional data. A new set of experiments using Poly(methylmethacrylate) (PMMA) samples were performed to measure mass loss rate and surface temperature. Circular PMMA samples with a diameter of 95.25 mm and a thickness of 30 mm are subjected to parabolic pulses of irradiation ranging from 25 to 45 kW/m^2 and time to peak between 160 and 480 s. The dual threshold is proven to be valid, and neither thresholds are reached in the cases where ignition does not occur. The surface temperatures show excellent agreement in most cases, with the maximum difference being 10%. For mass loss rate, the model predicts the peak value with an average error of 58 %, which is acceptable given the experimental difficulty of measuring such small mass loss rates (< 3 g/m^2-s). Qualitatively, the mass loss rate measurements serve to validate the blind predictions. Thus, the dual threshold for ignition provides a conservative indication of when ignition will occur.This work is part of the 2017 PhD thesis of Dr Izabella Vermesi at Imperial College London: "Computational study of material fire behaviour under transient irradiation" http://hdl.handle.net/10044/1/5914