Abstract: Influence of sample thickness and four prescribed constant heat fluxes on surface temperature, mass loss rate, ignition time and critical temperature of clear PMMA (Polymethyl Methacrylate) was investigated. Inversing modelling was utilized incorporating experimental measurements to determine the thermodynamics and kinetics of PMMA, which were employed as inputs of the numerical model to simulate the other scenarios. Accuracy and reliability of the numerical model were verified by comparing with experimental results and analytical correlations. The results showed that the declining rate of the ignition time decelerates with increasing heat flux. When the thickness of PMMA was less than 3 mm, the surface temperature and mass loss rate decrease with increasing thickness. As the thickness gets larger than 3 mm, the average ignition temperature was （628±20） K, and no sensible dependency on thickness was detected, indicating that the ignition temperature can be employed as ignition criterion.

Key words: ignition characteristics; finite thick PMMA; inversing modelling; numerical model; theoretical analysis; critical temperature