Fire behaviors of vertical and horizontal polymethyl methacrylate slabs under autoignition conditions

In this study, horizontal and vertical black polymethyl methacrylate (PMMA) samples were investigated experimentally and theoretically under 25‐70 kW/m 2 heat flux with autoignition conditions. An empirical model was developed to predict the autoignition time, where the significance of this model is to unify the prediction for both horizontal and vertical samples. Both horizontal and vertical samples show similar phenomena, such as burning stages and no higher than 2 cm sample expansion. It is obtained that the vertical samples show averagely 21% lower CO yield and also longer ignition time than those of the horizontal samples, while the CO 2 yield increases about 24%. However, the vertical samples show a relatively high fire risk. This can be reflected by higher surface temperature, bigger average heat release rate ( HRR ) and mass loss rate ( MLR ), higher peak MLR , longer bubbling stage, higher CO 2 yield, where the differences become even bigger under a higher external heat flux. A linear relationship was found between the average HRR of vertical and horizontal samples, and also between the ignition temperature and external heat flux for both orientated samples. The research outcomes provide a practical basis and guide for the risk assessment of combustible materials under both vertical and horizontal orientation.