Effects of backwall on inner thermal structure in opposed-flow horizontal flame spread of thick PMMA panel

Temperature fields in the condensed phase for the PMMA panel on aluminum backwall. [Display omitted] •Effects of backwall on detailed inner thermal structure of flame spread are calculated.•A mushy zone is introduced to describe the melting in burning PMMA panel.•Thicker molten liquid will be generated for PMMA flame spread on aluminum board.•PMMA flame spread faster on metal board than on gypsum one.•Heat accumulation at preheated region is enhanced by the metal wall. Flame spread on solid fuel is an important aspect of fire safety engineering. In this work, an unsteady numerical model was developed to investigate the opposed-flow horizontal flame spread mechanism of thick polymethyl methacrylate (PMMA) on backwall with different thermal properties. The temperature fields in both the condensed fuel and backwall material, the fuel gas distribution and the pyrolysis profiles along the fuel bed were analysed in detail. The results showed that the thermal properties of the backwall play an important role in the structure of the melting interface of the PMMA flame front. The molten layer will not penetrate the sample obviously for the case of gypsum backwall. However, the molten liquid penetrates the PMMA sample with more molten liquid generated on the aluminium backwall, thus resulting the higher possibility of molten droplets. It is also indicated that the heat accumulation at the preheated region was enhanced by the heat transfer through the aluminium backwall from the flame and pyrolysis region. On the other hand, the aluminium wall behind the flame tip normally remain at relatively high temperatures, which in turn improved the pyrolysis rate, and lead to higher gas temperature. Therefore, the flame spread of polymer would be accelerated obviously for the case of metal backwall. This work provides important information for the acceleration mechanism of metal backwall on the opposed-flow horizontal flame spread of polymer and the design of polymer components, and also provides an important basis for engineering application to evaluate the fire risk of materials by considering the thermal performance of the backwall.