Bench-scale tests on simultaneous ignition of two different plastics through "bridge-mixing"

"Bridge-mixing" was proposed earlier to explain why different combustibles might be ignited simultaneously during flashover. Different fuel vapors gasified from those combustibles were ignited by thermal radiation first. Thermal feedback from flames above the combustibles would then ignite the solid. This point is further studied in this article by bench-scale tests with a cone calorimeter. Sample cubes of two materials with different ignition temperatures were tested at a high thermal radiative heat flux of 70 kWm-2. Materials selected are polymethylmethacrylate (PMMA) and polyvinyl chloride (PVC). The two sample cubes were placed on the cone tray at different separation distances. In this way, different mixing of gasified fuel vapors released from the plastics can be achieved. Those two combustibles were found be ignited simultaneously at shorter separation distances. This might explain why combustible items of different ignition temperatures appear to be ignited at roughly the same time during flashover for some scenarios. On the basis of this study, the mixing of gasified fuel vapors from different combustibles by the fire-induced airflow is a key factor in igniting each item. Geometry of the rooms, combustibles, and ventilation provisions are important factors affecting simultaneous ignition.