| Summary: | To explore the flame spread mechanism over non-charring materials, the downward flame spread over polymethyl methacrylate (PMMA) samples is studied experimentally under the spacing scenarios of 2 mm, 7 mm, 13 mm, 19 mm and 25 mm. The experimental results show that: (1) As the spacing increases, the flame height, the length of the preheating zone and mass loss rate all increase first and then decrease. When the spacing is 13 mm, each value reaches the maximum. (2) As the spacing increases, the flame spread speed increases first and then decreases, approaching the single burning PMMA slab finally. In this study, a heat transfer model is proposed to examine the spacing effect over PMMA slabs. According to experimental results, a correlation between the flame spread rate and spacing is derived. Besides, experimental data agree well with the theoretical model. Keywords: Spacing, Downward flame spread, Thermally-thin PMMA, Pyrolysis zone temperature, Flame spread rate
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