| PUBLIC SAFETY |
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Effects of the backboard on downward flame spread over polymethyl methacrylate |
| LI Dayu1, ZHAO Kun1, ZHOU Kuibin1, SUN Penghui1, WU Jinmo2 |
1. College of Safety Science and Engineering, Nanjing Tech University, Nanjing 211816, China;
2. Hefei Institute for Public Safety Research, Tsinghua University, Hefei 230601, China |
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Abstract Polymethyl methacrylate (PMMA) is one of the main architectural decoration materials and is widely used in different types of buildings. Once ignited, the flammable PMMA can cause serious building fire accidents. In building decoration, PMMA is used alone or attached to the noninflammable wall. Consequently, the PMMA could burn in a free condition or under the wall effect in a building fire. However, experimental studies on the effect of the backboard on PMMA downward flame spread are rare. In this study, the effect of the backboard was experimentally studied on the downward flame spread for PMMA of 2- and 5-mm thicknesses using a self-designed experimental setup. The experimental setup holds a noninflammable backboard that can be dismantled for the downward flame spread of PMMA in the free condition. Experimental observation showed significant changes in the flame color and pyrolysis front shape with and without the backboard. As compared to the free condition, the airflow induced by the entrainment of the burning flame might cause a flow boundary layer across the backboard, which helped to qualitatively explain the difference in the flame color and the pyrolysis front shape between the two thickness conditions. The experimental measurements also showed that the backboard had a negative effect on the downward flame spread rate, particularly for the 2-mm thick PMMA, which was characterized by an increase in the pyrolysis front angle and a drop in the average flame height, mass loss rate, and flame spread rate. The heat transfer analyses for the steady flame spread under the backboard and free conditions were conducted using the geometrical feature of the pyrolysis zone, and thus a comparison between the two conditions in terms of total heat feedback was conducted. There are four main conclusions. (1) The backboard caused a darker flame of PMMA and a blue flame of the 2-mm thick PMMA, as compared to those in the free condition. (2) Under the backboard effect, the pyrolysis front showed a "-" shape when the 2-mm thick PMMA burned in a steady stage, while the 5-mm thick PMMA held the pyrolysis front of an inverted "V" shape with a larger front angle than that in the free condition. (3) During the steady stage of flame spread, the backboard reduced the mass loss and flame spread rates by decreasing the total heat feedback to the pyrolysis zone. (4) The reductive effect was more significant on the mass loss rate and the flame spread for the 2-mm thick PMMA than the 5-mm thick PMMA because the former could be fully located in the flow boundary layer, whereas the latter partially located in the flow boundary layer in terms of the thickness. In summary, the qualitative and quantitative analyses of the experimental results show that the backboard can slow down the downward flame spread rate by restricting the air entrainment and reducing the heat feedback, and the inhibiting effect is more obvious for the thinner PMMA.
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| Keywords
downward flame spread
pyrolysis front
flame spread rate
mass loss rate
heat transfer
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Issue Date: 23 April 2023
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2023,
Vol. 63
