Upward Flame Spread Over an Array of Discrete Thermally-Thin PMMA Plates
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Upward Flame Spread Over an Array of Discrete Thermally-Thin PMMA Plates Fu-Hai Gou and Hua-Hua Xiao*, State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027 Anhui, China Lin Jiang, School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210014 Jiangsu, China Mi Li, Man-Man Zhang and Jin-Hua Sun*, State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027 Anhui, China Received: 1 August 2020/Accepted: 18 November 2020
Abstract. Experiments and theoretical analysis were conducted to investigate the upward flame spread over a homogenous PMMA plate and an array of discrete thermally thin PMMA elements. In the experiment, a digital video camera was used to record the flame spread process. An electronic balance and thermocouples were adopted to monitor the mass loss and pyrolysis front position, respectively, as a function of time. In the theoretical analysis, the mass loss rate of PMMA was correlated to the heat transfer during flame spread. The experimental results show that the flame spread rate peaks in the case of discrete PMMA elements with a fuel coverage around 80% rather than 100% (the homogenous case) because the gap with an appropriate spacing between neighboring elements accelerates the flame spread. However, the flame cannot spread over an array of discrete fuels at a coverage of 50% or smaller where the gap is too large to allow effective heat transfer required for flame spread. A smaller coverage of PMMA results in a larger mass loss rate per area since the gaps between elements can entrain more air to promote the burning. A logarithmic relation, that can well describe the mass loss rate as a function of PMMA coverage, was proposed based on the theoretical analysis and the fitting of experimental measurements. Keywords: Thermally thin material, Discrete PMMA element, Flame spread rate, Mass loss rate List of Symbols A B F Ff g h H
Burning area of PMMA plate, m2 Spalding B number PMMA coverage View factor of flame to PMMA surface Earth normal gravitational acceleration, m s-2 Convective heat-transfer coefficient, W m-2 K-1 Flame height, m
*Correspondence should be addressed to: Hua-Hua Xiao, E-mail: [email protected]; JinHua Sun, E-mail: [email protected]
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Fire Technology 2020 Dhv Hp k L m_ 00 Nu Pr q_ 00r q_ 00cv q_ 00rr q_ 00misc Ra S t Tf Tfilm Tp T¥ Vf W x b a t r ef ep
Heat required to convert PMMA at its surface temperature to gas phase, J g-1 Pyrolysis height, m Thermal conductivity of gas phase, W m-1 K-1 Length of PMMA plate, m Mass loss rate per unit area, g s-1 m-2 Nusselt number Prandtl number Radiative heat flux from the flame to PMMA surface, J s-1 m-2 Convective heat flux from the flame to PMMA surface, J s-1 m-2 Radiative heat flux lost from PMMA surface to ambient environment, J s-1 m-2 Conductive heat flux from thermal baffle to air, J s-1 m-2 Rayleigh number Length of spacing, m Time, s Flame temperature, K Film temperature, K Pyrolysis temperature, K Ambient temperature, K Flame spread r
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