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.

中文翻译：

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向上的火焰蔓延到一系列离散的热薄 PMMA 板

进行了实验和理论分析以研究在均质 PMMA 板和离散热薄 PMMA 元件阵列上向上蔓延的火焰。在实验中，使用数码摄像机记录火焰蔓延过程。采用电子天平和热电偶分别监测质量损失和热解前沿位置，作为时间的函数。在理论分析中，PMMA 的质量损失率与火焰蔓延过程中的传热相关。实验结果表明，在燃料覆盖率约为 80% 而不是 100%（均质情况）的离散 PMMA 元件的情况下，火焰蔓延速率达到峰值，因为相邻元件之间具有适当间距的间隙会加速火焰蔓延。然而，火焰不能以 50% 或更小的覆盖率在一系列离散燃料上蔓延，其中间隙太大而无法实现火焰蔓延所需的有效热传递。较小的 PMMA 覆盖率会导致单位面积的质量损失率较大，因为元件之间的间隙会夹带更多空气以促进燃烧。基于理论分析和实验测量拟合，提出了一种对数关系，可以很好地描述作为 PMMA 覆盖率函数的质量损失率。