基于两种热流吸收模式的聚合物热解及火蔓延机理研究

Surface and in-depth absorption of external heat flux of polymers, employed as boundary condition and the energy source term in conservation equations respectively, affect the pyrolysis process greatly. The existing literatures with single and extreme absorption hypothesis of heat flux lack scientific and reasonable understanding of the absorption process. In this project, the entire thermal degradation process of different types of polymers is divided into several stages, and the demarcation criteria are studied experimentally for charring and non-charring polymers according to the mass loss rate. Then, the combination mode of the surface and in-depth absorption, namely the percentage of each absorption mode, is studied by termination experiments of different pyrolysis stage in which the transmitted heat flux through polymers with fixed thickness is measured quantificationally with water-cooled method. A multi-stage and combined pyrolysis model is established based on the experimental study, in which both absorption modes are considered, to enrich the understanding of the thermodynamics and chemical kinetics process of pyrolysis of polymers. Subsequently, a flame spread model, combining the proposed solid pyrolysis model and the feedback heat flux from flame measured accurately by experiments, is developed in which the heat feedback process in pyrolysis and preheat region of previous flame spread model is modified and the effects of absorption modes, char layer and soot layer on the surface of polymers are also considered. Based on the experiments, the flame spread mechanism is studied for finite dimension materials and the criterion between 2D and 3D condition is obtained.

聚合物材料表面和深度热流吸收模式分别作为热解模型中边界条件和控制方程中能量源项对其热解过程有着重要影响，已有研究中采用单一极端的热流吸收模式缺乏对吸收过程科学合理的理解。本项目首先针对聚合物材料不同热解阶段两种热流吸收模式的组合吸收，以热解失重速率为依据实验研究碳化和非碳化聚合物材料的各阶段划界标准，通过终止各阶段热解实验，利用水冷条件下测材料固定厚度透射热流的方法，定量测量各阶段两种吸收模式百分比，并以此为基础建立综合考虑两种吸收方式的分段组合热解模型，深入了解材料热解的热力学和化学动力学过程。其次，利用建立的固相热解子模型，结合通过实验方法精确测量的火焰反馈热流，探索材料表面的火蔓延过程，对传统和经典理论中预热区和热解区的热反馈过程进行改进，考虑吸收模式、碳层和材料表面烟颗粒层的影响；接着结合实验，研究材料有限尺寸条件下的火蔓延规律，得到二三维划界标准并建立相应蔓延模型。

聚合物热流表面和深度吸收分别作为热解模型中边界条件和控制方程中能量源项对热解过程有重要影响，已有研究中采用单一极端的热流吸收模式缺乏对吸收过程科学合理的理解。本项目针对聚合物不同热解阶段两种热流吸收模式的组合吸收，以热解失重速率为依据实验研究聚合物的各阶段划界标准，通过终止各阶段热解实验，利用水冷条件下测材料固定厚度透射热流的方法，定量测量各阶段两种吸收模式百分比，并以此为基础建立综合考虑两种吸收方式的分段组合热解模型。此外，利用该模型，结合火焰反馈热流，探索材料表面的火蔓延过程，对传统理论中预热区和热解区的热反馈过程进行改进。最后结合实验，研究有限尺寸条件下的火蔓延规律并建立相应蔓延模型。.研究发现：1）常热流表面吸收时，热穿透层内温度梯度较大，传热过程受热传导控制；深度吸收时，热穿透层内温度分布均匀，传热过程靠深度吸收实现；相同热流下，表面吸收的表面温度较深度吸收高；表面温度随外部热流增大而增大；发展了一种以临界质量流率为着火标判据的高热流下聚合物热解着火的解析模型；2）变热流下，建立了考虑表面吸收和深度吸收的热解着火分析和数值模型，采用临界质量流率作为着火判据。半透明材料的热解和着火行为随深度吸收系数的增大接近于不透明固体；建立了增长热流下聚合物着火的近似解析模型，该模型分别采用了临界温度和临界质量流率，考虑了深度吸收及表面热损失的影响；3）实验研究了聚合物向下火蔓延过程角度的影响，并进行了理论分析。当角度小于过渡值时，火蔓延速率为常数且与倾斜角度正弦的立方根成正比，当角度大于过渡值时，火蔓延速率随角度增加而增大。给出了不同角度下火蔓延速率的简化表达式。.本研究改善了聚合物热解和火蔓延研究不足的现状，丰富了聚合物材料热解和火蔓延相关参数的数据储备，深化了对热解和火蔓延机理的理解，对更加准确预测聚合物在火灾发生和发展阶段的热解和燃烧特性有重要的理论和现实意义。