熔流液火焰诱发的热塑性材料向下火蔓延加速机制研究

Flowing flames, which are generated during the downward flame spread of thermoplastics, generally play an important role in the spread process. The heat transfer and ignition behavior from the pyrolysis regimes to the preheated regimes will be enhanced by the downward flowing flames. It makes a rapidly accelerating downward flame spread of thermoplastics. The accelerating mechanism of the downward flame spread of thermoplastics induced by the flowing flames will be experimentally and theoretically studied in this project. Firstly, the formation mechanism and developing characteristics of the downward flowing flame of thermoplastics will be investigated in detail since it is the initiative source for the downward accelerating flame spread. And then, the accelerating mechanism of the flowing flames for the downward flame spread of thermoplastics will be further experimentally studied under the conditions of the ambient radiant heat fluxes and different inclined angles for sheets with a different thickness. In addition, there are some random characteristics for the flowing flames during the downward burning of thermoplastics, such as their locations and sizes. Therefore, theoretical works will be carried out for the effects of the flowing flames on the downward flame spread using the Monte Carlo method based on the classic flame spread models for solid fuels. A new downward accelerating flame spread model will be developed based on the above experimental and theoretical works in this project. It will be helpful and important for us to know more about the detailed downward accelerating flame spread characteristics of thermoplastics based on the works in this project. It will also show us the enhance mechanism for the downward flame spread of thermoplastics by the flowing flames. Some new flame spread models and application guidance for this kind of materials will be developed in the future.

热塑性材料向下火蔓延过程中，易形成熔流液火焰，从而强化热解火焰区对预热区的加热与引燃作用，形成加速发展的向下火焰传播过程。本项目针对此类材料向下火蔓延中的特殊行为，首先从熔流液火焰的形成机制与流动特性入手，深入研究这一导致热塑性材料向下加速火蔓延的诱发源，继而分析熔流液火焰对其向下加速火蔓延的驱动机制；接着，进一步分析材料厚度、倾斜角度和外界辐射热源等参数的影响，研究不同条件下熔流液火焰对热塑性材料向下火蔓延过程中传热传质特性的影响。最后，基于经典固体材料向下火蔓延模型，针对熔流液火焰的形成位置及其大小等具有一定的随机特性，采用蒙特卡罗方法对熔流液火焰进行模拟分析，发展适用于热塑性材料的向下加速火蔓延模型。 本项目的开展，有助于深入认识热塑性材料向下加速火蔓延现象，揭示熔流液火焰对其传热与传质过程的强化机理，为此类材料向下火蔓延模型的建立和在实际应用中的科学设计提供支撑。

热塑性材料向下火蔓延过程中，由于火焰前锋区域发生材料熔融滴落，与普通固体材料向下火蔓延易趋于稳定发展特性明显不同。项目执行四年以来，开展了熔流液火焰形成机制与流动特性、熔融滴落对热塑性材料向下火蔓延过程的影响机制、火焰前锋区域内部温度场诊断与传热特性、材料厚度与倾斜角度及环境预加热对其向下火蔓延特性的影响机制、熔流液子火焰随机性影响等方面的研究。提出了将该类材料向下火蔓延过程抽象为“固体燃料顶部窄条形熔融油池火的向下发展与移动”模型，提出了基于椭圆截面的火焰前锋区域熔融液相层增长模型，揭示了热塑性材料向下火蔓延过程中形成的熔融液体燃料仅20%在同时期燃烧消耗的特征，指出大量积聚于火焰前锋区域的熔融液体是其火蔓延行为突变和火灾突然加速的关键致因；提出了基于图像分割计算的热塑性材料向下蔓延子火焰细观发展特性分析新方法，揭示了子火焰蔓延速率与其火焰高度间的错峰发展特性，提出了无量纲延时发展模型，基于统计分析，确定了子火焰蔓延速率峰值与火焰高度峰值间的延时区间；提出了综合内部多点温度测量和火焰前锋实时位置测量的内部温度截面重构方法，揭示了热塑性材料向下火蔓延过程中固相预热区和液相熔融区的温度场特性，指出了“固-液”界面处由于熔融相变吸热对其中温度场的显著影响，阐明了热塑性材料厚度增大对于熔流液火焰滴落的若干促进因素；揭示了熔融液相层随着材料倾斜角度增大而变陡的规律，建立了热塑性材料向下火蔓延速率与其表面初始预热温度的相关性模型，将热塑性材料向下火蔓延过程划分为三个控制机理区：传热主控区、滴落主控区和传热滴落共同控制区。.本项目的开展，有力促进了我们对于热塑性材料向下火蔓延过程中火焰前锋区域发生的传热传质、温升相变、流动突变等细节特性的认识，推动了聚合材料燃烧模型的发展，为相关工程实际的火灾防治系统设计提供了支撑，也为此类材料火灾事故的过程重构提供了基础。