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。

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.