【作者】 龚俊辉；

【导师】 杨立中；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【摘要】 固体可燃物热解及火蔓延是火灾初始和发展阶段的重要分过程,决定着后续火灾的发展,因此一直是国内外众多学者重点研究的对象。热解又是火蔓延的子过程,两者有着密切的关系。非碳化聚合物材料作为现代建筑中广泛应用的装饰材料,其在火灾中表现出的诸多特殊蔓延行为越来越多地受到研究者的关注。在外部热流作用下,非碳化聚合物材料受热后发生热解反应,热解产生的可燃气通过形成气泡等方式从材料内部传输到材料表面进而析出到外界空气中。热解释放出可燃性气体与空气混合形成可燃预混气,经点燃后会在材料表面发生燃烧化学反应形成火焰。燃烧产生的热量一部分用来加热燃烧产物,一部分通过辐射散失到环境中,剩下的一部分则通过对流、辐射和热传导的方式反馈回材料表面形成反馈热流而对未燃材料进行预加热,当未燃材料被加热后发生热解又进一步释放出热解可燃气以维持气相燃烧,如此不断的循环进而形成材料表面持续的火蔓延过程。与传统的碳化材料(如纸张、木材等)不同,非碳化材料在热解的过程中基本没有碳层形成,不会阻碍外部热流对材料的预加热,因此其热解速率要远大于碳化材料的热解。此外,材料的熔融和流动特性也会增加材料的蔓延速率。本文首先在锥型量热仪氮气气氛下对PMMA (Poly(methyl methacrylate))、 ABS(Poly(acrylonitrile butadiene styrene))和HIPS(High Impact Polystyren)三种典型非碳化聚合物材料进行了不同热流下的热解实验研究,在此基础上对非碳化聚合物材料有限尺寸自然条件下逆流火蔓延过程进行了实验和理论研究,其中包括材料厚度、宽度、环境压力和三维效应等参数对逆流火蔓延过程的影响。具体内容概括如下：热解方面,在低、中、高三种热流下对PMMA、ABS和HIPS的一维热解过程进行了实验研究。通过对比热电偶和红外热像仪材料背面温度的测量结果,验证了非接触测温方法的可行性。利用实验所测得的材料背面温度和质量损失速率,并结合Stanislav I. Stoliarov的ThermoKin模型得到材料的导热系数与温度的分段线性关系。进而在实验的基础上建立了一个考虑材料表面热流吸收方式和热解化学反应的非碳化聚合物材料的一维热解模型,模型对文献中广泛采用的深度吸收和表面吸收这两种假设分别进行了研究和对比,结果表明不同吸收方式对材料表面温度和热穿透层内温度分布有较大影响,这种不同在空气气氛下体现在着火时间的明显差异。而在宏观热解方面,即不考虑材料内部热解细节的情况下,两者的模拟结果均在可接受的范围之内。通过对比失重速率和背面温度的实验和模拟结果验证了此一维热解模型的正确性。火蔓延方面,在热解研究的基础上对自然对流和不同环境压力下的逆流火蔓延过程进行了实验和理论分析。结果表明在材料宽度较小的情况下,逆流火蔓延速率随厚度的增大而增加,这与二维无限宽条件下的结论有较大差别。主要原因是有限尺寸条件下,材料两侧的燃烧过程加速了蔓延速率。在厚度固定情况下,火蔓延速率反比于材料宽度。在合肥、西宁和拉萨不同环境压力条件下,失重速率、火焰高度和火蔓延速率均随压力的增大而增大。在低压条件下,气相化学反应动力学和火焰反馈热流是影响火蔓延速率的主要因素,当表征气相化学反应的Damkohler数小于一个临界值会产生熄火现象。本文还从传热和传质角度出发对火蔓延过程进行了理论分析并得到简化模型,模型中材料的火蔓延速率是材料尺寸、火焰前沿角度和材料热物理参数的函数。另外此模型还提供了通过测量火蔓延速率估算热解区和预热区火焰反馈热流大小的方法。同样,模型与实验结果较好的一致性说明了简化模型的正确性。更多还原

【Abstract】 Pyrolysis and flame spread of solid combustible are important subprocedures in initial and development stage in fire which determine the subsequent process, and thus have been the focus of numerous researchers all over the world in the last decades. Pyrolysis is also the one of sub processes of flame spread, and mutual relation exists between both of them. More and more attention is paid by the investigators to the special spread behaviors in fire of non-charring polymers used extensively as structure and decorative materials in modern buildings. When sample is heated by external heat flux, thermal degradation reaction occurs in the non-charring material. Volatiles generated by the polymer are transferred from interior to the surface of samples by bubbles and then diffuse to the ambient environment. Combustible premixed gas fuel mixed by the volatiles and air is ignited at the surface of polymer on which combustion reaction take place and sustained flame is established over the material. Some of the energy produced by the combustion is used to heat the combustion products and some other is released to the environment by radiation. The rest, feedback heat flux, is used to preherat the virgin material by convection, radiation and conduction. Combustible fuel is released to maintain the gas phase combustion when the virgin polymer is heat. This continuous cyclic process leads to the sustained flame spread process over the polymer. The flame spread rate of non-charring polymer is much larger than that of charring polymer due to the absence of the char layer which emerges during the pyrolysis of traditional charring polymer, such as paper and wood, and block the external incident heat flux. Futhermore, the melt and flow characteristics of non-charring polymer will also enhance the spread rate.Experimental study and numerical simulation about pyrolysis of three typical non-charring polymers, Poly(methyl methacrylate)(clear PMMA), Poly(acrylonitrile butadiene styrene)(ABS) and High Impact Polystyrene (HIPS), under three different heat flux in cone calorimeter and nitrogen atmosphere are conducted in this thesis. Based on the study of pyrolysis, experimental and theoretical study on opposed flow flame spread over non-charring polymers under natural convection condition with finite dimension are performed, namely, the effect of thickness, width, ambient pressure and finite dimension on the flame spread process. Specific content is summarized as follows:For the prolysis, experimental investigation about one dimensional pyrolysis is conducted under low, medium and high heat flux in cone calorimeter and nitrogen atmosphere for PMMA, ABS and HIPS. The validity of the non-contact temperature measurement method is verified by the comparison between the bottom surface temperature measured by thermocouples and infrared thermal camera. The piecewise linear relationship between thermal conductivity of material and temperature is obtained through the measured bottom surface temperature and mass loss rate in experiments coupled with the ThermoKin model of Stanislav I. Stoliarov. Furthermore, an one dimension pyrolysis model of non-charring polymers, which considers the absorption methods of incident heat flux at the surface of sample and thermal degradation reaction in interior of solid, is developed based on the experiments. Investigation and comparison about surface and in-depth absorption hypotheses, commonly used in literatures, are conducted in the model, and the result indicates that different absorption methods have significant influence on top surface temperature and the temperature distribution in heat penetration layer, which can be reflected in the distinct discrepancy of ignition time in air atmosphere. Both simulation results with different absorption assumption are acceptable when the details of the micro pyrolysis in the polymers are not the focus. Also, the correctness of the established model is validated by the comparison between experimental and simulation results about mass loss rate and bottom surface temperature.For flame spread, experimental and theoretical analysis of opposed flow flame spread are performed under natural convection condition in different environmental pressure. The results show that the opposed flow flame spread rate increases with the increasing thickness when the width of sample is small, which is distinctly different with the conclusions of that two dimensional infinite width condition. The explanation is that the lateral combustion at the two sides accelerates the spread process for finite dimension material. With fixed thickness, the flame spread rate inversely proportional to the width of sample. In different environmental pressure, Hefei, Xining and Lhasa, the mass loss rate, flame height and spread rate increase with increasing ambient pressure. In low atmospheric pressure, chemical reaction kinetics and flame feedback heat flux in gas is the controlling mechanism for flame spread rate. When the Damkohle, which characterizes the gas phase chemical reactions, decreases below a critical value, no sustained flame can be maintained over the surface of polymer. A simplified model is developed in this thesis based on the theoretical analysis from the perspective of heat and mass transfer, in which the spread rate is a function of dimension of sample, leading edge angle and thermal and physical parameters of material. Meanwhile, the model provides the feedback heat flux measurement method for pyrolysis and preheated zone by measuring the flame spread rate and mass loss rate. Also, the good agreement between the experimental and analytical results verifies the validity of the simplified model.更多还原