节点文献

热解挥发份辐射衰减及流动特性对固体可燃物热解及着火影响研究

Research on Effect of Radiation Attenuation and Flow Characteristic of Pyrolysis Volatiles on Radiant Pyrolysis and Ignition of Solid Combustibles

【作者】 周宇鹏

【导师】 杨立中;

【作者基本信息】 中国科学技术大学 , 安全技术及工程, 2010, 博士

【摘要】 热辐射作用下固体可燃物的热解及着火过程是一个耦合固-气相传热传质、化学反应、流体流动的复杂过程,是火灾的初始阶段,并与其它后续的火灾过程紧密联系,因此一直是火灾科学研究者着力展开的一个研究主题。在外部热辐射作用下,当固体可燃物受热区域达到一定温度后发生热解并释放出可燃气体,热解可燃气体与空气中的氧气形成预混可燃气体,当其浓度和热力条件(温度)达到临界条件时着火发生。其中,热解生成的热解挥发份对入射辐射热流存在一个吸收衰减效应,它可以导致热解及着火过程的迟滞,已有的研究工作还不能充分地确定该效应在实际火灾热辐射工况下的重要性。同时,之前许多研究者把辐射着火简单地看作一个仅涉及固相的传热传质过程,然而在无外加引火源情况下,辐射距离变化导致的可燃热解挥发份气相温度场和浓度场的不同,将会造成相同入射热流下固体可燃物着火特性的差异,说明单一的固相部分简化存在局限性。此外,之前大量的相关研究都是在小尺寸工况展开的,其所做的假设以及所获得的结果能否有效地适应于更接近实际的火灾大尺寸工况也值得考虑。本文对热辐射作用下固体可燃物的热解及着火过程进行了实验研究和理论分析,主要工作围绕固体可燃物热解挥发份的辐射衰减、稀释及积聚效应展开,同时涉及气相传热对着火的影响,具体概括如下:通过火灾早期特性实验台电阻型辐射源模拟接近实际火灾工况下的热辐射作用,获得了典型固体可燃物(松木和有机玻璃)着火发生前的热解挥发份辐射衰减范围(6-14%),实验验证了该热辐射工况下辐射衰减效应的明显存在。耦合辐射衰减量值和失重速率,基于比尔定律提出了判定固体可燃物热解挥发份辐射衰减能力的更为合理的物理参量,指正了Kashiwagi等人相关研究结论的不足。进而在Kung和我们之前模型的基础上,成功地将辐射衰减效应引入到固体可燃物热解着火模型中;发现辐射衰减效应对于模型的主要计算结果(包括表面温度、失重速率、着火时间)都有着显著的影响,在考虑了辐射衰减效应后,模型预测的表面温度和着火时间结果更加接近于实验结果,指出了在当前热解着火模型中对热解挥发份辐射衰减效应考虑的必要性,较好地发展了当前相关模型。针对固体可燃物无引火源辐射着火研究中单一固相传热传质问题简化的不足,通过火灾早期特性实验台大功率辐射源展开实验,发现加热于同样大小表面入射热流(即统一固相热边界条件)时,固体可燃物的着火时间和着火临界热流都随辐射距离的增大而增大,这个之前未曾发现的无引火源着火特征,对修正小尺度实验相关结论具有重要的参考意义。通过对可燃热解挥发份浓度稀释特性与气相温度场非单调分布特性的获取,并合理地引入表征可燃热解挥发份浓度和温度的无量纲参量,成功地解释了上述由辐射距离造成的无引火源辐射着火特征差异,指出了气相传热传质过程对于无引火源辐射着火发生的重要性。实验对比研究了有、无引火源情况下样件尺寸对于固体可燃物着火特性的影响。发现在有引火源的情况下,样件尺寸对于着火时间的影响较小。而无引火源着火时间随着样件尺度的减小而显著增大,特别是在低热流区段(q"in<40kW/m2);相同入射热流下,无引火源着火时刻的质量流率呈现出随着样件尺寸的增大而减小的趋势。随着样件尺寸的增大,可燃热解气羽流的流动从小尺寸样件的向四周发散(稀释作用占主导地位)转变至大尺寸样件的向中心积聚(卷吸积聚作用占主导地位)变化。通过对实验样件中心区域热解可燃气浓度的实测,发现其均随着样件尺寸的增大而增大,验证了前人有关样件尺寸变化对其影响的猜测,解释了由于样件尺寸导致的无引火源着火时间的差异,指出对于热解着火模型中气相、部分的热解可燃气流动的一维假设存在一定的适用范围。

【Abstract】 Radiant pyrolysis and ignition of solid combustibles is a complex process with heat and mass transfer in solid and gas phase, chemical reaction and fluid flow. Radiant pyrolysis and ignition of solid combustibles is the early stage in fire and is closely connected with other followed important fire processes, so it is always an interesting subject in fire research. Subjected to external radiant heating, the heated area of solid combustible starts to decompose when a certain temperature attained, the pyrolysis volatile emanates from the solid combustible and mixes with fresh air. Once the concentration and the temperature of combustible mixed gas are sufficient, the radiant ignition is triggered. In radiant ignition process, the incident radiant heat flux is absorbed and attenuated by the pyrolysis volatiles. The attenuation will retard the thermal decomposition and delay the time to ignition. It is still not known whether the radiation attenuation effect is significant in real fire situations. The radiant ignition simply is treated as a heat and mass transfer process with thermal decomposition in solid phase by some people before. However, by the difference of radiant auto ignition at different incident distances subjected to a same heat flux, it is found the consideration with single solid phase for radiant ignition is limited. Furthermore, the most of radiant ignition studies before are all carried out in bench scale condition, the correlative assumption and conclusion should be verified in the larger fire conditions.The radiant pyrolysis and ignition of solid combustibles has been studied experimentally and theoretically in this paper. The main content includes:the radiation attenuation, dilution and accumulation effects of the pyrolysis volatiles, and effect of the heat transfer in gas phase on radiant ignition.The radiant heating in fire is simulated by a resistant radiant heater. The degree of radiation attenuation of incident heat flux of pine and PMMA before ignition is obtained (6-14%). Based on Beer’s law, a more reasonable parameter for determining the radiation absorptivity of pyrolysis volatiles of different fuels is presented, and the insufficiency of Kashiwagi’s correlative conclusion is indicated. Then, the radiation attenuation effect has been successfully introduced in an improved PDE radiant ignition model based on Kung and our model before. It is found that the main calculated results have all been influenced obviously by the radiation attenuation effect, which illustrates that the consideration of radiation attenuation by pyrolysis volatiles in radiant ignition models is necessary.For the insufficiency of the consideration with a single solid phase for radiant ignition, the quantitative effect of the radiant distance on radiant ignition of solid combustibles has been investigated experimentally. It is found the time to auto ignition and the critical incident heat flux to auto ignition both increase with the increase of radiant distance which doesn’t propose by other researchers before. Based on the quantitative concentration dilution characteristic of combustible pyrolysis volatiles and the non-monotonic distribution of gas phase temperature between radiant heater and sample surface obtained, the radiant auto ignition phenomenon above is well explained. The importance of the heat and mass transfer in gas phase for radiant auto ignition is pointed out.The effect of sample size on the radiant auto and piloted ignition of solid combustibles is studied experimentally. It is found the time to ignition decreases greatly with the increase of sample size, especially in low heat flux range (q"in<40kW/m2). For heating under a same heat flux, the mass flux in ignition decreases with the increase of sample size. As the increase of sample size, the flow mode of pyrolysis volatiles plume changes from the diluting to around with small size sample to accumulating to center with large one. It is found the measured concentration of pyrolysis volatiles in the sample center increases with the increase of sample size, which validates the corresponding hypothesize by other people before and emphasizes on the limited acceptance of the one dimension flow assumption of pyrolysis volatiles in the gas phase part of radiant ignition model.

节点文献中: 

本文链接的文献网络图示:

本文的引文网络