逆向阴燃传播过程和模型

为了解逆向阴燃传播的过程和特点,用聚氨酯泡沫材料在绝热封闭的装置中进行了实验.实验过程用热电偶进行温度的测量,通过调节气体流量观测风速对逆向阴燃的影响.聚氨酯泡沫材料的逆向阴燃可分为两个阶段,第一阶段泡沫材料阴燃变成多孔炭,材料的透过率很小,能进入材料内部参与反应的氧气量也少,气流增大,在增加氧气的同时也带走更多的热量,阴燃传播速度随风速先增大后减小.第二阶段多孔炭的燃烧,空气很容易进入到材料内部,传播速度随风速线性增大,燃烧达到的最高温度值也随风速增大.在第一阶段中逆向阴燃的传播是匀速的,以此建立了一维传播模型,模型模拟结果同实验的结果具有很好的一致性.     

木质材料用无机阻燃剂的性质及其阻燃机理(之二)

四、阻燃机理在通常的火灾下,木质材料的燃烧状态可分为阳燃(有焰燃烧)和阴燃(无焰燃烧)。为了维护阳燃,可燃物、热量、氧化剂和燃烧连锁反应是不可缺少的;而可燃物、氧化剂及热量对阴燃亦不可缺少。     

阴燃火灾的调查

论述了以香烟作为阴燃火灾最常见的起火源，由阴燃火转化为有焰燃烧的机理，以及阴燃在火灾中形成的痕迹特征。     

国际空间站环境中对二氧化碳灭火剂浓度的要求

在宇宙飞船上灭火是一个至关重要的问题。当前 ,国际空间站 (ISS,International Space Station)灭火时需要包括 5 0 %浓度的二氧化碳 (CO2 )灭火剂。这一要求与 NFPA 12一致 ,该管理标准适合于 CO2 灭火系统 ,它要求 5 0 %的浓度用于阴燃火灾、34%的浓度用于有焰燃烧。开发 NFPA12是用于地面环境灭火。本文论述材料在微重力下、特别是针对在 ISS环境中的燃烧 ,并估算在微重力下二氧化碳灭火剂所需浓度。在 ISS环境中通常有 2 0 .9%的氧气 ,NFPA的要求足以在轨道微重力条件下扑灭有焰燃烧。对于初始氧气浓度大于 2 5 %的环境 ,如果要求 CO2 浓度小于34% ,那么应进行材料实验。     

自然对流条件下垂直向上阴燃向有焰火转化的研究

为了解自然对流条件下垂直向上阴燃向有焰火转化的机制，对竖直放置实验体中的聚氨酯泡沫进行了阴燃实验研究。根据在此条件下的阴燃特点，建立了稳定阴燃时实验材料内部的气流速度模型和阴燃向有焰火转化时的临界风速判定表达式。实验分析得出：在稳定阴燃时实验材料内部的风速在0．001～0．0024m／s的范围内，在阴燃向有焰火转化时实验材料内部的临界风速在0．63～1．07m／s范围内。     

辐射板法测定铺地材料临界辐射通量试验装置研究

该成果主要应用于铺地材料的燃烧性能检测和评价。其技术原理如下：以一个与水平方向成３０°角，并面向试样的燃烧着的辐射板（以空气——燃烧气为原料）作为辐射热源，并通过对燃气量的调节，使其试验箱内沿水平方向产生特定的辐射通量分布。放置在试验箱底部的滑支平台上的铺地材料表面在此外部施加的特定强度辐射条件下，用小火焰点燃试样后火焰会沿着试样表面水平方向蔓延至一定位置后熄灭，由于不同的铺地材料的燃烧性能不同，所以火焰沿试样水平方向蔓延的程度也不同，测定火焰最远熄灭点的位置，再根据事先已标定好的位置与辐射通量…     

丙烷/氢气混合燃烧的火焰特征与辐射特性试验研究

针对丙烷和氢气混合气体燃烧的火焰特征与辐射特性开展试验研究。根据不同的混合比进行了10组试验,分析火焰高度、火焰中心线温度、火焰外部辐射通量、火焰内部辐射通量的变化。研究发现,随着混合气体中丙烷气体组分的比例不断下降,混合燃烧时的平均火焰高度逐渐下降。丙烷/氢气的混合比例大于50%时,火焰内部辐射通量随着总热释放速率的增大而显著下降;当丙烷/氢气的混合比例小于50%时,火焰内部辐射通量基本保持一致。     

不同氧浓度船舶机舱油池火灾热流场特性

搭建1.5 m×1.5 m×1.0 m的船舶封闭舱室火灾实验平台,以柴油池火为火源,对初始氧气体积分数为19%、20%、21%条件下的火灾演变特性进行研究。采用数值模拟和实验相结合的方法,分析舱内温度场和速度场的变化。研究结果表明:燃烧初期阶段,初始氧气浓度增加,靠近火焰根部的温升明显,而顶部区域的升幅较小;燃烧后期阶段,初始氧气浓度降低,火焰燃烧状态改变,燃烧速率和燃烧温度降低。初始氧气浓度的降低使舱内整体温度和空气流速下降,油池火燃烧速率随之下降。     

氧体积分数对阻燃三元乙丙橡胶燃烧特性的影响

阻燃三元乙丙橡胶广泛用于公共建筑、交通工具等领域,火灾时其周围的氧体积分数会发生变化。为研究此类材料在真实火灾中的情况,以铺地用的阻燃三元乙丙橡胶为样品,采用火焰蔓延仪和热重/差热同步分析仪,在不同氧体积分数的氧氮混合气氛下进行试验,研究氧体积分数对燃烧特性的影响。结果表明,有氧气氛下,氧体积分数降低时,失重速率、放热量和活化能均降低,但反应模型不变。样品能够发生明火燃烧的氧体积分数范围为15%～18%。有明火燃烧时,氧体积分数降低时,火焰降低,刺激性黑烟量减少,残留物表面灰白色区域和裂纹减少,裂隙变浅;临界热辐射通量、等效对流换热系数和引燃温度升高,HRR曲线第一个峰值和火势蔓延速率降低;CO产率第二个峰值以及CO/CO₂值升高。在无明火燃烧时,样品释放出大量灰烟,残留物中部凸起,表面中部为灰黑色、有金属光泽的,底部有黄褐色黏稠液体,CO和CO₂产率远低于有焰燃烧。     

不同密度FPUF阴燃特性及热解动力学分析

自制中尺度综合阴燃实验平台,以2、3、5L/min的气流速率对软质聚氨酯泡沫(FPUF)样品P1、P2、P3(密度为20、30、35kg/m3)进行阴燃实验,以扫描电镜和热重分析仪对其阴燃特性、微观形貌及热解和动力学特性进行表征,探讨密度对FPUF的阴燃、传播速率、热解过程和动力学参数的影响。结果表明,FPUF阴燃建立所需时间的长短依次为P2、P3和P1;阴燃传播速率随密度增大而增大,且随气流速率增大而更加明显;SEM结果表明,FPUF密度越小其孔径越大,越有利于气流通过;热重及TG动力学分析结果表明,FPUF在氮气气氛下的热解大致分为200~330℃和330~450℃两个阶段,密度对其影响主要体现在第二阶段。     

Smolder ignition of polyurethane foam: effect of oxygen concentration

Experiments have been conducted to study the ignition of both forward and opposed smolder of a high void fraction, flexible, polyurethane foam in a forced oxidizer flow. Tests are conducted in a small scale, vertically oriented, combustion chamber with supporting instrumentation. An electrically heated Nichrome wire heater placed between two porous ceramic disks, one of which is in complete contact with the foam surface, is used to supply the necessary power to ignite and sustain a smolder reaction. The gaseous oxidizer, metered via mass flow controllers, is forced through the foam and heater. A constant power is applied to the igniter for a given period of time and the resulting smolder is monitored to determine if smolder is sustained without the assistance of the heater, in which case smolder ignition is considered achieved. Reaction zone temperature and smolder propagation velocities are obtained from the temperature histories of thermocouples embedded at predetermined positions in the foam with junctions placed along the fuel centerline. Tests are conducted with oxygen mass fractions ranging from 0.109 to 1.0 at a velocity of 0.1 mm/s during the ignition period, and 0.7 or 3.0 mm/s during the self-sustained propagation period. The results show a well-defined smolder ignition regime primarily determined by two parameters: igniter heat flux, and the time the igniter is powered. These two parameters determine a minimum igniter/foam temperature, and a minimum depth of smolder propagation (char), which are conditions required for ignition to occur. The former is needed to establish a strong smolder reaction, and the latter to reduce heat losses from the incipient smolder reaction to the surrounding environment. The ignition regime is shifted to shorter times for a given igniter heat flux with increasing oxygen mass fraction. A model based on concepts similar to those developed to describe the ignition of solid fuels has been developed that describes well the experimental ignition results.     

棉被阴燃火灾实验研究

针对烟头引燃棉被的火灾案例,实验测量烟头的表面温度及热辐射强度,得到烟头表面最高温度可达289 ℃,很容易引燃棉被等可燃物.通过无被套棉被的阴燃实验和有被套棉被的阴燃转明火实验发现,在没有被套的情况下,棉絮间空隙较大,无法积聚热量,只能阴燃而无明火;有被套情况下,产生的热量大部分被阻隔在棉被内部,散热速度较慢,提供了棉被阴燃转明火所需的热量.     

聚氨酯和聚苯乙烯热解前后的结构演变

以软质聚氨酯泡沫和聚苯乙烯泡沫颗粒两种典型多孔材料为试验对象,对其孔隙率、孔径分布、表观密度和堆积密度等结构特性参数及表面微观形貌进行了测算和表面扫描电镜(SEM)表征,研究不同热解程度材料结构特性演变与阴燃建立和传播趋势之间的关系。结果表明:软质聚氨酯泡沫结构尺寸更大,整体更为疏松,受热成炭后呈现轮廓清晰的细削骨架,孔隙硕大且畅通,氧气完全自由输运,热量传递更直接,更易建立阴燃、维持传播并实现向明火的转化;相对而言,聚苯乙烯泡沫颗粒孔径较为细小,受热后成炭显著,孔径尺寸减小,空气气流量和氧气容纳量大大减少,不易建立阴燃过程。     

Radiant smoldering ignition of plywood

This paper investigates the role of self-heating in the smoldering ignition of 18 mm (three-quarter inch) thick maple plywood exposed to radiant heat fluxes between 6 and 15 kW/m² in the cone calorimeter for up to 8 h. The minimum heat flux for smoldering ignition was experimentally determined to be 7.5 kW/m². This compares favorably to predictions made using classical self-heating theory. The role of self-heating was explored via temperature measurements distributed within the specimens. Elevated subsurface temperature profiles indicated self-heating was an important ignition factor resulting in ignition at depth with smolder propagation to the surface and into the material. The ignition depth was shown to be a function of the heat flux with the depth moving towards the surface as the heat flux increased.     

Results from a Full-Scale Smoke Alarm Sensitivity Study

A series of 24 full-scale experiments was conducted to examine the effects of alarm type (photoelectric, ionization, and dual sensor), alarm location, fabric type (100% cotton and 100% polyester), polyurethane foam density, ignition scenario, and room configuration, on smoke alarm performance. A two-level, fractional factorial design of eight experimental configurations was developed around the five factors: fabric type, foam density, fire location, ventilation, and ignition scenario. A structure, designed to represent a single-story home or apartment, was constructed inside the Large Fire Laboratory at the National Institute for Standards and Technology for the experiments. The fire source was a chair mockup consisting of a seat and back cushion of a specific cover fabric and foam density, weighing between 5.5 kg and 8.3 kg. It rested on a metal frame and was subjected to a small propane gas flame, or an electric cartridge heater to initiate smoldering. Each experimental configuration was replicated three times. Smoldering fires were allowed to progress until they naturally transitioned to flaming fires except for one test that was terminated early due to time constraints. The smoldering to flaming transition times ranged from (81 to 182) min. Each fire progressed for a time sufficient to produce multiple hazards (smoke, heat, and toxic gases). All alarms tested were purchased from retail outlets and activated at their preset levels. Photoelectric, ionization, and dual photoelectric/ionization alarms were co-located at multiple locations to facilitate comparisons of each alarm type, and different designs of the same type of alarm. For smoke alarms in the room of fire origin, it was observed that each of the five factors had an effect on the measured alarm times that was primarily a result of fire growth rate (fabric type, foam density, and ignition scenario), or smoke dilution and transport (fire location and ventilation). The photoelectric alarm responded quicker on average than ionization alarm in two of four smoldering fire configurations, responding before the ionization alarm in all 6 trials, while the ionization alarm responded before the photoelectric alarm in two of three trials for the other two configurations. The ionization alarm responded quicker on average than photoelectric alarm in all four flaming fire configurations, and responded before the photoelectric alarm in all 12 flaming fire trials. One dual alarm had the fastest average alarm time for all four smoldering fire configurations, and responded first in 11 of the 12 trials. It also yielded faster average alarm times than the other dual alarm in seven of eight configurations, and was the first dual alarm to respond in 22 out of 23 trials where dual alarms were present.     

Persistence of Sonic Deposition on Smoke Alarms in Forensic Fire Investigations

Smoke alarms have been shown to develop sonically-deposited regions of acoustically agglomerated soot particles when they sound in smoke-filled air. These sonic depositions can be examined forensically post-fire to determine if the smoke alarm sounded during the incident. However, it is not clear how these sonic depositions are affected by common firefighting and post-fire actions. To determine the effects of post-fire forensic smoke alarm testing and environmental conditions on the persistence of the existing sonic deposition of soot on the horns of a smoke alarm, sixty (60) smoke alarms were subjected to smoke from fires of several different fuel types and common post-fire conditions or actions. Initially, each alarm was exposed to smoke in a small-scale experimental fire to develop sonic deposition around the horn. The fuel types for the fires were smoldering wood, flaming toluene–heptane, smoldering polyurethane foam, flaming polyurethane foam, and a combination of smoldering and flaming polyurethane foam. The alarms were then subjected to four common post-fire actions: pressing the test button, exposure to synthetic canned smoke, exposure to standing water, and exposure to running water. Each detector was visually inspected before and after the post-fire action. Results varied from no soot removed to almost all soot removed depending on the fuel type and post-fire test. An objective evaluation system was used to rank the degree to which soot was removed from the alarm horns: 0 (no soot removed), 1 (some soot removed), and 2 (all soot removed) based on visual inspection. The smoldering wood and smoldering polyurethane foam fires left behind a sticky resin that was essentially unaffected by any of the post-tests. The flaming foam and flaming toluene–heptane fires left powdery soot on the horn which could be easily wiped off. This soot was almost completely washed off by running water (1.067 average degree of removal) while the canned smoke and standing water post-tests removed a significant portion of the soot (0.533 and 1.000 average degrees of removal, respectively), which could lead an investigator to an errant sounding determination. Pressing the test-button appeared to make minimal impact on the amount of soot around the alarms horns regardless of the fuel type (0.067 average degree of removal).     

柔性聚氨酯泡沫燃烧产物毒性评估

介绍了对弹性聚氨酯泡沫燃烧产物毒性的相关研究,并对其是否能用于毒性危害分析进行了评估。研究显示,暴露到聚氨酯燃烧产物中的人会导致中毒。窒息气体氮气有效剂量法能有效预测受害人因暴露于聚氨酯燃烧产物而导致的行为能力丧失情况。同时,也证明有效剂量法不能正确预测聚氨酯阴燃时情况。该方法不适用于热分解。研究发现,目前很多经验方法具有很大的局限性,对毒性危害研究造成了很多负面的影响。烟气粒子携带毒物的吸入和传播以及化学合成复杂的毒物,研究和考虑得很少。因此,很有必要对复杂毒性有机体的火灾燃烧产物进行更深入的物理和化学分析研究以及动物模型研究。     

Effect of Smoke Source and Horn Configuration on Enhanced Deposition, Acoustic Agglomeration, and Chladni Figures in Smoke Detectors

A series of UL/EN based test fires was conducted in a two room/corridor enclosure to investigate the viability of methods for determining whether a smoke detector sounded under a variety of smoke conditions and to see if this methodology could be applied to a detector with a different horn configuration. The presence of enhanced deposition in the form of a black or orange-brown ring and agglomerates around the central opening of a smoke detector horn was found to be a reliable indicator that the horn sounded when it was exposed to smoke from eight standardized, single-substrate fuel sources including hydrocarbon pool, flaming polyurethane foam, and smoldering polyurethane foam fires. Determinations could generally not be made for detectors exposed to white or gray smoke generated by flaming paper, smoldering paper, flaming wood, smoldering wood, and smoldering cotton wick due to a general lack of visible soot deposition within the detector. Therefore, it is not recommended to use the absence of a black or orange-brown ring of enhanced deposition, in and of itself, as an indicator that the horn did not sound. Nevertheless, this conclusion can be reached when the absence of enhanced deposition is combined with evidence supporting the presence of flaming fuels that produce black, sooty smoke. Test series were conducted using two different smoke detector brands, each having a different horn configuration. Findings suggest that the same type of methodology for determining whether the detector sounded is applicable to both models. Chladni figures were not found on any of the smoke detectors, whether they sounded or not; hence, the absence of a Chladni figure was not an indicator that the detector did not sound. A smoke flow visualization technique was used to determine the mechanism that caused the observed enhanced deposition and agglomerates on horns that sounded during a smoke exposure. Additionally, a smoke box test series showed that the extent of observed soot deposition increased with increasing smoke exposure.     

Critical mass flux for flaming ignition of wet wood

Wood is a common building material and can constitute the bulk of the fuel load in structures. Cellulosic, woody material is also the fuel in a wildland fire. Wood and forest fuels are porous and hygroscopic so their moisture content varies with the ambient temperature and relative humidity. A complete understanding of both structural and wildland fire thus involves understanding the effect of moisture content on ignition. The ignition criterion considered in this work is critical mass flux – that a sufficient amount of pyrolysis gases must be generated for a diffusion flame to establish above the surface. An apparatus was built to measure the critical mass flux for sustained flaming ignition of woody materials for varying environmental conditions (incident heat flux and airflow (oxidizer) velocity). This paper reports the variation of measured critical mass fluxes for poplar with externally applied incident radiant heat flux, airflow velocity, and moisture content. The critical mass flux is seen to increase with increasing levels of moisture content, incident heat flux, and airflow velocity. Future work will focus on modeling these experiments and exploring the changes in critical mass flux with species, thickness, and live fuels.