Absorption of thermal energy in PMMA by in-depth radiation

An experimental technique is developed to quantify the absorption of thermal energy in black PMMA (Polycast) by in-depth radiation in semi-transparent media. In-depth heating occurs when non-reflected incident heat flux enters the solid without first being absorbed at the exposed surface. Transient conduction due to temperature gradients occurs within the solid in response to this in-depth absorption. An analytical model is developed for predicting time to ignition for such in-depth heating situations. Using the measured absorption coefficient, κ, the analytical prediction for time to ignition is found to be in excellent agreement with data from experiments of Saito and Delichatsios.     

XPS挤塑板燃烧特性实验研究

进行XPS挤塑板燃点温度测定和辐射引燃实验。研究表明:XPS挤塑板点燃温度约为355℃;0.019 4kW/(m2.s)的辐射热流增量不足以引燃挤塑板试样,当辐射热流大于0.060 4kW/(m2.s)时挤塑板试样能够被引燃,引燃温度分别为362、385℃。辐射引燃实验过程中挤塑板表面温度最大值分别为975、996℃。辐射引燃过程中XPS保温板质量损失速率呈现三个阶段:平缓减少阶段、急剧骤减阶段和相对稳定阶段。质量骤减阶段保温板质量呈明显的线性变化。     

Characterization of thermal properties and analysis of combustion behavior of PMMA in a cone calorimeter

This paper deals with the thermal degradation of a black poly(methyl)methacrylate (PMMA) in a cone calorimeter (CC) in air with a piloted ignition. The influence of several heat fluxes (11 kW m⁻² and 12 kW m⁻², and ten values from 15 to 60 kW m⁻² in steps of 5 kW m⁻²) on PMMA sample degradation and the decomposition chemistry has been studied. Thus, thermal properties have been deduced and calculated from ignition time and mass loss rate (MLR) curves. During our experiments, among compounds quantified simultaneously by a Fourier transformed infrared (FTIR) or gas analyzer, five main species (CO₂, CO, H₂O, NO and O₂) have been encountered, regardless of the external heat flux considered. The main product concentrations allow calculation of the corresponding emission yields. Thus, mass balances of C and H atoms contained in these exhaust gases were able to be compared with those included in the initial PMMA sample. Using the standard oxygen consumption method, heat release rate (HRR), total heat release (THR) and effective heat of combustion (EHC) have been calculated for each irradiance level. Therefore, these different results (thermal properties, emission yields, HRR, THR and EHC) are in quite good accordance (same order of magnitude) with those found in previous studies.     

辐射热流下有限厚度PMMA着火特性研究

探究透明PMMA（聚甲基丙烯酸甲酯）厚度和4种不同恒定热流对材料表面温度、质量损失率、着火时间及着火温度的影响。利用反演模型并结合部分实验数据得到PMMA的热物性参数,将其他工况模拟结果与实验测量值及理论分析值相互对比,验证了数值模型和理论分析的准确性和可靠性。结果表明,PMMA着火时间的下降趋势随热流的增大而逐渐变缓。PMMA厚度小于3 mm时,表面温度和质量损失率随厚度增大而减小。PMMA厚度大于3 mm时,着火温度的平均值为（628±20） K,其着火特性几乎不随厚度的变化而变化,故着火温度可作为PMMA着火判据。     

Contribution to flashover modelling: Development of a validated numerical model for ignition of non-contiguous wood samples

A computational model of flashover is presented that closely follows the experimental setup at CNRS-ENSMA-Poitiers. A propane burner with thermal power of 55 kW is used as a primary source of fire and square beech wood samples (30 mm×30 mm×5 mm) as fire spread targets. The computational model describes the wood pyrolysis with a progress variable. Using the conservation of heat fluxes at the solid–gas interface, the thermal diffusion in the wood samples is coupled with the convective and the radiative heat transfer in the ambient gas phase. The incoming heat flux at the upper surface of the wood samples reaches values between 20 and 30 kW/m². With the ignition and subsequent combustion of the pyrolysis volatiles, the heat flux increases by approx. 12 kW/m². The results show that the ignition of the wood samples is triggered at an approx. surface temperature of 650 K. Due to large local variations in incident heat flux, significant differences in the ignition times of the wood samples are observed. The comparison of the calculated and the experimentally measured temperature shows a good agreement for the first wood sample and the model predicts the ignition time very well. But for the second and the third wood samples the model overpredicts the temperature, which leads to a premature ignition of these wood samples.     

热塑性高分子材料强制点燃过程模拟与分析

采用锥形量热仪对典型热塑性高分子材料——聚甲基丙烯酸甲酯(PMMA)在不同外部入射热通量下进行了强制点燃的实验研究。在气相反应和固相反应动力学及传递过程分析的基础上，建立了热塑性高分子材料强制点燃过程的数学模型，通过数值分析的方法将点燃数据相关联，得到PMMA强制点燃的点燃时间与临界表面温度的表达式，并计算了PMMA不同外部入射热通量下强制点燃时间及点燃的临界表面温度。模拟计算结果与实验结果的比较表明，二者基本吻合。采用所得到的数据关联式对强制点燃过程的影响因素进行了定量分析。     

气承膜式会议中心防火分隔措施研究与数值模拟分析

针对某气承膜式会议中心结构特点难以在建筑内采用实体防火墙、防火卷帘等方式进行防火分隔的问题,通过膜材燃烧实验,对燃烧破洞临界温度进行了测定。以临界破洞温度与单位面积膜材表面引燃所需要的辐射热流量作为膜结构火灾安全判定准则,提出防火分隔措施方案。根据火灾动力学原理,分析了火源附近气膜表面烟气温度和热辐射流量。结果表明该防火分隔措施基本可以达到防止火灾蔓延、保护气膜结构安全的消防安全目标。     

室外空气温度测试中低成本自制防辐射罩的优化研究

室外热环境的研究热度越来越高,室外空气温度的精确测试对其研究具有重大影响。百叶箱被认为是室外空气温度测量中的标准仪器。但由于百叶箱体积过大、笨重难以携带,许多学者为了各自的研究目的或考虑成本自制了很多辐射罩。由于室外空气温度测试中采用不同的防辐射罩,得到的结果不同。为了提高室外空气温度测试的精度,减少防辐射罩所带来的误差,通过对某种常用的防辐射罩进行优化,为以后室外空气温度测试提供参考意见。通过问询、网上调查问卷等调研方法进行统计发现,最常用的低成本自制辐射罩为外表面为铝箔的水平圆筒。因此,选择该装置作为研究对象,考虑到风速和太阳辐射对室外空气温度测试的精度的影响,通过增强空气的对流作用,减少进入防辐射罩的太阳辐射,在实地实验测试中,通过改变圆筒长度与其直径比例和圆筒直径与仪器直径比例,改变内外表面的搭配,改变其安装角度,对该防辐射罩进行优化,为防辐射罩的优化提供意见。结果表明:1圆筒长度与其直径比例宜选择6倍。2圆筒直径与仪器直径比例宜选择6倍。3外表面不适宜选择吸收率过大的材料,内表面不适宜选择反射率过高、吸收率过小的材料。内外表面均优先选择白色。4水平筒的安装角度既要考虑太阳辐射,又要考虑风向。昼间测试时,分时段调整水平筒的角度的方案可行,建议每一时间段的调整范围在40°左右。     

Onset of smoldering in cotton: Effects of density

The ignition temperature for smoldering in cotton has been determined for several densities. Experiments and consistent theoretical calculations show that an increase in density leads to decrease in ignition temperature. Experimentally it is found that repeated heating of a cotton sample results in a higher ignition temperature than for samples heated once, as a result of partial decomposition during heating. For the lowest density investigated, smoldering occurs but is not self-sustained. The smoldering front moves through the cotton with a velocity independent of the density. The mass loss rate due to smoldering increases with density. It can be concluded that density affects both the ignition temperatures and the mass loss rates in a systematic way.     

Polyamide 6 and Polyurethane Used as Liner for Hydrogen Composite Cylinder: An Estimation of Fire Behaviours

The most advanced gas storage cylinders are composed of a high molecular weight polymeric liner and fibre reinforced composite. The goal of this paper is to study in ISO 5660 cone calorimeter the thermal behaviours of carbon fibre/epoxy composite covered by a liner made of polyamide 6 (PA6) or polyurethane (PU). Time-to-ignition, amount and rate of mass loss, heat release rate, total heat release rate and effective heat of combustion were measured and calculated at three irradiance levels (20, 40 and 60 kW m^?2). The main exhaust gaseous species evolution as well as oxygen consumption were also quantified during the thermal decomposition process. The transient temperatures were measured at middle-thickness of composite layer and at composite/liner interface by using K-type thermocouples. Indeed, these technical data play a significant role to choose the adequate liner to be used for full-composite cylinder application. Results show that the liner type has no effect on flaming ignition of exposed composite as well as the temperature profiles within materials. Comparing to PA6, the PU liner presents a faster melting and decomposition rate (i.e. with a lower thermal resistance), a lower heat release rate levels and low major gas (i.e. CO, CO₂ and NO) emission yields (i.e. a lower gaseous product toxicity). Based on the comparison of the fire-to-reaction properties, the PU thermoplastics are recommended to be used as liner to cover gas storage composite cylinder.     

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.     

Temporal trends in gaseous mercury evasion from the Mediterranean seawaters

Mercury evasion from seawaters is considered to be one of the main natural sources of mercury released to the atmosphere. The temporal evolution of this mechanism is related to biotic and abiotic processes that produce mercury in its elemental form and as DGM. The efficiency of these processes depends upon the intensity of the solar radiation, the ambient temperature of the air parcel above the seawater, and the water temperature. In the Mediterranean region, the magnitude of these mechanisms are particularly significant, due to favorable climate conditions and to the presence of large cinnabar deposits that cross the whole region; all these synergic factors yield significant evasional fluxes of mercury from the surface water during most of the annual period. In this work, mercury fluxes were measured by using a floating flux chamber connected to an atomic absorption analyzer. Photosynthetic active radiation (PAR) and UV components of the solar radiation were measured using the same system adopted in the EC 'ELDONet project'. The measurements of the mercury evasional fluxes were carried out at three sites of the northern Tyrrhenian Sea during 1998. Two sites were located at unpolluted and polluted coastal areas, and the third was an offshore site. The evasional flux showed a typical daily trend, highest at midday when the ambient temperature and solar radiation were at the maximum, and lowest, near to zero, during the night. Besides the day-night behavior, a seasonal trend was also observed, with minimum values during the winter period (0.7-2.0 ng/m2 h) and maximum values during the summer (10-13 ng/m2 h).     

Numerical study of under-ventilated fire in medium-scale enclosure

In an enclosure, as all the air inflow is consumed in burning with the excess fuel, the internal fire enters the decay phase, and such process is said flame exhaust. The complicated multistage process from an initial fire growth up to a flame exhaust followed by an external burning is investigated by means of a Large-Eddy-Simulation (LES). Turbulent combustion process is modelled by an Eddy Break-Up concept by using two sequential, semi-global steps for CO prediction. The numerical model solves three dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The critical fuel supply rate needed for flame to exhaust and the time period from the fuel ignition to the appearance of an external flaming in medium-scale facilities are previously obtained experimentally by Chamchine AV, Graham TL, Makhviladze GM, et al. [Experimental studies of under-ventilated combustion in small and medium-scale enclosures. In: Proceedings of the fourth international seminar on fire and explosion hazards; 2003. p. 97–107.], and the general trends predicted by the numerical model follow closely their experimental observation. This model is capable of adequately describing the essential simultaneous phenomena (flame height, soot generation, CO production, convection and radiation) occurring in a room fire. The distinct transient stages of fire development prior to flame exhaust and scenarios of the exhaust are analysed. An external burning is followed after the flame exhaust inside enclosure, and the flame height, H_f, past the ceiling is approximately in an order of the opening height. Even though the flame exhaust takes place under the critical conditions, the heat transferred from the hotter gases and the external fire source poses significant threat to people inside enclosure, and potentially induces an ignition of fuel package exposed near the opening of an enclosure.     

柴油机缸内燃烧过程对羰基物排放的影响

采用生物柴油、柴油及其调合油进行柴油机台架试验,测量了缸内瞬变压力随曲轴转角的变化情况,通过放热规律计算,结合柴油机羰基物排放的测量结果,分析了缸内燃烧过程与羰基类污染物生成之间的内在关系。研究结果表明,预混燃烧阶段发生燃料氧化反应,形成甲醛、C7、C5、CO、C2H2等燃烧中间产物;随着负荷的增加,扩散燃烧期延长,成为羰基类燃烧中间产物再次氧化分解的主要阶段;随着负荷的增大,缸内温度、压力提高,滞燃期缩短,在高温、高压区域的滞留时间延长,羰基类污染物被再次氧化、转化几率增加;高负荷时,排气温度较高,有利于羰基类物质发生再次氧化。     

An experimental evaluation of critical surface temperature as a criterion for piloted ignition of solid fuels

With a view to developing a simple engineering method for the prediction of piloted ignition, the validity of the critical surface temperature criterion for piloted ignition is examined experimentally for seven thermoplastic materials. The results indicate that the surface temperature at piloted ignition for each material studied varies by ±15 K or less. As such, the surface temperature criterion appears to be suitable for engineering calculations.

Analysis of time-surface temperature histories shows that the radiant heat source temperature has a significant effect on the material heating over the range of source temperatures utilized (700–1050 K). The variations in material heating are of sufficient magnitude to cause changes in ignition times by a factor of two or more for different heat sources present in typical fire scenarios.

Our current level of understanding of piloted ignition is shown to be insufficient to support extrapolation procedures to determine the minimum incident radiant flux required for piloted ignition. An experimental approach to determination of the minimum radiant flux required for piloted ignition is demonstrated to be feasible.     

Coupled analytical approach to predict piloted flaming ignition of non-charring polymers

Classical thermal theory of piloted ignition is extended by coupling the heat balance at the exposed sample surface and the finite-rate pyrolysis in the material volume. Approximate analytical solutions for the sample temperature are obtained for an arbitrary sample thickness, with the external radiative heating, surface re-radiation, heat of gasification, and the convective heat flux corrected for blowing taken into account. The volatile mass flux is evaluated by integrating the pyrolysis rate throughout the layer, with the assumption of high activation energy limit. Critical mass flux of combustible volatiles is used as the ignition criterion. This enables the ignition temperature to be evaluated instead of being pre-assumed as is done in the classical thermal theory. Coupled analytical approach proposed in this work is verified by comparisons to the numerical solution obtained by the Pyropolis model for the same problem setup. This approach has also been validated by comparisons to published experimental data (ignition temperatures and times to ignition) for three non-charring thermoplastics: polymethylmethacrylate, polyethylene and polypropylene.     

Ignition of wood under time-varying radiant exposures

Many studies have utilized a small-scale experimental apparatus such as the cone calorimeter to investigate the piloted ignition of wood exposed to constant levels of incident heat flux; however, there is a deficiency of similar studies related to the non-piloted ignition of wood exposed to time-varying heat fluxes which might represent more realistic fire exposures. In this study, a method was established for producing well-controlled, time-varying exposures using the conical radiant heater of a cone calorimeter. Experiments were conducted in which the incident flux, time to non-piloted ignition, and back-surface temperature of spruce wood were measured. Measured data were used in combination with a numerical heat transfer model to compute the time-dependent temperature distribution through each specimen, and thereby deduce the surface temperature at ignition. From the 30 specimens tested, the average surface temperature for non-piloted ignition of wood was determined to be 521±10 °C. From this surface temperature range, the heat transfer model was used to predict the range of time over which non-piloted ignition was likely to occur for a given time-varying exposure. This procedure was found to produce excellent predictions of ignition time for the time-varying exposures considered in this study. In addition, several existing ignition models were considered, and their suitability for predicting the non-piloted ignition of wood was assessed.     

Rigid foam polyurethane (PU) derived from castor oil (Ricinus communis) for thermal insulation in roof systems

This paper discusses the response of the thermal insulation lining of rigid foam polyurethane (PU) derived from castor oil (Ricinus communis) in heat conditions, based on dynamic climate approach. Liners have been widely used, because the coverage of buildings is responsible for the greatest absorption of heat by radiation, but the use of PU foam derived from this vegetal oil is unprecedented and has the advantage of being biodegradable and renewable. The hot wire parallel method provided the thermal conductivity value of the foam. The thermogravimetric analysis enabled the study of the foam decomposition and its lifetime by kinetic evaluation that involves the decomposition process. The PU foam thermal behavior analysis was performed by collecting experimental data of internal surface temperature measured by thermocouples and assessed by representative episode of the climatic fact. The results lead to the conclusion that the PU foam derived from castor oil can be applied to thermal insulation of roof systems and is an environmentally friendly material.     

Experimental study of radiation and free convection in an enclosure with a radiant ceiling heating system

Participation of the radiation and free convection in the heat transferred from the ceiling surface of a room to other internal surfaces has been investigated in this study. A model enclosure representing a room was constructed and equipped with a radiant ceiling heating system. In order to have a thermal map over both internal and external surfaces of the enclosure, 108 elements were specified over the walls, floor and ceiling of the enclosure. Temperatures at both sides of the elements were measured using an infrared thermometer and k-type thermocouples under steady state heat flow condition. Using the measured temperatures, conductive heat transfer through the compartment elements was first calculated. A model based on the net-radiation method was employed to compute the radiation exchanges between internal surfaces of the elements. Convection participation was also specified using radiation and conduction for each element. Based on the results, more than 90% of the heat is transferred by the radiation from the heated ceiling to the other surfaces of enclosure. The participation of the radiation increases slightly as the ceiling temperature is increased.     

Exchange of oxygen and carbon dioxide across the water surface during algal blooms in a pond

Exchange of O₂ and CO₂ across the water surface during algal blooms was studied in an experimental pond from measurements of O₂ and TCO₂, pH and chlorophyll-a. Production and consumption rates of O₂ due to biological activity were also measured using light and dark bottles.Algal blooms occurred twice during the experimental period of 22 days. The trajectory on the phase plane showing the time changes in concentration of O₂ and TCO₂ made a clockwise, converging loop. A new concept and quantity called total oxygen (TO₂), which is the sum of the concentrations of O₂ and TCO₂, is proposed in order to distinguish the role of gas exchange from that of deposits. From the analysis of the trajectory, the total amount of TO₂ was found to decrease during the production period, and increase during the decomposition period. This was due to the difference between O₂ and TCO₂. The mass transfer coefficient of O₂ was found to be (3.6 ± 1.9) × 10⁻⁴, and that of TCO₂ (1.0 ± 0.5) × 10⁻⁴ cm s⁻¹.During the production period, 60% of produced O₂ was lost to the atmosphere, and 20% of consumed TCO₂ was supplied from the atmosphere. During the decomposition period, 80% of consumed O₂ was supplied, and 35% of produced TCO₂ was lost.