少量聚氨酯泡沫样品从阴燃到有焰燃烧的变化研究

试验主要观察处于阴燃状态的少量聚氨酯泡沫样品的流速、氧气浓度和辐射热通量对气／固界面的影响。因为此试验研究对象是少量聚氨酯汽沫，所以阴燃蔓延以厦转化到有焰燃烧时必须借助于降低热损失并同时增加其氧气浓度。试验中，我们把呈平行六面体的样品竖向放置在风道中。样品的其中三个侧边处在高温状态，第四边暴露在上升气流和辐射中。结果发现，随着气流流速的降低以度氧气浓度的增加，或者增加辐射通量，都会加快其变成有焰燃烧的过程，减少这种变化的延误时间。试验结果表明，炭化部位内部的有焰变化因阴燃作用而出现滞后，这已经通过超声波穿透样品内部得到了证实。笔者这里给出了简化了的分析，证明这种变化可以作为一个气相燃烧程序进行处理。     

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.     

Ignition,Heat Release Rate and Suppression of Elastomeric Materials

The focus of this paper is to determine flammability characteristics of rubber materials that are common to vehicle tires, conveyor belts, and electrical power cable insulation and to compare the thermal magnitude of cargo quantities of these materials to other fuels that are publicly transported. Although a literature review was performed, very little data was found on this topic. Standard flammability test procedures were used to measure the critical flux for ignition, critical ignition temperature, and heat release rates (HRR) of rubber compounds common to tire tread materials and conveyor belt covers. Both the intermediate scale calorimeter: ISO 14696, ASTM E-1623 (ICAL) and the cone calorimeter: ISO E-5660, ASTM 1354 (Cone) provided the bulk of the data. Critical ignition flux and vertical flame spread data for rubber based electrical insulations were determined using a radiant panel from a modified ASTM flame spread apparatus: ASTM E-162. thermogravimetric analysis was also used to evaluate thermal decomposition progression of selected test materials. Further, suppression tests were conducted on tire piles to evaluate agents to extinguish and control tire fires. Also, the HRR of the tire piles were measured and compared to work performed by others. Results confirm that the area heat release rate of rubber materials is directly proportional to exposure flux intensity. The critical exposure flux for ignition of a variety of rubber-based materials is approximately 20 kW/m² to 30 kW/m² and the critical temperature for piloted and non-piloted ignition were independent of exposure intensity at ~400°C and ~600°C respectively. In large quantities, rubber tire loads have total HRR comparable to the heat released from similar areas of liquid hydrocarbon spills.     

Controlled atmosphere pyrolysis apparatus II (CAPA II): A new tool for analysis of pyrolysis of charring and intumescent polymers

A new gasification apparatus has been developed to enable a comprehensive analysis of pyrolysis of charring and intumescent materials. This apparatus provides well defined boundary conditions and highly resolved measurements of mass, temperature and sample profile evolution of a disk-shaped 0.07 m diameter material sample exposed to radiant heat. All measurements are collected simultaneously, in a single experiment, and recorded as a function of time. The oxygen concentration in the pyrolysis zone is controlled and can be reduced below 1 vol% to ensure that the measurements are free of oxidation effects. The radiation from an external conical heater has been carefully characterized to account for changes in the sample surface position, including the surface's angular orientation. Using an empirical expression, the radiation heat flux can be predicted with less than 2% error based on the known surface position and heat flux set point. The NIST Fire Dynamics Simulator (FDS) has been utilized in the direct numerical simulation mode to investigate convective losses from the sample surfaces. The convective heat transfer coefficient computed for the top (radiation exposed) surface has been found to be dependent on the surface position; its space-averaged value has been validated against experimental measurements. The capabilities of the apparatus are demonstrated using poly(vinyl chloride). It is shown that the apparatus provides repeatable data necessary for modeling of transport processes inside pyrolyzing intumescent solids. Non-one-dimensional nature of these processes is discussed.     

Ignition and Opposed Flow Flame Spread Using a Reduced Scale Attachment to the Cone Calorimeter

A reduced scale ignition and flame spread technique (RIFT) was implemented in the cone calorimeter test system to generate thermal and combustibility material response parameters normally associated with an intermediate scale test, LIFT. The magnitude of the parameters derived using RIFT compare favorably with those obtained using the standard LIFT apparatus. The results of the modified cone calorimetric system or RIFT can be used to generate more specific material combustibility characteristics associated with materials burning over time, helping to create and refine engineering models of solid phase combustion. When validated, such models would help us predict and control flame spread with more certainty.     

The effect of ignition source exposure and specimen configuration on the fire growth characteristics of a combustible interior finish material

The potential fire hazard presented by plastic based combustible interior finish materials is discussed. Compartment fire experimental methods and apparatus based on Uniform Building Code Standard No. 42-2 were used to study the effect of ignition source exposure and specimen configuration on the actual fire growth characteristics of polyvinyl chloride (PVC) foam wall covering. The results obtained are qualitative in nature, clearly demonstrating two important characteristics of the fire behavior of PVC foam: (i) the PVC foam has a ‘critical’ ignition source strenght of 64–75 kW; (ii) preheating the PVC foam greatly increases its peak net rate of heat release.     

Conductive and Radiative Heat Transfer in Ceramic and Metal Foams at Fire Temperatures

In addition to the multiple actual or possible applications of metal and ceramic foams in various technological fields, their thermal properties make them a good candidate for utilization as fire barriers. Several studies have shown experimentally their exceptional fire retardance due to their low apparent thermal conductivity. However, while the thermal properties of this porous material have been widely studied at ambient temperature and are, at present, well-known, their thermal behaviour at fire temperatures remains relatively unexplored. Indeed, at such temperatures, the major difficulties are not only due to the fact that thermal measurements are rendered fussy since heavy equipments are required but also stem from the fact that a significant part of the heat transfer occurs by thermal radiation which is much more difficult to evaluate than conductive heat transfer. Therefore, the present chapter is written with a view to report progress on the knowledge of heat transfer in open cell foams and to enlighten the reader on the mechanisms of heat transfer at high temperatures. A first part is devoted to the review of the prior published works on the experimental or theoretical characterisations of radiative and conductive heat transfers from ambient to high temperatures. By taking inspiration from the concepts and models presented in these previous works, we propose, in a second part, a model of prediction of the conductive and radiative contributions to heat transfer at fire temperatures. This analytical model is based on numerical simulations applied to real foams and takes into account the structure of the foam and the optical and thermal properties of the constituents. In a third part, we propose an innovative experimental technique of characterization of heat transfer in foams at high temperatures which permit to evaluate independently the radiative and conductive contributions from a unique and simple measurement. The experimental results obtained on several metal and ceramic foams are compared to the results predicted by our numerical model. The good adequacy between experimental and theoretical results show the consistency of both approaches.     

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.     

Effect of stove asymmetric radiation field on thermal comfort using a multisegmented bioheat model

The multinode multisegment bioheat model of Salloum et al. [Salloum M, Ghaddar N, Ghali K. A new transient bio-heat model of the human body. In: Proceedings of the ASME 2005 summer heat transfer conference, 17–22 July 2005, San Francisco, Paper no. HT2005-72303] is integrated with a space heat model to study human thermal response when subjected to radiant asymmetry in stove-heated domestic spaces in Lebanon. For any given person position, the overall comfort level is based on Frank et al. model correlations [Frank SM, Srinivasa NR, Bulcao CF, Goldstein DS. Relative contribution of core and cutaneous temperatures to thermal comfort and autonomic responses in humans. Journal of Applied physiology 1999;86(5):1588–93]. The assessment of local comfort level is based on the maximum deviation of the clothed segments skin temperature from the mean skin temperature and its relation to the radiant temperature asymmetry.

Experiments were run on human subjects at steady-state conditions to measure the variation of the skin temperature at different locations of the human body segments while standing in an asymmetric thermal radiation field generated by a stove-heating unit. The experiments were conducted to validate the applicability of the bioheat model in predicting skin temperature in asymmetric conditions. The measured skin temperature of various body segments and the radiative asymmetry agreed within ±5% of values predicted by the bioheat model [Salloum M, Ghaddar N, Ghali K. A new transient bio-heat model of the human body. In: Proceedings of the ASME 2005 summer heat transfer conference, 17–22 July 2005, San Francisco, Paper no. HT2005-72303].

The space heat model and the bioheat model are applied to a case study to predict both overall thermal comfort and local thermal discomfort in a typical radiant heat space at different standing positions of the person. Strong thermal discomfort exists within the vicinity of the stove high-temperature surface. The local discomfort is considered at values of maximum SD>1.1 °C derived from consideration of Fanger et al. [Fanger PO, Ipsen BM, Langkilde G, Olesen BW. Comfort limits for asymmetric thermal radiation. Energy and Buildings 1985;8(3):225–36] data of comfort limits and skin temperature measurements for asymmetric thermal radiation.     

Smoke and heat release and ignitability as measures of fire hazard from burning of carpet tiles

A series of 22 commercial carpet tiles, covering the range of backings found in the marketplace, and with the same face material (nylon) was chosen for fire testing. All the carpets were tested in the cone calorimeter rate of heat release apparatus. They were also all tested in the NBS smoke density chamber, in the flaming mode. A selection of samples was further tested using the flooring radiant panel. A preliminary investigation was made to choose the optimum radiant incident flux to be used, which was determined to be 25 kW/m².

It was found that the carpets showed a wide range of fire performance, including ranges of peak rate of heat release and of time to ignition of c. 3 and of smoke factor of c. 8. It was not found possible to correlate the results of the NBS smoke chamber or radiant panel tests with any of the results obtained from the cone calorimeter. A classification scheme was proposed to determine fire performance of carpets, based on the ratio of time to ignition (in seconds) and peak rate of heat release (in kW/m²). According to this scheme, four categories of fire performance would be expected:

4.

     

Radiant Ignition of Polyurethane Foam: The Effect of Sample Size

Although smouldering ignition of upholstery items remains a leading cause of residential fire deaths, relatively little research is conducted on the topic. An experimental investigation of the effect of sample size on the ignition and spread of smouldering and flaming in polyurethane foam under natural flow conditions is reported here. Polyurethane foam samples are used because this is a common material in modern, residential environments and one for which there exists significant quantities of previous experimental data in the literature. Samples of different square cross-section size and a fixed height of 150 mm are insulated on all sides except the top which is exposed to a radiant heat flux and is open to the air. Samples with side lengths of 50 mm, 100 mm, and 140 mm are studied. Ignition and spread dynamics are diagnosed using thirteen thermocouples located along the vertical centre line. The onset of smouldering ignition (13  $\hbox{kW}\, \hbox{m}^{-2}$ , 8  $\hbox{kW}\, \hbox{m}^{-2}$ and $7\,\hbox{kW}\, \hbox{m}^{-2}$ for 50 mm, 100 mm and 140 mm sample sizes respectively) is observed at significantly lower heat fluxes that flaming (45  $\hbox{kW}\,\hbox{m}^{-2}$ , 32  $\hbox{kW}\,\hbox{m}^{-2}$ and $30\,\hbox{kW}\,\hbox{m}^{-2}$ respectively). Critical heat fluxes for smouldering and flaming ignition increase with decreasing sample size, with smouldering ignition being significantly more sensitive to sample size than flaming ignition under the size range studied. Smouldering spread rates are measured in the range from 3  $\hbox{mm}\, \hbox{min}^{-1}$ to $25\,\hbox{mm}\, \hbox{min}^{-1}$ and found to be a strong function of the heat flux and depth of the smoulder front. The effect of sample size on smouldering has been theoretically proposed before but this is the first time that this effect has been demonstrated experimentally for ignition. The fact that large samples result in the lowest critical heat flux could have implications for testing procedures and translation of results from small-scale testing to real-scale in the built environment.     

Steel foam for structures: A review of applications,manufacturing and material properties

The objective of this paper is to provide a state-of-the-art review for the structural application, manufacturing, material properties, and modeling of a new material: steel foam. Foamed steel includes air voids in the material microstructure and as a result introduces density as a new design variable in steel material selection. By controlling density the engineering properties of steel components may be altered significantly: improvement in the weight-to-stiffness ratio is particularly pronounced, as is the available energy dissipation and thermal resistivity. Full-scale applications of steel foams in civil structures have not yet been demonstrated. Therefore, existing applications demonstrating either proof-of-concept for steel foam, or full-scale use of aluminum foams in situations with clear civil/structural analogs are highlighted. Adoption of steel foam relies on the manufacturing method, particularly its cost, and the resulting properties of the steel foam. Therefore, published methods for producing steel foam are summarized, along with measurements of steel foam structural (modulus, yield stress, etc.) and non-structural (thermal conductivity, acoustic absorption, etc.) properties. Finally, existing models for predicting foamed steel material properties are summarized to highlight the central role of material density. Taken in total the existing research demonstrates the viability of steel foams for use in civil/structural applications, while also pointing to areas where further research work is required.     

泡沫钢的应用、制作及属性

对泡沫钢的结构应用、制作、属性及模拟方法进行说明。泡沫钢内部含有许多微小空隙,故提出紧密度,作为泡沫钢的设计变量。改变紧密度,泡沫钢结构的工程特性可能发生重大变化:质量与刚度之比显著提高,有效能耗和热电阻也发生较大变化。在土木工程领域中,泡沫钢的实际应用尚未见报道。因此,在类似结构研究比较充分的前提下,着重研究现有泡沫钢的理论或泡沫铝的实际应用。泡沫钢的应用取决于制作方法,特别是成本费用及泡沫钢的性能。总结了已公开的泡沫钢生产方法、结构属性(模态、屈服应力等)或非结构属性(导热性、隔音性)。总结了泡沫钢属性的现有测量模型,突出了材料紧密度的重要作用。现有研究证明了泡沫钢在结构工程应用中的有效性,并指出进一步的研究方向。     

Wall flame heights with external radiation

An existing flame heat transfer fire testing apparatus was used to study the upward flame spread potential of two kinds of wall materials: (1) PMMA (Polymethylmethacrylate) and (2) Douglas Fir Particle Board. PMMA is noncharring whereas Douglas Fir Particle Board is a charring material. Various levels of external radiant heat flux ranging from 1.8 W/cm² to 3.4 W/cm² were imposed onto the wall samples in order to measure the flame heights as a function of energy release rate. Flame height measurements were established visually by a review of video recordings. The results for these wall flames correlate flame height to the 2/3 power of energy release rate per unit sample width. The wall results are generally higher than data from gas burner line fires against a wall for a range of 10 to 200 kW/m.Note: This paper is a contribution of NIST and is not subject to copyright.     

Evaluation of heat flux reduction provided by the use of radiant barriers in clay tile roofs

In Brazilian towns and cities the greatest thermal gain occurs through the roof of single-storey buildings. In this regard, the use of thermal radiation barriers has the function of minimizing the heat flux through the roof. Even though the use of this type of thermal insulation has increased in recent years; there are still no technical standards which address the subject. Thus, many products have become available on the market which have the appearance of a radiant barrier, but without low emissivity, and not functioning properly as thermal insulation. The objective of this study is to analyze the efficiency of some types of radiant barriers found on the civil construction market, as well as to analyze the efficiency of sheets made from the joining together of a solid urban waste, long-live carton packaging, in loco and in the laboratory. The in loco measurements were carried out in a roof of a residence in the city of Florianópolis, where the heat flux, surface temperatures of the tiles and the ceiling, and the internal and external temperatures, were monitored. The laboratory experiments were carried out with an apparatus which simulates the thermal resistances of a real roof. The results allowed the verification of which is the best type of radiant barrier, that is, which achieves the greatest reduction in heat flux through the roof. This study also allowed a comparison between the in loco and laboratory results.     

Heat Transmission and Thermal Energy Storage in Firefighter Turnout Suit Materials

This paper describes research conducted to develop a better understanding of the effects of turnout materials on heat transmission and thermal energy storage in moisture preconditioned samples exposed to low level radiant heat. It utilized a newly developed laboratory apparatus and testing procedures to generate data on a range of materials used in the construction of firefighter turnouts.     

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.     

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.     

Effect of moisture on the burn potential in fire fighters' gloves

The effect of moisture on thermal protection in fire fighters' glove materials was determined during radiant and conductive heat/pressure exposures. Typical glove materials were studied with and without a Gore-Tex^TM moisture barrier. During exposures to a radiant load of 2.3 cal/cm² sec, wetted samples provided more thermal protection than dry samples. Different results were obtained using conductive heat/pressure tests (500°C at 0.28 kg/cm² pressure): dry Gore-Tex^TM mitts provided more thermal protection than totally wetted samples, while the reverse was true of glove materials without Gore-Tex^TM. Reference: James H. Veghte, Effect of Moisture on the Burn Potential in Fire Fighters' Gloves,Fire Technology, Vol. 23, No. 4, November 1987, pp 313–322.     

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.