Sensitivity and uncertainty analyses for ignition of fiber‐reinforced polymer panels

The ignition characteristics of combustible solids are affected by many factors such as material properties, external heating source, and surrounding environmental conditions. In practice, these factors can vary significantly from one application to another. Thus, it is important to evaluate the sensitivity and uncertainty aspects of the effect of these factors on ignition. This study attempts to achieve this goal through sensitivity and uncertainty analyses on the piloted ignition of fiber‐reinforced polymer (FRP) composite panels. A Monte Carlo simulation using the Latin hypercube sampling method was employed to conduct sensitivity and uncertainty analyses. An integral model combining a general thermal thickness model with a heating rate‐related ignition temperature criterion was used as the ignition prediction model. Time‐to‐ignition was evaluated as the output parameter against the variations of input parameters such as material properties, external heating source, and surrounding environmental conditions. In addition, to identifying important sensitivity factors and uncertainty ranges of piloted ignition, a critical thermal thickness was found for the composite panels. These findings can serve as guides for the fire safety design of FRP composite materials for various applications. Copyright © 2015 John Wiley & Sons, Ltd.     

聚甲基丙烯酸甲酯材料强制点燃研究

使用锥形量热仪研究了聚合物材料PMMA(Poly(methyl methacrylate))在不同热通量下的强制点燃,得到其相应的点燃时间和临界表面温度。同时考虑到强制点燃过程中气、固相的能量和质量传递及化学动力学过程,给出了一种描述聚合物材料强制点燃的气相和固相数学模型,并对实验结果进行了计算,同时与文献数据进行了比较,其计算结果与实验值吻合较好。     

A simplified model for predicting the ignition of FRP composites with validation using intermediate‐scale fire experiment data

This study presents a simplified theoretical model to predict the ignition of FRP composites of general thermal thickness (GTT) subjected to one‐sided heating. A simplified GTT heat transfer model to predict the surface temperature of GTT composite panels was developed, and the exposed surface temperature was used as ignition criterion. To validate the GTT model, intermediate scale calorimeter fire tests of E‐glass fiber reinforced polyester composite panels at three heat flux levels were performed to obtain intermediate‐scale fire testing data in a controlled condition with well‐defined thermal boundary conditions. The GTT model was also verified by using results from finite element modeling predictions. This model can be used to estimate the surface temperature increase, time‐to‐ignition, and mass loss of FRP composites for fire safety design and analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

A theoretical basis for flammability properties

Theoretical formulations are presented for the fire growth processes under external radiant heating. They included ignition, burning and energy release rate, and flame spread. The behaviour of these processes with external heating is described along with the critical conditions that limit them. These include the critical heat fluxes for ignition, flame spread and burning rate. It is shown how these processes and their critical conditions depend on a limited number of properties measurable by a number of standard test methods. The properties include heat of combustion, the heat of gasification, ignition temperature and the thermal properties of the material. Alternatively, the properties could be related to parameters easily found from data; namely: (1) the critical heat flux (CHF) for ignition; (2) the slope of the energy release rate with externally imposed flux, defined as heat release parameter (HRP); and (3) the ignition parameter, defined as thermal response parameter (TRP). It is further shown that the flame heat flux differences between small laminar flame ignition sources and larger turbulent flames can affect flame spread due to heat flux and ignition length factors. Finally, it is found that the critical energy release rates theoretically needed for ignition, sustained burning, and turbulent upward flame spread are roughly 13, 52, and 100 kW/m², respectively, and independent of material properties. Copyright © 2005 John Wiley & Sons, Ltd.     

丁羟复合推进剂的辐射点火

采用CO2激光点火装置,对丁羟复合推进剂的点火过程进行了实验研究,利用描述固体推进剂物化现象的一维传热模型对复合推进剂的辐射点火特性进行了理论分析。通过最小二乘法拟合实验数据得到了丁羟复合推进剂的点火准则。结果表明,丁羟复合推进剂的点火过程主要包括惰性加热及气相点火过程,惰性加热时间和点火延迟时间随热流密度的增大而减小,且随着热流密度的增大,热流密度的影响逐渐降低。固相传热数学模型能够比较准确地描述复合推进剂的辐射点火特性。     

Ignition studies of cerium nitrate treated towels

This study evaluated the ignitability of cotton towel material saturated with an oxidizer solution of 0.5 N cerium nitrate in 2 N nitric acid. Four types of ignition testing were performed in this work: self‐heating oven tests, hot object ignition tests, radiative smoldering ignition tests, and piloted flaming ignition/burning rate tests. Results indicate that cerium nitrate significantly enhances the ignitability of the towels. Self‐heating properties of cerium nitrate treated towels were measured using the standard constant temperature oven method described by Bowes. Based upon these self‐heating properties, self‐heating is not a hazard for storage scenarios other than bulk storage (depths of several meters) of cerium nitrate treated towels at room temperature. Surface ignition of hot objects was observed for object temperatures as low as 250°C placed upon room temperature cerium nitrate treated towels. Ignition for hot objects buried within a pile of towels occurred for object temperatures as low as 230°C. Radiant heating tests of cerium nitrate treated towels showed initiation of smolder at heat fluxes as low as 3 kW/m² at surface temperatures as low as 175°C. This compares with ordinary cellulosic materials that require 7–8 kW/m² heat fluxes and temperatures of 250°C. All four scenarios demonstrate enhanced ignitability and burning rates of cerium nitrate treated towels. Copyright © 2005 John Wiley & Sons, Ltd.     

Thermal-balanced integral model for pyrolysis and ignition of wood

The pyrolysis and ignition of wood is of great importance to understand the initial stage of combustion, helping control the occurrence and spread of unwanted building and forestry fires. The development of a thermal-balanced model is introduced for examining the analytical relationship between the ignition time and external heat flux. The critical heat flux, one of the important fire-retardant characteristics of combustible solid, is determined from a correlation study between the ignition time and external heat flux. One of the thermal-balanced integral models, considering the effect of surface heat losses, average absorptivity and moisture content, is employed to give the prediction of surface temperature rise, ignition time and ignition temperature of the Aspen. The results show that the model readily and satisfactorily predicts ignition temperature and ignition time of wood with different moisture contents.     

The piloted transition to flaming in smoldering fire retarded and non‐fire retarded polyurethane foam

The piloted transition from smoldering to flaming, though a significant fire safety concern, has not been previously extensively studied. Experimental results are presented on the piloted transition from smoldering to flaming in non‐fire retarded (NFR) polyurethane foam and the fire retarded polyurethane foam Pyrell^®. The samples are small blocks, vertically placed in the wall of an upward wind tunnel. The free surface is exposed to an oxidizer flow and a radiant heat flux. The smolder product gases pass upwards through a pilot. The experiments on NFR foam show that the smolder velocity and peak smolder temperature, which increase with the oxygen concentration and heat flux, are strongly correlated to the transition to flaming event, in that there are minimum values of these parameters for transition to occur. The existence of a minimum smolder velocity for ignition supports the concept of a gaseous mixture reaching a lean flammability limit as the criterion for the transition to flaming. To compensate for the solid‐ and gas‐phase effects of the fire retardants on the piloted transition in Pyrell, it was necessary to increase the oxygen concentration and the power supplied to the smolder igniter and the pilot. The piloted transition is observed in oxygen concentrations above 17% in NFR foam and above 23% in Pyrell. The results show that although Pyrell is less flammable than NFR foam, it is still susceptible to smoldering and the piloted transition to flaming in oxygen‐enriched environments, which is of interest for special applications such as future space missions. Copyright © 2008 John Wiley & Sons, Ltd.     

Investigations on the self‐ignition of deposits containing combustibles

Self‐ignition of deposited combustibles is a possible reason for excessive fires occurring on deposits for recycling materials or on waste dumps. Two series of experiments were performed to assess the hazard of self‐ignition: hot storage tests with different homogeneous mixtures of combustible material and inert material and tests with pockets of combustible material embedded in inert matter. In the first test series considerable exothermicity (ΔT>60K) was observed for mass fractions of combustible material as low as 2.5%. In the second series it could be shown that the heat transfer from a pocket of burning material through the inert matter can ignite a second pocket of combustible matter. Based on the experimental data, numerical simulations were performed to predict self‐ignition on real‐scale waste deposits. For a deposit of specific size and shape, the influence of the ambient temperature on the occurrence of self‐ignition has been investigated. Copyright © 2008 John Wiley & Sons, Ltd.     

Approximate solutions for the ignition of a solid as a function of the Biot number

The ignition of a finite planar 1‐dimensional solid is addressed for constant applied radiant heat flux. An ignition temperature criterion is used, with constant properties, and linearized reradiation. An approximate integral analysis leads to analytic formulas to predict the ignition time in terms of Biot (Bi) number and heat flux. The accuracy of the integral solution is found to be good compared to the exact infinite series solution for the conduction equation. The integral model is then used to assess the accuracy of common ideal thick and thin formulas for ignition. There is a domain of heat flux and Bi where the ideal formulas are not accurate. An example is given where the integral model is accurately applied to ignition data to illustrate the range where the ideal formula is not accurate.     

Selecting an ignition criterion methodology for use in a radiative fire spread submodel

Research is ongoing to increase the functionality of the fire zone modeling software BRANZFIRE, by converting it from a deterministic to a probabilistic model. One component of this work is the development of a radiative fire spread submodel for which a suitable ignition criterion method is needed. This paper provides details of that ignition criterion procedure and its implementation into the submodel. A list of requirements that the ignition methodology had to satisfy was established. Of the many different piloted ignition models available, the Flux–Time Product technique, and its associated ignition criterion, was selected to be incorporated into the fire spread submodel. This method provides a practical engineering approximation of when a secondary fuel item that is subjected to incident radiation will ignite that is commensurate with the accuracy of the overall model. Primarily to demonstrate the use of the technique in the submodel, a series of ignition experiments were conducted on a single example of upholstered furniture using the Cone Calorimeter apparatus, with specimens tested in both the horizontal and vertical orientation, under piloted and auto ignition conditions. The experimental incident radiation and time‐to‐ignition data, for the piloted ignition mode, was analyzed using a modified Flux–Time Product correlation procedure. To deal with the auto ignition mode, an empirical approximation, based on the modified Flux–Time Product procedure, is proposed. Data for use in the submodel was therefore also derived for the auto ignition mode, based on an experimental determination of the minimum ignition flux. Copyright © 2010 John Wiley & Sons, Ltd.     

Piloted ignition of wood: A review

The physical phenomenon of piloted ignition of a material is described. A number of mathematical models of this phenomenon are presented in order of decreasing complexity. The most sophisticated models include gas-phase phenomena. Simple models neglect all chemical effects and are purely thermal. The most commonly used criteria for piloted ignition are discussed. Correlations used in piloted ignition studies from the past 40–50 years are summarized. Many investigators have been successful in correlating piloted ignition data of wood using a simplified thermal model in combination with a critical surface temperature criterion. Emphasis of this review is therefore on this approach. The paper concludes with a detailed analysis of various factors affecting piloted ignition of wood. Some of the factors are related to the experimental conditions, others are associated with the characteristics of the material.     

Thermal Decomposition and Ignition of PBXN‐110 Plastic‐Bonded Explosive

Due to safety requirements, insensitive behavior in slow or fast thermal heating (cook‐off) conditions is a desired behavior for today’s munitions. The ignition time of munitions under slow or fast cook‐off conditions is an important parameter in the design of insensitive munitions. The critical temperature, which mainly depends on the chemical, physical, and the geometrical properties of the energetic material, is the determining factor whether the material will end up with thermal initiation or not, when it is exposed to an external heat source. In this study a slow cook‐off test setup is designed and developed and the tests for a generic munition containing PBXN‐110 plastic‐bonded explosive are performed in order to obtain temperature distribution in the test item, ignition time, ignition temperature, and ignition location. In this paper the development procedure and the experimental results of the slow cook‐off tests are explained. Moreover, the kinetic parameters such as activation energy and pre‐exponential factor for the plastic‐bonded explosive obtained from the TGA tests are presented.     

抚顺油页岩半焦燃烧特性

为了提高油页岩半焦燃烧特性对以油页岩半焦氧化为主要热源的抚顺油页岩炼油工艺效率,利用扫描电镜与热重分析对450、550、650℃三种不同制焦温度下的抚顺油页岩半焦(J1、J2、J3),进行了表面形态和燃烧特性分析。结果表明,由于挥发分的析出,半焦表面结构变得粗糙,羽化现象严重。由于可燃物质随制焦温度上升析出较多缘故,半焦着火温度随制焦温度的上升而增高,在20℃/min升温速率下,着火温度由J1焦样的384.7℃升高到J3焦样的408.8℃。半焦的活化能在低转化率比在高转化率时要小,这主要是由于在高转化率下,可燃物减少,灰分热阻增加。     

Ignition performance of new and used motor vehicle upholstery fabrics

This paper examines the standards for fire safety in transport systems and in particular the test method for the flammability of materials within passenger compartments of motor vehicles. The paper compares data from ignition tests conducted in the cone calorimeter and the FIST apparatus with tests conducted using the FMVSS 302 horizontal flame spread apparatus. Ten materials were selected as representative of those used as seat coverings of private and commercial passenger vehicles. The time to ignition of new and used materials subject to exposure heat fluxes between 20 kW/m² and 40 kW/m² was measured. The results from the ignition tests were analysed using thermally thick and thermally thin theoretical models. The critical heat flux for sustained piloted ignition was determined from the time to ignition data using the thermally thin approach. Derived ignition temperatures from both the thermally thick and thermally thin methods were compared with measurements using a thermocouple attached to the back surface of materials in selected tests. The flame spread rates in the FMVSS 302 apparatus were determined and a comparison was made between the performance of the materials in the flame spread apparatus, the cone calorimeter and the FIST. The results suggests that a critical heat flux criterion could be used to provide an equivalent pass/fail performance requirement to that specified by the horizontal flame spread test although further testing is needed to support this. Copyright © 2004 John Wiley & Sons, Ltd.     

An alternative way for the ignition times for solids with radiation absorption in‐depth by simple asymptotic solutions

We present a new simplified method to calculate the ignition times when both conduction and in‐depth radiation absorption in an externally heated solid are significant by deriving an integral equation for the surface temperature history. We achieve this by decomposing the problem into two simple additive problems and in addition, we also provide asymptotic solutions for the surface temperature history at short and long times. These solutions allow to find the relations between the ignition times at different degrees of in‐depth energy absorption and the relations between the thermal solid parameters and the in‐depth absorption coefficient. In addition, these results are useful in interpreting experimental results on ignition in order to assess when and to what extent in‐depth radiation affects the ignition times. Copyright © 2011 John Wiley & Sons, Ltd.     

正丁烷/氢气/空气混合物催化着火的实验研究

通过实验研究了正丁烷/氢气/空气混合物在Pt催化剂表面上的催化着火特性.实验发现,氢气在正丁烷/空气混合物的催化着火过程中既存在热作用,也存在化学作用.当化学作用为主导时,可燃混合物的化学催化着火温度明显低于热着火温度.实验结果表明,只有当氢气在可燃混合物的含量大于某个临界值时,化学催化着火才能发生,而且该临界值的大小以及催化着火温度均与可燃混合物的当量比有关.     

Extension and evaluation of the integral model for transient pyrolysis of charring materials

In most numerical simulations of fire growth and fire spread, pyrolysis models are required to calculate the reaction of the solid material to an incident heat flux. Important results of the pyrolysis model are the mass release rate of combustible pyrolysis gases and the surface temperature. In this paper an integral model is evaluated for the prediction of pyrolysis of charring materials. An existing integral model is extended with a finite and semi‐infinite cooling state. In this state both char and virgin material are present but the pyrolysis reactions have been interrupted due to insufficient heat supply. The results show that such a cooling state can occur in flame spread calculations. Simulations with the integral model are further compared with the results of a moving grid model, which has the same physical basis. Unlike the integral model, the moving grid model does not require any assumption for the temperature profile in the solid. The influence of the quadratic assumed temperature profile in the integral model on the accuracy of the predictions of the mass release rate of pyrolysis gases is evaluated for several cases. It is shown that the integral model has problems with sudden variations of the external heat flux. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of external heat flux on the thermal diffusivity and ablation performance of carbon fiber reinforced novolac resin composite

Ablation is an effective and reliable method largely used in aerospace structures and other high temperature conditions to protect the payload from the damaging effects of external high heat flux. In an ablation process, the high heat fluxes are dissipated by the material through a series of endothermic processes. This finally leads to the loss and the consumption of the material itself. The ablative material keeps the surface temperature within a certain range, and as a consequence an increase of the heat flux will not cause a consistent temperature rise, but will bring about an increase of the surface recession rate. The objective of this work is to give information on the effect of the external heat flux to evaluate effective thermal diffusivity behavior and ablation performance of carbon fiber reinforced composite based on novolac resin. Here, we calculate the effective thermal diffusivity of this composite at different heat flux conditions using inverse solution technique of conservation equations of mass and energy. The ablation performance evaluation is based on experimental transient ablation rate measurement in oxyacetylene flame test. The results of this work explained the ablation process and thermal diffusivity behavior of this composite as a high performance heat shield at high external heat fluxes.     

泡沫金属强化石蜡相变蓄热过程可视化实验

相变材料的低热导率是限制潜热蓄热广泛应用的重要原因。将相变材料石蜡真空条件下注入到泡沫金属铜内制备泡沫金属铜-石蜡复合相变材料,通过铜的高热导率及高孔隙材料的大面体比来强化相变换热过程。采用DSC示差扫描量热法对石蜡进行热物性测量获得准确的石蜡相变温度及相变潜热。以管壳式相变蓄热结构为对象,提取对称结构进行可视化设计,对比纯石蜡及泡沫金属铜-石蜡复合材料在相同运行条件下的相变过程,追踪二者熔化过程的相界面位置随时间的演化过程并布置热电偶准确测量材料内部的温度分布。结果显示加入泡沫金属后的复合材料的内部温差明显减小,温度分布均匀,蓄热热通量显著增大,有效缩短相变时间并缓解了自然对流造成的顶部过热和底部不熔化现象。