Kinetic and mechanism of the thermal degradation of a plywood by using thermogravimetry and Fourier-transformed infrared spectroscopy analysis in nitrogen and air atmosphere

The thermal degradation of plywood was investigated using thermogravimetric analysis (TGA) device. The TGA experiments were conducted between ambient temperature and 1000 °C for seven different heating rates: 5, 10, 15, 20, 30, 40 and 50 °C min⁻¹. The experiments were realized under inert and air atmospheres in order to characterize the plywood thermal decomposition occurring during the pyrolysis and oxidative processes. Throughout all the tests, the gaseous emissions were continuously monitored using a Fourier transformed infra-red spectrometer (FTIR). The progress in the mass, the mass-loss rate (MLR) and gas emissions data allow to propose a mechanism of the thermal decomposition of plywood with six different stages. The reactions (stages) of this mechanism is of a rate represented by a modified Arrhenius law containing four unknown kinetic parameters (A, E_a, n and ν) for each reaction. These 24 unknown parameters are determined by using the inverse optimization method of the genetic algorithms. The model developed is validated regardless of the heating rate and atmosphere (inert or air) chosen. A very good agreement is obtained between the experimental and the numerical mass loss rate evolutions.     

Outlining the mechanism of flame retardancy in polyamide 66 blended with melamine-poly(zinc phosphate)

Glass-fiber reinforced polyamide 66 is flame retarded with a mixture of melamine-poly(zinc phosphate), (Safire^®400) and diethyl aluminium phosphinate. The performance of this synergistic combination of additives is multi-modal and a comprehensive investigation is undertaken to elucidate the underlying flame retardancy mechanism. The strategy was to characterize the different chemical species responsible for flame retardancy that are generated in gas and condensed phases under different fire scenarios. Following heat release rate (HRR) curve of flame retarded polyamide formulations obtained by mass loss calorimeter, samples in different stages of degradation are collected and investigated. Further flame retardants and formulations were degraded in tubular furnace whose temperature protocol relied on thermal degradation profile obtained from thermogravimetric analysis (TGA). In either case, species generated in condensed phase were studied by solid state nuclear magnetic resonance spectroscopy (magic angle spinning (MAS) NMR; ²⁷Al, ³¹P and ¹³C), Fourier transform Infra-red spectroscopy (FTIR), X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and optical microscopy, whereas TGA coupled FTIR, and pyrolysis gas chromatography mass spectrometry (Py/GC/MS) were utilised to investigate species released in gas phase. Flame retardancy mechanism is elaborated based on the identification of the chemical species in both gas and condensed phases and their specific contributing role.     

DECOVALEX_IV TASK_D项目的热-水-力耦合过程的数值模拟

核废料地下处置系统研究的国际合作项目DECOVALEX(development of coupled models and their validation against experiments) 致力于高放射性核废料地质处置系统围岩中多个物理场的耦合过程研究和工程屏障的可行性评估.中国研究小组第一次参加DECOVALEX计划.介绍了其子课题Task_D的情况和阶段性研究结果,包括2种处置方式--瑞士FEBEX和美国Yucca Mountain类型的1×104 a以上的预测模拟及其比较分析.建立一套复杂的非饱和多孔介质中热液力(THM)耦合过程的非等温流动和形变控制方程,涉及到固、液、气三相和四种组分(固体骨架,水,干空气和水蒸汽).其表现为相对独立同时又相互交叠的4类模型(1) 考虑有效应力、热膨胀应力和膨润土的水膨胀应力变形模型;(2) 考虑水与蒸汽在蒸发凝结时物质交换的流动模型;(3) 包含蒸汽/干空气相对运动的蒸汽分子扩散和对流模型;(4) 三相局部非等温过程的热交换模型.在此基础上根据Task_D的设定要求简化为较为实用的方程,并发展了相应的程序对1×106 a THM耦合行为进行了预测模拟,其结果在2005年2月Task_D讨论会和DECOVALEX工作会议与多个国家相互独立的研究结果进行对比,吻合程度很高,这给模型的建立和程序发展都带来了信心.     

Experimental study on the influence of different thermal insulation materials on the fire dynamics in a reduced-scale enclosure

Four scaled (1:5) fire experiments with two identically classified types of commercially available sandwich panels incorporating either stone wool (SW) or polyisocyanurate (PIR) foam as cores were conducted using a modified version of the ISO 13784-1 (Reaction to fire tests for sandwich panel building systems — Part 1: Small room test) standard. This was to assess the suitability of scaled experiments for assessing sandwich panel fire behavior. In the modified version of the test standard (scaled and full experiments), the fire severity was increased to simulate fires that could occur in commercial premises. This was achieved by prolonging and doubling the heat release rate output of the gas burner at the end of the experiments. Furthermore, non-structural damages such as screw-hole damages were applied to the enclosures to reflect real life observations.The results showed differences in the fire behavior, depending on whether the enclosures were constructed of panels filled with SW or PIR insulation material. The mass losses of the insulation materials showed significant contribution from the PIR cores, regardless of fire load and the non-structural damage.The qualitative behavior with respect to the “flashover” failure criterion, as stated in the ISO 13784-1, was successfully obtained in all of the scaled experiments. As such, the scaled experiments mimicked the behavior of the full scale SW experiments to a satisfactory degree. However, the PIR compartments failed considerably earlier in the full scale tests than in the scaled experiments. Therefore, it can be concluded that when the energy contribution from the core material remained negligible compared to the gas burner, the measured parameters matched quite well. Therefore, if the insulating core material does not dominate the fire dynamics of the compartment and the energy from the gas burner dictates the fire scenario then the scaled set-up will predict the temperature in the full scale compartment. Based on this and with further development with respect to, especially, time, this kind of scaled experiments could be a valuable testing method for assessment of the behavior of sandwich panel, and therefore merit further studies and eventually increased use.     

Effect of Dissolved Gases on Spray Evaporative Cooling with Water

An experimental investigation of the effect of nondegassed water used to cool a solid surface is presented. The solid surface is subjected to thermal radiant input from three panels positioned above it. The water is deposited on the surface in the form of a sparse spray with droplets of about 10 µl. Previous experiments with degassed water are compared to a new set of experiments. In addition, the effect of dissolved gases (air) is quantified in terms of the overall transient thermal behavior of the solid. A lower steady-state average temperature is achieved when gases remain in the water. This result suggests that the configuration of the liquid droplets on the surface and the radiant heat input into the droplet are altered by the gas bubbles in the deposited droplet. This information provides insight into fire control mechanisms by automatic sprinkler systems.     

Investigation of thermal degradation of pine needles using multi-step reaction mechanisms

The objective of this study is to assess the relevance of several multi-step reaction mechanisms to describe the mass loss and the mass loss rate of pine needles in TGA at different heating rates in inert and oxidative atmospheres. The kinetic parameters of the different reactions were optimized using the Shuffled complex evolution (SCE) technique. Model results show that both mass loss and mass loss rate should be considered in order to evaluate properly the mechanism. The drying process is described accurately by a single reaction with a well-established set of kinetic parameters. The conversion of dry pine into char requires a five-step reaction mechanism that is combined of three reactions to describe the pyrolysis under inert atmosphere and another two reactions to describe the oxidative process. Less detailed mechanisms were found to be unable to reproduce the mass loss rate. In particular, the one-step reaction mechanism, widely used to model the pyrolysis process in wildland fire simulations, should be used with care. Finally, the char oxidation process can be described with a single step-reaction mechanism. The final complex mechanism is comprised of one reaction for drying, five reactions for the conversion of dry pine into char, and one reaction for the char oxidation, is promising. Further studies are required for its validation in large-scale experiments.     

The simulation of fire sprinklers thermal response in presence of water droplets

When a fire occurs, the sprinkler closest to the location of the fire typically activates first and releases water droplets into the rising plume of hot gases. Part of these droplets is entrained by the plume and may impact on adjacent sprinklers providing evaporative cooling and thus delaying their activation. The model of the thermal response of sprinklers in these conditions suggests the introduction of the concept of equivalent cylindrical links: a solid metallic cylinder is said to be equivalent to a given fire sprinkler link if it reaches the activation temperature of the sprinkler at the same time, both in dry conditions and in presence of water droplets carried by the hot gas flow. Tests are conducted on both fire sprinklers and equivalent cylindrical links to validate this theoretical approach. The results compare favorably both in dry and wet conditions for the range of parameters considered in this study. Therefore, this approach enables the transient quantification of the sprinkler thermal response in an actual fire scenario such as a large-scale fire test.     

A study of non-flashover and flashover fires in a full-scale multi-room building

Realistic fire environments in a prototype multi-room apartment in a multi-storey building are studied. The fires are designed as non-flashover and flashover types, using standard polyurethane mattresses as fuel. A comprehensive set of experimental data is presented. The measured results include flame spread velocity, mass release rate, gas temperature, radiation heat flux and gas analysis. A computational fluid dynamics (CFD) model, called a CESARE-CFD fire model, has been used to simulate these polyurethane slab fires. The CFD model is described by three-dimensional transport equations for mass, momentum and enthalpy. The turbulence flow was modelled using the k−ϵ model. A soot formation model and a flame spread model were incorporated into the CFD model. The flame spread velocity and the mass release rate of the polyurethane slab fires were predicted in this study. It was found that the CFD model provided reasonable predictions of the magnitude and trends for the experiments both in the non-flashover and flashover fire cases.     

The contribution of flames under ceilings to fire spread in compartments

An experimental investigation has been made of flames spreading beneath both combustible and non-combustible ceilings. Experiments were performed in a model representing the ceiling of a corridor with a fire at one end: a gas burner was used to represent the fire, this was replaced by wooden cribs in experiments to be described in later part of this report.

The flames rising vertically from the fire in effect drew air up into the horizontal layer of flames and gases beneath the ceiling. Depending on the rate of flow of fuel gas (or rate of burning of the cribs) this air could be sufficient for complete combusion of the fuel gases and the flame length was then apparently determined by mixing processes within the layer. When this air was not sufficient, the remaining air for combustion was entrained vertically into the horizontal layer from the cool air beneath and the flames became much longer.

Correlations of lengths of horizontal flames beneath non-combustible ceilings have been derived and related to the much shorter lengths of vertical flames. Relationships have been derived from the experimental data from which it is possible to estimate the radiation downwards from a hot non-combustible ceiling and the gases beneath it to the floor with a view to estimating the contribution to fire spread on the floor. A heat balance of the ceiling gases was satisfactory, so confirming the validity of the calculations. Horizontal flames radiate more of the heat produced at a level sufficient to assist fire spread than do vertical ones.

A combustible ceiling lining results in longer flames, an increase in the distance over which heat radiated downwards at an intensity sufficient to promote fire spread and a faster rate of increase of radiation than a non-combustible one with similar thermal constants.

The aspect of performance which best related to the results of BS 476 tests was the rate of increase of radiation downwards in the early part of the experiments. The radiation downwards was apparently partly determined by extraneous factors such as the detachment of the board from its holding nails and whether the decomposition products were emitted as jets.

The rate of spread of fire along a narrow strip of wood on the floor beneath a burning ceiling lining has been calculated and the results related to the index of performance on the Fire Propagation Test.     

Extinction of surface stabilized gaseous diffusion flames: Part I Simplified numerical model and implications for solid fuels in fires

In this work a previously proposed empirical and analytical criterion for extinction is numerically extended and validated for varying fuel dilution, oxidant dilution, strain rate, and surface temperature. The output of this work is presented in two parts: the current Part I uses simple kinetics and constant thermal transfer properties and Part II uses detailed kinetics, varying thermal transfer properties, flame radiation feedback and flame suppression agents in order to demonstrate that conclusions from the simplified model are still valid. In addition this work goes beyond the concept of critical flame temperature or mass flux for extinction by including the influence of slow chemical kinetics through the Damkohler number which becomes even more important for commonly used fire retarded materials.Extinction of flames on solid fuels is modeled by decoupling the pyrolysis chemistry from the gas-phase combustion chemistry using the flame energy feedback versus pyrolysis rate curves and an energy balance at the surface. This approach has the advantage of identifying and deducing key materials properties for solid and gaseous phase from experiments. Simulations are performed in a planar stagnation-point flow diffusion flame configuration using one-step Arrhenius chemical kinetics and a simplified transport model with Lewis number equal to unity. Only quasi-steady conditions are considered for the gaseous phase even if the pyrolysis rate of the solid is transient because the response time for the solid phenomena is, in general, much larger than the response (diffusion) time for the gaseous phenomena.It is found that at high pyrolysis rates and low straining rates (infinitely fast kinetics regime) there is no leakage of oxygen to the surface of solid fuel. However, as the solid fuel extinction is approached, oxygen leakage occurs because the effective air to fuel mass stoichiometric ratio becomes less than one owing to fuel dilution near the surface. At high straining rates, solid combustion cannot be sustained at any pyrolysis rate. In the infinitely fast kinetics regime, an appropriate scaling has been developed which collapses the convective heat flux curves onto a single one. In general, the critical pyrolysis fuel mass flux exhibits a universal behavior for variation of various model parameters when plotted versus a modified Damköhler number, and becomes constant when the latter is sufficiently high. Comparison with experiments is discussed, and the implications of the criterion for characterizing ignition flammability properties of solid fuels are also discussed.     

Computational analysis of reduced-mixing personal ventilation jets

In this paper we develop a detailed computational fluid dynamics (CFD) model of a personal ventilation (PV) setup comprising a PV nozzle, seated thermal manikin and floor diffuser, then use experimental velocity and tracer gas concentration data for the same setup to validate the CFD model. Specifically, we compare CFD results with the experimental results obtained with both a conventional round nozzle and a novel low-mixing co-flow nozzle directing a PV fresh air jet toward the breathing zone (BZ) of a seated thermal manikin in a thermally controlled chamber ventilated also by a floor diffuser behind the manikin. The CFD model shows excellent agreement with the experimental data. We then exercise the CFD model to study the effect of nozzle exit boundary conditions such as turbulence intensity and length scale, flow rate and temperature, and manikin temperature on the air quality in the BZ of the heated manikin. It is shown that the air quality of the novel PV system is sensitive to the nozzle exit turbulence intensity and flow rate, and insensitive to jet temperature within the 20–26 °C range, and to body temperature within a clo range of 0–1. A companion paper presents in detail the experimental set up and results used to validate the CFD model discussed in this paper.     

A novel artificial neural network fire model for prediction of thermal interface location in single compartment fire

Thermal interface is the boundary between the hot and cold gases layers in a compartment fire. The height of the interface depends predominantly on the mass of air entrained into the fire plume. However, the analytical determination of the air mass flow rate is complicated since it is highly nonlinear in nature. Currently, computer models including zone models and field models can be applied to predict fire phenomena effectively. In the zone model computation, the compartment on fire is commonly divided into two layers to which conservation equations are applied to evaluate the fire behaviour. However, the locations of the fire bed and the openings are ignored in the computation. Computational fluid dynamics techniques may be employed, but a major shortcoming is the requirement for extensive computational resources and lengthy computational time. A unique, new and novel artificial neural network (ANN) model, denoted as GRNNFA, is developed for predicting parameters in compartment fires and is an extremely fast alternative approach. The GRNNFA model is capable of capturing the nonlinear system behaviour by training the network using relevant historical data. Since noise is usually embedded in most of the collected fire data, traditional ANN models (e.g. feed-forward multi-layer-perceptron, general regression neural network, radial basis function, etc.) are unable to separate the embedded noise from the genuine characteristics of the system during the course of network training. The GRNNFA has been developed particularly for processing noisy fire data. The model was applied to predict the location of the thermal interface in a single compartment fire and compared with the experiments conducted by Steckler et al. (Flow induced by fire in a compartment, NBSIR 82-2520, National Bureau of Standards, Washington, DC, 1982). The results show that the GRNNFA fire model can predict the location of the thermal interface with up to 94.5% accuracy and minimum computational times and resources. The trained GRNNFA model was also applied to rapidly determine the height of the thermal interface with different locations of fire on the compartment floor and different widths of the opening against field model predictions. Among the five test cases, four of them were predicted well within the minimum error range of the experiment results. It also demonstrated that the prediction accuracy is related to the amount of knowledge provided for network training.     

An analysis of heat flux induced arc formation in a residential electrical cable

The interaction between an energized size 14 American Wire Gauge (1.6 mm conductor diameter) nonmetallic-sheathed cable and a fire simulated using radiative heating was investigated. The time from the beginning of radiative exposure to an electrical arcing event was measured as a function of the heat flux and nominal alternating current voltage in air and nitrogen environments. The trends observed in these experiments were captured with simple mathematical expressions. Highly time-resolved voltage and current readings were collected around the time of the arcing events to better understand the dynamics of arc formation. These observations, together with cable insulation resistance measurements, were used to gain insight into the mechanism of heat flux induced cable failure. It was also demonstrated that a numerical pyrolysis model describing transient thermal degradation of cable insulation can be used to extrapolate the time of arc formation measurements conducted in this work to fire scenarios not realized in the cable testing experiments.     

Fire whirls in forest fires: An experimental analysis

This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale. A particularity of the study is the use of typical forest fuels instead of fossil fuels as seen in some of previous studies on this topic. The forest fuels tested in the experiments were dead needles of Pinus pinaster, straw of Avena sativa, dead leaves of Eucalyptus globulus and a mix of shrubs mainly composed by heather (Erica australis) and gorse (Pterospartum tridentatum). The experimental results of the tests with and without forced flow inside a fire whirl generator were compared with tests in similar conditions out of the generator. It was possible to evaluate the effects of fuel bed size, bulk density and external vorticity on several parameters like flame height and diameter, mass decay and heat release rate. The results show that forced flow increases dramatically the burning rate and reduces the time needed to achieve a high rate of energy release. Comparison with results of other sources show that the flames that are generated in the present fire whirl generator are in a transition from fire whirl to pool fire regime and that it is possible to scale up some flow and thermal properties of field scale fire whirls and to derive predictive models on the basis of laboratory scale experiments.     

Numerical Studies on Thermally-Induced Air Flow in Sloping Tunnels with Experimental Scale Modelling Justifications

Study of smoke movement or air flow due to fire in sloping tunnels is important in designing smoke control systems. In contrast to a horizontal tunnel, there is an acceleration along the longitudinal axis due to smoke buoyancy. This phenomenon together with thermal radiation would lead to a complicated heat transfer mechanism of the ceiling jet in sloping tunnels. In the present work, thermally induced air flow arising from fire in sloping tunnels was studied via numerical simulations using the Computational Fluid Dynamics code FLUENT. Prior to the application of FLUENT in simulating the air flow under different conditions, scale model experiments were carried out and the results were compared with simulation results, to establish the reliability of FLUENT in simulating fires in sloping tunnels. For this purpose, a tunnel section model of length 3 m, width 0.8 m and height 1 m was constructed, with a 1.5 kW electrical heating source to model fire. Hot air movement pattern driven by the electric heater was studied with the tunnel inclined at 0°, 10°, 20° and 30° to the horizontal. Four cases of the same configuration as the scale tunnel experiments were simulated using FLUENT, with predicted results agreeing well with experimental results. Having established the suitability of FLUENT in simulating air flow due to fire in sloping tunnels, numerical simulations were carried out to study air flow in sloping tunnels with different scenarios, that is, for tunnels with different gradients and with fire located at different positions in the tunnel. Macroscopic number on heat transfer, including the Rayleigh number Ra, the average and local Nusselt number Nu_ave for sloping tunnels were also studied from the measured results. The correlation between Nu_ave and Ra, which shows the effect of hydrodynamic properties on relative dominance of heat transfer in tunnel fire, was also discussed.     

聚丙烯腈织物热稳定性与燃烧特性研究

对聚丙烯腈(PAN)织物在空气气氛下的热稳定性和3种热辐射强度下的燃烧特性进行研究。结果表明,PAN织物在空气气氛中的热分解过程主要包括3个失重阶段;随着热辐射强度的增大,PAN织物点燃时间有所提前,热释放速率和产烟率的峰值均得到了一定程度的提高,到达峰值的时间均有不同程度的提前,质量损失率增加,且初始热分解时间提前;当热辐射强度为25 kW/m2时,PAN织物的燃烧不充分,烟密度最大;热辐射强度越大,烟气扩散越快,且PAN织物的火灾性能指数值减小,火灾增长指数值增大;PAN织物具有较高的火灾危险性。     

Characterizing gaseous air cleaner performance in the field

As part of an ongoing effort to better understand the performance of indoor air cleaners in buildings, the National Institute of Standards and Technology (NIST) has completed a series of gaseous air cleaner field tests and model simulations. This paper focuses on experiments to measure the removal of decane with a sorption-based in-duct gaseous air cleaner and a sorption-based portable air cleaner in a single-zone test house. Due to the lack of standardized gaseous air cleaner field testing protocols, a field test method was developed using semi-real-time concentration measurements and mass balance analysis. A total of 24 experiments were completed with directly measured single-pass removal efficiencies ranging from 24% to 56% and removal efficiencies based on a transient whole building mass balance ranging from 30% to 44%. Experimental results revealed important factors affecting field performance such as air cleaner contaminant loading for the in-duct air cleaner and room air mixing for the portable air cleaner. An additional six tests were conducted to evaluate the predictive capability of the indoor air quality model CONTAM.     

Comparison of Numerical and Experimental Results of Fire Induced Doorway Flows

The ability to calculate the changes to vent flows when a sprinkler activates can lead to improved predictions of fire environments outside of the room of origin in sprinklered occupancies, ultimately leading to an engineering design tool based on numerical simulations. Hence, for the current study, numerical calculations using NIST Fire Dynamics Simulator (FDS) are compared with real scale compartment experimental data for unsprinklered and sprinklered cases. Mass flow rate and temperature are typical parameters used to quantify the flow induced by a fire in a compartment. Hence, numerical results for doorway mass flow rate and temperature are compared with the experimental data for three fire sizes in order to validate the numerical model. Then, using current experimental data for sprinkler characteristics, numerical calculations for doorway mass flow rate and temperature are compared with the experimental data for the three fire sizes of the sprinklered case.     

LNG空温式气化器管束传热特性

液化天然气(LNG)空温式气化器由翅片管管束组成,以节能环保的优势在LNG中小型气化站得到广泛应用。空温式气化器管束中的单根翅片管传热会受到相邻翅片管的影响,与大空间中的单根翅片管的传热过程存在差异,传统的传热设计中往往忽略管束中翅片管的传热差异,带来较大误差。针对空温式气化器管束传热特性展开研究,对涉及流固耦合传热问题的翅片管管束进行整场建模与求解,采用混合物模型和Lee模型求解管内LNG气化传热,得到了管束中不同位置翅片管的空气侧温度场分布及传热系数,建立了空气侧传热差异系数拟合公式,并用管束空气侧传热特性实验对拟合公式进行了验证。     

Definition and experimental evaluation of the smoke “confinement velocity” in tunnel fires

An experimental study is carried out on a reduced scale tunnel model (scale reduction is 1:20). The main objective is to evaluate the longitudinal velocity induced into a tunnel when a fire plume continuously released is confined and extracted between two exhaust vents located on both sides of the fire source. For the experimental simulations, fire-induced smoke is simulated by an air and helium mix release. Smoke flow is symmetrical as regards the fire location and experiments are realized for an half tunnel with only one vent activated downstream the source. The vent extraction flow rate is step by step increased and the length of the stratified smoke layer downstream the vent as well as the longitudinal fresh air flow induced, are measured. A confinement velocity is then associated to the minimum value of the longitudinal air flow needed to prevent the smoke layer propagation downstream the vent. This velocity is evaluated for several values of the fire heat release rate and finally compared with the corresponding critical velocity obtained for a longitudinal ventilation system.