Heat Transfer Model of Flame Resistant Fabrics During Cooling After Exposure to Fire

Protective clothing is used in many industries to protect firefighters and other workers from fire and other hazards. While skin burns can occur during a fire, protective fabric temperatures remain high for some time even after a fire ends. Therefore, skin burn injuries can occur during the time in which a fabric is cooling. A heat transfer model has been developed that can predict inherently flame resistant fabric temperatures and skin burn injuries during this cooling phase. This paper describes the heat transfer model, including methods used to calculate the apparent heat capacity and the convection heat transfer coefficient as the fabric cools. The new model has been validated using data from bench top tests of Kevlar®/PBI fabric specimens. Parametric studies using the model demonstrate the importance of selected thermal properties and boundary conditions on fabric temperatures and bench top test results.     

A Heat Transfer Model for Firefighters' Protective Clothing

An accurate and flexible model of heat transfer through firefighter protective clothing has many uses, including investigating the degree of protection, in terms of burn injury and heat stress, of a particular fabric assembly and analyzing cheaply and quickly the expected performance of new or candidate fabric designs or fabric combinations.This paper presents the first stage in developing a heat transfer model for firefighters' protective clothing. The protective fabrics are assumed to be dry, which means no moisture from perspiration, and the fabric temperatures considered are below the point of thermal degradation, such as melting or charring. Many firefighter burns occur even when there is no thermal degradation of their protective gear. A planar geometry of the fabric layers is assumed with one-dimensional heat transfer. The forward-reverse model is used for radiative heat transfer. The accuracy of the model is tested by comparing time-dependent temperatures from both within and on the surface of a typical fabric assembly to those obtained experimentally. Overall, the model performed well, especially inside the garment where the temperature difference between the experiment and the stimulation was within 5°C. The predicted temperature on the outer shell of the garment differed most from experimental values, by much as 24°C. This was probably due to the absence of fabric-specific optical properties, such as transmissivity and reflectivity, used for model input.     

Heat transfer in a cylinder sheathed by flame-resistant fabrics exposed to convective and radiant heat flux

A numerical model is developed which investigates heat transfer in a cylinder sheathed by flame-resistant fabrics when suddenly exposed to convective and radiant heat flux from simulated pre-flashover fire radiation. The column inside the cylinder system simulating the human body is assumed to keep at a constant temperature. This model incorporates characteristics of the heat-induced changes in flame-resistant fabrics and dry air thermo-physical properties. Temperature distribution was calculated with the help of a one-dimensional radial heat transfer model. A skin burn equation is quoted to predict second-degree skin burn injuries based on the numerical model. The effects of air gap thickness on mean incident heat flux to the skin simulant surface are also discussed. Results from the numerical model contribute to a better understanding of the heat transfer process within flame-resistant fibrous materials and fabrics in intensively high-temperature environment. At the same time, the method in the paper also helps to establish a systematic method for analyzing heat transfer in other cylindrical applications.     

Material fire properties and predictions for thermoplastics

Ignition and burning rate data are developed for nylon 6/6, polyethylene, polypropylene and black polycast PMMA in a cone calorimeter heating assembly. The objective is to examine a testing protocol that leads to the prediction of ignition and burning rate for thermoplastics from cone calorimeter data. The procedure consists of determining material properties, i.e. thermal inertia, specific heat, thermal conductivity, ignition temperature, heat of gasification and flame heat flux from cone data, and utilizing these properties in a model to predict the time to ignition and transient burning rate. The procedure is based on the incident flame heat flux being constant in the cone calorimeter which occurs for flames above the top of the cone heater. A constant net flame heat flux of approximately 20 kW/m² for nylon 6/6, 19 kW/m² for polyethylene, 11 kW/mP² for polypropylene and 28 kW/m² for black PMMA is obtained for irradiation levels ranging from 0 to 90 kW/m². The burning rate model is shown to yield good accuracy in comparison to measured transient burning in the cone assembly.     

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.     

Compressive Response of Composites Under Combined Fire and Compression Loading

The thermal buckling of an axially restrained composite column that is exposed to a heat flux due to fire is studied by both analytical and experimental means. The column is exposed to fire from one-side and the resulting heat damage, the charred layer formation and non-uniform transient temperature distribution are calculated by the thermal model developed by Gibson et al. (Revue de l’Institute Francais du Petrole 50:69–74, 1995). For the thermal buckling analysis, the mechanical properties of the fire-damaged (charred) region are considered negligible; the degradation of the elastic properties with temperature (especially near the glass transition temperature of the matrix) in the undamaged layer, is accounted for by using experimental data for the elastic moduli. Due to the non-uniform stiffness and the effect of the ensuing thermal moment, the structure behaves like an imperfect column, and responds by bending rather than buckling in the classical Euler (bifurcation) sense. Another important effect of the non-uniform temperature is that the neutral axis moves away from the centroid of the cross section, resulting in another moment due to eccentric loading, which would tend to bend the structure away from the fire. In order to verify the mechanical response, the compressive buckling behavior of the same material subjected to simultaneous high intensity surface heating and axial compressive loading were investigated experimentally. Fire exposure was simulated by subjecting the surface of rectangular specimens to radiant heating in a cone calorimeter. Heat flux levels of 25 kW/m², 50 kW/m² and 75 kW/m² were studied. All specimens exhibited buckling and subsequent catastrophic failure, even at compressive stresses as low as 3.5 MPa under a surface heat flux of 25 kW/m². Details of the experimental procedure, including modifications made to a cone calorimeter to allow simultaneous mechanical loading are presented.     

Heat Transfer in Thin Fibrous Materials Under High Heat Flux

A heat-transfer model has been developed for two common, inherently flame-resistant fabrics, Nomex® IIIA and Kevlar®/PBI, when subjected to the high heat fluxes used in bench top tests, such as the thermal protective performance (TPP) test, ASTM D 4108. The apparent heat capacity method was used to model thermochemical reactions in these materials with information from thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests. Also included were in-depth radiation absorption, variable thermal properties, and heat transfer across an air space from the fabric to a test sensor. The finite element method was used to solve the resulting equations. Absolute temperatures predicted by this relatively simple model fall within 4% of those measured by an infrared thermometer. Estimated times to the Stoll second-degree burn criterion are within 6% of those derived from actual tests.     

Predicting Mechanical Strength of In-Use Firefighter Protective Clothing Using Near-Infrared Spectroscopy

The exact lifespan of in-use firefighter protective clothing is difficult to predict due to the large variations in use between individual garments. Furthermore, testing methods used to evaluate new protective clothing are destructive in nature and could not be applied to in-use garments. Various non-destructive techniques have been proposed for the evaluation of in-use clothing, each possessing its own advantages and disadvantages. The ability of near-infrared spectroscopy to predict the tensile strength of thermally aged fabrics used in protective clothing for wildland firefighters and other workers is investigated here. Fabrics were exposed to heat fluxes from 10 kW/m² to 40 kW/m² for various durations using the cone calorimeter, after which the tensile strength of the fabrics was measured. Temperatures measured during the exposures and results of thermal gravimetric analysis tests were used to interpret changes in tensile strength. Multivariate linear regression was used to develop correlations between the tensile strength and the reflectance values measured between 1500 nm and 2500 nm for new and thermally aged fabrics. It was found that models based on reflectance measurements made at as few as three wavelengths could be used to estimate the tensile strength of the thermally aged specimens.     

Protective Performance of Environmentally Stressed Fabrics Containing Melamine Fiber Blends

In this study, the mechanical properties critical to the protective performance of firefighter turnout gear were evaluated for environmentally stressed outer shell (OS) fabrics containing melamine fiber blends. Environmental stress factors that affect the durability of turnout gear include temperature, ultraviolet (UV) radiation, moisture, abrasion, and laundering. The effect of fiber blend, fabric construction, and finishing processes including water repellent coatings and pigmented melamine-containing OS fabrics were also studied. Melamine-containing OS fabrics show comparable thermal protective performance and have superior tear resistance when compared to the traditionally used polyaramid blends. This study reveals that the thermal protective protection (TPP) rating of fabric assemblies incorporating environmentally stressed OS fabrics containing melamine fiber blends is well above the NFPA minimum TPP requirement of 35 cal/cm². However, the tear strength (measured using ASTM D 5587 standard test method) of all melamine-containing OS fabrics exposed to environmental stresses was observed to have significantly deteriorated, and most OS fabrics, depending on fiber blend and fabric structure, would fail to meet requirements of NFPA 1971 standard. The study thus suggests that environmental stressing has a more detrimental impact on the tear strength than the thermal protective performance of OS fabrics. Deterioration in tear strength of all UV exposed OS fabrics is largely due to photodegradation of constituent fibers. Changes in tear strength of OS fabrics subjected to thermal exposures and laundering is cumulative effect of loss in tensile strength of single yarns and dimensional stability of the fabric itself. Furthermore, finishing treatments affect performance properties of fabric by increasing fiber packing factor in yarn, changing yarn crimp and yarn spacing thereby making dimensional changes to the fabric. Surface coatings alter tear resistance of fabric by influencing yarn slippage and fabric rigidity. Fabrics dyed with black and dark blue dyes cause less UV degradation of fibers than bright yellow and brown dyes.     

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。     

Transmission Through and Breakage of Multi-Pane Glazing Due to Radiant Exposure

A series of small and large-scale tests were performed to measure the radiant transmission of energy and the window breakage characteristics of seven different multi-plane glazing samples. The samples tested included both double and triple-pane glazing specimens with a laminate interlayer between panes for additional strength. These test series were designed to provide the information necessary to assess the hazard from radiant energy to building occupants and contents due to a large fire in close proximity to a structure with a large amount of exterior windows. For incident heat fluxes 30 kW/m² or lower, the triple-pane glazing samples had a total transmittance less than 10% of the incident heat flux, back-side surface temperatures did not exceed 100°C, and the back-side heat flux did not exceed 4 kW/m². For double-pane laminates, the total transmittance was less than 25% of the incident heat flux, the back-side temperature did not exceed 220°C, and the back-side heat flux did not exceed 5 kW/m². For incident heat fluxes greater than 30 kW/m², the glazing samples degraded very quickly, generally buckling and losing integrity. The time for the first pane to crack decreased with increasing incident flux level. A number of tests included a water deluge system, which served to maintain sample integrity for extended exposures. In these cases, the total transmittance was less than 6% of the incident heat flux, back-side surface temperatures did not exceed 45°C, and the back-side heat flux did not exceed 1 kW/m².     

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.     

ICARUS: A code for evaluating burn injuries

A computer code, ICARUS (Injuries CAused by Radiation Upon the Skin), has been developed for evaluating time to second-degree burn injury caused by thermal radiation. This paper introduces the modeling methodology incorporated in ICARUS and illustrates the code's validity and application. ICARUS enables studies of the effects of thermal radiation on the skin and benefits assessments of the shielding effects of clothing layers. ICARUS uses a unique method of solving the complex heat transfer problem associated with simultaneous radiation, conduction, and convection in a multilayered diathermanous clothing/skin assembly, which is especially useful when coupled with the thermal responses (moisture loss, charring, shrinking, and so on) of the clothing fabrics themselves. The code is designed to run on an IBM-PC compatible.     

阻燃织物的辐射热防护性能研究

介绍织物的辐射热防护性能 RPP 测试原理。选用 25 种阻燃面料进行测试,分析辐射热源强度与阻燃织物 RPP 值 的关系。借助于 SEM 测试对受热实验后织物微观变化进行表 征,分析实验现象及实验后织物的宏观变化。结果表明：在热流密 度较低的情况下,影响织物辐射热防护性能的主要因素是织物物 理结构参数;本征型阻燃织物的 RPP 值随着热流密度的增加而 减小,非本征型阻燃织物的 RPP 值随着热流密度的增加出现减 小、增加交替现象。热流密度增大时,烧伤时间减小的幅度大于辐 射热流密度增加的幅度,RPP 值总体上呈减小趋势。RPP 值不 宜作为评价织物在宽分布辐射热流量下的辐射热防护性能的唯一 指标。随着辐射热源强度的增加,本征型阻燃织物外观几乎没有 变化,而后整理织物和混纺织物炭化分解逐渐加深。     

EcoSmart Fire as Structure Ignition Model in Wildland Urban Interface: Predictions and Validations

EcoSmartFire is a Windows program that models heat damage and piloted ignition of structures from radiant exposure to discrete landscaped tree fires. It calculates the radiant heat transfer from cylindrical shaped fires to the walls and roof of the structure while accounting for radiation shadowing, attenuation, and ground reflections. Tests of litter burn, a 0.6 m diameter fire up to 250 kW heat release under a Heat Release Rate (HRR) hood, with Schmidt-Boelter heat flux sensors in the mockup wall receiving up to 5 kW/m² radiant flux, in conjunction with Fire Dynamic Simulator (FDS) modeling verified a 30% radiant fraction, but indicated the need for a new empirical model of flame extinction coefficient and radiation temperature as function of fire diameter and heat release rate for use in ecoSmartFire. The radiant fluxes predicted with both ecoSmartFire and FDS agreed with SB heat flux sensors to within a few percent errors during litter fire growth. Further experimental work done with propane flame heating (also with 30% radiant fraction) on vertical redwood boards instrumented with embedded thermocouples validated the predicted temperature response to within 20% error for both models. The final empirical correlation for flame extinction coefficient and temperature is valid for fire diameters between 0.2 and 7.9 m, with heat release rates up to 1000 kW. From the corrected radiant flux the program calculates surface temperatures for a given burn time (typically 30 s) and weather conditions (typically dry, windy, and warm for website application) for field applications of many trees and many structural surfaces. An example was provided for a simple house exposed to 4 burning trees selected on a Google enhanced mapping that showed ignition of a building redwood siding. These temperatures were compared to damage or ignition temperatures with output of the percentage of each cladding surface that is damaged or ignited, which a homeowner or a landscaper can use to optimize vegetation landscaping in conjunction with house exterior cladding selections. The need for such physics-based fire modeling of tree spacing was indicated in NFPA 1144 for home ignitability in wildland urban interface, whereas no other model is known to provide such capability.     

On the Protective Performance of Firefighters’ Garments: Air Gaps Between Fabric Layers

Municipal firefighters count on their protective garments to avoid skin burns caused by thermal and flame exposures. Typical firefighting garment consists of three layers of different fire-resistant fabrics named as outer shell, moisture barrier and thermal liner. This paper employed a numerical heat transfer model for firefighters’ garments, which paid more attention to modeling air gaps bounded between garment’s layers. The paper explored and compared the influences of air gaps bounded between garment’s layers on its protective performance. Specifically, the paper investigated the effect of a variation in the air gaps between the garment layers from 1 mm to 6 mm, a variation in the backside emissivity of the outer shell and moisture barrier layers from 0.9 to 0.1 and a variation in their typical thicknesses from 50% to 200% on the protective performance of garment. The results showed that increasing the width of the gap between the moisture barrier and the thermal liner, reducing the outer shell backside emissivity and increasing the moisture barrier thickness improves the protective performance of firefighters’ garments more than does increasing the width of the gap between the moisture barrier and outer shell, reducing the moisture barrier backside emissivity and increasing the outer shell thickness, respectively.     

Mechanical Property Degradation of Naval Composite Materials

The effect of heat and fire on the mechanical properties and failure of polymer composite materials used in naval ship structures is investigated. Coupled thermal-mechanical models are presented for predicting the loss in strength and failure of load-bearing polymer laminates when heated from one-side. The thermal component of the models predicts the temperature and decomposition rate of a laminate. Using this information, several mechanical models based on progressive softening analysis or laminate analysis can be used to predict the reduction in strength and time-to-failure. A coupled thermal-mechanical model that is solved using finite element analysis is also presented. Experimental fire-under-load tests are performed on several types of polymer laminate materials to evaluate the accuracy of the models. The tests were performed at different heat flux levels between 10 kW/m² and 75 kW/m², which is equivalent to surface temperatures between about 250°C and 700°C. The temperature, mass loss and char formation of a laminate can be accurately predicted for a wide range of thermal conditions using the models. The models can also predict the time-to-failure of laminates under static tension or compression loading. The models presented in this chapter are considered useful analytical tools for naval architects to estimate the loss in mechanical performance and time-to-failure of composite ship materials in fire.     

典型汽车内饰材料的燃烧特性研究

采用锥形量热仪实验对涤纶面料丙纶玻璃纤维板、涤纶面料丙纶麻纤维板和 PVC 革丙纶麻纤维板 3 种典型汽车内饰材料在 25、35、50 kW/m2 热辐射强度下的点燃时间、质量损失率、热释放速率等燃烧特性参数进行研究,并选取点燃预测模型计算材料的临界热辐射强度,使用轰燃倾向指数和热释放总量评价其潜在火灾危险性。结果表明,在实验热辐射强度下,涤纶面料丙纶麻纤维板质量损失百分率最大,结构完整性最差;涤纶面料丙纶玻璃纤维板平均点燃时间最短,临界热辐射强度最小,最容易被引燃;PVC 革丙纶麻纤维板热释放速率峰值最大,火灾性能指数最小,发生轰燃的可能性最大。     

Flame spread limits (LOC) of fire resistant fabrics

Selecting fabrics based on their fire resistance is important for professions with substantial fire risk such as firefighters, race car drivers, and astronauts suits. Generally, fire resistant materials are tested under standard atmospheric conditions. However, their flammability properties can change when the ambient conditions deviate from standard atmospheric conditions. Particularly in high altitude locations, aircraft, and spacecraft, the pressure and oxygen concentrations are different than in a standard atmosphere. Also, the presence of external radiation (i.e. overheating component or nearby fire) can reduced the fire resistance of a material. In this work, an experimental study was conducted to analyze the influence of environmental variables such as oxygen concentration, ambient pressure, and external radiant heat flux on the flame spread limits of two different fire resistant fabrics: Nomex HT90-40 and a blend made of Cotton/Nylon/Nomex. Ambient pressure was varied between 40 and 100 kPa and ambient oxygen concentrations were decreased until the Limiting Oxygen Concentration (LOC), limiting conditions which would permit flame propagation, were found. Experiments were conducted using no external radiant flux or a radiant flux of 5 kW/m² to examine the influence of the presence of a nearby heat source. Among the results, it was found that as ambient pressure is reduced the oxygen concentration required for the flame to propagate must be increased. The external radiant heat flux acts as an additional source of heat and allows propagation of the flame at lower oxygen concentrations. An analysis of the propagation limits in terms of the partial pressure of oxygen suggest that the LOC of a material is not only determined by heat transfer mechanisms but also by chemical kinetic mechanisms. The information provided in this work helps characterize increased flammability risk of materials when in environments different from the standard atmospheric conditions at which they are typically tested.     

Predicting the structural response of a compartment fire using full-field heat transfer measurements

Inverse heat transfer analysis (IHT) was used to measure the full-field heat fluxes on a small scale (0.9 m×0.9 m×0.9 m) stainless steel SS304 compartment exposed to a 100 kW diffusion flame. The measured heat fluxes were then used in a thermo-mechanical finite element model in Abaqus to predict the response of an aluminum 6061-T6 compartment to the same exposure. Coupled measurements of deflection and temperature using Thermographic Digital Image Correlation (TDIC) were obtained of an aluminum compartment tested until collapse. Two convective heat transfer coefficients, h =35 W/m²-K and h =10 W/m²-K were examined for the thermal model using the experimentally measured heat fluxes. Predictions of the thermal and structural response of the same compartment were generated by coupling Fire Dynamics Simulator (FDS) and Abaqus using the two values for h, h =35 W/m²-K and h from convection correlations. Predictions of deflection and temperature using heat fluxes from IHT and FDS with h=35 W/m²-K agreed with experimental measurements along the back wall. The temperature predictions from the IHT-Abaqus model were independent of h, whereas the temperature predictions from the FDS-Abaqus model were dependent on h.