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.     

Numerical Modeling of Heat and Moisture Through Wet Cotton Fabric Using the Method of Chemical Thermodynamic Law Under Simulated Fire

The paper deals with numerical modeling of heat and moisture transfer behavior of a fabric slab during combined drying and pyrolysis. The model incorporates the heat-induced changes in fabric thermo physical properties and the drying process is described by a one-step chemical reaction in the model. The new model has been validated by experimental data from modified Radiant Protective Performance (RPP) tests of fabrics. Comparisons with experimental data show that the predictions of mass loss rates, temperature profiles within the charring material and skin simulant, and the required time to 2nd skin burn are in reasonably good agreement with the experiments. It is concluded that moisture increases the time to 2nd degree skin burn for fabrics exposed to low intensity heat flux of 21 kW/m², but under high heat flux exposures, such as 42 kW/m², moisture tend to increase heat transfer through the thermal protective fabric system and the tolerance time of the same fabrics will reduce. The model can find applications not only in thermal protective clothing design, but also in other scientific and engineering fields involving heat transfer in porous media.     

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.     

Skin Burn Translation Model for Evaluating Hand Protection in Flash Fire Exposures

A model is described for use in translating measured heat flux to predict second and third degree hand burn injury in fire exposures. The model adapts a burn translation algorithm for estimating burn injuries used in established instrumented fire test manikin technologies. It facilitates more accurate prediction of burns to human hands by accounting for the cylindrical geometry of the fingers, bone tissue beneath the skin, and different skin thickness data that represents the different areas of the hand. A numerical modeling approach is used to demonstrate the response of the skin burn model for predicting hand burn injury in heat exposures encountered in fire manikin testing.     

Numerical analysis and experimental validation of heat transfer in ground heat exchangers in alternative operation modes

A finite element numerical model has been developed for the simulation of the ground heat exchangers (GHEs) in alternative operation modes over a short time period for ground-coupled heat pump applications. Comparisons between the numerical and analytical results show that the finite line-source model is not capable of modeling the GHEs within a few hours because of the line-source assumption. On the other hand, the experiments with respect to the alternative cooling and heating modes have been undertaken during a short-time period. The comparisons show a reasonable agreement between the numerical and the measured data. The results illustrate that the finite element numerical model can be used to simulate the heat transfer behavior of the GHEs in short time scales instead of the typical finite line-source model. Finally, the variation of the U-tube pipe wall temperatures demonstrates that the discontinuous operation mode and the alternative cooling/heating modes can effectively alleviate the heat buildup in the surrounding soil.     

用“消防假人”测定消防员人体烧伤程度

本文阐述了人体皮肤烧伤的分度和严重度的分类,以及皮肤烧伤与温度和时间的关系,详细说明了如何运用“消防假人”测得的温度分布状况来计算和判定消防员人体皮肤的烧伤程度。     

An integrated numerical simulator for thermal performance assessments of firefighters’ protective clothing

Research and development of firefighters’ protective clothing relies on a large number of fire disaster experiments in order to assess the thermal performance. It would be substantially advantageous to substitute a virtual numerical experiment for a real one in terms of time, cost and safety. The present article reports the development of an integrated numerical simulator that makes possible the estimation of burn injuries originating from fire disasters. In the simulator, a general-purpose computational fluid dynamics program computes the fluid flow and heat transfer in an in situ fire event, while a one-dimensional program calculates the radiative–conductive heat transfer through the clothing and human skin. A data interface combines the two simulations by loose coupling so as to give the real-time burn injury progress output. The predicted surface heat fluxes and burn degrees agree with experimental measurements reasonably well. Possible numerical error sources are discussed that call for potential improvements in the future.     

感温火灾探测器的预警时间

文章结合建筑火灾的特性和感温火灾探测器的工作原理,分析了感温火灾探测器的换热机理,建立其换热数学模型,并对此模型作详细的求解计算和实例分析,确定了影响感温火灾探测器预警时间的主要因素,计算出感温火灾探测器的预警时间。     

Low-cost wool-based fire blocking inter-liners for upholstered furniture

New Zealand is reluctant to implement mandatory fire safety regulations for domestic furniture because of the cost/benefit even though up to 30% of household deaths and injuries over the past 20 or more years can be attributed to soft furnishings. Work has been carried out to develop a low-cost inter-liner using fire retardant treated wool with altered proportions of different fire resistant synthetic fibres that may be attractive for voluntary inclusion by manufacturers. Bench- and full-scale tests have been conducted to characterise the fire performance of composites of polypropylene upholstery fabric, different inter-liner materials and a polyurethane foam core. Inter-liner blends that reduced the peak rate of heat release, extended the time to peak rate of heat release and reduced other combustion product output were identified. A blend of 75% Zirpro treated fire retardant wool and 25% Panox fibre was found to be the best candidate material when fire performance, cost and practical application were considered.     

汽车车身燃烧残留痕迹特征的研究

汽车发生火灾后，车身面板受到火焰作用形成明显的燃烧痕迹。对车身面板进行了火焰作用模拟试验，通过热分析（TG／DTA）、宏观形貌和微观形貌（SEM）等分析方法对车身燃烧残留痕迹的形成机理及其变化规律进行研究，提出了关于车身火灾燃烧残留痕迹的勘查方法，有效地应用于汽车火灾原因调查之中。     

地铁站厅中杂货亭火灾场景特性研究

采用地铁站厅中的杂货亭模型实体火灾试验,研究地铁站厅杂货亭的火灾场景,得出了模拟站厅杂货亭在火灾中的热释放速率、烟气浓度、温度、烟密度的变化规律,站厅杂货亭发生轰燃的时间、最大热释放量。火灾中烟气是首先弥漫整个房顶,然后再向下蔓延。     

Study of Water Droplet Behavior in Hot Air Layer in Fire Extinguishment

Evaporation of water droplets while traveling in hot air layer will be studied. The air-droplet system is analyzed by solving the mass, momentum and energy conservation equations for each phase. The droplet phase is described by the Lagrangian approach. Two conditions of air flow in the smoke layer are assumed. Firstly, as commonly used in modeling fire suppression by water spray, the smoke layer is assumed to be quiescent. Secondly, both gas cooling effect and air entrainment in the water spray cone are included. The properties of gas phase related to evaporation are specific heat capacity, thermal conductivity and dynamic viscosity. All these are evaluated by the one-third rule. The Runge–Kutta algorithm is used to solve the ordinary differential equation group for the droplet motion with heat transfer. Droplet positions, velocities, temperatures and diameters are calculated while traveling in the hot air reservoir. The effects of air temperature, water vapor mass fraction, thickness of hot air reservoir, and initial diameter on the droplet behavior are analyzed. The quantity of heat absorbed by a single droplet is calculated. Results are then calculated for a water spray by taking it has many droplets. The cooling effect of the water vapor produced is considered. Water spray consisting of small droplets should absorb more heat while acting on the hot air layer. The ratio of the heat for vaporization to the total heat absorbed by water can go up to 0.9 when all the droplets are evaporated. Limited experimental data are selected to verify the mathematical model. Predicted results are useful for studying fire suppression by water mist system.     

Tragwiderstand von Betonbauteilen nach dem Brand

Load carrying capacity of concrete structures after fire exposure. The mechanical and thermal properties of building materials change at elevated temperatures. This change of material properties has an important influence on the load carrying and deformation behaviour in case of fire. The rate of increase of temperature through the cross section in a concrete element is relatively slow and so internal zones are protected against heat. Only a small part of the cross‐section is affected by the temperature action (loss of strength and stiffness). For these reasons reinforced concrete structures with adequate structural detailing usually reach high fire resistance without any additional fire protection. However after the fire has been extinguished the heat penetration into the cross section continues for hours. Additionally calcium hydroxide reformats during the cooling phase widening up micro cracks. The combination of these two phenomena can lead to a significant reduction of the compressive strength of concrete after the fire. This paper analyses the influence of the loss of strength during and after the cooling on the load carrying capacity of concrete elements. The numerical calculations presented in this study showed that after the cooling phase concrete elements can exhibit a load carrying capacity lower than during the fire. The design of concrete structures based on equivalent time of standard fire exposure without cooling phase can lead to unsafe results. For this reason it may be important to take into account the cooling phase of the fire for the calculation of the load bearing capacity of concrete structures.     

Using adiabatic surface temperature for thermal calculation of steel members exposed to localized fires

This paper provides a new way to calculate the temperature of steel components in localized fires. A newly developed concept named adiabatic surface temperature (AST) on fire-structural interface research has been used in the calculation to represent the fire exposed boundary conditions. Based on AST, a simple heat transfer model with the capability of considering heat conduction among different parts of the steel sections has been proposed. The popular CFD code FDS (version 5.3.0), which can output ASTs, has been adopted to simulate the heating of a steel beam and a steel column in a localized fire. Steel temperatures calculated by the proposed model agree very well with the results from CFD simulation. Studies also obviously show that using the surrounding gas temperatures for thermal calculation of steel members exposed to fire might yield unreasonable and unsafe results.     

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.     

Numerical study on temperature distribution of high-strength concrete-filled steel tubes subjected to a fire

High-strength concrete-filled steel tube (CFST) columns offer a number of benefits and are widely used in high-rise building construction. This paper presents a brand new thermal modeling approach in comparison with tremendous test data for the heat transfer analysis. A heat transfer model has been developed to predict the thermal response of high-strength CFST columns under standard fire conditions with consideration of a number of parameters: steel and furnace emissivity, thermal interface conductance and concrete strength. The verified numerical models discussed the variation of emissivity of steel surfaces and thermal interface conductance in fire. It can be concluded that the influence of emissivity and thermal interface conductance is considerable in the numerical analysis. It is also demonstrated that thermal behavior of high-strength CFST columns subjected to fire during heating and cooling stages, and providing the guidance on predicting thermal response of high-strength concrete-filled steel tube columns.     

双层玻璃幕墙的传热特征研究

展示了一个通过计算流体动力学(CFD)计算机模拟获得双层玻璃幕墙传热特征的方法。通过CFD模拟计算的结果,拟合得到了双层玻璃幕墙系统和单层玻璃幕墙系统的室内得热与太阳辐射和室内外温差两个变量之间的线性近似公式。该关系表征了幕墙系统的传热特征:太阳得热系数和有效传热系数(或U值)。用该公式估算夏冬季透过幕墙的室外冷(热)负荷十分方便,工程实用性很高。     

Application of field model and two-zone model to flashover fires in a full-scale multi-room single level building

This paper presents a comparison of the results from a computational fluid dynamics (CFD) model and a two-zone model against a comprehensive set of data obtained from one flashover fire experiment. The experimental results were obtained from a full-scale prototype apartment building under flashover conditions. Three polyurethane mattresses were used as fuel. The CFAST two-zone model (version 2.0) was also used to predict results for this flashover fire test. The mass release rate, gas temperature, radiation heat flux and gas compositions (O₂, CO₂ and CO) were measured. A CFD program, CESARE-CFD Fire Model, has been developed and was used also to predict results for polyurethane-slab fire. A simple flame spread model was incorporated into the CFD program to predict the mass release rate and heat release rate during the fire instead of providing it as an input as is required for most zone and CFD models. It was found that the CFD model provided reasonable predictions of the magnitude and the trends for the temperatures in the burn room and the species concentrations, but over-predicted the temperatures in the adjacent enclosures. From a life safety perspective, the CFD model conservatively predicted the concentrations of CO and CO₂. The predicted temperatures from the CFAST fire model agreed well with the experimental results in most areas. However, the CFAST model under predicted the temperature in the lower layer of the room of fire origin and the concentration of CO in most areas.     

Analytical behavior of eccentrically loaded concrete encased steel columns subjected to standard fire including cooling phase

A nonlinear 3-D finite element analysis (FEA) model was developed to predict the behavior of eccentrically loaded concrete encased steel (CES) columns subjected to ISO-834 standard fire including heating and cooling phases. The finite element model has been validated against published tests conducted at elevated temperatures. Comparisons between the predicted results and the test results show that this model can accurately predict the behavior of CES columns under fire. The FEA model was then used to investigate the typical temperature-time curve and mid-height lateral deformation-time curve of eccentric compression CES columns in a complete loading history including initial loading, heating and cooling. It is shown that the temperature delay is obvious at the inner layers of concrete. The fire resistance of a CES column should be checked for the full process of fire exposure until temperatures everywhere in the column start to decrease. The lateral deformation of the column still gradually increases during the cooling phase and the column may fail during that phase. There is a large residual deformation after the fire exposure. Furthermore, the variables that influence the behavior of the CES columns under fire were investigated in parametric studies. It is found that the main parameters which influence the lateral deformation-time curve of the column during the full process of fire exposure are load ratio, slenderness ratio, duration time, depth to width ratio and steel ratio, and the main parameters which influence the residual deformation ratio of the column after fire are load ratio, duration time, cross-sectional depth and steel ratio.