An extended travelling fire method framework for performance-based structural design

This paper presents the extended travelling fire method (ETFM) framework, which considers both energy and mass conservation for the fire design of large compartments. To identify its capabilities and limitations, the framework is demonstrated in representing the travelling fire scenario in the Veselí Travelling Fire Test. The comparison between the framework and the test is achieved through performing a numerical investigation of the thermal response of the structural elements. The framework provides good characterization of maximum steel temperatures and the relative timing of thermal response curves along the travelling fire trajectory, though it does not currently address a non-uniform fire spread rate. The test conditions are then generalized for parametric studies, which are used to quantify the impact of other design parameters, including member emissivity, convective heat transfer coefficient, total/radiative heat loss fractions, fire spread rate, fire load density, and various compartment opening dimension parameters. Within the constraints of this study, the inverse opening factor and total heat loss prove to be the most critical parameters for structural fire design.     

Radiative heat transfer in circulating fluidized bed furnaces

Three methods of estimating the effective emissivity of a gas-particle suspension are compared and the radiative heat transfer coefficient of an isothermal suspension is defined. Heat flux measurements obtained from circulating fluidized bed combustors are examined. Radiation from a particle suspension with core temperature dominates the radiative heat transfer in the upper part of the furnace, where the particle density is low and no substantial particle boundary layers are formed. Over the lower parts of the heat transfer surfaces, where significant thermal and particle boundary layers are present, the radiative heat flux is dominated by emission from the relatively low temperature particle layer in the vicinity of the heat receiving surface.     

The term ‘heat flux’ is used ambiguously. Clear definitions are needed on how to express thermal exposure

The expression ‘heat flux’ without any qualifier like ‘to a surface at ambient temperature’ as frequently used in fire safety science and engineering literature and standards is ambiguous and misleading. Boundary conditions in fire safety engineering problems cannot be expressed as a given heat flux (or net heat flux), as the heat flux depends on and varies with the exposed surface temperature and thereby the properties of the target body. Therefore, it is important that the terminology is reviewed and that an agreement is reached on how to express thermal exposure in a well‐defined and unambiguous way. A proposal is given on how the boundary conditions can be defined in a consistent way that is applicable to fire resistance and reaction‐to‐fire problems. Copyright © 2014 John Wiley & Sons, Ltd.     

Flame extension and the near field under the ceiling for travelling fires inside large compartments

Structures need to be designed to maintain their stability in the event of a fire. The travelling fire methodology (TFM) defines the thermal boundary condition for structural design of large compartments of fires that do not flashover, considering near field and far field regions. TFM assumes a near field temperature of 1200°C, where the flame is impinging on the ceiling without any extension and gives the temperature of the hot gases in the far field from Alpert correlations. This paper revisits the near field assumptions of the TFM and, for the first time, includes horizontal flame extension under the ceiling, which affects the heating exposure of the structural members thus their load-bearing capacity. It also formulates the thermal boundary condition in terms of heat flux rather than in terms of temperature as it is used in TFM, which allows for a more formal treatment of heat transfer. The Hasemi, Wakamatsu, and Lattimer models of heat flux from flame are investigated for the near field. The methodology is applied to an open-plan generic office compartment with a floor area of 960 m² and 3.60 m high with concrete and with protected and unprotected steel structural members. The near field length with flame extension (fTFM) is found to be between 1.5 and 6.5 times longer than without flame extension. The duration of the exposure to peak heat flux depends on the flame length, which is 53 min for fTFM compared with 17 min for TFM, in the case of a slow 5% floor area fire. The peak heat flux is from 112 to 236 kW/m² for the majority of fire sizes using the Wakamatsu model and from 80 to 120 kW/m² for the Hasemi and Lattimer models, compared with 215 to 228 kW/m² for TFM. The results show that for all cases, TFM results in higher structural temperatures compared with different fTFM models (600°C for concrete rebar and 800°C for protected steel beam), except for the Wakamatsu model that for small fires, leads to approximately 20% higher temperatures than TFM. These findings mitigate the uncertainty around the TFM near field model and confirm that it is conservative for calculation of the thermal load on structures. This study contributes to the creation of design tools for better structural fire engineering.     

Simulating the thermal response of the flame manikin with different materials exposed to flash fire by CFD

To investigate the differences of thermal response between heat flux sensors and human skin on the flame manikin, a three‐dimensional heat transfer model was developed and validated by the flame manikin system. The initial temperature of the model with sensor material was set to 300 K, and the model with skin material was set as the real condition. Simulated results validated the effectiveness of heat flux measured by the sensor. The incident heat flux through the measured surface was influenced by the different emissivity of the human skin and experimental sensors. Significant difference was found for the temperature response of these two kinds of materials within 4‐s fire exposure. The heat flux measured by sensor or the simulated results with actual human skin parameters could be used as the input boundary condition of the skin heat transfer model for Henriques's skin burn prediction. It is necessary to study the actual skin thermal response by experiments, where the 3D model established in this study could be used as the supplementary means for skin simulant sensor development. These findings will also be adopted in our following study of skin burn prediction module in the 3D full‐scale simulation platform. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Overall stability analysis of oversized steel‐framed buildings in a fire

Our present paper summarizes the shortcomings in the current fire‐resistant design of oversized steel structures and proposes a method for overall stability analysis of steel structures in the event of fire. The Fire Dynamics Simulator (FDS) software platform–based large‐eddy simulation technology can accurately reflect the environment in a fire scenario and correctly predict the spatial–temporal change in the smoke temperature field within an oversized space. Adopting the FDS software and finite element structural analysis (ANSYS) coupling can fundamentally overcome the natural defect of adopting the International Organization for Standardization (ISO) standard curve (or other indoor homogeneous temperature increase curves) that substitutes a point for the overview of a field. They reflect the structural additional internal force and internal force redistribution incurred by the gradient temperature difference of the spatial–temporal changing nonhomogeneous temperature field and both theoretically and technically realize the analysis of structural heat transfer and mechanical properties in a natural fire. Furthermore, a modified model to predict the steel temperature curve in localized fire is also proposed. The localized fire in large spaces can be treated as a point fire source to evaluate the flame thermal radiation to steel members in the modified model. Copyright © 2014 John Wiley & Sons, Ltd.     

非均匀热流边界条件下螺旋管内流动传热的场协同分析

建立了螺旋管内流动换热的物理数学模型,对均匀和非均匀热流边界条件下螺旋管内湍流传热进行了数值模拟。结果表明：当对螺旋管表面施加相同的加热功率时,均匀热流边界条件下湍流传热系数高于非均匀热流边界条件下的湍流传热系数,且均匀热流边界条件下螺旋管内的场协同角低于非均匀热流边界条件;非均匀热流边界条件时,在相同的De下,曲率较小的螺旋管传热系数大,且曲率较小的螺旋管内场协同角较小;同时,随着管径的增大,螺旋管内的传热系数也随之下降,但螺旋管内的场协同角随之增大。     

炉壁黑度对炉膛辐射换热影响的研究

分析了炉壁黑度对炉膛辐射换热流的影响，分别对火焰直接加热炉和间接加热炉进行了讨论。结果表明：炉壁黑度对制品表面净辐射热流几乎没有影响，因而增大或减少炉壁黑度对强化炉内辐射换热没有意义。     

Thermal response characteristics of fire blanket materials

The thermal response characteristics of over 50 relatively thin (0.15–3.7 mm) fire blanket materials from four different fiber groups (aramid, fiberglass, amorphous silica, and pre‐oxidized carbon) and their composites have been investigated. A plain or coated fabric sample was subjected to a predominantly convective or radiant heat flux (up to 84 kW/m²) using a Meker burner and a cone heater, respectively. In addition to conventional thermal protective performance ratings for protective clothing, two transient thermal response times (for the fabric back‐side temperature to reach 300 °C and for the through‐the‐fabric heat flux to reach 13 kW/m²) and a steady‐state heat‐blocking efficiency (HBE) were introduced for both convective and radiant heat sources. For most woven fabrics, the HBE values were approximately 70 ± 10% for both convection and radiation and only mildly increased with the fabric thickness or the incident heat flux. Nonwoven (felt) fabrics with low thermal conductivity exhibited significantly better insulation (up to 87%) against convective heat. Highly reflective aluminized materials exhibited exceptionally high HBE values (up to 98%) for radiation, whereas carbon and charred aramid fabrics showed lower HBEs (down to 50%) because of efficient radiation absorption. A relatively thin fire blanket operating at high temperatures can efficiently block heat from a convective source by radiative emission (enhanced by its T⁴‐dependence and high surface emissivity) coupled with thermal insulation and from a radiant heat source by surface reflection while the aluminum surface layer remains. Copyright © 2013 John Wiley & Sons, Ltd.     

聚合物表面性能对强化冷凝传热的影响

对离子束动态混合注入(DIMI)技术制备的黄铜、紫铜、不锈钢和碳钢管基聚四氟乙烯(PTFE)表面的冷凝传热实验发现,用不同加工条件制备的表面具有不同的化学组成、不均匀的表面状态以及不同的物理化学性质,从而导致不同的冷凝成滴面积和传热性能,而且表面加工条件对滴状冷凝传热的寿命有至关重要的作用,不同基体材料应有不同的最佳制备工艺条件.不同工艺条件下制备黄铜基PTFE表面水蒸气竖直管外冷凝传热通量比相应的膜状冷凝提高0.3-4.6倍,冷凝传热系数提高1.6-28.6倍.实验结果也表明冷凝表面基体材料对冷凝传热性能有一定的影响.     

3D heat transfer modeling and parametric study of a human body wearing thermal protective clothing exposed to flash fire

Understanding the heat transfer mechanism through a clothed man system in extreme environments is of great significance for human thermal protection. Three‐dimensional heat transfer models were developed based on the computational fluid dynamics considering a real‐shape human body in this study. The influences of air gap width, clothing thickness, and emissivity on the heat transfer within a dressed manikin exposed to flash fire were investigated. Simulated results indicated that the heat transfer in the air space was more complicated on the garment level than the fabric, because of the varying relative positions between different body segments and the heat source, as well as the ventilation openings. Increasing the fabric thickness was an effective method to reduce the transferred energy, which could remarkably lower the skin temperature. Decreasing both of the surface and backside emissivity from 0.9 to 0.3 could increase the protective performance, where surface emissivity reduction was recommended since the decrease of clothing temperature could minimize the risks of fabric degradation. The purpose of assuming the uniform air gap was to perform the parametric study, which was unrealistic indeed. The model with real clothing shape will be developed and investigated in the near future.     

基于自润湿溶液的纳米表面传热性能

采用阳极氧化法在钛板表面制备出TiO₂纳米管阵列,并以其为加热表面。以含不同浓度丁醇的自润湿溶液为实验工质,考察了自润湿溶液浓度变化对系统临界热流密度和传热系数的影响,并从气泡行为的不同分析了两者耦合强化传热的机理。结果表明：相比于光滑表面和蒸馏水的常规组合,TiO₂纳米管表面和自润湿溶液耦合传热使得系统的临界热流密度大幅度提高,随自润湿溶液浓度的升高,传热系数依次降低。具有超亲水性和较大粗糙度的纳米管表面与1%（质量分数,下同）自润湿性溶液相耦合时,其最大传热系数和临界热流密度分别为11.963 kW·m^-2·℃^-1与623.706 kW·m^-2,比常规组合传热分别提高了84.1%和143.8%。由气泡可视化可知,耦合传热在沸腾过程中产生的气泡细小,脱离速度快,不易团聚,合并后的气泡易破碎,易形成微气泡,从而使系统进入剧烈的微气泡沸腾状态。气泡的高脱离频率和特殊有效的液体补充路径,是提高系统传热系数和临界热流密度的主要原因。     

Experimental and analytical protocol for ignitabiilty of common materials

In this study, a protocol was developed to increae accuracy, generality and efficiency when determining piloted ignition properties. A new procedure for calibrating the radiative and convective heat flux protiels on exposed speciments, such as Douglas-fir plywood, has been implemented for the lateral ignition and flame spread test (LIFT) apparatus. The boundary conditions needed for heat transfer anylysis are made unambiguous by including a simple, direct measure f surface emissivity. A new aluminum foil shutter improves accuracy for measuring ignition time. A recently developed theroy of ignitanility provides a formula to account for the transition form thick to thin thermal behaior, allowing specimens of finite thicknesses and a fuln range of test irradiances.     

Temperature of post‐flashover compartment fires—Calculations and validation

This paper describes and validates by comparisons with tests a one‐zone model for computing temperature of fully developed compartment fires. Like other similar models, the model is based on an analysis of the energy and mass balance assuming combustion being limited by the availability of oxygen, ie, a ventilation‐controlled compartment fire. However, the mathematical solution techniques in this model have been altered. To this end, a maximum fire temperature has been defined depending on combustion efficiency and opening heights only. This temperature together with well‐defined fire compartment parameters was then used as a fictitious thermal boundary condition of the surrounding structure. The temperature of that structure could then be calculated with various numerical and analytical methods as a matter of choice, and the fire temperature could be identified as a weighted average between the maximum fire temperature and the calculated surface temperature of the surrounding structure as a function of time. It is demonstrated that the model can be used to predict fire temperatures in compartments with boundaries of semi‐infinitely thick structures as well as with boundaries of insulated and noninsulated steel sheets where the entire heat capacity of the surrounding structure is assumed to be concentrated to the steel core. With these assumptions, fire temperatures could be calculated with spreadsheet calculation methods. For more advanced problems, a general finite element solid temperature calculation code was used to calculate the temperature in the boundary structure. With this code, it is possible to analyze surrounding structures of various kinds, for example, structures comprising several materials with properties varying with temperature as well as voids. The validation experiments were accurately defined and surveyed. In all the tests, a propane diffusion burner was used as the only fire source. Temperatures were measured with thermocouples and plate thermometers at several positions.     

Radiative heat flux measurement uncertainty

As part of an effort to characterize the uncertainties associated with heat flux measurements in a fire environment, an uncertainty analysis example was performed using measurement data from a room corner surface products test that followed the guidelines of ISO 9705. Equations to model the heat transfer at the surface of a Schmidt‐Boelter (thermopile) type total heat flux gauge were selected for use to calculate the incident radiative flux from a total heat flux measurement. The effects of the heat flux measurement uncertainty sources were evaluated by conducting an uncertainty propagation on the resulting equation for incident radiation. For the model equations and the example conditions selected, the free‐stream temperature estimate and the heat flux gauge calibration constant were determined to be major uncertainty contributors. The study demonstrates how to systematically identify major sources of uncertainty for the purpose of reducing total uncertainty and thereby enhancing experiment design. Published in 2003 John Wiley & Sons, Ltd.     

Computational study of heat transfer in a bubbling fluidized bed with a horizontal tube

A combined approach of discrete particle simulation and computational fluid dynamics is used to study the heat transfer in a fluidized bed with a horizontal tube. The approach is first validated through the good agreement between the predicted distribution and magnitude of local heat transfer coefficient with those measured. Then, the effects of inlet fluid superficial velocity, tube temperature and main particle properties such as particle thermal conductivity and Young's modulus are investigated and explained mechanistically. The relative importance of various heat transfer mechanisms is analyzed. The convection is found to be an important heat transfer mode for all the studied conditions. A large convective heat flux corresponds to a large local porosity around the tube, and a large conductive heat flux corresponds to a large number of particle contacts with the tube. The heat transfer is enhanced by the increase of particle thermal conductivity while it is little affected by Young's modulus. Radiative heat transfer becomes increasingly important as the tube temperature is increased. The results are useful for temperature control and structural design of fluidized beds. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

The role of flame flux in opposed-flow flame spread

The flame spread process is driven by the net heat flux to the specimen surface, including the flux from the flame itself. This flame flux is important since it comprises a major part of the driving force causing flame fluxes were obtained. The values which are reported do not appear consistent and show more deviation among materials than would be anticipated. The most common fire test used for obtaining engineering data on flame spread (ASTM E 1321) also is not formulated in terms of flame flux as a driving force. This motivated an experimental programme, whereby six materials have been studied using the flame spread geometry of the ASTM E 1321 test, but with additional instrumentation for recording heat fluxes. The flame fluxes obtained experimentally in this study show much less variation among materials than the comparable data from the literature survey.     

竖直管气固鼓泡流化床传热机理的CPFD模拟

针对细颗粒气固鼓泡流化床中床料与竖直传热管壁面间的传热行为,在前期实验的基础上,采用计算颗粒流体力学(CPFD)方法从颗粒在传热壁面更新的角度,深入分析了传热特性与壁面气固流动行为之间的关联性。结果表明,模拟得到的传热管壁面颗粒更新通量和基于颗粒团更新模型的颗粒团平均停留时间均能很好解释实验测得的传热系数变化规律,这证实颗粒团更新是影响传热过程的控制性因素。模拟还发现随加热管从床层中心向边壁的移动,加热管周向方向上颗粒更新通量和传热系数的不均匀性都呈增大趋势。随着表观气速的增大,气泡行为导致床层颗粒内循环流率增大,这是导致颗粒团在加热管壁面上的更新频率增大以及床层与壁面间传热系数增大的根源。     

An experimental and modeling study of heat radiation characteristics of inclined ceiling jet in an airplane cargo compartment

A series of pool fires were carried out in an airplane cargo compartment to investigate the effect of the pressure on the heat radiation flux (HRF) of the inclined ceiling jet fire. During the tests, different static chamber pressures ranging from 50 to 101 kPa were controlled by the air flow in and out; both free fire and inclined ceiling jet fire were conducted with five different heat release rates (HRRs), which were produced by a 17‐cm square porous gas burner using propane as fuel. Vertical flame temperature, thermal plume temperature beneath the inclined ceiling, and HRF to the horizontal floor were measured and analyzed; at the same time, the flame shape was recorded by a video. It was found that the HRF was increased with the HRR, and there was a sudden rise for these fire impinging on the ceiling. The flame radiation fraction had a weak correlation with the environment pressure, while the flame emissivity was increased with the increasing ambient pressure. Besides, on the basis of the assumption that the flame emissivity is equaled in both free flame and the inclined ceiling jet fire, HRF calculated model was established and compared with the experimental results.