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.     

Assessment of draft ISO radiant panel spread-of-flame test

A versatile radiant panel apparatus for studying flame propagation in materials used as interior finishes on floors, walls and ceilings is described. Comparative trials with this apparatus and other tests based on relatively very small sources of heat were made on a wide variety of floorcoverings. The results showed that fibre composition and specimen orientation have a marked effect on flame spread. Materials which gave favourable (low flame spread) results in the panel test also gave favourable results in the small heat source tests. On the other hand, some materials which gave favourable results in the small heat source tests gave unfavourable results in the panel test. It was concluded that although small heat source tests will continue to play an important role in flammability testing, the radiant panel test gives an additional assurance of the flame spread resistance of materials in general and should be strongly recommended for materials intended for high risk areas. The effects on flame spread in the panel test of various factors concerned for materials intended for high risk areas. The effects on flame spread in the panel test of various factors concerned with the structure and conditions of test specimens and the apparatus are discussed. A small modification of the radiant panel apparatus is described which enables it to be temporarily altered to simulate a standard French radiant panel test.     

Experimental studies on the thickness of upward flame over poly(methyl methacrylate) slabs

Experiments were performed to investigate the flame thickness of the upward flame on the poly(methyl methacrylate) slabs with width of 100‐400 mm. The results indicated that the flame thickness exhibited an increase first and then decrease trend in the upright orientation, and the maximum thickness location was approximately equal to the pyrolysis front location. The thickness of the flame along the lateral side of slab was less than that of the interior flame. The flame maximum thickness as a function of pyrolysis height and width was obtained, which showed the maximum thickness provided a power law increase with the pyrolysis height and width. Furthermore, the maximum thickness exhibited a power law increase with the total heat release rate as well. Based on the obtained flame thickness, the radiation heat flux at the pyrolysis height was estimated by using a simplified model. Comparison with the calculated convective heat flux revealed approximated pyrolysis heights for upward flame transition from convective heat flux controlled to radiation heat flux controlled.     

An exercise in obtaining flame radiation fraction from the cone calorimeter

The radiant fraction of the heat of combustion emitted by flames of burning fuels is an important quantity needed to predict the thermal radiation from pool fires to remote targets and as a local flame parameter in CFD models. Although there are data for radiant fraction of gas flames, there are little data for this parameter for burning solid materials. The sole source of these data is Archie Tewarson, who used the FMGlobal Fire Propagation Apparatus to compute the radiant fraction of the heat of combustion from energy losses associated with enthalpy flow and duct heat losses. This paper describes a similar approach to obtain the radiant fraction of the heat from flames of burning solids using the cone calorimeter. In the present work, the cone calorimeter is calibrated using a Meeker burner with a premixed methane/air flame that is small and blue and has minimum flame radiation. A heat loss correction factor due to thermal conductance from the duct to the ambient air is determined from the calibration by measuring the temperature of the combustion stream in the duct at the gas sampling location. That factor was found to be 13 ± 2 W/K by calibration compared with a theoretical estimate of 9.3 W/K. The effect of the heat capacity of the duct walls is accounted for by de‐convoluting the duct temperature history. The necessary measurements to compute the radiant fraction then become the heat release rate by oxygen consumption, the mass flow rate in the duct, and the gas temperature in the duct at the sampling location. Results were obtained for 15 polymers, eight of which could be compared with data for nominally similar materials obtained by Tewarson. In addition, results are found to be in good agreement with a correlation by Tewarson in terms of combustion efficiency. Copyright © 2016 John Wiley & Sons, Ltd.     

Premixed gas combustion stabilized in fiber felt and its application to a novel radiant burner

Premixed gas combustion stabilized in a unique ceramic fiber felt has been investigated. Our aim was to better understand the flame structure and flame stabilization mechanisms in the porous felt medium in order to develop a new radiant burner. A novel recuperative radiant burner was designed and constructed. A flame stabilizes near the downstream interface of the porous medium that is an excellent selective thermal emitter. The burner was developed for use as a gas-fired light source. The combustion performance of the burner at various operating conditions and the effect of heat recuperation are presented. Combustion modes on the fiber felt were examined carefully. An optimal flame structure for the premixed gas combustion is attained and the flame stabilizes in the porous fiber felt at radiant mode combustion over a wide range of firing rates. The burner emits desired spectral radiation and generates fairly intense light at the conditions of heat recuperation. The light radiant burner could be used as an alternative low-glare light source in an integrated heat and light system in which the light is distributed through light pipes.     

The simulation of an existing ethane/dehydrogenation reactor

This paper is concerned with the mathematical representation of an existing box-type furnace for the thermal dehydrogenation of ethane. The furnace is composed of a preheating convection section and a radiant heated section. The two models considering (i) the tube-side calculation and (ii) the radiant heat transfer in the combustion chamber are independent of each other. A study is made of the effects of pressure, heat flux level and “steam/hydrocarbon” ratio on the effluent composition. A significant improvement in reactor operation if a more uniform heat flux could be achieved is predicted. From the radiant heat transfer computations an unexpected variation in flame temperature in the furnace under study was detected.     

Study of vertical upward flame spread on charring materials—Part II: Numerical simulations

Simulation results, obtained by means of application of an enthalpy‐based pyrolysis model, are presented. The ultimate focus concerns the potential of the model to be used in flame spread simulations. As an example we discuss vertically upward flame spread over a charring material in a parallel plate configuration. First, the quality of the pyrolysis model is illustrated by means of cone calorimeter results for square (9.8 cm × 9.8 cm exposed area), 1.65 cm thick, horizontally mounted MDF samples. Temperatures are compared at the front surface and inside the material, for different externally imposed heat fluxes (20, 30 and 50 kW/m²), for dry and wet samples. The mass loss rate is also considered. Afterwards, vertically upward flame spread results are reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high), vertically mounted face‐to‐face, for different horizontal spacings between the two plates. The simulation results are compared to experimental data, indicating that, provided that a correct flame height and corresponding heat flux are applied as boundary conditions, flame spread can be predicted accordingly, using the present pyrolysis model. Copyright © 2010 John Wiley & Sons, Ltd.     

航空煤油不同尺寸池火热流及温度特性研究

为研究航空煤油池火燃烧过程中热流与温度特性,搭建了油料燃烧模拟实验台架。通过分析实验图像与数据,发现航空煤油池火辐射热通量随径向距离和高度的增大逐渐减小,其中辐射热通量对高度增加更为敏感,随高度升高下降幅度更快;热辐射先于热对流达到强度最大值,随后在稳定燃烧阶段后期热对流强度存在跃升现象,强度超过热辐射强度,成为该阶段主要传热方式;航空煤油池火中心线上,下部区域火焰连续燃烧、氧气浓度低,上部区域油火卷吸空气、氧气浓度较高,是导致不同高度最高温度到达时刻不同的主要原因;对不同尺寸油池火来说,其热通量峰值随着油池尺寸增大而增大,同时方形油池热通量峰值显著高于圆形油池。     

A theoretical basis for flammability properties

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

Steady states of the surface of a nonadiabatic flame near the limits

Within the framework of a weakly linear model, which describes a nonadiabatic flame near the limit of its propagation caused by heat losses, steady states of the combustion-wave from are studied. Three-dimensional structures of the wave front are formed because of diffusion-thermal instability of the planar flame. The limits of propagation of a curved flame front are shown to expand if the diffusion-thermal instability is taken into account: a cellular flame can exist at heat losses higher than the critical value for the two-dimensional flame. The stability of steady solutions, which describe the cellular flame near the limits of its propagation, is studied. For sufficiently high heat losses, steady solutions for a nonadiabatic flame with front discontinities are obtained. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 4, pp. 3–11, July–August 1999.     

Unsteady combustion of solid propellants subject to dynamic external radiant heating

A theoretical and experimental investigation is conducted on the effect of a dynamic external radiant heat flux on the combustion of energetic materials. These studies have illustrated the need for including the effect of the mean radiant heat flux and in-depth absorption. Also, a method for obtaining the linear frequency response function over a range of frequencies from a single test using series of radiant pulses is demonstrated. Experimental results have been obtained for an AP/HTPB propellant.Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign. Published in Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 31–36, May–June, 1993.     

Laminar flame in fine-particle dusts

A simple conductive model of the laminar flame in fine-particle dusts, with particles burning in the diffusion mode, is presented. The model is based on the assumption about wide zones of combustion in dusts; therefore, the main contribution to the formation of the heat flux toward the pre-flame zone is made by the heat released on burning particles near the interface between the burning and pre-flame zones. The normal flame velocity is demonstrated to increase with decreasing particle size and with increasing fuel and oxidizer concentrations. The maximum velocities are reached under the condition of identical volume heat capacities of the solid and gas phases, which requires the fuel concentration to be several times higher than the stoichiometric value. The calculated results are validated by experimental data on the normal flame velocity as a function of the fuel concentration for gas suspensions of magnesium, aluminum, zirconium, iron, and boron particles.     

An investigation into the effects of gravity level on rate of heat release and time to ignition

Two experiments were performed on board an aircraft flying repeated parabolic trajectories to generate free-fal conditions. The first experiment investigates the way in which rate of heat release (RHR) varies with gravity for a candle flame under an imposed low-speed flow. In line with previous studies of flame spread rate it has been shown that rate of heat release drops significantly in imcrogravity. The heat loss due to radiation decreases by a larger proportion than that due to non-radiative processes indicating a lower flame temperature. The RHR from a microgravity flame is flow rate dependent, increasing for increased flow rate at air speeds under 0.03 ms^?1. For the geometry used in this experiment hypergravity caused only a small increase in RHR. The second experiment studied the ignitability of thermoplastics under an imposed radiant heat flux. The ignition test apparatus consists of a conical spiral heating element positioned horizontally above the sample, a continuous spark ignition source and a removable heat shield. Experiments were conducted in a sealed pressure chamber on samples of either PMMA or POM, 1.5 mm thick, with a ceramic backing. There is some indication that gravity influences the time to ignition for some materials.     

Effect of flame heat flux on thermal response and fire properties of char‐forming composite materials

This study evaluates the effect of flame heat flux on the prediction of thermal response and fire properties of a char‐forming composite material. A simplified two‐layer flame model was developed and incorporated into a heat transfer thermal model to predict the thermal response and fire reaction characteristics of a burning material. A typical char‐forming material, E‐glass reinforced polyester composite, was used in the study. A cone calorimeter was used to measure the fire reaction characteristics of the composite. The flame heat flux in a cone calorimeter test setup was estimated using the simplified flame model. Thermal response and fire property predictions with and without the effect of flame heat flux were compared with experimental data obtained from the cone calorimeter tests. Results showed that the average flame heat flux of the composite in a cone calorimeter was 19.1 ± 6 kW/m² from model predictions. The flame had a significant effect on the thermal response and fire properties of the composite around the first heat release peak but the effect decreased rapidly afterwards. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigation of the thermal behavior effect of the surrounding material environment on the swirling flame structure

This paper is focusing on the numerical simulation of a swirling flame, resulting from the interaction of multiple fires, evolving in a free and unlimited environment. A typical system, formed by a central fire source surrounded by four heat sources, is used. Since the thermal characteristic of the surrounding sources is the main engine for the rotation of flame, a detailed study is performed by varying the heating flux of these sources. This study shows that an increase of the heating flux of surrounding sources has as a result an intensification of the penetrating air puffs through the openings between the surrounding four heat sources. These puffs tangentially drive the central flame, thus producing a marked improvement on the angular momentum. Moreover, this study shows that the flame height is strongly affected by the flame rotation. Moreover, two different aspects of the flame height evolution are observed from the flow visualization and the thermal and dynamic fields for the different cases studied.     

A theory for flame extinction based on flame temperature

The extinction and suppression of diffusion flames is examined theoretically. The effects of oxygen reduction and external heat flux are examined compared to data in the literature. An application of extinction in compartment fires is also examined. The theory developed is based on a critical flame temperature, and that theory includes transient effects and the addition of water as well. Copyright © 2004 John Wiley & Sons, Ltd.     

Influences of specimen size and heating mode on the ignitability of polymeric materials in typical small‐scale fire test conditions

Small‐scale fire tests including the Underwriters Laboratories 94 (UL94) vertical burning test and the cone calorimeter test are widely used. In this paper, the ignition times of materials heated by the conical heater of a cone calorimeter and the UL94 flame were measured. It was found that for polymer bars heated by the UL94 flame, the ignition time is relatively short and increases with the specimen thickness. But the contribution of the specimen thickness to the delay of the ignition time is limited. The intrinsic properties of materials play a more important role in the ignition time than the specimen thickness. In addition, respectively corresponding to one‐dimensional, two‐dimensional, and three‐dimensional heat transfer, three heating modes of the UL94 flame were presented and compared with the conical heater. It was found that whether the heat source is the conical heater or the UL94 flame, the ignition time depends on the heat flux and the multidimensional heat transfer. The ignition time decreases with the increasing heat flux, and the magnitude order of the ignition time might drop when the heating mode changes from one‐dimensional to multidimensional heat transfer. Copyright © 2011 John Wiley & Sons, Ltd.     

高压可燃气体泄漏动力学过程与喷射火热灾害分析

以高压可燃气体泄漏诱发的喷射火为研究对象,对泄漏过程中喷口处稳态气流状态参数变化及其被点燃之后的喷射火火焰形态与辐射热流场预测模型研究进展进行了综述。归类分析了基于理想气体状态方程和Abel-Noble状态方程的高压气体泄漏模型及其适用情况,并对不同浮力控制和动量控制范围的几种喷射火火焰几何尺寸模型进行了概括性总结,同时对不同火焰形态下的点源、多点源、固体以及线源4种热辐射模型进行了汇总,最终提出将3种模型耦合联用以建立适用于不同泄漏条件下喷射火热灾害分析预测方法。分析表明,该分析预测方法具有很好的适用性。     

Experimental evaluation of flame and flamelet spread over cellulosic materials using the narrow channel apparatus

Originally conceived as an apparatus to study near‐limit flames and their breakup into flamelets and later modified to function as a microgravity simulation apparatus, the narrow channel apparatus serves also as a facility for examining long time flame spread and material flammability in on‐earth (terrestrial) applications. These applications include flame spread in narrow gaps, persistence of flame in heat‐loss environments, and flame‐to‐flamelet front transition. The narrow channel apparatus tests described here measure behavior of the spreading flame and features of the flame‐to‐flamelet transition. Measured quantities include flow, flame and flamelet velocities in normal and inverted tests, flow deceleration and acceleration rates with associated flame or flamelet response, flame‐to‐flamelet transition times, and influences of fuel thickness. The principal goal of this research was to ascertain the capacity of the narrow channel apparatus to produce data for phenomena observed in both (1) simulated microgravity flame spread and (2) terrestrial flame spread in narrow gaps and channels. Copyright © 2012 John Wiley & Sons, Ltd.