基于CFD的开放空间油池火燃烧速率和热辐射研究

采用混合组分燃烧模型和有限体积辐射模型,通过液体表面蒸发模型对液态燃料和火羽流进行耦合,建立开放空间油池火模型.利用CFD方法分别对不同直径的庚烷油池火进行模拟,研究其燃烧速率、热释放速率随直径的变化以及火焰中轴上的温度和单位体积热释放速率(HRRPUV)分布,并得出油池表面的热辐射反馈以及油池外部水平和垂直方向的热辐射强度分布规律.部分模拟结果与实验进行对比,验证该模型的适用性和有效性.     

Effect of scale and fuel type on the characteristics of pool fires for fire fighting training

This paper examines the effects of pool size and fuel type on the characteristics of pool fires. The fuels studied include hydrocarbon solvents, alcohol and their blends. The large-scale experiments were conducted on 0.445 and 1 m diameter pools in two enclosures: 41 m long×5.4 m wide×2.4 m high and 25 m long×2.7 m wide×2.4 m high, under a ventilation rate of 1–1.2 m/s. In these tests, measurements of the fuel pyrolysis mass loss rate, heat release rate (from CO₂ and CO concentrations as well as oxygen depletion), smoke extinction area (from laser attenuation), total heat flux emanating from the flame, and the yields of CO₂ and CO were made. Bench-scale tests were conducted using the same fuels and fuel blends on 100 mm× 100 mm pool fires in a Cone Calorimeter—a well-known fire test method.

The hydrocarbon+alcohol fuel blends are used to create large pool fires for fire fighting training. The aim of this study was to: (1) Formulate a fuel blend which will produce a “hot” fire for realistic fire fighting training while generating a minimal quantity of smoke to comply with the pollution regulations. (2) Investigate the effect of fuel type and scale on the properties of the pool fires. (3) Make comprehensive measurements on pool fires in tunnels which can be used for validating mathematical fire models.

This study has resulted in the formulation of a fuel blend which satisfies the requirements of fire fighting training as well as Environment Protection Authority of New South Wales. The bench and the large-scale results correlate well showing good agreement between the heat of combustion, smoke extinction area (SEA) and the yields of CO₂ and CO. The SEA increases with the CO and CO₂ yields which in turn increase with the carbon fraction and the aromatic content of the fuel/fuel blend. The SEA correlates strongly with the CO and CO₂ yields and in both the correlations, the entire data collapse on a single curve. These correlations indicate that the SEA and the CO₂ and CO yields vary mainly with the fuel type, and not so much with the pool size. The flame heat flux increases with the carbon content in the fuel.     

Measurement of Flame Emissivity of Hydrocarbon Pool Fires

Flame emissivity is an important parameter in the study of pool fires. A series of pool fire experiments are carried out with four different fuels namely diesel, gasoline, hexane and kerosene for pool diameters of 0.10 m, 0.13 m and 0.20 m. Flame emissivity at a height of 0.25 times the pool diameter from the base is measured by observing the flame with reference to a black body using infrared camera. Influence of pool diameter (0.3 m, 0.34 m, 0.5 m, 0.7 m and 1.0 m) on flame emissivity at a height of 0.25 times the pool diameter is studied with diesel as the fuel. Variation of flame emissivity with the height of the flame along the center of diesel pool fire is investigated for diameters of 0.3 m, 0.5 m, 0.7 m and 1.0 m. It is observed that the flame emissivity is less at the tip of the flame in comparison with that at the base of the pool fire. The measurement of flame emissivity by observing flame with reference to a black body using infrared camera is corroborated with the measurements conducted with reference to an electrically heated black body for diesel pool fires with diameters 0.3 m, 0.5 m and 0.7 m. Flame emissivity is also inferred from the mass burning rate measurements for diesel oil pool fires of 0.3 m, 0.5 m, 0.7 m and 1.0 m diameters. Flame emissivities are independent of the measurement method. Temperature and surface emissive power distributions of the diesel pool fires for diameters 0.3 m, 0.5 m, 0.7 m and 1.0 m are computed using infrared thermography.     

Heat feedback to the fuel surface of a pool fire in an enclosure

As a part of an effort to determine the energy balance at the pool fire surface in compartments, a series of fire experiments were conducted to study heat flux of the flame in a vitiated environment formed with air and combustion products gases. This paper presents experimental results of the burning behaviour of a heptane pool fire in a reduced scale compartment equipped with a mechanical ventilation network. Measurements of heat fluxes, fuel mass loss rate, oxygen concentration and temperature are performed for heptane fires of 0.26 and 0.3 m diameter pans at different ventilation flow rates. An original method to separate effects of the radiant heat flux of the flame and of the external heat feedback to the fuel surface is developed. This was achieved by using an additional heat flux measurement located under the pool fire. A correlation was also developed to determine the temperature rise on the plume centerline in the compartment as a function of the heat release rate. The results indicate a decrease in the fuel mass loss rate, flame temperature and heat fluxes to the fuel surface as the oxygen concentration measured near the fuel decreases by varying the air refresh rate of the compartment. The flame radiation fraction shows a similar behaviour, whereas the convective fraction of the flame heat flux increases when oxygen concentration decreases. Based on these experimental findings, it was discussed that any classification of the burning regime of a pool fire should consider both the effects of pan diameter and the burning response to vitiated air.     

A Physical Model and Improved Experimental Data Correlation for Wind Induced Flame Drag in Pool Fires

The flames over a burning liquid fuel are observed to spill over the downwind edge of the liquid pool in a wind. Empirical correlations in the literature relate the total base dimension of the fire (diameter + the spill over) with the wind Froude number. This leads to erroneous and physically incorrect (negative) value for the flame spillover at low wind speeds and/or in large diameter fires. The data from laboratory scale (0.1–0.6 m) to field scale (up to 35 m) fires of different hydrocarbon fuels on the wind induced flame “drag” or “spillover” were re-examined. The ratio of the flame spillover distance with the pool diameter is seen to vary in direct proportion to the square root of the Froude number but with different proportionality constants for different fuels. A physical model was developed to analyze the phenomena that occur at the base of a pool fire subject to a wind. This model indicates that the non-dimensional downwind flame spillover distance is directly proportional to the square root of the Froude number, inversely proportional to the square root of the dimensionless heat of combustion of the fuel and directly proportional to the 1/4th power of the ratio of vapor density to air density. Available experimental data are synthesized into a single correlation when plotted on the basis of the non-dimensional parameters from the model. This correlation includes the Froude number, the Damkohler number (dimensionless heat of combustion of the fuel), the wind flow Reynolds number and the fuel vapor-to-air density ratio.     

Thermal Radiation from Fire Whirls: Revised Solid Flame Model

The conventional solid flame model for calculating the radiation from pool fires involves a constant flame surface emissive power. In this paper, in terms of the fact that the flame shape of fire whirl can be simulated by combination of a cylinder and a cone, a new formula is proposed to characterize the vertical profile of the flame surface emissive power of fire whirl, by which a revised solid flame model is presented for calculating the radiant heat flux from the flame, in the horizontal and vertical directions. In comparison to the conventional solid flame model, the revised model agrees better with the experimental data of radiant heat flux, especially for heat release rates below a certain critical range. By the revised model it is indicated that the profile of radiant heat flux significantly varies with the distance away from the pool centerline. The effect of plume radiation and flame pulsation are suggested to be considered for further improving the radiation model of fire whirl.     

Determination of Flame Emissivity in Hydrocarbon Pool Fires Using Infrared Thermography

A methodology is proposed for the determination of flame emissivity in hydrocarbon pool fires using infrared thermography. Experiments on gasoline and diesel oil pool fires with diameters of 0.13, 0.18 and 0.5 m were carried out, determining the flame emissivity for these types of fires in transition regime and turbulent regime. Also analysed was the evolution of the flame emissivity depending on the pool diameter, and a new correlation for the estimation of the emissivity depending on the above mentioned diameter was obtained.     

Thermal Analysis of Hollow Steel Columns Exposed to Localised Fires

In fires in large compartments like enclosed car parks, airport terminals and industrial halls, the uniform distribution of gas temperature of post-flashover stages are unlikely to occur; in these cases, the thermal actions of a localised fire must be taken into account. In order to design steel structures for a localised fire, very detailed data concerning the development of temperatures in steel is required. EN 1991-1-2 presents a simplified model for calculating the temperatures in ceiling slabs and in the beams that may support such slabs; however, no simplified calculation model for the heat transfer in vertical elements, such as columns, is yet available. There is a need for more experimental data on real scale structures exposed to localised fires. A research project on the evaluation of temperatures in steel columns exposed to localised fires was carried out at the University of Coimbra. Full-scale natural fire tests were used to test columns, instead of conducting the usual furnace tests. This paper presents and discusses the results of the experimental tests on unprotected hollow steel columns exposed to localised fires, each of them simulating a distinct fire scenario according to different fire loads, positions and ventilation conditions. During the fire tests, real measurements showed flame heights and burning times different to those preliminarily estimated: flame heights had been conservatively predicted; while, the duration of the burning had been significantly underestimated.     

Attenuation of Swirling Motion of a Fire Whirl in a Vertical Shaft

Fire Technology - Experiments on fire whirls generated by a gasoline pool fire in a vertical shaft were carried out. Vortex motions of swirling flame induced by buoyancy above the pool fires were...     

Natural smoke filling in atrium with liquid pool fires up to 1.6 MW

Experimental studies on natural smoke filling in an atrium induced by a liquid pool fire up to 1.6 MW were carried out. The new full-scale burning facility, the PolyU/USTC Atrium constructed at Hefei in China, was used. Five sets of hot smoke tests with diesel pool fires of 2×2 m placed on the floor were carried out. All openings were closed, except leaving a small vertical vent of 0.2 m high for supplying fresh air. Transient variations on the mass of the burning fuel, the vertical temperature distributions and the smoke layer interface heights were measured. Results compiled from the tests were compared with those predicted by a smoke filling model developed from plume equations; the NFPA smoke filling equation; and a model developed by Tanaka and co-workers.     

Bending of wind-blown flames from liquid pools

The bending of a flame by wind influences the amount of heat transferred by radiation and convection, the fuel burning rate, and the flame spread rate. To what extent will a flame be bent by wind? The author presents correlations of data taken from liquid pool fires, which enable us to predict flame bending and trailing for large fires.     

Investigations on the Characteristics of Radiative Heat Transfer in Liquid Pool Fires

This work numerically investigates the radiative flux and flame shape from heptane liquid pool fires with small to medium sized pool fires (14–38 cm), grid sizes, spectral bands and solid angles. Experimental results were originated for comparison. The results show that calculated flame shapes and radiative fluxes are in good agreement with the measurements for suitable grid resolution. Besides, the effect of the intermittency of a flame on thermal radiation was performed. Experimentally, a shield was used to shade the radiant power from a persistent flame, and the radiative flux was measured from an intermittent flame on 30 cm diameter pool fires. The results show that 36% of the radiative flux measured is emitted from the intermittent flame when the radiometer is located at the base of the pool fire. As the radiometer is moved upwards, the radiative fluxes measured from the intermittent flame increase gradually, even to 50%.     

Statistical Analysis of Effects of Nanoparticles on Hydrocarbon Pool Fire

The effects of pool diameters, height, fuel type, and presence of nanoparticles on flame temperature are tested. Nanoparticles were added and mixed with the fuel and its effect on flame spread time over the liquid fuel surface is studied. The results showed that pool diameter has the maximum effect on the increase of flame temperature. Nanoparticles reduce the flame temperature from 10 to 20% while flame spread time is increased 15% to 30% for kerosene and 6% to 26% for diesel oil. Experimental measurement of flame temperature distribution in a pool fire is carried out. Statistical analysis of flame temperature, flame spread time was made using Yates and ANOVA methods.     

A review of physics and correlations of pool fire behaviour in wind and future challenges

This paper reviews the physics and correlations for the burning behaviour of pool fires in wind, discussing also challenges for future research on this topic. In the past decades, the burning behaviour of pool fires in still air, which is solely buoyancy driven, has been extensively studied. These studies are primarily focused on scale, radiation, soot, pressure and gravity effects. However, these phenomena and physics change significantly with much more complexity in the presence of wind, with regard to heat feedback and burning rate; flame morphological characteristics; flame turbulence, soot and radiation emission. Remarkable progress has been made in understanding the behaviour of the heat feedback and burning rate, flame tilt, flame length and flame base drag of wind-blown pool fires. Several semi-empirical correlations have been proposed for these quantities, based on experimental data and the physically dimensional analysis. However, for wind-blown pool fires, the flame soot and radiation emission coupling with complex flow turbulence scales due to the interaction of buoyancy with wind still require more basic research. All these processes are more challenging especially for wind-blown large scale pool fires, which require knowledge and understanding of the physics, especially for establishing evaluation methodologies of their hazard and adverse impact.     

高高原机场环境下可燃液体燃烧特性研究

选择航空煤油和正庚烷为燃料,在不同油盘直径工况下,研究其在高高原机场低压环境下的燃烧速率、热释放速率和火羽流中心温度变化等典型特征参数的变化规律。结果表明：航煤和正庚烷整个燃烧过程大致分为初期燃烧、稳定燃烧、衰减熄灭三个阶段;同一工况下,正庚烷的热释放速率大于航煤,燃烧时间更短,最大热释放速率与油盘直径有一定的指数关系,可以预测低压环境下油池火的最大热释放速率;火羽流中心温升与火源高度、功率存在指数关系,对理想火羽流模型进行了修正并应用于康定高高原低压环境,修正系数为8.43。     

Wildland fire spot ignition by sparks and firebrands

Wildland and Wildland Urban Interface (WUI) fires are an important problem in many areas of the world and may have major consequences in terms of safety, air quality, and damage to buildings, infrastructure, and the ecosystem. It is expected that with climate changes the wildland fire and WUI fire problem will only intensify. The spot fire ignition of a wildland fire by hot (solid, molten or burning) metal fragments/sparks and firebrands (flaming or glowing embers) is an important fire ignition pathway by which wildfires, WUI fires, and fires in industrial settings are started and may propagate. There are numerous cases reported of wildfires started by hot metal particles from clashing power-lines, or generated by machines, grinding and welding. Once the wildfire or structural fire has been ignited and grows, it can spread rapidly through ember spotting, where pieces of burning material (e.g. branches, bark, building materials, etc.) are lofted by the plume of the fire and then transported forward by the wind landing where they can start spot fires downwind. The spot fire problem can be separated in several individual processes: the generation of the particles (metal or firebrand) and their thermochemical state; their flight by plume lofting and wind drag and the particle thermo-chemical change during the flight; the onset of ignition (smoldering or flaming) of the fuel after the particle lands on the fuel; and finally, the sustained ignition and burning of the combustible material. Here an attempt has been made to summarize the state of the art of the wildfire spotting problem by describing the distinct individual processes involved in the problem and by discussing their know-how status. Emphasis is given to those areas that the author is more familiar with, due to his work on the subject. By characterizing these distinct individual processes, it is possible to attain the required information to develop predictive, physics-base wildfire spotting models. Such spotting models, together with topographical maps and wind models, could be added to existing flame spread models to improve the predictive capabilities of landscape-scale wildland fire spread models. These enhanced wildland fire spread models would provide land managers and government agencies with better tools to prescribe preventive measures and fuels treatments before a fire, and allocate suppression resources and issue evacuation orders during a fire.     

妙峰山林场针阔枯叶可燃物地表火行为影响因素研究

为了解火行为指标影响因素,采用点烧试验方法,对妙峰山林场可燃物油松和栓皮栎枯叶地表火行为进行了研究。结果显示：火焰蔓延速率、火焰长度和火线强度的显著影响因素均为风速和坡度（P<0.05）;而可燃物厚度和针阔叶比例对火焰蔓延速率、火焰长度和火线强度影响较小,未达到显著性水平（P>0.05）,且在风速、坡度、可燃物厚度和针阔叶混合比例4 个因素中,风速是最主要影响因素。在以风速4 m/s 进行试验时,最大蔓延速率、火焰长度和火线强度分别可以达到：2.357 m/min、67.329 cm和119.622 kW/m。     

The role of temperature on carbon monoxide production in compartment fires

The objective of this study was to assess the effect of temperature on carbon monoxide production in compartment fires in order to resolve the difference between global equivalence ratio-yield correlations obtained in simplified upper layer environments and more realistic compartment fires. The chemical reactivity of upper layer gases was studied using a detailed chemical kinetics model. An analysis of the modeling and experimental data in the literature provided insights into the effect of temperature on carbon monoxide production.

The effect of changing temperature on compartment fire upper layer composition is twofold: (1) the generation of species in the fire plume is changed; and (2) oxidation of post-flame gases in the layer is affected. Elevated compartment temperatures correlate with increased fire plume temperatures and more complete oxidation of the fuel to CO₂ and H₂O within the plume. The layer temperature dictates post-flame oxidation in the layer. For most situations, upper layer temperatures below 800K indicate chemically unreactive layers. As such, combustion within the fire plume dictates final CO production in the compartment. Reactions in the upper layer dictate final CO levels when upper layer temperatures are about 900K and higher.     

Burning Rate of Liquid Fuel on Carpet (Porous Media)

The occurrence of a liquid fuel burning on carpet has been involved in many incendiary and accidental fires. While the research on a liquid fuel fire on carpet is still limited, much work on porous media has been performed using sand or glass beads soaked with liquid fuel. In this study, a heat and mass transfer theory was first developed to analyze the burning process of liquid on carpet, and then several small-scale tests were performed to validate the theory. This analysis is valid for pool fires intermediate in size (5–20 cm. in diameter). The experimental apparatus consisted of a circular pan (105 mm) and a load cell. Varying amounts of fuels (heptane, kerosene and methanol) were spilled onto the carpet, which was allowed to burn in a quiescent environment. It was found that due to the different controlling mechanisms, the liquid burning rate could be less or more than that of a similarly spilled free-burning pool fire. For the worst-case scenario in fires, the maximum enhancement of the burning rate due to the porous media is predictable through the physical properties of the fuel. This analysis is valid for both combustion and evaporation. Several similar results in the scientific literature are analyzed to further describe the trend. This work explains the role of carpet in liquid pool fires and also helps to explain special risks related to the presence of carpet involved in arsons and will be useful in reconstruction of the early development of an incendiary or accidental fire.