Extinction limit of a pool fire with a water mist

This work describes an experimental investigation of fire extinction limit and enhancement for a gasoline pool fire interacting with a water mist. A downward-directed nozzle produces a fine water spray over a small-scale opposed pool fire. The fire extinction limit is obtained from minimum nozzle injection pressure measured when the fire extinguish takes place. The burning rate of the fuel is also measured using a verified technique. For the conditions tested, it is shown that there are two distinct regions in the relationship between the distance from the nozzle to the fuel pan and the injection pressure, i.e. a fire extinction region and a fire enhanced region. The effective water flux is shown to be a more useful parameter than the injection pressure for the fire extinction limit. It is also revealed that the larger the spray thrust the larger the burning rate is in the fire enhanced region.     

Extinction of gas and liquid pool fires with water sprays

Extinction in open space of flames from pool fires by downwardly directed water sprays has been investigated on two linear scales, one three times larger than the other. Circular pool fires were employed as fire sources, mostly in the form of gas discharge (methane) from a horizontal sand surface but also, to a limited extent, in the form of heptane pools. The results are presented in normalized plots based on scaling theory verified in a previous study. Extinction data from the methane fires are insensitive to the initial spray angle of the nozzle discharge. The data are consistent with an engineering relation showing extinction water flow rate approximately proportional to an effective nozzle diameter, and to the 0.4-power of both nozzle height and freeburn heat release rate. This result has been interpreted to indicate that spray-induced dilution of the flammable gas is a major factor in extinguishing fires from gaseous discharge. Extinction data of liquid pool fires from this study (n-heptane) and previous investigations (gasoline, JP-5) are consistent with the methane data, except for somewhat higher water rates at extinction.     

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.     

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.     

Experimental study on the extinction of liquid pool fire by water droplet streams and sprays

This paper provides evidence about the interaction between the water droplet stream and the flame, and explains how the interaction affects the suppression effectiveness. Two purpose-built gasoline pools were used to generate different open fires. The mono-disperse water droplet streams and water sprays were used as the flame suppressant. The first pool with a circular shape was equipped with a concentric pipe to allow the droplet stream to pass through the flame without impinging the gasoline. The second pool with a long narrow shape was equipped with expandable sides and allowed to extend the fire size. The passing ways of the droplet stream were systematically varied. The results clearly show two modes of flame inhibition; one is by blocking or interfering with the mixing of gasoline vapor and fresh air, and the other by cooling down the flames. For the stream case, the direction of the stream passing through the flame can affect the effectiveness of the suppression which increases as the angle is changed from vertical to horizontal. Also, there is an optimum distance between the stream axis and gasoline surface for flame inhibition. Moreover, the ability can be affected by the droplet size. On the same volume flow rate, the larger the droplet size, the more effective the flame suppression. For the water spray passing through the flame in the long groove pool, whenever the quantity of water vaporization reaches a critical value, the effectiveness of flame suppression by combining the obstructing and cooling effects becomes better.     

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。     

An experimental study of complex fuel burning behavior with water application

This work reports a series of small-scale experiments conducted to characterize the burning behavior of selected complex fuels with water application. The main objective of this work is to provide suitable data for the development and validation of fire suppression sub-models for commodities with multiple fuels and complex geometry (complex fuels). The water transport process and burning behavior of two representative complex fuels are examined using three fuel surface conditions including a regular-closed face (CF), a side-open face (SF) and a top-side-open surface (TF). The fuels are exposed to two water application rates at three different water application times with respect to ignition. The results of water collection rates under no fire condition show that the water transport is delayed in the open surface cases (SF and TF). In the free-burn experiments, the surface conditions are shown to have little impact on the fire growth rates. The results of the fire suppression experiments show that the water application time has the most significant impact on controlling the fire, and that the open-surface conditions tend to adversely affect the suppression outcome.     

Assessment of the Burning Rate of Liquid Fuels in Confined and Mechanically-Ventilated Compartments using a Well-Stirred Reactor Approach

The objective of this work is to provide a ‘support tool’ to assess the burning rate of a pool fire in a well-confined and mechanically-ventilated room using a single-zone model based on conservation equations for mass, energy and oxygen concentration. Such configurations are particularly relevant for nuclear facilities where compartments are generally sealed from one another and connected through a ventilation network. The burning rates are substantially affected by the dynamic interaction between the fuel mass loss rate and the rate of air supplied by mechanical ventilation. The fuel mass loss rate is controlled by (i) the amount of oxygen available in the room (i.e. vitiation oxygen effect) and (ii) the thermal enhancement via radiative feedback from the hot gas to the fuel surface. The steady-state burning rate is determined by the ‘interplay’ and balance between the limiting effect of oxygen vitiation and the enhancing effect of radiative feedback. An extensive sensitivity study over a wide range of fuel areas and mechanical ventilation rates shows that a maximum burning rate may be obtained. For the studied HTP (Hydrogenated Tetra-Propylene) pool fires, the maximum burning rate is up to 1.75 times the burning rate in open air conditions.     

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.     

Smoke production,radiation heat transfer and fire growth in a liquid-fuelled compartment fire

A detailed investigation is described of the interaction between fire development, smoke production and radiative exchange in a half-scale ASTM compartment in which the source is a heptane pool fire. Measurements of heat flux, fuel mass loss rate, ventilation flow rates, temperature and soot volume fraction are reported for the compartment for varying door widths. Data from the compartment are compared with open pool fire measurements using the same equipment. The confined geometry is shown to exert a strong influence on pool fire development and suggests that considerable caution is needed in employing open pool fire data as boundary conditions for CFD simulation. Numerical simulations based on the direct calculation of radiative exchange between the liquid fuel surface, the smoke-laden environment and bounding walls do reproduce the behaviour observed when combustion, soot production and radiation are modelled in detail and finely resolved spatially.     

Thermal exposure from large scale ethanol fuel pool fires

The growing use of ethanol as fuel for combustion engines has dramatically increased the need for large scale storage of ethanol in tanks. There are new risks related to fires in storage tanks having larger volumes. Very little experimental data exist to support risk assessments regarding emitted radiation and burning rate for large pool fires. Experience from small scale tests show that the exposure to nearby surroundings is less for alcohols than for hydrocarbon fuels like gasoline and these results are often extrapolated to fires of large sizes. This paper describes the results of two pool fires conducted within the frame of the ETANKFIRE project, one with 97% ethanol and 3% gasoline and the other with 85% ethanol and 15% gasoline, both with a surface area of 254 m². The results show, contrary to experience from small scale pool fires, that the exposure to nearby surroundings is much larger for ethanol-rich fuels compared to the calculated radiative heat flux from a pure gasoline fire of same fuel area.     

材料性质对浸汽油固体着火及燃烧特性影响

以报纸、纸板和碎纸屑等常见固体可燃物为对象,考察汽油添加量对其引燃时间、火焰传播速率、火灾痕迹及烧损速率的影响,分析固体材料性质对浸油固体着火及燃烧特性的影响机理。结果表明:随着汽油含量增加,报纸和纸壳的引燃时间缩短,表面火焰传播速率加快,超过一定值后有闪火出现,烧损速率增加,而碎纸屑的火焰传播速率则随汽油含量的增加呈先增大后减小的趋势。浸油纸壳燃烧时能够明显地区分出汽油燃烧和纸壳炭化两个速率。     

A compartment burning rate algorithm for a zone model

A model is presented that dynamically predicts the mass loss rate of fuel in a compartment as a function of ventilation, thermal feedback, fuel type and scale. Without a loss of generality, a floor-based fuel is considered. The effect of ventilation is included in the model through the ambient oxygen concentration in the ambient surrounding the fuel at the floor. A mixing model associated with the inlet airflow at the vent is developed to determine this oxygen concentration. An extinction criterion for the flame is based on a critical flame temperature for a diffusion flame associated with the ambient conditions surrounding the flame at the floor. The model is executed in BRI2002, a zone model, capable of computing species and thermal conditions in the upper and lower compartment gas layers. Computations show good agreement with small-scale compartment data for heptane pool fires. The results can accurately portray many regimes of burning including extinction, combustion oscillations, reduction in the flaming area, and quasi-steady burning.     

单、双喷头细水雾对正庚烷池火灾的抑制作用

为了研究单、双喷头细水雾抑灭正庚烷池火灾的效能和机理,在半体积飞机模拟货舱中开展了单、双喷头细水雾雾滴粒径测试和抑灭20 cm 正庚烷池火灾的实验研究。结果表明,双喷头细水雾协同工作会导致雾滴之间相互碰撞发生二次破碎,有助于雾化效果的提升。通过对燃料表面温度、火焰区平均温度和舱内氧气浓度的测量和计算,对比分析了单、双喷头细水雾抑灭火的主导机理。结果表明,单喷头细水雾灭火的平均时间为283.14 s,耗水量约为3.54 L,燃料表面冷却是其抑灭火的主导机理。双喷头细水雾灭火的平均时间为212.22 s,耗水量约为5.31L,火焰冷却是其抑灭火的主导机理。     

甲醇和异丙醇池火泡沫灭火试验研究

对直径2.5 m的甲醇和异丙醇池火发展过程、冷却保护和泡沫灭火进行了试验研究.结果表明,异丙醇池火的发展较甲醇池火迅速;采用冷却水保护储罐罐壁能够显著抑制池火的发展和热辐射;受燃液表面和火焰的破坏作用,灭火泡沫释放至燃液表面后需经历一定延迟时间才能对池火产生影响,该延迟时间随泡沫混合液供给强度的增大而缩短;推算出直径2...     

Burning behavior of sedan passenger cars

Four full-scale fire experiments using 4-door sedan passenger cars were carried out. The cars were ignited either at the splashguard of the right rear wheel or at the left front seat in the passenger compartment with a gasoline spill. The temperature inside the burning car and the mass loss rate were measured. The burning of the 4-door sedan was composed of three compartmental fires: the engine compartment, the passenger compartment, and the rear part inclusive of the fuel. In the experiments where ignition was initiated at the splashguard, the flame spread in the following order: to the rear part of the car, to the passenger compartment, and to the engine compartment. Breakage of the window glass markedly affected the spread of fire into the passenger compartment. The quantity of gasoline in the fuel tank also affected the speed of spread of the fire, because the gasoline ignited at an early stage of the fire. In the experiment where ignition was initiated in the passenger compartment, the fire gained force after the windshield was broken entirely. The flame spread in the following order: to the passenger compartment, to the engine compartment, and to the rear part of the car. The temperature within the passenger compartment peaked at 1000 °C. The heat release rate (HRR) curves showed several peaks depending on the burning of the three compartments. The HRR increased markedly when the fire spread to several different parts of the car at the same time. The HHR peaked at 3 MW when the passenger compartment and fuel (gasoline) burned simultaneously. The measured HRR curves were characterized by superposition of a Boltzmann curve and a Gaussian curve in order to obtain a model, which allowed us to make a more precise prediction of the fire spread probability from a burning car to nearby structures. The HRRs of burning cars were described by the sum of HRR from each compartment.     

Numerical simulation of the penetration capability of sprinkler sprays

Computational models have been utilized to investigate the penetration capability of sprinkler sprays directly above a fire source with respect to water flow rate, spray drop size, and spray momentum. The spray models are generated by assigning a representative drop size, mass flow rate, discharge speed, and discharge angle for each of 275 trajectories in such a way that they produce computed results which match the measured water flux distribution and spray momentum in the absence of a fire. The spray/fire plume interaction models are created by combining the spray models using a Lagrangian particle tracking scheme with free-burn fire plume models. Actual delivered densities and penetration ratios are computed through the interaction simulations at six flow rates, three fire sizes, and two ceiling heights. Drop sizes and spray momentum at two flow rates are increased by 25 and 50% from the original values without changing the other spray characteristics in order to investigate the effects of each parameter on penetration capability independently. The study indicates that there is an optimal flow rate for a given sprinkler that gives the highest penetration ratio within a practical flow range. It is also shown that increasing drop size is a much more effective way for obtaining a higher penetration ratio compared to increasing spray momentum.     

不同工况下细水雾灭火效能影响的数值模拟

采用FDS对单室火灾中细水雾与火焰相互作用过程进行数值模拟分析,探讨细水雾与火焰相互作用过程中不同区域的细水雾灭火机理,分析粒径分布、速度和雾化角度对细水雾灭火产生的影响.模拟结果表明:在细水雾与火焰相互作用过程中粒径分布对灭火效能影响显著;细水雾在粒径小于100 μm时不能实现有效灭火;当粒径为200～400 μm时细水雾能有效抑制火焰发展并熄灭火源;在细水雾灭火机理中,相对于气相冷却和隔氧窒息,细水雾的表面冷却作用起到主导作用;细水雾喷射速度对灭火效果影响较大,细水雾动量不小于火羽流动量是火灾发展得到有效控制的重要前提;细水雾有效雾通量随着雾化角度增大而逐渐减小,雾化角度增大不利于细水雾灭火效能提高.     

Experimental Study on Fire Behaviors of Kerosene/Additive Blends

The widely used fuels in practical are blended fuel whose combustion characteristics is more complex than those of the single-component fuel in real fire scenarios. The fire behaviors of aviation kerosene/diethylene glycol dimethyl ether (DGM) blends (R-D) and aviation kerosene/ethanol (R-E) blends were studied using a cone calorimeter. The parameters of pool fires, including the ignition time, burning rate, fuel temperature, heat release rate and combustion yield, were investigated. Janssens’ method was adopted to analyze the ignition times of the two blends. Two types of representative burning processes for blended fuel pool fires were identified. For R-D blends, the burning process is similar to that for typical pure fuels. The process for R-E blends, however, is novel, having two obvious burning processes due to the appearance of an intermediate decay stage. The fuel exhaust mass fraction (approximately 15%) was found to be almost constant throughout the intermediate decay stage. The fuel temperature during the experiment indicated that the liquid surface boiling temperature of R-D blends ranges from 162°C to 200°C depending upon the composition of these blends. For R-E blends, the initial boiling temperature is affected by the ethanol ratio, while the boiling temperature in the second process is equal to the boiling temperature of pure RP-3 kerosene. When the ethanol ratio is lower than 40%, the initial boiling temperature of R-E blends is approximately 120°C; when the ethanol ratio is higher than 40%, the boiling temperature is equal to the boiling point of ethanol. A method for calculating the burning rate of each component in the burning processes of the two blends is put forward, with the results agreeing well with the interpretation of the two burning processes. The ratio of the combustion yield CO₂/CO and the carbon conversion ratio increase with the oxygenated fuel ratio, indicating that the combustion is more complete when oxygenated fuel is added. These results will be useful for fire hazard assessment and firefighting in terms of fuel storage and transportation.     

The burning of large pool fires

A non-intrusive method for obtaining the spatial distributions of radiative properties (i.e.) absorption-emission coefficients and radiation temperatures) in pool fires is described. The method consists of measuring the lateral transmittance and radiance of the fire and performing an Abel inversion on the measurements to obtain absorption-emission coefficients and local radiation temperatures in the fire. Local radiative properties so obtained are used to calculate the flame radiation flux—the dominant heat transfer mode —to the pool surface. The computed flux is in good agreement with the flux inferred from experimentally measured burning rates of the fire. These experiments are performed on a 0.73 m diameter PMMA pool fire. The results presented here show that large pool fires tend to be significantly non-uniform in temperature and species concentrations and the non-uniformities play an important role in determining the burning rate of these fires.