Burning rate of a pool fire with downward-directed sprays

The present study investigates how water sprays affect fire intensity, the burning rate of fuel and the relationship between droplet size and degree of water penetration. Downward-directed sprays which interact with a small-scale opposed gasoline pool fire are experimentally investigated in an open environment. It is shown that the burning rate of fuel is always greater under this opposed spray-fire plume arrangement compared to the freely burning condition, i.e. without water sprays, when fire extinction does not arise. These results imply that water sprays are able to enhance an oil fire. Furthermore, very small droplets are shown to be ineffective for fire extinction by cooling because they do not reach the fuel surface through fire plumes. Therefore, within a small-scale gasoline pool fire in an open environment, the mechanism of the fire extinction by water sprays is concluded to act via the cooling of the fuel surface, which will lead to the suppression of fuel evaporation, rather than the cooling of the fire plume itself.     

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.     

小尺度变压器油池火灾燃烧特性的实验研究

开展了直径20～50 cm 的圆形变压器油池火实验,分别测量并研究了火焰形态、燃烧速率、火焰轴心温度等燃烧特性。结果表明：在变压器油燃烧的3 个阶段中,稳定燃烧阶段持续时间最长,而衰减熄灭阶段要明显短于起始燃烧阶段。在燃烧速率方面,油池的直径越大,变压器油燃烧速率越快,燃烧时间越短,变压器油的液位下降速度也越快。变压器油的单位面积燃烧速率与经典辐射模型具有良好的一致性,单位面积燃烧速率与油池直径呈现幂函数形式。在火焰温度方面,油池中心线温度呈现距离液面的垂直高度越大其火焰温度越低的趋势,液面上方火焰温度最高可达700～800 ℃,同时油池火尺度对火焰最高温度的影响不大。     

Extinction condition of counterflow spray diffusion flame with polydisperse water spray

The effect of polydisperse water droplet size distribution on the burning behavior and extinction condition of counterflow spray diffusion flame was investigated experimentally in this study. N-heptane as liquid fuel spray and nitrogen as a carrier gas were introduced from the lower duct while water spray and oxidizer consisting of oxygen and nitrogen was issued from the upper duct. The burning behavior of spray flame for different fuel droplet size with and without water spray was observed and the extinction condition of counterflow spray diffusion flame was characterized by oxygen concentration at extinction. The results show that the minimum value of oxygen concentration at extinction for counterflow spray diffusion flame with water spray is similar to the extinction condition without water spray for higher mean droplet diameter of water. The minimum value of oxygen concentration at extinction shifts to the smaller fuel droplet size when decreasing the water droplet size. For fuel droplet size higher than 48 μm, the optimum of water droplet size for suppressing counterflow spray diffusion flame was smaller than gaseous flame. The explanation of optimum water droplet size based on the coupled effect of Stokes number and vaporization Damköhler number can be used for prediction of the effectiveness of water droplet on the suppression of counterflow spray diffusion flame.     

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.     

侧向喷射条件下细水雾扑灭水平表面流淌火研究

为探讨侧向喷射条件下细水雾扑灭流淌火的技术可行性,针对水平表面流淌火,在考虑火焰辐射、燃料与垫层之间的对流换热基础上,建立细水雾在开敞空间扑灭流淌火理论模型,实验研究细水雾喷头排数、倾斜角度对火焰形状、火焰温度、灭火机理和灭火时间的影响。结果表明：细水雾冲击燃料加快其流淌时可以增强灭火能力;采用单排喷头水平喷射时可有效抑制一侧火焰且对火焰拉伸作用最强;采用双排喷头水平喷射时,可有效抑制两侧火焰且灭火速度最快;当喷头向上倾斜15°时,细水雾冷却燃料能力减弱并使灭火时间变长。     

一种低压中速离心式水雾喷头的开发和实验研究

根据对喷头结构参数的理论计算，设计了一种低压中速离心式水雾喷头，并与目前常见的低压高速水雾喷头进行了灭火实验效果比较，结果表明新设计的水雾喷头能更好抑制和扑灭A类火、B类火及AB类混合火，并节省用水量。     

Experimental study on oscillating behaviour in a small-scale compartment fire

To investigate fire behaviour in a compartment, the effect of ventillation factor and fuel surface area was studied experimentally by using methanol as a fuel. A limit of stable combustion appeared at a small ventillation factor. As the fuel surface became larger, the stable combustion limit increased. Oscillating combustion was observed in both an unstable combustion region and a stable combustion region near the boundary of these regions. In the unstable combustion region, extinction followed the oscillating combustion. On the other hand, in the stable region the oscillating combustion lasted until the fuel was exhausted. The period of the oscillating combustion was 1·–1·5 s in the stable combustion region.     

Large-Scale Water Spray and Water Mist Fire Suppression System Tests for the Protection of Ro–Ro Cargo Decks on Ships

This paper summarises a series of large-scale fire suppression tests conducted to simulate a fire in the trailer of a heavy goods freight truck on a roll-on roll-off (ro–ro) cargo deck. The tests were conducted with a traditional deluge water spray system as well as a deluge high-pressure water mist system. Parameters such as the water discharge density, the system operating pressure, the nozzle K-factor and whether the fire was fully exposed to the water spray or shielded were varied. The total and convective heat release rate of the fire was measured in order to determine the fire suppression and fire control capabilities of the tested systems. Test results indicate that a water discharge density of at least 10 mm/min is necessary to provide fire suppression of a fire in a heavy goods freight truck, whilst 5 mm/min would provide fire control. Furthermore, the test results indicate that a high-pressure water mist system would require higher flow rates as compared to a traditional water spray system in order to provide fire control.     

Fine-spray (water mist) protection of shipboard engine rooms

Twenty-three fire tests were conducted to determine the ability of current fine-spray (water mist) technologies to extinguish fires specified in the International Maritime Organization (IMO) fire test procedure for engine rooms greater than 3000 m³. The tests were conducted using nozzle installed at a 5 m height and 1·5 m spacing in a large test facility (2800 m² area and 18 m height). Two types of nozzles were tested: a low pressure nozzle operating between 1·2 and 1·5 MPa with flow per nozzle between 12·0 and 13·41/m and a multi-nozzle high-pressure prototype consisting of seven nozzles operating at 6·9 MPa flowing at 5·3 1/m per prototype. These nozzles were selected because they had been shown to extinguish IMO fire tests in enclosures with a protected area of 83 m² and a ceiling height of 4·5 m. The fire tests selected from the IMO procedure included 6 MW diesel spray fires on top of the IMO engine mock-up, a shielded 6 MW diesel spray fire adjacent to the mock-up, a 1 MW shielded diesel spray fire adjacent to the mock-up, and a wood crib within a 2 m² pan filled with heptane. In tests in which no additional enclosure surrounded the nozzles other than the test facility, fires were not significantly affected by the water mist using either nozzle. To further investigate mist-system capabilities, a ceiling was then placed directly over the nozzles at a 5 m height covering an area of 188 m². Using 90 high-pressure prototype nozzles, the test fires were not extinguished. A 940 m³ enclosure was then formed by dropping tarpaulins to the floor from the ceiling. A 4 m² vent was placed in the wall. With the 90 high-pressure prototype nozzles, the 6 MW spray fire on top of the mock-up was extinguished. When the 6 MW fire was shielded beside the mock-up, the fire was not extinguished. With the vent closed, the 6 MW shielded spray fire was extinguished. Under the same test conditions, a 1 MW shielded diesel spray fire and a 0·1 m² heptane pool fire were not extinguished. The fire test results indicated that protection of engine rooms with volumes of about 1000 m³ is possible by optimizing current fine-spray technology while significantly larger volumes will require improved discharge characteristics.     

水喷雾雾化特性实验研究

对水喷雾的雾化特性进行分析,通过自主搭建有限空间实验系统测量不同影响因素下的喷雾粒径,进而分析喷嘴安设角度、喷嘴直径和喷雾压力对喷嘴雾化特性的影响规律。结果表明：喷嘴安设角度为0̊、10̊、20̊、30̊时,特征粒径D10、D50、D90 以及体积平均粒径与喷雾压力呈负相关、与喷嘴直径呈正相关,4 MPa 时的特征粒径值最小。综合分析可知,喷嘴安设角度为10̊时为最优安设角度。     

Physical scaling of water mist fire extinguishment in industrial machinery enclosures

Froude-based scaling relationships had previously been theoretically extended to, and experimentally validated in the laboratory for, water mist suppression of fires in open environment and in enclosures, which were shown applicable to gas, liquid and solid combustible fires. Before applying these relationships to real-world settings, their applicability needs to be further evaluated for the intended protection. This paper presents such an evaluation on scaling water mist fire extinguishment in an industrial machinery enclosure. In this evaluation exercise, a full-scale water mist protection set-up tested for a 260-m³ machinery enclosure was selected as the benchmark. A ½-scale machinery enclosure test replica was then constructed, together with a ½-scale nozzle whose orifices were geometrically similar to those of the full-scale nozzle. Spray measurements indicated that the ½-scale spray closely met the scaling requirements, in terms of discharge K-factor, water mist flux, droplet velocity and droplet size distribution. Two spray fires and one pool fire, which were scaled with the respective full-scale fires, were used to challenge the water mist protection in the ½-scale enclosure. At least five replicated tests were conducted for each of the four tested fire scenarios. Overall, the instantaneous local gas temperature and oxygen concentration measured inside the ½-scale enclosure for each fire scenario agreed reasonably well with those measured at the corresponding locations inside the full-scale enclosure, meeting Froude modeling's requirement that scalar quantities be preserved in different scales. The fire extinguishment times obtained from the ½-scale tests for each fire scenario were also statistically consistent with that observed in the corresponding full-scale test. Based on the obtained results, it is concluded that, for machinery enclosures and other similar occupancies, the previously laboratory-validated scaling relationships for water mist fire suppression can be used to determine the fire extinguishing performance of a full-scale water mist protection in a ½-scale test facility.     

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。     

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.     

AFFF 对航空煤油油池火的抑灭有效性研究

按照国家标准灭火试验的要求,采用不同的泡沫喷射率对不同尺寸航空煤油油池火开展了灭火效果试验,分析油盘尺寸、泡沫喷射率对灭火效果的影响,验证固定喷射率条件下,能够扑救的最大尺寸的油盘。结果表明：当油盘面积为0.8 m2 时,泡沫喷射率为2.9 L/min 时的灭火时间比为1.3 L/min 时缩短近60 s;当泡沫喷射率为1.3 L/min 时,最大可控制1.0 m2 航空煤油油池火的90%燃烧,未能将其完全扑灭;当泡沫喷射率为2.9 L/min 时,可完全扑灭2.0 m2 及其以下的航空煤油油池火。     

Gravimetric measurement of solid and liquid fuel burning rate near and at the low oxygen extinction limit

A precision mass balance coupled with a variable oxygen flow tunnel/nozzle enables us to measure the burning rate of solid and liquid fuels as a function of ambient oxygen percentage all the way to the extinction limit. Two sample configurations have been studied. The first is a liquid fueled wick flame (ethanol tea lamp). The total burning rate (mass/time) is measured as a function of wick length and oxygen percentage. Near the low oxygen limit, limit-cycle flame oscillation has been found that can last for many minutes to hours. The averaged mass burning rate of the oscillatory flame is about one-half that of the steady flame occurring at slightly higher oxygen. In the second configuration, local burning rate (mass/area/time) of poly(methyl methacrylate) spherical shell samples have been measured in the flame stabilization zone. Each sample has a different amount of heat loss and a different oxygen limit. The critical burning rate at their respective oxygen limits are different. This implies that critical burning rate is not a property of the material alone and it should not be used as the only criterion to judge the extinction or the ignition of materials.     

间接蒸发冷却用气-水雾化喷嘴特性实验研究

为了提高间接蒸发冷却器换热效率,提出了将气－水雾化喷嘴应用于间接蒸发冷却器改进换热器表面水膜的均匀性,实验研究了扇形两相(气－水)喷嘴在不同空气压力和水压下的特性,测出了不同压力时喷嘴雾化角和气水质量流量值,得出了其变化规律,确定出了最佳的喷雾气水压力比和雾化角等参数。     

细水雾系统参数对有障碍物液体火灾灭火效果的影响

利用FDS 数值模拟软件，采取控制变量法设置4 组数值模拟，分别研究细水雾系统水滴粒径、雾化角度、喷水强度、喷头高度对火源周围存在无法燃烧或尚未开始燃烧的障碍物情况下灭火效果的影响。研究发现，当火源设置为乙醇池火时，存在障碍物条件下，细水雾系统水滴粒径设置在250~400 μm，雾化角度设为120°，喷水强度设为2.0~2.5 L/（min·m2），喷头高度距障碍物1 m 时灭火效果较为理想。     

Interaction of a water mist with a buoyant methane diffusion flame

This work describes observations and measurements from the interaction of a fine water spray from a hollow cone nozzle, with purely buoyant diffusion flames from a natural gas ceramic-plate burner located directly underneath the nozzle. The burner plate was instrumented with thermocouples cemented on its upper and lower surfaces to assess the influence of the spray on the burner temperature. A set of thermocouples was also used to measure plume centerline temperatures above the burner plate. An imaging system was used to record the presence of droplets near the burner surface, and a narrow angle total radiation detector was used to measure changes in local flame radiation. A limited number of measurements of the steady state O₂ and CO concentrations along the plume centerline were also carried out.

For the conditions tested, the plume-to-spray thrust ratio was large, resulting in negligible direct penetration of the droplets into the fire region. A consequence of the low spray thrust was an almost droplet-free region above the flame. The observed cooling of the ceramic burner when the spray was applied was due to decreased radiant emission from the flame as well as deposition and evaporation of droplets entrained into the plume near the burner. The centerline plume temperatures did not change significantly upon application of the spray, at least within the error limits of thermocouple measurements. However, there was a significant decrease in O₂ and an increase in CO concentrations along the plume centerline upon application of the spray. An energy balance on the ceramic-plate burner, together with the experimental data, yielded estimates of the water deposition rate on the burner surface.     

在受限和通风防火分区内对燃烧速率特性的试验研究

在核安全研究框架内对在受限和通风防火分区内油池火燃烧速率进行了试验研究。在实体火灾试验基础上，此研究为在受限和通风火灾场景下的燃烧速率机理提供了新的信息。描述了在自由条件和空气受限条件下所进行的试验，对试验装置、仪器以及火源进行了详细叙述。在相同场景（0．4m^2TPH油池火）下，对自由条件和空气受限条件的试验情况下的燃烧速率进行了对比。在空气受限情况下，燃烧速率与时间的变化曲线显示出三个不同阶段：自由条件和受限燃烧速率相同；不稳定阶段，空气受限条件下燃烧速率高于自由条件下的燃烧速率;稳定阶段。从图像分析看，不稳定阶段显示，动荡和间歇火焰大大提高了燃烧速率。介绍了通风速率和油池面积对此现象的影响。试验结果为理解有限区域内燃烧速率提供了新的试验信息。