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。     

Simulation of water mist suppression of small scale methanol liquid pool fires

The focus of this paper is on numerical modeling of methanol liquid pool fires and the suppression of these fires using water mist. A mathematical model is first developed to describe the evaporation and burning of liquid methanol. The complete set of unsteady, compressible Navier–Stokes equations are solved along with an Eulerian sectional water mist model. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modeled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol.The interaction of water mist with pulsating fires stabilized above a liquid methanol pool and steady fires stabilized by a strong co-flowing air jet are simulated. Time-dependent heat release/absorption profiles indicate the location where the water droplets evaporate and absorb energy. The relative contribution of the various suppression mechanisms such as oxygen dilution, radiation and thermal cooling is investigated. Parametric studies are performed to determine the effect of mist density, injection velocity and droplet diameter on entrainment and suppression of pool fires. These results are reported in terms of reduction in peak temperature, effect on burning rate and changes in overall heat release rate. Numerical simulations indicate that small droplet diameters exhibit smaller characteristic time for decrease of relative velocity with respect to the gas phase, and therefore entrain more rapidly into the diffusion flame than larger droplet. Hence for the co-flow injection case, smaller diameter droplets produce maximum flame suppression for a fixed amount of water mist.     

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 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.     

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.     

Suppression of fuel and air stream diluted methane–air partially premixed flames in normal and microgravity

The effects of fuel and air stream dilution (ASD) with carbon dioxide on the suppression of normal and microgravity laminar methane–air partially premixed coflow jet flames were experimentally and numerically investigated. Experiments were conducted both in our normal-gravity laboratory and at the NASA Glenn Research Center 2.2 s drop tower. Measurements included flame topology and liftoff heights of diluted flames, critical diluent mole fractions for flame blowout, and the radiant heat loss from flames. The flames were also simulated using an axisymmetric unsteady numerical code that utilizes detailed chemistry and transport models. In addition, counterflow flame simulation results were used to examine similitude between the counterflow and coflow flame suppression, and further characterize the effectiveness of fuel stream versus ASD on flame extinction. A smaller relative fuel stream dilution (FSD) extinguishes partially premixed flames (PPFs) with increasing premixing as compared to dilution of the air stream. Conversely, smaller ASD is required to extinguish PPFs as they become less premixed and approach nonpremixed (NP) behavior. Fuel stream diluted PPFs and air stream diluted NP flames extinguish primarily through a reactant dilution effect while fuel stream diluted NP flames and air stream diluted PPF are extinguished primarily by a thermal cooling effect. Normal gravity flames lift off and blow out with a smaller diluent mole fraction than microgravity flames. The difference between the fuel and ASD effectiveness increases as the gravitational acceleration is reduced. Radiation heat losses are observed to increase with increasing diluent mole fraction and decreasing gravity.     

A numerical study of water-mist suppression of large scale compartment fires

The focus of this paper is on simulating water mist suppression of fires in large enclosures. A two-continuum formulation is used in which the gas phase and the water-mist are both described by equations of the Eulerian form. The water-mist model is coupled with previously developed codes based on the multi- block Chimera technique for simulating fires. Computations are performed to understand the various physical processes that occur during the interaction of water-mist and fires in large enclosures. Droplet sectional density contours and velocity vectors are used to track the movement of water-mist and to identify the regions of the fire compartment where the droplets evaporate and absorb energy. Parametric studies are performed to optimize various water-mist injection characteristics for maximum suppression. The effects of droplet diameter, mist injection velocity, injection density, nozzle locations and injection orientation on mist entrainment and flame suppression are quantified. Numerical results indicate that for similar injection parameters such as mist injection density, injection velocity and droplet diameter, the time for suppression was smallest for the top injection configuration. Water-mist injection through the side walls, the front and rear walls and through the floor were found to be less efficient than the top injection configuration. These results are compared with our earlier predictions on water-mist suppression of small scale methanol pool fires and other experimental studies.     

A computational fluid dynamics study of wood fire extinguishment by water sprinkler

A Computational Fluid Dynamics (CFD) model is developed to predict extinguishment times of an array of wood slats by water sprinkler. The model predicts flow field, combustion of wood volatiles and radiation transfer. The gas-phase model is coupled with the wood pyrolysis model to predict a volatile release rate. A sprinkler water spray is modelled using a Lagrangian particle tracking procedure, coupled with the gas flow model by a Particle-Source-In-Cell algorithm. A simple model of instant droplet evaporation at the burning surface is employed.

The experimental program includes full-scale experiments in a fire gallery with a commercial sprinkler system installed in the roof. In some tests a water restrictor is used to vary the water flow rates. Water droplet size and velocity distributions are measured to serve as inputs to the spray model. A vertical array of wood slats is ignited uniformly in a slight draft of about 0·7 m/s. A few minutes after self-sustained burning is developed, the sprinkler is activated. Thermocouple and heat flux measurements in the vicinity of the flame, as well as a video record, are used to determine flame shape and to provide data for validation of the CFD model. Burning rates are measured by load cell and by CO₂ measurements.

Extinguishment happens primarily due to fuel cooling, which is indicated by long extinguishment times (two orders of magnitude longer than for plastic materials).

The predictions of burning rate and flame shape are reasonably accurate. Extinguishment times are modelled for different water flow rates. The dependence on water flow rate is found to be weak because the extinguishment process is controlled by the thermal time constant of the whole wood sample.     

含添加剂高压细水雾抑灭乙醇池火性能研究

为探究含NaCl、KCl、Na2CO3、K2CO3、NaHCO3、KHCO3六种碱金属盐以及SDBS、CAB35、APG0810三种起泡剂对高压细水雾抑灭乙醇池火性能的影响,在空间尺寸为6 m×6 m×4 m的受限空间内开展纯高压细水雾与含不同添加剂的高压细水雾对乙醇池火抑灭性能的对比实验。结果显示：相比纯高压细水雾灭火,添加碱金属盐与起泡剂的高压细水雾的灭火时间明显缩短,并且缩短的主要是处在火焰被撕裂抑制、火焰向周围游走阶段的时间;其他条件不变时,灭火时间随着添加剂浓度的增加先减小后增大,与纯高压细水雾相比,两类添加剂中各自灭火时间最短的分别为含3%KHCO3、3%SDBS的高压细水雾,可分别缩短灭火时间57.4%、64.6%;平均降温速率随着添加剂浓度的增加先增大后减小,降温速率最高的为含3%SDBS的细水雾,相较于纯高压细水雾提高了217.54%;含不同碱金属盐与起泡剂的高压细水雾对乙醇池火的抑灭性能各不相同,但与纯高压细水雾相比都得到显著提高。     

水喷雾系统抑制电动汽车火灾有效性实验研究

为解决现有消防手段难以有效扑灭电动汽车火灾的问题,设计了水喷雾隔热阻火系统和拖车式车载水喷雾灭火降温系统.对模拟电动汽车火灾进行灭火有效性全尺度实验.结果表明,在现有工况条件下,水喷雾隔热阻火系统能有效抑制车辆底盘的射流火焰,可防止火灾向相邻车辆蔓延,但受安装位置限制,该系统对驾驶舱内部火焰的抑制和降温效果较差,灭火后...     

A model for radiation evaluation and cooling system design in case of fire in tank farms

The effects on surrounding tanks of the heat radiated by a pool fire occurring over a storage tank of liquid hydrocarbons are studied by means of a solid flame model. The irradiation to the surroundings is calculated with a set of empirical correlations for flame parameters and by means of a complete 3D view factor model, implemented in a self-built code, which solves the view factor problem regarding the infinitesimal surfaces that describe the source and target surfaces. The calculation of irradiation on target surfaces is then applied to evaluate the local and overall amount of cooling water needed to prevent the fire from propagating to the surroundings. The cooling water, which protects the tanks facing the flame, is expected to be distributed as a film moving downward from a pipe around the top of the vessel, while tank top is equipped with foam systems. Once validated in terms of available literature data, the present model is applied to a series of test cases regarding different separation distances between tanks in order to evaluate the mass flow rates of water needed for cooling the target tank side for a variety of operating conditions.     

Interaction of water mists with a diffusion flame in a confined space

This paper describes the study of the interaction of water mists with a diffusion flame in a confined space with proper ventilation control. Water mist was generated by a single pressure nozzle and diffusion flames were produced from ethanol and pine samples, respectively. The LDV/APV system was employed to determine the water mist characteristics. The Cone Calorimeter was used to measure the heat release rate, oxygen and carbon monoxide concentrations and other important parameters of the interaction under various conditions. The test results showed that water mist suppressed the diffusion flame in the confined space through oxygen displacement, evaporative cooling and heat radiant attenuation, and enhanced the combustion through expansion of the mixture and chain reaction as well. Suppression played the dominating role when the water mists with enough volume flux were applied to the diffusion flame in confined space. The poorer was the ventilation, the easier the suppression. The water mists had a more complex effect on the solid sample than the liquid, and affected the smoke release rate and movement.     

含磷酸二氢铵细水雾灭火有效性研究

采用小尺度灭火实验,在标准受限空间下研究磷酸二氢铵添加剂不同添加量对细水雾扑灭汽油池火的有效性,分析磷酸二氢铵的加入细水雾灭火机理的不同。受限空间尺寸3 m×3 m×3 m,油盘直径33 cm,工作压力2 MPa。结果表明:加入磷酸二氢铵后,火焰温度明显降低,且大大缩短了细水雾的灭火时间;随着磷酸二氢铵浓度的不断增大,细水雾的灭火时间先减小后增大,存在一个最短的灭火时间。     

超细水雾影响小型火焰在固体可燃物上蔓延的数值模型

开发了CFD模型，用来预测细水雾中火焰沿固体燃料蔓延的特性。采用气体和固体燃料作为对比试验，利用不稳定两维保守公式描述自持性火焰蔓延情况。因为的分析重点主要放在火焰前锋火焰的熄灭机理（火焰前锋完全暴露在细水雾中），所以考虑了有限率（finite—rate）化学反应。火焰基本传播数据也可虑了用于细水雾和蒸气质量的公式，这包括水蒸发造成的能量消耗。还对在细水雾喷洒下聚合材料做成的厚燃料床上火焰的水平传播进行了试验。结果显示，火焰热释放区内自持性能量守恒对外部能量消耗非常敏感。在本试验中，主要是水滴蒸发造成的能量消耗。因此，在火焰前锋，火焰在细水雾的喷洒下挣扎着，要么按几乎相同的速度（没有水喷雾情况下）继续传播，要么被完全熄灭。本文还研究了水滴直径大于30μm细水雾的灭火特性。本文获得了在不同条件下火焰蔓延情况下，灭火需要的细水雾质量分率的关键浓度。     

A numerical study of the interaction of water spray with a fire plume

Water spray-based fire extinguishing equipment such as sprinklers has been widely used in fire suppression and control. However, the fire extinguishing mechanism in such devices is not well understood due to the complexity of the physical and chemical interactions between water spray and fire plume. Currently, quantitative approaches (e.g. numerical modeling) to estimate the performance and effectiveness of water spray systems have not been developed to a stage where they can be used to optimize the design for different operating environments and types of fire. In the present work, a numerical simulation approach is introduced to provide a quantitative analysis of the complex interactions occurring between water spray and fire plume. The effects of several important factors (namely water spray pattern, water droplet size and water spray flow rate) on the fire suppression mechanism are investigated. The simulations show that the water spray with solid cone pattern and finer water droplet size is more effective in extinguishing fires than the one with hollow cone pattern and coarse water droplet size. To suppress a fire, the water spray flow rate has to be more than a certain critical value. However, using too high water spray flow rate does not increase fire suppression efficiency but only leads to increased operational cost because of the excess water flow rate. In the current paper, the principles of fire suppression with water spray are also discussed, which are useful in designing more effective water spray fire suppression systems.     

Modeling of fire suppression by fuel cooling

Fire suppression with water spray was investigated, focusing on cases where fuel cooling is the dominant suppression mechanism, with the aim to add a specific suppression model addressing this mechanism in Fire Dynamics Simulator (FDS), which already involves a suppression model addressing effects related to flame cooling. A series of experiments was selected, involving round pools of either 25 or 35 cm diameter and using both diesel and fuel oil, in a well-ventilated room. The fire suppression system is designed with four nozzles delivering a total flow rate of 25 l/min and injecting droplets with mean Sauter diameter 112 μm. Among the 74 tests conducted in various conditions, 12 cases with early spray activation were especially considered, as suppression was observed to require a longer time to cool the fuel surface below the ignition temperature. This was quantified with fuel surface temperature measurements and flame video recordings in particular. A model was introduced simulating the reduction of the pyrolysis rate during the water spray application, in relation to the decrease of the fuel local temperature. The numerical implementation uses the free-burn step of the fire to identify the relationship between pyrolysis rate and fuel surface temperature, assuming that the same relationship is kept during the fire suppression step. As expected, numerical simulations reproduced a sharp HRR decrease following the spray activation in all tests and the suppression was predicted in all cases where it was observed experimentally. One specific case involving a water flow rate reduced such that it is too weak to allow complete suppression was successfully simulated. Indeed, the simulation showed a reduced HRR but a fire not yet suppressed. However, most of the tests showed an under-estimated duration before fire suppression (discrepancy up to 26 s for a spray activation lasting 73 s), which demonstrates the need for model improvement. In particular the simulation of the surface temperature should require a dedicated attention. Finally, when spray activation occurred in hotter environments, probably requiring a combination of fuel cooling and flame cooling effects, fire suppression was predicted but with an over-estimated duration. These results show the need for further modeling efforts to combine in a satisfactory manner the flame cooling model of FDS and the present suggested model for fuel cooling.     

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.     

Extinguishment of Cooking Oil Fires by Water Mist Fire Suppression Systems

A series of full-scale experiments were conducted in a mock-up commercial cooking area to study extinguishing mechanisms and effectiveness of water mist against cooking oil fires. The impact of water mist characteristics, such as spray angle, droplet size, flow rate, discharge pressure and type of nozzle, on the effectiveness of water mist against cooking oil fires was investigated. A series of oil splash experiments were also conducted to determine if the oil was splashed by water mist. In addition, the change in oil composition during heating and fire suppression was determined using Fourier Transform Infrared (FTIR) technique.The study showed that cooking oil fires were very difficult to extinguish, because they burned at high temperature and re-ignited easily due to changes in oil composition during heating and fire suppression. The water mist systems developed in the present work effectively extinguished cooking oil fires and prevented them from re-ignition. The spray angle, discharge pressure, and water flow rate were important factors to determine the effectiveness of water mist in extinguishing cooking oil fires.     

汽油池火热辐射量化模型预测准确性分析

分析池火热辐射半经验模型的预测精度随汽油池火油池直径的变化。运用MATLAB 分析了各组合模型热辐射通量预测值与实验值之间的误差,筛选出用于预测汽油池火的热辐射通量的最优组合模型。结果表明,采用本文拟合的汽油热辐射系数ηrad能提高固体火焰模型热辐射通量的预测准确性,采用经典固体火焰模型及本文提出的热辐射系数公式的组合模型为最优组合模型。当池火直径为0.3～22.3 m 时,最优组合模型的预测值与实验值的归一化均方误差低于0.05,该组合模型的预测准确度高于Mudan 模型与Shokri-Beyler 模型等其他半经验模型。     

Study of Water Droplet Behavior in Hot Air Layer in Fire Extinguishment

Evaporation of water droplets while traveling in hot air layer will be studied. The air-droplet system is analyzed by solving the mass, momentum and energy conservation equations for each phase. The droplet phase is described by the Lagrangian approach. Two conditions of air flow in the smoke layer are assumed. Firstly, as commonly used in modeling fire suppression by water spray, the smoke layer is assumed to be quiescent. Secondly, both gas cooling effect and air entrainment in the water spray cone are included. The properties of gas phase related to evaporation are specific heat capacity, thermal conductivity and dynamic viscosity. All these are evaluated by the one-third rule. The Runge–Kutta algorithm is used to solve the ordinary differential equation group for the droplet motion with heat transfer. Droplet positions, velocities, temperatures and diameters are calculated while traveling in the hot air reservoir. The effects of air temperature, water vapor mass fraction, thickness of hot air reservoir, and initial diameter on the droplet behavior are analyzed. The quantity of heat absorbed by a single droplet is calculated. Results are then calculated for a water spray by taking it has many droplets. The cooling effect of the water vapor produced is considered. Water spray consisting of small droplets should absorb more heat while acting on the hot air layer. The ratio of the heat for vaporization to the total heat absorbed by water can go up to 0.9 when all the droplets are evaporated. Limited experimental data are selected to verify the mathematical model. Predicted results are useful for studying fire suppression by water mist system.