Extinction of counterfiow diffusion flames with halon replacements

Extinction of counter/low diffusion flames on liquid fuels was investigated, to confirm the superiority of the counterflow diffusion flame over the cup burner method for measuring flame extinguishing concentrations of fire suppressants, and to examine the fire suppression effects of halon replacements. The flame extinguishing concentration for the counterflow flame was less sensitive to the burner size than that for the cup burner method. Furthermore, the flow velocity of the fuel vapor had no change when the suppressant concentration in the oxidizer mixture of the counterflow diffusion flame was varied, whereas it changed remarkably in the case of the cup burner flame. The flame extinguishing concentrations of nitrogen, carbon dioxide, halon 1301 (CF₃Br), and three kinds of hydrofluorocarbons (HFC) and perfluorocarbon (FC) for n-heptane or ethanol counterflow flames were measured at various strain rates. Adiabatic flame temperatures at the extinction concentrations were calculated using the flame extinguishing concentrations measured for counterflow flames, assuming various equivalence ratios. The results suggest that HFC-23 (CHF₃) suppression exhibits a higher contribution to the chemical suppression effect than other HFC or FC.     

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.     

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.     

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 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.     

细水雾灭火机理探讨

本文研究了细水雾水滴直径与蒸发时间的关系,不同可燃物燃耗氧量的关系,并分析了水滴蒸发吸热量和水蒸汽分压增加对灭火效果的贡献,结合实验研究结果提出细水雾灭火的机理主要是水滴迅速汽化形成的水蒸气层阻碍了氧气向燃烧区域的扩散,而使可燃物燃烧耗尽局部区域氧气窒息熄灭。     

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.     

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.     

Modeling of SO(2) scrubbing in spray towers

The present article aims at developing simple realistic models in order to describe the gaseous removal process of SO(2) by absorption with and without chemical reaction in spray towers. Effects of droplet size, droplet velocity, superficial gas velocity, liquid flow rate and tower height on the performance of such a system are theoretically predicted. Model calculations bring out some very interesting facets of gas scrubbing as functions of droplet diameter and velocity. Four distinct regimes, viz. droplet lean, dense droplet, rigid droplet and droplet inertia controlling regimes, are found important in spray scrubbing process. Model calculation also elucidates the existence of rigid droplet (sphere) for a distinct droplet size at a specific droplet velocity. Theoretical considerations reveal that best performance can be achieved in the droplet inertia-controlling regime. Effect of turbulence on scrubbing is also considered for modeling. The model development and experimental data are limited to use of water-soluble alkaline scrubbing. However, the predicted values agree reasonably well with the available experimental data at lower gas and liquid flow rates for relatively smaller droplets. Models can also be applied to any gas-liquid spray absorption process subject to the assumptions and conditions necessary to describe the specific physico-chemical hydrodynamics of the system. However, incorporation of various droplet interactions can further refine the models for better prediction of removal efficiency.     

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.     

Acoustic extinction of laminar line-flames

A systematic study was conducted to elucidate the effects of acoustic perturbations on laminar diffusion line-flames burning in air, and to determine the conditions required to cause acoustically-driven extinction. Line-flames were produced from the fuels n-pentane, n-hexane, n-heptane, and n-octane using fuel-laden wicks. The wicks were housed inside a burner whose geometry produced line-flames that approximated a two-dimensional flame sheet. The acoustics utilized ranged in frequency from 30 to 50 Hz, and acoustic pressures from 5 to 50 Pa. Prior to acoustic testing, the unperturbed mass loss rates and flame heights were measured. These quantities were found to scale linearly, which is consistent with the Burke-Schumann theory. The mass loss rates associated with hexane-fueled flames experiencing acoustic perturbations were then studied. It was found that the strongest influence on the mass loss rate was the speed of oscillatory air movement experienced by the flame. It was also found that the average mass loss rate increased linearly with the increasing air movement speed. Finally, acoustic perturbations were imposed on the flames from all fuels to determine acoustic extinction criterion. To ascertain if the observed phenomenon was unique to the alkanes tested, flames fueled by JP-8 (a kerosene-based fuel) were also examined. Using the data collected, a model was developed which characterized the acoustic conditions required to cause flame extinction. The model was based on the ratio of a modified Nusselt number to the Spalding B number of the fuel. It was found that at the minimum speaker power required to cause extinction, this ratio was a constant (independent of the chemical nature of the fuel).     

Effect of initial conditions of modeled PDFs on droplet characteristics for coalescing and evaporating turbulent water spray used in fire suppression applications

Parametric studies were conducted for a coalescing and evaporating turbulent water spray using a stochastic separated flow technique that includes submodels for droplet dynamics, heat and mass transfer, and droplet–droplet binary collisions. While the initial droplet size distribution, in general, is not known due to the difficulty in the optical access to the nozzle exit region, the size distribution is modeled using the analytical PDFs (probability density functions) such as log-normal, Rosin–Rammler, Gaussian, and Nukiyama–Tanasawa distribution model. Standard deviation of the PDFs is varied and their effects on droplet size and speed distribution in the downstream are reported. The arithmetic mean droplet size at the nozzle exit, which is used as input for simulations, was extrapolated using the existing experimental data obtained at downstream locations.     

Pulsation Behavior of Pool Fires in a Confined Compartment with a Horizontal Opening

This study reveals the flame pulsation behavior of pool fires in a confined compartment with a horizontal opening. A series of tests were carried out in a small-scale compartment with a central horizontal opening to obtain parameters, such as the mass loss rate (MLR), oxygen concentration, flame height, flame pulsation frequency. Results showed that the average MLR in the steady burning period varied with the opening size, whereas the oxygen concentration value at extinction remained constant for larger horizontal opening size. The mean flame height as obtained by image processing was compared with the predicted correlation for different horizontal openings. The flame pulsation frequency was obtained by performing a fast Fourier transform on the flame height. The influences of the pool fire diameter and horizontal opening size on the flame pulsation frequency were considered. The empirical correlation with exponent of ?0.5 between the pulsation frequency and diameter was verified for confined compartments, as well as the effect of the horizontal opening size on the flame pulsation frequency. In addition, the flame pulsation frequency could be characterized by a scaling relationship between the Strouhal number and Froude number.     

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.     

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.     

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 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.     

Absorptance and transmittance of water spray/mist curtains

Water spray is a highly effective fire-protection agent, because evaporating droplets extract energy from the flame and combustion products as well as attenuate thermal radiation by absorption and scattering. The main objective of this paper is to characterize radiative transfer in a homogeneous water spray curtain (layer) and to calculate spectral transmittances and absorptances of a curtain. The spectral and total absorptances and transmittances reported in the paper provide fundamental understanding of radiation attenuation by water curtains. The empirical equation developed for the scaled spectral extinction coefficient of water sprays may be useful for modeling radiative transfer in spray curtains using CFD. Since the spray acts as a shield that attenuates radiation such global radiation characteristics can readily be used in simple, preliminary design analyses for mitigating and controlling fires.     

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。