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.     

Combustion properties of large liquid pool fires

Data from a number of large liquid pool fire experiments, recorded at the Fire Research Institute of Japan over more than a dozen years, were gathered and compiled. Burning rate and radiative outputs of gasoline, kerosene, and heptane were of primary interest. It is difficult to deduce the behavior of large fires from data on small fires because of the likelihood of decreasing combustion efficiency and heavy soot generation with resulting blockage of radiation. The data compiled here is possibly the largest body of data available for study on large pool fires.     

Heat transfer mechanisms in liquid pool fires

The heat transfer mechanisms in liquid pool fires were studied. The heat balance in the combustion system which includes both the liquid fuel and the burner vessel was discussed. The convective heat from the vessel wall to the fuel is dominant in small scale pool fires, and was calculated using the finite difference program. The heat flux between the wall and the fuel during flame spreading over the fuel below its flash point in the vessel of small width was calculated. From the results, it was found that the poorer the thermal conductivity of the vessel the larger the heat flux from the wall to the fuel immediately below the fuel surface. The liquid level at which a flame self-quenches was measured for various materials and wall thicknesses of the vessel. The concentration gradient of fuel at the self-quenching liquid level was found to be nearly constant for a given fuel.     

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.     

Study and prediction of boilover in liquid pool fires with a water sublayer using micro-explosion noise phenomena

An experimental simulation was conducted to investigate the mechanisms of boilover phenomena in liquid pool fires with water sublayers. Observations of fire behavior and instrumentation of the local temperature history in oil/water layers revealed that a typical boilover process can be divided into three phases, i.e. a quasi-steady period, premonitory period and boilover period. Our attention was mainly focused on the premonitory phenomena of boilover, which is generally considered as one of the most important factors leading to the occurrence of boilover. Experimental examinations demonstrated that boilover only happens after the fuel/water interfacial temperature has reached the boiling point of water, and that it was the violent seething of water at the interface which brought about boilover. The emission of micro-explosion noise, one of the most prominent premonitory phenomena of boilover was examined in detail and found to be a result of the water boiling. The investigation on the premonitory micro-explosion noise of boilover illustrated that it is a possible means for early and remote detection of the occurrence of boilover in liquid pool fires. However, in a real fire situation, the micro-explosion is always contaminated by the environmental noise. Hence, pattern recognition techniques should be used to differentiate the micro-explosion noise from the unwanted background noise, and a predictive model should be used to evaluate the status of oil burning and predict the occurrence of boilover. A set of noise features and a practical model have been presented for these purposes.     

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.     

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.     

Extinction of fires in hyperbaric chambers

Properties of water spray injected into compressed atmospheres were studied in order to design an effective fixed spray system for use in a hyperbaric chamber. Fire extinction tests were conducted in such atmospheres with water spray from single nozzles. Extinction tests with nozzles fixed at various locations in the pressure vessel will be reported in Part II. Severe tragedies in compressed and oxygen-enriched atmospheres increasingly used in medicine, space, and undersea research emphasize the necessity of the installation of a suitable fire extinguishment system.     

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.     

细水雾熄灭受限空间油池火主导机理分析

通过对细水雾熄灭油池火实验过程中温度场的分析，得出了池火火焰存在核心反应区、通风控制区和燃料控制区。通过对烟气成分和池火燃烧熄灭过程中的能量分析可知，细水雾吸热膨胀、稀释氧气和窒息对熄灭油池火所起的作用非常微弱。细水雾对油池火的作用过程可分为主导机理不同的两个阶段，初期主导机理为气相冷却和衰减热辐射，后期主导机理为表面冷却。     

Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments

In this paper we perform predictive simulations of liquid pool fires in mechanically ventilated compartments. We show that steady state burning rates are accurately predicted using a detailed model for the liquid phase heat transfer. The effect of lowered oxygen vitiation on the burning rate of pool fires is correctly captured. Simulations were done using the Fire Dynamics Simulator and the experiments considered were conducted in the OECD PRISME project. The main difference between the present study and previous simulation studies is the use of a detailed liquid evaporation model and the direct calculation of the vitiation and thermal environment interactions through the CFD solver.     

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.     

Interaction effects of multiple pool fires

What are the effects of flame behavior of a number of fires burning in close proximity to one another? The results of measurements of burning rates, heat feedback, flame height, and flame trailing are reported for fires involving liquid pools.     

Suppression of sodium fires with liquid nitrogen

Sodium has unusual fire hazards, including autoignition when heated in air or exposed to liquid water. Owing to limitations of existing suppression agents for sodium pool fires, suppression using liquid nitrogen (LN₂) is examined here. Sodium pools of 5–80 g were heated in stainless steel beakers. At about 290 °C, pool surface autoignition occurred and caused a rapid pool temperature increase. Vapor phase combustion occurred when the pools reached 320–450 °C, ultimately leading to pool temperatures up to 700 °C. For suppression tests, LN₂ delivery (at 2.7 g/s) began when the fires became fully-developed, near a pool temperature of 600 °C. Liquid nitrogen was found to be an effective suppression agent. The minimum amount of LN₂ required to suppress a fully-developed sodium pool fire was found to be about three times the initial sodium pool mass.     

Some aspects of structures of turbulent pool fires

The results of an experimental study of the burning of JP-5 and methanol pools emphasize structural differences between JP-5 and methanol fires as well as the importance of radiant feedback of energy to the pool surface in controlling rates of burning.     

水雾在扑灭大型工业燃油炊具油池火的应用

大型商用燃油炊具面积巨大，并涉及到大量热油，灭这种大型商用燃油炊具火灾非常困难。利用水雾系统对大型商用燃油炊具保护进行了研究，从理论和试验对水雾系统的灭火机理以度扑灭大型油池火灾需要达到的相关标准进行了研究。根据灭火机理和相关标准，设计了两套水雾灭火系统，进行了一系列的实体火灾试验。研究表明，两套水雾灭火系统成功地扑灭了大型油池火灾，有效预防了复燃。其灭火性能取决于水雾系统类型、喷射压力以及炊具罩的位置。     

Air entrainment and thermal radiation from heptane pool fires

An experimental study was made to explore the air entrainment into heptane pool fires and the smoke blocking effect on thermal radiation from fire. Heptane was burned in five different sized tanks 0.3 m, 0.6 m, 1 m, 2 m, and 6 m in diameter, and its burning rate, flame temperature, vertical hot gas velocity, and thermal radiation were measured. It was found that the total amount of air entrainment is about 5 times the stoichiometric air requirement. A smoke block effect appeared at tank diameter of 2 m or more.     

Extinction of fires by halogenated compounds — A suggested mechanism

The author suggests the halogenated compounds extinguish diffusion flames by promoting recombination of reactive oxygen atoms to form less reactive molecular oxygen; and he discusses data that tend to support his suggestion. Note: This paper is a contribution of the National Bureau of Standards not subject to copyright.     

Extinction of lithium fires: Thermodynamic computations and experimental data from literature

As described in the literature, various substances have been tried as extinguishants for metallic lithium fires. For many of these substances, the equilibrium temperature was computed from the reaction of the substance with hot (1040°C) lithium. For some of these substances, the computed equilibrium temperature was sufficiently low that the substances could be considered good extinguishants, and this was generally borne out by literature data. Some of the more promising substances for use as lithium fire extinguishants are silicon carbide (SiC), magnesium oxide (MgO), liquid argon, and boron nitride. Compounds found both by computation and in the literature to enhance combustion include carbon dioxide, Teflon^TM, Halon 1211, and carbon tetrachloride.     

The extinction of fires in aircraft jet engines — Part IV,extinction of fires by sprays of bromochlorodifluoromethane

The extinction of a liquid surface diffusion flame of kerosine burning in air has been studied using sprays of bromochlorodifluoromethane produced by swirl-type nozzles.