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Fire extinguishing technologies of fine water mist containing additives have good application prospects. Additives that are efficient, nontoxic, nonpolluting, widely used, economical and reliable provide a reference for the application and promotion of fine water mist fire extinguishing systems containing alkali metal salts. In this paper, six potential alkali metal salt additives were selected: potassium carbonate (K 2 CO 3), sodium carbonate (Na 2 CO 3), potassium oxalate (K 2 C 2 O 4 ·H 2 O), potassium acetate (CH 3 COOK), potassium chloride (KCl), and potassium bicarbonate (KHCO 3); these additives were delivered into the fires as mists of their solutions at certain concentrations, the extinguishing time was recorded, and the minimum fire extinguishing concentration (MEC) of fine water mist containing alkali metal salt was tested by using a cup burner experimental apparatus. In addition, different types of alkali metal salts were compared to determine the anions and cations with better fire suppression performance levels. The suppression efficiencies of different ratios of additive packages for hydrogen-doped natural gas diffusion flames were studied to determine a specific ratio of additive packages. Based on the results of the experiment, in contrast to the pure water mist, the suppression efficiency of alkali metal salt greatly improved. The cation extinguishing ability was K+>Na+, and CO 3 2- and CH 3 COO− were better than other anions in alkali metal salts. An efficient compound additive was identified as 1.5% K 2 CO 3 + 0.5% CH 3 COOK. With increasing hydrogen addition, the MEC of fine water mist with additives increased. • The addition of alkali metal salts improved the suppression efficiency. • With the increase of H 2 addition, the MEC of fine water mist with additive increased. • An efficient compound additive was identified as: 1.5% K 2 CO 3 + 0.5% CH 3 COOK. [ABSTRACT FROM AUTHOR]

The aim of this paper was to study the suppression influence of water mist on oil pool fires, taking diesel fires and n-heptane fires as experimental objects. The effects of spray pressure and temperature on water mist suppression were examined, and an experimental platform for the suppression of water mist in a small space was set up. Their fire prevention performance and fire extinguishing mechanisms were analyzed by comparing the flame temperature and extinguishing time of diesel and n-heptane pool fire. Three types of spray pressure were set. Water mist was designed at different temperatures and design experiments were carried out for this purpose. The change process of smoke concentration, thermocouple temperature, and flame combustion under different working conditions were analyzed, and the factors affecting the fire extinguishing effect of water mist on oil pool fire were discussed. The results show that 20 °C water mist is more effective at medium and high pressure than at low pressure. Moreover, 80 °C water mist at 9 MPa is more effective in extinguishing n-heptane fire. The flame extinction time is about 10 s, which is more than 40 s higher than that of cold water. [ABSTRACT FROM AUTHOR]

In this work, a new type of compound additive and water mist compatible fire extinguishing method was designed, and the effects of its suppression on a 18650 LiMn204/Li(Ni0.5Co0.2Mn0.3)02 lithium-ion battery fire was investigated. To do so, a self-designed experimental platform was used to study the fire extinguishing capabilities and influence of the composite additives containing water mist on a lithium-ion battery fire. Several cell parameters were measured to evaluate the suppression effect of the fire extinguishing method, such as extinguishing time, maximum temperature, and heat release rate. The results show that both physical and chemical additives can play significant physicochemical roles in extinguishing fires and are more effective than pure water mist. The physical additives enhance the heat absorption and cooling as well as radiation heat barrier and oxygen asphyxiation mechanisms by reducing the surface tension and droplet size in the fog field. Furthermore, the chemical additives enhance the fire extinguishing efficiency by decomposing the active gases CO2 and H2O in the fire field and capturing the free radicals of the flame in the battery combustion reaction. [ABSTRACT FROM AUTHOR]

Single-fire and double-fire extinguishments by water mist were conducted to assess the similarities and differences. The double-fire extinguishment requires higher applied volume flux of the water mist and spray momentum, while it is achieved faster than the single-fire extinguishment, owing to the smaller size of the single flames of the double-fire. Both the air entrainment restriction among the two flames of the doublefire and the typical re-ignition phenomenon have a significant effect on the double-fire extinguishment. A double-fire merging from the burner surface is more difficult to be extinguished in terms of the critical operating pressure since the air entrainment restriction among the two flames of the double-fire is more intense. A double-fire with a smaller flame spacing requires a longer extinguishing time since re-ignition happens more easily. The equivalent diameter (De) is applied to calculate the plume-spray thrust ratio of the double-fire and evaluate the competition between the double-fire and the water mist quantitatively. © 2022 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved. [ABSTRACT FROM AUTHOR]

Summary: The suppression effectiveness of an intrinsically clean hybrid fire suppressant consisting of nitrogen and water mist was experimentally studied by a cup burner method with methane/air coflowing flame. A series of hybrid agents with different weight ratios of nitrogen and water mist (Sauter mean diameter: 12.6 μm) were tested and compared with nitrogen and water mist used alone. The minimum mass‐based extinction concentration (MEC) of neat nitrogen and water mist was measured as 32.1% and 15.2%, respectively. When combined, MEC of the hybrid agents fell between the limiting values of nitrogen and water mist used alone. An approximately linear relationship was shown for the suppression behavior of nitrogen and water mist in combination. The flow field of suppressants during the flame suppression process was studied by particle imaging velocimetry (PIV) technique. A small portion of water droplets near the flame front evaporated, which contributed to flame extinction by heat absorption and oxygen dilution. The remaining bulk mist was entrained vertically with the air flow, showing little effect on the flame base. [ABSTRACT FROM AUTHOR]

The effectiveness of suppression of lithium ion battery, LIB, fires with water mist was investigated experimentally using a previously developed bench-scale wind tunnel. The experiments were conducted on lithium ion cell arrays constructed from twelve cylindrical (18,650 form factor), fully charged, lithium cobalt oxide cells, which were densely packed in a rectangular configuration without gaps. An electric heater was employed to initiate thermal runaway in one cell. Failure propagation was then tracked using thermocouples located at the bottom surfaces of the cells. Heat release due to flaming combustion was computed based on the oxygen consumption method. Experiments conducted at 640 l min−1 of air, 320 l min−1 of air, and 186 l min−1 of nitrogen tunnel purge flow were used as reference points. All reference point experiments underwent complete cascading failure. Addition of 1.0 and 1.6 g s−1 of water mist to 320 and 640 l min−1 of air produced agent concentrations of 14.1 and 11.1 wt%, respectively, which are slightly above the concentration of water mist recommended for suppression of traditional fires. Application of water mist at 1.0 and 1.6 g s−1 prevented cascading failure in 40% and 50% of the tests, respectively, and significantly reduced the rate of failure propagation in the other tests. At both water mist delivery rates, flaming combustion associated with burning of battery ejecta was inhibited, reducing the combustion efficiency below 50%. One of the key findings of this study is that suppression of flaming combustion is not sufficient to stop cascading failure. The array must be continually cooled with water mist until the temperature of the cells decreases below a certain threshold, which prevents chemical reactions between battery materials inside the cell casings. [ABSTRACT FROM AUTHOR]

Water mist has a good three-dimensional cooling effect; however, its extinguishing effect on oil fires is less than that of foam extinguishing agents. This study aimed to enhance the gasoline fire extinguishing efficiency of water mist by adding KBr, Tween-80, and a dissolved CO 2 solution as multicomponent additives with different suppression mechanisms. The fire extinguishing time was measured to evaluate the extinguishing efficiency of different additives. The test showed that KBr mainly works near the flame base and makes an unstable gap between the flame base and the gasoline surface; the Tween-80 mist and dissolved CO 2 mist make the flame height lower under the test conditions. By analyzing the different extinguishing phenomena, temperature and efficiency of each additive, the multicomponent additives were proposed to enhance the extinguishing efficiency based on different mechanisms. The results showed the water mist with KBr, Tween-80 and dissolved CO 2 resulted in a remarkable improvement in the ability to extinguish a gasoline fire compared with pure water mist. [ABSTRACT FROM AUTHOR]

• Compared to pure magnesium dust, the flame preheating zone of the magnesium dust/fine mist water hybrids has widened, with the emergence of a red radiative zone. • The explosion products of the magnesium dust/fine water mist hybrids generate "cubic" oxide products with different structure from those of pure magnesium dust explosions. • This study established the explosion mechanism for the magnesium dust/fine water mist hybrid explosions, providing a research foundation for exploring the three-phase explosion of magnesium dust/fine water mist/hydrogen gas. The primary purpose of this paper is to deeply investigate the impact of water mist on the explosive characteristics of magnesium dust and to broaden the inerting and suppression strategies for magnesium dust explosions. The explosion characteristics and flame propagation behaviors of magnesium dust/water mist mixtures were examined in a 16.2-L visualized closed vessel. The explosion products were further analyzed using SEM and XRD. The results indicate that with the addition of water mist, the explosivity of low-concentration magnesium dust (≤600 g/m3) increases. The explosive strength of three higher concentrations of magnesium dust (800 g/m3, 1200 g/m3, 1600 g/m3) notably decreases, with the P max dropping by 69.86%, 54.08%, and 46.22% respectively compared to pure magnesium dust. The leading edge of the preheat zone exhibits red radiation, the fluctuation range of the instantaneous flame propagation speed decreases, and cubic particles appear in the explosion products. These changes are mainly attributed to the hydrogen produced in the explosion of the magnesium dust/water mist mixture. Under the combined effects of hydrogen and steam, the preheat zone significantly expands. Moreover, as the concentration of hydrogen increases, its presence weakens the suppressing effect of the water mist, manifested by the evident slowing of the average flame speed, the maximum flame speed, and the rate of decrease in explosive strength. The suppression of magnesium dust explosions by water mist mainly relies on physical actions such as heat absorption, blocking thermal radiation, and diluting oxidizer concentration. [ABSTRACT FROM AUTHOR]

Recent research has shown that acoustics can be used to suppress flames from a liquid fuel source. The results of these experiments indicated that acoustics alone are insufficient to control flames beyond the incipient stage. Recent research has also shown that variations in the delivery of water mist to a fire can enhance the mist’s efficiency. Therefore, the addition of acoustics to water mist may be an effective means of enhancing an established fire protection technology. For the first time, acoustics and water mist have been combined and studied as a flame suppression strategy. A series of experiments were conducted that explored the potential for coupling acoustics with water mist as means of flame suppression. Heptane fueled flames were created from two different sized ceramic fiber wicks: 30 mm × 50 mm with 5 mL of fuel, and 60 mm × 100 mm with 20 mL of fuel. The flames were then exposed to water mist delivered at a constant rate, which was found to be incapable of suppressing the flames. Next, low frequency sound waves at 62 Hz and 80 Hz were used to suppress flames from both wicks, with each frequency being generated by a different resonator. Finally, acoustics from both resonators were combined with water mist, and used to suppress flames from both wicks. The results showed that a combination of acoustic waves and water mist suppressed the flames more effectively than each individual technique on its own. This finding opens the possibility of developing more efficient ways to use water mist technology. [ABSTRACT FROM AUTHOR]

On account of the fire development rules with great complexity and uncertainty in hot and high humid confinement spaces, fire suppression effects and mechanisms by water mist of different pressures in confined space with one opening at surroundings of different temperature (40 o C/26.5 o C) and different humidity (RH 90% / 37.5%) were simulated by FDS. The flame and the smoke layer temperature, the heat radiation received by the floor and the oxygen near the flame were analyzed. It is concluded that under the experimental and simulated conditions in this paper, in hot and high humid environment, the flame cooling effect by the water mist is less significant than that in normal surroundings, and decreasing with the increase of the water mist pressure. The reason is the stronger perturbation to airflow near the flame root by the higher pressure water mist, so the average flame temperature decreases slowly. With the increase of pressure, the cooling effect on smoke layer is enhancing. At hot (40 o C) and high humidity (RH90%) environment, the cooling effect on smoke layer by water mist is weaker than that in normal temperature and humidity environment, but the gap is reduced with the increase of water mist pressure. The heat radiation attenuation effect is more significant in hot and high humid surroundings than that in normal environment, and increasing with the increase in pressure. With the increase of water mist pressure, the temperature and humidity of surroundings, the oxygen concentration reduction rate will be greater, and the oxygen suffocation effect will be more significant. The researches have important theoretical and application value for application of water mist fire suppression technology. [ABSTRACT FROM AUTHOR]

The presence of liquid fuel droplets in a flammable mixture causes cellular instabilities on the flame surface, which significantly enhances the flame speed when compared to the fully vaporized case. The prediction of the mechanisms responsible for the onset of cellularity for two-phase mixtures is essential to better understand spray combustion. The present study considers an innovative experimental strategy to isolate and investigate any potential mechanisms. The fuel droplets were replaced by inert water droplets in order to amplify the thermal sink effect, characterized by the absorption of part of the heat released by the flame, and to suppress the local enrichment of fuel formed around droplets. Spherical expanding flames with narrow-size distribution droplets were used and qualitative comparisons of the flame structure were performed with a shadowgraph system. The results have shown that the heat sink has no significant effect, whereas the local enrichment of fuel appears as a key phenomenon, which suggests that in the case of fuel droplet aerosols the onset of cellularities is triggered in the inhomogeneous part of the gaseous phase. [ABSTRACT FROM AUTHOR]

Transient simulations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow hydrogen diffusion flame suppressed by ultrafine water mist (UFM). As UFM was added incrementally to the oxidizer stream, the flame experienced a series of destabilization process, i.e., detachment, drifting and blowoff. The simulations predicted that the critical mass flow rate of 10-μm UFM was 6 g/min, which is in agreement with the value calculated by a perfectly stirred reactor model and the value measured by the experiments. The critical mass flow rate exhibited a plateau region as the diameter increased from 5 μm to 20 μm. The optimal diameter for UFM was ≈ 10 μm. A scrutiny on the extinguishing mechanisms reveals that both the chemical kinetic effect and latent heat play important roles in determining the optimal diameter in this configuration. For the chemical kinetic effect, water molecule inhibits the flame through 1) enhancing the chain-terminating reaction H + O 2 (+M) = HO 2 (+M) and 2) subsequently scavenging free radicals in the flame. An energy equation was used to investigate the relative importance of extinguishing mechanisms for UFM. It shows that the thermal cooling outweighs the chemical kinetic effect in terms of contributions to flame inhibition although the chemical kinetic effect is obviously enhanced compared with N 2 . [ABSTRACT FROM AUTHOR]

The influence of water mist on explosion of H 2 -air mixtures is studied experimentally in a closed channel with rectangular cross section. The investigated range of H 2 concentrations covers slow flames, accelerating flames, transition to detonation and detonations. Water loading ratios particularly relevant for severe accident scenarios in nuclear power plants are examined. A system for water mist generation and injection was designed such that the experimental conditions are well defined. The water loading ratio is determined by extinction measurement in each experiment. Laser diffraction measurements provide the droplet size distribution. Water mist injection results in lower overpressure and retarded transition to detonation compared to dry H 2 -air mixtures. The effect of a 0.11–0.12 kg/m 3 water loading ratio is comparable to a decrease in H 2 concentration by 0.6%vol in the deflagration regime and by 1–2%vol in the detonation regime, while transition to detonation is retarded more significantly. [ABSTRACT FROM AUTHOR]

Water mist has been recognized as an alternative of halogenated hydrocarbon fire suppressants due to its high thermal and chemical effects on the flame speeds. In the present study, the effects of fine water mist on laminar flame speeds of propane-air mixtures are investigated both experimentally and numerically. In experiments, the laminar flame speeds are measured using a single jet-plate configuration for the cases with and without water mist. The numerical simulation is also performed using the PREMIX and OPPDIF codes in CHEMKIN package. To include the phase change with evaporation, the evaporation process is assumed as a chemical reaction of which rate constant follows the Arrhenius law. The laminar flame speeds without water mist increase with stretch rate for all the equivalence ratios tested and are in fairly good agreement with the previous experiments. When the water mist is added, the numerical simulation predicts the positive dependence of flame speed on stretch rate similar to the case without water mist, whereas in the experiments the flame speeds decrease with stretch rate. In the stagnation flow field, the large radial acceleration of the flow induces the mist droplet accumulation around the stagnation stream line, leading to the negative dependence of flame speed on stretch rate. Numerical simulation reveals the thermal, dilution and chemical effects of water mist on laminar flame speed, and the chemical effect is found to be small but cannot be neglected. The water mist reduces the rates of chemical reactions involving the radicals such as O, H and OH, which have the positive sensitivity of flame speed. Furthermore, three-body chain terminating reactions involving H 2 O are enhanced. These reactions have large negative sensitivities of flame speed due to high chaperon efficiency of H 2 O. [ABSTRACT FROM AUTHOR]

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60 μm. When water mist is not added, the critical stretch rate at extinguishment is 439 s −1 as compared to the theoretical value of 460 s −1 . For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D 32 from 10 μm to 60 μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist. [ABSTRACT FROM AUTHOR]

Water mist, a replacement for Halon gaseous agents in fire fighting, has been studied for decades. However, the fire-extinguishing reliability of water mist is debated. For example, there are significant differences in extinguishing times between tests conducted under the same conditions, and water mists have difficulty extinguishing small fires. To date, no study of the probability distribution of extinguishing times has been reported. In this study a statistical analysis of the extinguishing time distribution of pool fires extinguished using water mist is presented. The fire sources were circular/square stainless steel pans with gasoline, diesel, ethanol or daqing RP-3 as fuel. Two types of extinguishing scenarios were observed. In one situation, the fire was extinguished via a blow off process, when the flames had not yet been suppressed. Flame cooling is the primary fire extinguishing mechanism; the mass loss rate and combustion heat of the fuel are two key factors. In the other situation, the fire was initially suppressed and subsequently extinguished after a long suppression stage. Surface cooling is the primary fire extinguishing mechanism; the flash point of the fuel is the key factor. [ABSTRACT FROM AUTHOR]

Abstract: Experiments in a real-scale room were done on water mist application to a pool fire. A fire produced with fuel oil in a 35cm cylindrical pool was used, with a heat release rate reaching 75kW in stationary conditions. Water application was studied with a nominal flow rate equal to 25l/min provided by a set of four nozzles, injecting droplets with mean Sauter diameter equal to . Observations of fire suppression in these conditions showed two behaviors, which were analyzed and detailed with the help of numerical simulations conducted with FDS.v5. On one hand, a fast suppression (about 10s required) was observed when water mist was applied to a developed fire. In this case, droplets were injected into a hot environment and thus evaporated strongly, generating a significant vapor concentration and resulting in a fast gas cooling and in an inerting effect. On the other hand, when the mist was applied early, fire growth was controlled, but its suppression required a longer application (about 1min) and only occurred after a significant cooling of the flame and the liquid pool. These two mechanisms were detailed numerically through mass and energy balances for both the gas and the liquid phases and could help to derive suppression model improvements. [Copyright &y& Elsevier]

Abstract: Intensification of pool fire will occur under the application of water mist. The phenomenon was discovered in previous studies; however, there has been no unified conclusion for the reason of this phenomenon. Studying mechanisms of the intensification has a very important significance in the engineering application of water mist. According to previous studies, the intensification can be divided into two categories, one is the fire intensification in gas flame; the other is the interaction of water mist with the heated fuel, which lead to the increase of mass loss rate. In this paper, Mechanisms of the second type of fire intensification are studied through theoretical analysis and experiment. Research shows that the primary mechanism of fire intensification by water mist is boil over. [Copyright &y& Elsevier]

Water mist for fire suppression and control has received considerable attention as one of the potential measures for replacing HALON. Information on volume mean diameter and coordinate component velocity of two types of water mist has been obtained by using LDA/APV system and the mechanism of fire suppression using water mist been explored with the aid of Cup Burner system. A series of experimental study has been conducted to observe the effectiveness of CH4/air fire suppression using water mist. The results show that water mist consisting of 10 ~ 100 jMin diameter droplets is an effective flame suppressant in a co-flow cup burner flame. The diameter and mass fraction of water mist significantly affect the temperature distribution of the methane CH4/air flame. The effect of oxygen dilution is more prominent in the upper part of the flame, whereas, endothermic heat exchange with the droplets plays a critical role in the lower half of the diffusion flame. [ABSTRACT FROM AUTHOR]

Abstract: Extinguishment of methane and polymethylmethacrylate (PMMA) diffusion flames by gaseous and water-mist fire-extinguishing agents has been studied experimentally and computationally using a cylindrical burner inserted downwardly into an upward coaxial air stream. Transient computations were performed for methane flames with full chemistry and a simple water-mist model to reveal the flame structure and suppression processes. For methane, as a gaseous agent (CO2 or N2) or water mist (≈35μm number mean diameter) was added incrementally to the air stream: (1) a diffusion flame enveloping a porous bottom surface of the cylinder extinguished first, resulting in a flame with its base (edge) anchored at the leading edge of the side of the porous cylinder; and (2) the flame base oscillated, detached, drifted downstream, and extinguished eventually. For PMMA, the flame base attached to the fuel surface more closely than methane flames until detachment led to blowoff. The volume fractions of CO2 or N2 in the oxidizer stream at detachment and extinguishment were independent (or mild functions) of the mean oxidizer velocity (U ox). In computations, the initial envelope flame extinction led to extinguishment at the limits between the measured detachment and extinguishing conditions. The measured water flow rate at extinguishment was independent of U ox; thus, the water mass fraction decreased dramatically with increasing U ox, suggesting that the water mist was transported to the flame zone independently of the air flow. The calculated water mass fraction at extinguishment, by contrast, decreased only mildly with U ox because water mist was modeled as a gaseous species. The computation of the near-limit flames revealed that: (1) extinguishment occurred when the peak temperature decreased to ≈1600K; (2) adding CO2 or H2O exhibited chemical effects on the flame structure; and (3) the stagnation-point-flow structure was nearly identical to that of a near-extinction counterflow diffusion flame. [Copyright &y& Elsevier]

Finely atomized water mist (diameter <100 μm) is a promising fire suppressant showing high effectiveness and no toxicity. The effectiveness can be improved further by mixing a small concentration of alkali compounds with pure water. The present work investigates the effectiveness of alkali compounds in counterflow diffusion flames of methane and LPG (38% propane, 62% n-butane by mass). In methane flames the inhibitors ranked by suppression effectiveness follows: KHCO 3 ≈ CH 3 COOK > K 2 CO 3 > K 2 C 2 O 4.H 2 O ≈ Na 2 CO 3 > NaHCO 3 > CH 3 COONa > NaC 3 H 5 O 3 > NaBr > C 4 H 5 NaO 6.H 2 O. The effectiveness of the inhibitors increases linearly with solution concentration. Only the potassium salts were tested in LPG flames, and the ranking obtained is CH 3 COOK > KHCO 3 ≈ K 2 CO 3 > K 2 C 2 O 4.H 2 O. The numerical models qualitatively capture the experimental trends; however, they underpredict the suppression effectiveness. Detailed sensitivity analysis and reaction pathway diagram analysis suggest that the rate constants of the recombination reactions, species entropy, and L-J collision diameters need careful consideration for improving the model predictions. [ABSTRACT FROM AUTHOR]

Based on the tubular burner, the burning velocities, flame stretch and inhibition rules influenced by co-flow water mists were studied using a high-speed schlieren system. Moreover, the variation rules of the flame critical extinction in our burner equipment were also obtained by analyzing the process and mechanism of flame extinction and inhibition. It is shown that the flame stretch is related to the fuel concentration, co-flow fluxes and water mist diameters. For droplets with a larger diameter, the smaller the co-flow fluxes, the more obvious the flame stretch. When the water mist loading rate is rather smaller, for fuel-rich premixed flame with Le>1, the flame with larger burning rate tends to backfire more easily. Under the same water mist conditions, for fuel-lean premixed flame with Led, the smaller the gas concentration, the easier the flame is extinct. [ABSTRACT FROM AUTHOR]

Experiments were performed to quantify the effects of droplet concentration and air velocity on the suppression and extinguishment of a boundary layer flame with ultrafine water mist (UFM). Unlike the traditional nozzles, UFM has very small droplet size (Sauter mean diameter, 3 µm) and forms laminar flow with low momentum. The results show that the UFM mass fraction needed to extinguish the flame decreases linearly by a factor of 7 as the air velocity is doubled. About 12% UFM is needed to extinguish the flame when the data are extrapolated to zero air velocity. Below the extinction concentration, the flame temperature is reduced slightly, but the local burning rate is suppressed significantly. In contrast, the spray-nozzle-mist (SMD range 15 to 50 µm, high momentum) enhanced the local burning rate at water concentrations below the extinction limit. Furthermore, the extinction concentrations are significantly lower with the spray-nozzle-mist than with UFM. The precise reasons for this are not clear but it may be due to better penetration of high-momentum droplets into the flame than the UFM. Unlike the UFM, the nozzle spray induces turbulence in the flame. Despite these differences, the UFM and nozzle mist data show that the extinction concentration exhibits a shallow minimum in the range of 25 to 40 µm as the droplet size is increased. [ABSTRACT FROM AUTHOR]

A mapping is presented of the temperature and monochromatic extinction coefficient at the base region of a small-scale heptane pool fire before and during a water mist addition. A methodology, coupling extinction measurements at two wavelengths, permits to separate the respective contribution of soot particles and water droplets to these extinction measurements. The primary objective of this work is to provide new information about the flame structure at the base of the flame, region which has an essential role in anchoring the flame and in its stabilization and extinction. The results obtained could also be used, in further work, as a data base for the prediction of radiant heat flux to the fuel surface. The detailed measurements made show that the customarily described structure of the steady flame does not exist longer when mist is added. Due to the rapid evaporation of the droplets and the subsequent expansion effect, the flame is totally disrupted. With a more or less regular frequency, it seems “explode” with random lateral projection of reactive pockets. During this phase of lateral expansion, the flame appears to get wider in its central zone with clearances from part or the whole of the liquid surface. The flame looks like an annular flame anchored to the pan edge. Clearances are followed by re-ignition but sometimes a full clearance by extinction, depending on the characteristics of the mist application. It is especially seen that water mist has primarily for consequence to cool the flame and to push water vapor onto the fuel surface but also to increase significantly the level of temperature and its fluctuation in this zone. This suggests that extinction is rather obtained by a rapid and total clearance of the liquid, than from the reduction of the burning rate. [Copyright &y& Elsevier]

In the present study, the enhancement of the liftoff and blowout of a propane/air coflowing diffusion flame by water mist was experimentally investigated in the subsonic range. When the fuel injection velocity U f is low, an attached laminar diffusion flame is stabilized at the burner rim with a premixed flame base. An increase of U f leads to the liftoff of the attached flame, and eventually to the blowout of the lifted flame. The upstream laminar flame disappears at the liftoff and a turbulent lifted flame is stabilized in the downstream region. The stability of the lifted flame deteriorates with the increase of the coflowing air velocity U a and the water mist flow rate Q m. The stable lifted flame can be achieved by the dynamic balance of the local mean gas velocity U g ¯ and the mean turbulent flame propagation speed U p ¯ of the premixed flame formed at base of the lifted flame, and U p ¯ was estimated by the measurements of U g ¯ by a laser Doppler velocimeter. U p ¯ decreases with Q m and is of the order of 1 m/s at the blowout independently of U a and the mass fraction of water mist Y 0. This value is nearly equal to 2.5 S L 0 , where S L 0 is the adiabatic laminar flame speed of a stoichiometric propane-air mixture. The liftoff height L increases both with U f and Q m. For the case without water mist, a unique correlation has been proposed in the previous studies between L and U f in a non-dimensional form using S L 0. By extending this correlation to the diffusion flame in the water-mist-laden air stream, the laminar flame speed influenced by water mist was appraised and compared with the simulation previously reported. Resulting flame speed reduction is 65% of that predicted by numerical simulation, presumably due to the partial evaporation of water mist. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries (LiBs) are a proven technology for energy storage systems, mobile electronics, power tools, aerospace, automotive and maritime applications. LiBs have attracted interest from academia and industry due to their high power and energy densities compared to other battery technologies. Despite the extensive usage of LiBs, there is a substantial fire risk associated with their use which is a concern, especially when utilised in electric vehicles, aeroplanes, and submarines. This review presents LiB hazards, techniques for mitigating risks, the suppression of LiB fires and identification of shortcomings for future improvement. Water is identified as an efficient cooling and suppressing agent and water mist is considered the most promising technique to extinguish LiB fires. In the initial stages, the present review covers some relevant information regarding the material constitution and configuration of the cell assemblies, and phenomenological evolution of the thermal runaway reactions, which in turn can potentially lead to flaming combustion of cells and battery assemblies. This is followed by short descriptions of various active fire control agents to suppress fires involving LiBs in general, and water as a superior extinguishing medium in particular. In the latter parts of the review, the phenomena associated with water mist suppression of LiB fires are comprehensively reviewed. [ABSTRACT FROM AUTHOR]