The Effects of Droplet Size and Injection Orientation on Water Mist Suppression of Low and High Boiling Point Liquid Pool Fires

Ndubizu, Chuka C.; Ananth, Ramagopal; Tatem, Patricia A.

doi:10.1080/00102200008947310

Cited by 48 publications

(20 citation statements)

References 19 publications

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“…Water mist suppression effectiveness is often quantified in terms of suppression in burning rate [19], reduction in burn-time [20] or reduction in the fraction of original sample burned [21]. In the current experiments with 20 cm long, 0.95 cm diameter cable only about 30 % of the sample is combustible.…”

Section: Resultsmentioning

confidence: 99%

Mechanism of Suppression and Extinguishment of Communication Cable Fire by Ultra Fine Water Mist in Cross-Flow

Ndubizu¹,

Ananth²,

Rouson³

et al. 2006

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Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Water mist fire suppression experiments were performed in cross-flow on bare (without outer jacket) communication cables to simulate a worst-case scenario. As fine water droplets are injected at low inlet velocities, an initial envelope flame that engulfed the circumference of the cable recedes and forms a wake flame stabilized behind the cable. At high mist concentration and/or high air velocity, the flame is extinguished by flame shrinking rather than by flame blow-off, as is the case in flat plate boundary layer flames. Ultra fine mist (UFM), with droplet diameter (d ~3 μm), is more effective in reducing the extinguishment time than the high-pressure spray nozzle mist (d ~ 20 μm), which introduces sprayinduced turbulence. Extinguishment proceeds rapidly after a threshold concentration is exceeded, and the threshold concentration decreases with air velocity. Finally, the copper cylindrical mesh, which is part of the cable, significantly enhances the effectiveness of UFM in flame extinguishment. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S)Office

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Section: Resultsmentioning

confidence: 99%

Mechanism of Suppression and Extinguishment of Communication Cable Fire by Ultra Fine Water Mist in Cross-Flow

Ndubizu¹,

Ananth²,

Rouson³

et al. 2006

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“…The Grashof number, Gr, is of the order of 2x10 8 based on an average temperature of 1500 K and the diameter of the outer tube. This is much bigger than the square of the injected air Reynolds number of 4 x 10 5 (Re=650).…”

Section: Theoreticalmentioning

confidence: 99%

“…Most previous fire suppression studies [2][3][4] with water droplets have considered droplets from sprinklers and high pressure spray nozzles [5][6]. Ndubizu et al [7] conducted experimental studies of water mist suppression of co-flow, methane diffusion flames formed in a slot burner (Wolfhard-Parker burner) and for pool fires [8]. They used high-pressure nozzles to form mists with Sauter mean diameter (SMD) of 30 to 66 μm.…”

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confidence: 99%

Extinction Dynamics of a Co-flow Diffusion Flame by Very Small Water Droplets Injected into the Air Stream

Ananth¹,

Mowrey²

2008

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Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Computations are performed to examine the effectiveness of mono-disperse water droplets in extinguishing a co-flow, propane diffusion flame by injecting the droplets into the air stream. The calculations show that the droplets entrained into the reaction kernel at the flame base are crucial for extinction. The reaction kernel detaches from the burner rim and blows-off when the droplet concentration is increased to a critical value (extinction concentration). At the critical value, the maximum chain-branching reaction (H2+O=OH+H) rate in the reaction kernel was found to be reduced by a factor of 5 in our computations. A large decrease in the reaction rate indicates that the maximum heat generations rate and Damkholer number are too low to sustain the flame, and cause the flame blow-off. Large drops are more effective than small drops, and the extinction concentration of water increases from 10.5% to 15% by mass as the size is reduced from 32 to 4 micro m. As the droplet size is increased further (>32 micro m), the trend will reverse as the evaporation rates get too small despite increased penetration of flame core by the drops. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S) 28-07-2008Memorandum Report

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“…Computational simulation, based on a hybrid Eulerian-Lagrangian formulation for gas and droplet phases, was performed for the analysis of effects of water droplets in extinguishing diffusion flames and the thermal effects, mainly through the latent heat of evaporation, were shown to significantly influence flame extinguishment [8][9][10][11]. The effect of thermal cooling and oxygen dilution on suppression of the large scale pool fires were reported qualitatively [12,13].…”

Section: Introductionmentioning

confidence: 99%

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

Yoshida¹,

Uendo²,

Takasaki³

et al. 2011

Fire Safety Science - Proceedings of the 10th International Sym

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The behavior of fine water droplets was investigated in a methane-air counterflow diffusion flame, specifically when the water droplets were added to the inlet air feed and used to extinguish the diffusion flame. The water droplets studied had a relatively broad size distribution (from 1 to 60 m), with the number mean diameter of 15 m and Sauter mean diameter of 25 m. With an increase in the velocity gradient, the flame approaches towards the stagnation plane and flame thickness decreases. The flame properties, such as flame location and flame thickness are insensitive to the water mass loading. Along the stagnation stream line, the velocity decreases towards a local minimum just before the flame front. In contrast, the velocity increases in the flame zone due to the thermal expansion, and then decreases towards the stagnation point. In the decelerating zone, the droplet mass flux decreases on approach to the flame zone due to a non-equilibrium in velocity of large droplets between liquid and gas phases. Furthermore, as the droplet-laden air approaches the flame front, the flow stream begins to diverge in the counterflow field. Since the small droplets move away from the burner axis, the divergence of the air flow acts to reduce the water droplet mass flux along the stagnation streamline. Large droplets moving faster than the air flow "catch up" to slower ones just before the flame front, and the mass flux of the droplets tends to increase, resulting in the droplets accumulating just before the flame zone. Measurements of the velocities of individual droplets show that large droplets move faster than the small droplets which are in equilibrium (in velocity) between liquid and gas phases. Droplet behavior was found to be controlled by the Stokes number. Equilibrium in velocity is re-established just in front of the flame zone. Closer to the flame front, evaporation occurs and the mass flux of the droplets decreases drastically. On the other hand, thermophoresis acts on extremely small droplets to move them along the steep temperature gradient and these droplets are forced from the high to low temperature regions against the convection velocity. The thermophoretic velocity was estimated in the present study to be compared with the measured convection velocity and it was concluded that the thermophoretic effect is negligibly small over all sizes of droplets.

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The Effects of Droplet Size and Injection Orientation on Water Mist Suppression of Low and High Boiling Point Liquid Pool Fires

Cited by 48 publications

References 19 publications

Mechanism of Suppression and Extinguishment of Communication Cable Fire by Ultra Fine Water Mist in Cross-Flow

Mechanism of Suppression and Extinguishment of Communication Cable Fire by Ultra Fine Water Mist in Cross-Flow

Extinction Dynamics of a Co-flow Diffusion Flame by Very Small Water Droplets Injected into the Air Stream

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

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