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Interaction of water mists with a diffusion flame in a confined space
Fire Safety Journal 33 (1999) 129}139
Interaction of water mists with a di!usion #ame in a con"ned space Bin Yao*, Weicheng Fan, Guangxuan Liao State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, People's Republic of China Received 1 April 1998; received in revised form 29 March 1999; accepted 6 April 1999
Abstract This paper describes the study of the interaction of water mists with a di!usion #ame in a con"ned space with proper ventilation control. Water mist was generated by a single pressure nozzle and di!usion #ames 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 di!usion #ame in the con"ned 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 #ux were applied to the di!usion #ame in con"ned space. The poorer was the ventilation, the easier the suppression. The water mists had a more complex e!ect on the solid sample than the liquid, and a!ected the smoke release rate and movement. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Interaction; Water mists; Di!usion #ame; Con"ned space
1. Introduction The use of water mist for "re extinguishment and control is taken as one of the e!ective candidates for halon replacement, and much work has been done to develop this technique [1}22]. The water mist "re suppression systems have been reviewed
* Corresponding author. E-mail address: [email protected] (B. Yao) 0379-7112/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 9 9 ) 0 0 0 2 0 - X
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elsewhere [5}8]. The water mists, which have been de"ned as sprays which have 99% of the volume of water droplet less than 1000 lm in diameter [4], have di!erent suppression mechanism to that of the halon agent [9}21]. The extinguishing capacity is determined by the drop size distribution, spray location, spray momentum, enclosure geometry, obstructions within the space and the type of fuel [22]. It is very important to study the interaction of water mists with a #ame. Most #ames are di!usive, involving a variety of fuels [23,24], and occur in con"ned spaces. The study of the interaction of water mists with a di!usion #ame in a con"ned space will enhance the knowledge of such processes and be useful for developing the water mist "re suppression systems, improving the "re suppression and control e$ciency and extending their "eld of application. Various aspects of the interaction of water mists with a di!usion #ame in con"ned spaces have been investigated [9}14,29], but information on how water mists can enhance combustion and a!ect the smoke release rate and movement under some circumstances is currently very limited. The present work was carried out to study the mechanisms by which water mists suppress or enhance di!usion #ames in con"ned spaces, as well as the e!ect of water mists on smoke release rate from solid fuels. The Laser Doppler Velocimetry or Adaptive-Phase Doppler Velocimetry system (LDV/APV system) can be used to measure the velocity and drop size of the multiphase #ow simultaneously. The measurement techniques and the system con"guration have been described in detail before [25,26]. In this paper, the characteristics of the water mists were measured mostly with this system. The cone calorimeter can be used to measure the rate of heat release from a burning surface and analyze the combustion products when a constant #ow of air is provided into the con"ned space. The combustion characteristics such as heat release rate, combustion e$ciency, burning delay time and gas concentration can be determined [27]. This is a suitable apparatus in which to study the interaction of water mists with a di!usion #ame in a con"ned space as the combustion characteristics can be measured before and after the water mist actuation. Experiments were performed with the liquid and solid fuel samples, and the results compared with other similar experiments were possible.
2. Experimental apparatus The experiments were performed "rst with the use of the LDV/APV system, and the characteristics of the water mists were measured. The water mist was generated by using a single pressure atomizer. Two operating pressures were used, 0.3 and 1.2 MPa, respectively. The water #ow rates under the above pressures were 0.8 and 1.6 ml/s, respectively. The cone angle of the nozzle was 603 and the volume mean diameter of the mist was about 80 lm. The interaction experiments were performed in the 0.6 m;0.6 m;0.7 m glasswalled enclosure of the cone calorimeter shown schematically in Fig. 1. The fuel sample was located on the electronic balance, and there was a 20 kW/m radiant conical heater over the sample to keep the combustion stable. The combustion products were all collected by the hood and transferred for measurement and analysis. The rate of
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Fig. 1. Schematic of the experimental apparatus. 1 } Exhaust; 2 } Pressure and temperature measure; 3 } Gas sampling tube; 4 } Hood; 5 } Thermal video camera; 6 } Radiometer; 7 } Electronic balance; 8 } Fuel sample; 9 } Radiant conical heater; 10 } Water mist nozzle; 11 } Water tank; 12 } High pressure nitrogen; 13 } Pressure regulator; 14 } Pressure gauge; 15 } Blower.
#ow of fresh air into the con"ned space could be altered by means of the blower installed as part of the cone calorimeter. A radiometer was located 25 cm away from the #ame centerline, and thermocouples were arrayed along the centerline and radius to calibrate the Thermal Video System though it was only suitable for the solid surface. The Thermal Video System (TVS-2000) located 75 cm from the centerline was used to visualize the #ame thermal "eld. The water mists measured with LDV/APV system were injected downwards into the con"ned space with the nozzle placed 30 cm over the fuel sample. The small pool diameter was nearly 7.5 cm and the large one was 12.5 cm. The liquid fuel samples were both ethanol. One weighed 34 g, and the other weighed 100 g. The liquids were approximately 10 mm deep. The solid fuel sample was a 100 g piece of pine, with 10% water content. The surface area was the same as the 100 g liquid fuel sample. The system began to acquire data every 5 s after the automatic ignition started, and the water mists were injected into the con"ned space at 160 s after ignition. For the liquid fuel sample, the water mist generated under the 1.2 MPa pressure were applied continuously, while for the solid fuel sample, the water mists were applied for only 100 s to study the pyrolysis and gas"cation after the water mist application. All the raw data were saved and processed automatically by computer.
3. Results and discussion The characteristic parameters of the water mists determined their "re extinguishing capacity. Some APV results measured at 20 cm away along the nozzle axis, before
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Fig. 2. The typical characteristics of the water mists applied to the di!usion #ame in the con"ned space (1.2 MPa). The units of velocity and size are m/s and lm, respectively.
Fig. 3. The radial distribution of the axial-mean velocity of water mists.
water mist was injected into the con"ned space, are shown in Fig. 2 and the operation pressure was 1.2 MPa. Fig. 2A shows the relationship between the drop size and the nozzle axial velocity. Fig. 2B and 2C show the axial velocity and drop size histogram, respectively. The water mist was injected downward, while the positive axis in the measuring system was de"ned upward, so the velocity in Fig. 2 is negative. The absolute velocity values are shown in Fig. 3. Figs. 3 and 4 show the radial distribution of the axial mean velocity and volume mean diameter of the water mists under the 0.3 and 1.2 MPa, respectively. The mist velocity and diameter were relatively small, so the plume-to-spray thrust ratio was relatively large. To the di!usion #ame in the con"ned space with large plume-to-spray thrust ratio, the direct penetration of the droplet into the #ame region was negligible [16], and the volume #ux of the water mists mostly determined the
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Fig. 4. The radial distribution of the volume-mean diameter of water mists.
interaction process. Fig. 5 shows the di!erent interaction process with the di!erent water mist characteristics. Fig. 5A shows the thermal "eld visualization of the above large liquid fuel sample before the water mist application with the Thermal Video System, although the temperatures are not accurate. Fig. 5B and 5C show the thermal "eld visualization after the water mist application under di!erent pressures. The water mist #ow rate was 0.8 ml/s under 0.3 MPa pressure, and 1.6 ml/s under 1.2 MPa, respectively. The #ame was enhanced to a larger size under the lower volume #ux of water mists, while the #ame was suppressed signi"cantly under the higher volume #ux. The water mists enhance combustion through the evaporative expansion which enhances the mixing of the fuel vapor and air, while the water vapor interacts with the chain reactions to suppress soot formation, and hence reduce the radiant heat loss [28]. On the other hand, water mists suppress gas-phase combustion through cooling, oxygen displacement and the attenuation of thermal radiation. The two opposing mechanisms compete when the water mists are applied, which is more complex than the halon agents such as halon 1301. The combustion enhancement becomes insigni"cant when water mists with the proper characteristics such as adequate volume #ux are applied to the di!usion #ame in a con"ned space. Fig. 6 shows the heat release rate of the two liquid fuel samples for a period of approximately two-and-a-half minutes after activation of the water mist under the 1.2 MPa pressure. Before the water mist application, though the larger ethanol sample produced more heat than the smaller one, its heat release rate per unit surface area was lesser than the smaller one. When the water mist was applied, the heat release rate decreased immediately to a constant value under those conditions. The constant value decreased with the lower oxygen supply, lower radiant heat and an increased water mist volume #ux. The heat release rate of the larger sample decreased more quickly than the smaller one. Fig. 7 shows the carbon monoxide concentration of the ethanol sample products. The oxygen concentration of the combustion products shown in Fig. 8 decreased after ignition and increased quickly after the water mists were injected. The carbon monoxide concentration increased quickly when the water mists were applied. When the water mists were injected into the con"ned space, they
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Fig. 5. The thermal "eld visualization for the large liquid fuel sample before and after the water mist application under di!erent pressure.
absorbed the heat more quickly than large drops and slowed the combustion intensity so that the total oxygen consumption decreased. When the mists changed into vapor at the reaction zone, their volume increased by almost 1600 times, which decreased the fuel vapor and oxygen concentration in the #ame. Under these conditions, combustion was less e$cient and so more carbon monoxide was produced. The more oxygen that was required to support the combustion, the more easily the #ames were suppressed with water mists, and the more carbon monoxide was produced. These results are supported by direct measurements of oxygen and carbon monoxide concentration which have been made in the reaction zone with gas-sampling techniques [16]. The ventilation a!ected the water mist extinguishing capacity
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Fig. 6. Heat release rate of ethanol samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 7. CO concentration of ethanol sample products before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 8. Oxygen concentration of ethanol samples products before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
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Fig. 9. Smoke temperature of ethanol and pine samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 10. Fuel mass of the ethanol and pine samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
signi"cantly in the con"ned space. Under the present experimental conditions, the large scale #ames were more easily suppressed by the water mists than the small ones in the con"ned space with poor ventilation. The radiation heat #ux decreased after the water mist application because of not only the reaction rate decreasing, but also the radiant attenuation of the mists and their vapor [12,20]. The thermocouples located along the #ame centerline also indicated that the temperature decreased in the reaction zone and the thermal plume. The curves of smoke temperature (measured in the duct) vs. time are shown in Fig. 9. Experiments have also been made to compare the behavior of the solid fuel with the liquid during the water mist application period. Here, the rate of fuel mass loss was the proper parameter: the results are shown in Fig. 10. The two samples had the same weight, and were burnt under the same conditions. During the stabilized combustion before the water mist activation, both fuels lost mass at constant rates, but the solid sample lost mass more slowly than the liquid one. When the water mists were injected into the con"ned space, the #ame of the solid sample was suppressed at once.
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Fig. 11. CO concentration and smoke release rate of pine wood sample before and after the start of water mist application (160 to 260 s after ignition, discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Although the radiant conical heater was still working, the solid sample did not appear to lose any more mass. The application of water mist was stopped after 100 s (i.e. at 260 s) for the solid sample which then continued to lose mass under imposed radiant heater but at a lower rate, consistent with a smouldering process rather than #aming. It was found that much more carbon monoxide and smoke were produced, which would be harmful to man and the environment, and that re-ignition could occur. The carbon monoxide concentration and the smoke release rate of the solid sample before and after the application of water mist are both shown in Fig. 11. Initially, because a lot of mass was volatilized from the sample under the radiant heater before the vapors were ignited, the smoke release rate reached a peak and then decreased after ignition. During the period of water mist application, the carbon monoxide concentration increased and the smoke release rate decreased. The carbon monoxide concentration and the smoke release rate after the water mist application both increased with the power of the radiant heater and with the water mist application rate, but only within a certain range. Further work should be conducted to investigate the above phenomena. Because smouldering "res are less dependent on the oxygen concentration, the water mists could not slow down the reaction rate so easily, unless they were injected directly at the fuel. However, the water mists could prevent the transition from smouldering to #aming. The experiments were also made to study in#uences such as ventilation, spray angle, volume #ux, degree of obstruction and disturbance. In the con"ned space, proper spray angle and volume #ux would send the mists into the #ame and displace the oxygen to suppress the "res in a short time and with very little water. Obstructions within the apparatus were observed to a!ect the suppression capacity signi"cantly, especially on small scale "res, because the weaker buoyancy forces could not entrain enough mist into the #ame to suppress the "re. If the interaction of the #ame with mist was kept in a relatively unstable equilibrium, some physical disturbance perhaps could help extinguish the "re. For example, changing the spray angle or the #ow rate could break the balance and extinguish the "re.
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4. Conclusions The interaction of water mists with a di!usion #ame in a con"ned space was studied. In a con"ned space, fuel characteristics and ventilation condition signi"cantly a!ected the water mist suppression capability. If water mist with enough volume #ux was applied to the di!usion #ame, suppression played the dominating role. The lower the ventilation rate to the con"ned space, the more easily the "re could be suppressed and more carbon monoxide was produced. Water mist had a more complex e!ect on the solid fuel than the liquid, and more smoke and toxic gas would probably be produced during or after the water mist application. Future work will consider (1) various nozzle and fuel types, (2) various radiant heat #uxes, and (3) smoke components and movement during and after the water mist application. Acknowledgements The authors appreciate the support of this work by the `211 Project: 103-101a and National Nature Science Foundation of China (59876038) and would like to thank Mr. Fei Ge and Associate Prof. Jun Qin for their useful suggestion, discussion and necessary help. References [1] Rasbash DJ. The extinction of "res by water sprays. Fire Research Abstracts and Reviews 1962;4:24}53. [2] Alpert RL. Incentive for use of misting sprays as a "re suppression #ooding agent. Water Mist Fire Suppression Workshop Proceedings, NISTIR 5207, National Institute of Standards and Technology, Gaithersburg, 1993, p. 31}5. [3] Heskestad G. The role of water in the suppression of "res. J Fire Flammability 1980;12:254}62. [4] National Fire Protection Association, NFPA 750 Standard for Water Mist Fire Suppression Systems, NFPA, Quincy, MA, USA, 1996. [5] Jones A, Nolan PF. Discussions on the use of "ne water sprays or mists for "re suppression. J Loss Prevention Process Industries 1995;8(1):17}22. [6] Mawhinney JR, Richardson JK. A state-of-the-art review of water mist "re suppression research and development } 1996. Internal Report No. 718, National Research Council Canada, 1996. [7] Mawhinney JR. A closer look at the "re extinguishing properties of water mist. Proceedings of the 4th International Symposium on Fire Safety Science, International Association for Fire Safety Science, Boston, MA, 1994, p. 47}60. [8] Mawhinney JR. The role of "re dynamics in design of water mist "re suppression systems. Proceedings of the Seventh International Fire Science and Engineering Conference, 26}28, March, 1996, Cambridge England, p. 415}424. [9] Ames SA. et al. Cabin water sprays for "re suppression: an experimental evaluation. Civil Aviation Authority Paper 93009, Fire Research Station, UK, 1993. [10] Grosshandler WL. et al. Suppression within a simulated computer cabinet using an external water spray. Annual Conference of Fire Research: Abstracts, NISTIR 5499, National Institute of Standards and Technology, Gaithersburg, 1994, p. 75}76. [11] Chow WK, Cheung YL. Simulation of sprinkler-hot layer interaction using a "eld model. Fire and Materials 1994;18:359}79.
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[12] Yao B, Fan W, Liao G. Experimental study on the interaction of water mists with "res in the con"ned space. Proceedings of the Third Asia-Oceania Symposium on Fire Science and Technology, 10}12, June, 1998, Singapore. [13] Hadjisophocleous G, Knill K. CFD modeling of liquid pool "re suppression using "ne water sprays. Annual Conference of Fire Research: Abstracts, NISTIR 5499, National Institute of Standards and Technology, Gaithersburg, 1994, p. 71}2. [14] Kim MB. et al. Burning rate of a pool "re with downward-directed sprays. Fire Safety J 1996;27:37}48. [15] Alpert RL. Calculated interaction of sprays with large scale buoyant #ows. J Heat Transfer 1984;105:310}7. [16] Downie B, Polymeropoulos C, Gogos G. Interaction of a water mist with a buoyant methane di!usion #ame. Fire Safety J 1995;24:359}81. [17] McCa!rey BJ. Jet di!usion #ame suppression using water sprays * an interim report. Combust Sci Technol 1984;40:107}36. [18] Tamanini F. A study of the extinguishment of vertical wood slabs in self-sustained burning by water spray application. Combust Sci and Technol 1976;14:1}15. [19] Moghtaderi B. et al. E!ect of water spray on re-ignition characteristics of solid fuels. Proceedings of the 5th International Symposium on Fire Safety Science, International Association for Fire Safety Science, Boston, MA, 1997, p. 829}840. [20] Log T. Radiant heat attenuation in "re water sprays. Proceedings of the Seventh International Fire Science and Engineering Conference, 26}28, March, 1996, Cambridge England, p. 425}434. [21] Ravigururajan TE, Beltran MP. A model for attenuation of "re radiation through water droplets. Fire Safety J 1989;15:171}81. [22] Mawhinney JR. Characteristics of water mists for "re suppression in enclosures. Proceedings of Halon Alternatives Technical Working Conference, May 10}11, 1993, Albuquerque, NM, USA. [23] Fan W. An introduction to "re safety science. Hubei Science and Technology Press, 1993. [24] Drysdale D. An Introduction to Fire Dynamics, Second Edition. New York: Wiley, 1998. [25] Yao B, Qin J, Lino G. The hardware lay-out parameters to measure the large scale spray "eld by the 3D LDV/APV system and its application. J Univ Sci Techno China 1997;27(4):471}6. [26] Bachalo WD. Advances in spray drop size and velocity measurement capabilities for the characterization of "re protection systems. Proceedings of Water Mist Fire Suppression Workshop, March 1}2, 1993, Gaithersburg, MD, p. 75}92. [27] Redfern JP. Rate of heat release measurement using the cone calorimeter. J Thermal Analy 1989;35:1861}77. [28] Atreya A. et al. Basic research on "re suppression. Annual Conference of Fire Research: Abstracts, NISTIR 5499. National Institute of Standard and Technology Gaithersburg, 1994, p. 177}180. [29] Yao B, Liao G, Fan W. Experiment study on interaction of water mists with "res in a con"ned space. Fire Safety Sci 1997;6(2):48}52.
Interaction of water mists with a di!usion #ame in a con"ned space Bin Yao*, Weicheng Fan, Guangxuan Liao State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, People's Republic of China Received 1 April 1998; received in revised form 29 March 1999; accepted 6 April 1999
Abstract This paper describes the study of the interaction of water mists with a di!usion #ame in a con"ned space with proper ventilation control. Water mist was generated by a single pressure nozzle and di!usion #ames 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 di!usion #ame in the con"ned 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 #ux were applied to the di!usion #ame in con"ned space. The poorer was the ventilation, the easier the suppression. The water mists had a more complex e!ect on the solid sample than the liquid, and a!ected the smoke release rate and movement. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Interaction; Water mists; Di!usion #ame; Con"ned space
1. Introduction The use of water mist for "re extinguishment and control is taken as one of the e!ective candidates for halon replacement, and much work has been done to develop this technique [1}22]. The water mist "re suppression systems have been reviewed
* Corresponding author. E-mail address: [email protected] (B. Yao) 0379-7112/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 9 9 ) 0 0 0 2 0 - X
130
B. Yao et al. / Fire Safety Journal 33 (1999) 129}139
elsewhere [5}8]. The water mists, which have been de"ned as sprays which have 99% of the volume of water droplet less than 1000 lm in diameter [4], have di!erent suppression mechanism to that of the halon agent [9}21]. The extinguishing capacity is determined by the drop size distribution, spray location, spray momentum, enclosure geometry, obstructions within the space and the type of fuel [22]. It is very important to study the interaction of water mists with a #ame. Most #ames are di!usive, involving a variety of fuels [23,24], and occur in con"ned spaces. The study of the interaction of water mists with a di!usion #ame in a con"ned space will enhance the knowledge of such processes and be useful for developing the water mist "re suppression systems, improving the "re suppression and control e$ciency and extending their "eld of application. Various aspects of the interaction of water mists with a di!usion #ame in con"ned spaces have been investigated [9}14,29], but information on how water mists can enhance combustion and a!ect the smoke release rate and movement under some circumstances is currently very limited. The present work was carried out to study the mechanisms by which water mists suppress or enhance di!usion #ames in con"ned spaces, as well as the e!ect of water mists on smoke release rate from solid fuels. The Laser Doppler Velocimetry or Adaptive-Phase Doppler Velocimetry system (LDV/APV system) can be used to measure the velocity and drop size of the multiphase #ow simultaneously. The measurement techniques and the system con"guration have been described in detail before [25,26]. In this paper, the characteristics of the water mists were measured mostly with this system. The cone calorimeter can be used to measure the rate of heat release from a burning surface and analyze the combustion products when a constant #ow of air is provided into the con"ned space. The combustion characteristics such as heat release rate, combustion e$ciency, burning delay time and gas concentration can be determined [27]. This is a suitable apparatus in which to study the interaction of water mists with a di!usion #ame in a con"ned space as the combustion characteristics can be measured before and after the water mist actuation. Experiments were performed with the liquid and solid fuel samples, and the results compared with other similar experiments were possible.
2. Experimental apparatus The experiments were performed "rst with the use of the LDV/APV system, and the characteristics of the water mists were measured. The water mist was generated by using a single pressure atomizer. Two operating pressures were used, 0.3 and 1.2 MPa, respectively. The water #ow rates under the above pressures were 0.8 and 1.6 ml/s, respectively. The cone angle of the nozzle was 603 and the volume mean diameter of the mist was about 80 lm. The interaction experiments were performed in the 0.6 m;0.6 m;0.7 m glasswalled enclosure of the cone calorimeter shown schematically in Fig. 1. The fuel sample was located on the electronic balance, and there was a 20 kW/m radiant conical heater over the sample to keep the combustion stable. The combustion products were all collected by the hood and transferred for measurement and analysis. The rate of
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Fig. 1. Schematic of the experimental apparatus. 1 } Exhaust; 2 } Pressure and temperature measure; 3 } Gas sampling tube; 4 } Hood; 5 } Thermal video camera; 6 } Radiometer; 7 } Electronic balance; 8 } Fuel sample; 9 } Radiant conical heater; 10 } Water mist nozzle; 11 } Water tank; 12 } High pressure nitrogen; 13 } Pressure regulator; 14 } Pressure gauge; 15 } Blower.
#ow of fresh air into the con"ned space could be altered by means of the blower installed as part of the cone calorimeter. A radiometer was located 25 cm away from the #ame centerline, and thermocouples were arrayed along the centerline and radius to calibrate the Thermal Video System though it was only suitable for the solid surface. The Thermal Video System (TVS-2000) located 75 cm from the centerline was used to visualize the #ame thermal "eld. The water mists measured with LDV/APV system were injected downwards into the con"ned space with the nozzle placed 30 cm over the fuel sample. The small pool diameter was nearly 7.5 cm and the large one was 12.5 cm. The liquid fuel samples were both ethanol. One weighed 34 g, and the other weighed 100 g. The liquids were approximately 10 mm deep. The solid fuel sample was a 100 g piece of pine, with 10% water content. The surface area was the same as the 100 g liquid fuel sample. The system began to acquire data every 5 s after the automatic ignition started, and the water mists were injected into the con"ned space at 160 s after ignition. For the liquid fuel sample, the water mist generated under the 1.2 MPa pressure were applied continuously, while for the solid fuel sample, the water mists were applied for only 100 s to study the pyrolysis and gas"cation after the water mist application. All the raw data were saved and processed automatically by computer.
3. Results and discussion The characteristic parameters of the water mists determined their "re extinguishing capacity. Some APV results measured at 20 cm away along the nozzle axis, before
132
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Fig. 2. The typical characteristics of the water mists applied to the di!usion #ame in the con"ned space (1.2 MPa). The units of velocity and size are m/s and lm, respectively.
Fig. 3. The radial distribution of the axial-mean velocity of water mists.
water mist was injected into the con"ned space, are shown in Fig. 2 and the operation pressure was 1.2 MPa. Fig. 2A shows the relationship between the drop size and the nozzle axial velocity. Fig. 2B and 2C show the axial velocity and drop size histogram, respectively. The water mist was injected downward, while the positive axis in the measuring system was de"ned upward, so the velocity in Fig. 2 is negative. The absolute velocity values are shown in Fig. 3. Figs. 3 and 4 show the radial distribution of the axial mean velocity and volume mean diameter of the water mists under the 0.3 and 1.2 MPa, respectively. The mist velocity and diameter were relatively small, so the plume-to-spray thrust ratio was relatively large. To the di!usion #ame in the con"ned space with large plume-to-spray thrust ratio, the direct penetration of the droplet into the #ame region was negligible [16], and the volume #ux of the water mists mostly determined the
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Fig. 4. The radial distribution of the volume-mean diameter of water mists.
interaction process. Fig. 5 shows the di!erent interaction process with the di!erent water mist characteristics. Fig. 5A shows the thermal "eld visualization of the above large liquid fuel sample before the water mist application with the Thermal Video System, although the temperatures are not accurate. Fig. 5B and 5C show the thermal "eld visualization after the water mist application under di!erent pressures. The water mist #ow rate was 0.8 ml/s under 0.3 MPa pressure, and 1.6 ml/s under 1.2 MPa, respectively. The #ame was enhanced to a larger size under the lower volume #ux of water mists, while the #ame was suppressed signi"cantly under the higher volume #ux. The water mists enhance combustion through the evaporative expansion which enhances the mixing of the fuel vapor and air, while the water vapor interacts with the chain reactions to suppress soot formation, and hence reduce the radiant heat loss [28]. On the other hand, water mists suppress gas-phase combustion through cooling, oxygen displacement and the attenuation of thermal radiation. The two opposing mechanisms compete when the water mists are applied, which is more complex than the halon agents such as halon 1301. The combustion enhancement becomes insigni"cant when water mists with the proper characteristics such as adequate volume #ux are applied to the di!usion #ame in a con"ned space. Fig. 6 shows the heat release rate of the two liquid fuel samples for a period of approximately two-and-a-half minutes after activation of the water mist under the 1.2 MPa pressure. Before the water mist application, though the larger ethanol sample produced more heat than the smaller one, its heat release rate per unit surface area was lesser than the smaller one. When the water mist was applied, the heat release rate decreased immediately to a constant value under those conditions. The constant value decreased with the lower oxygen supply, lower radiant heat and an increased water mist volume #ux. The heat release rate of the larger sample decreased more quickly than the smaller one. Fig. 7 shows the carbon monoxide concentration of the ethanol sample products. The oxygen concentration of the combustion products shown in Fig. 8 decreased after ignition and increased quickly after the water mists were injected. The carbon monoxide concentration increased quickly when the water mists were applied. When the water mists were injected into the con"ned space, they
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Fig. 5. The thermal "eld visualization for the large liquid fuel sample before and after the water mist application under di!erent pressure.
absorbed the heat more quickly than large drops and slowed the combustion intensity so that the total oxygen consumption decreased. When the mists changed into vapor at the reaction zone, their volume increased by almost 1600 times, which decreased the fuel vapor and oxygen concentration in the #ame. Under these conditions, combustion was less e$cient and so more carbon monoxide was produced. The more oxygen that was required to support the combustion, the more easily the #ames were suppressed with water mists, and the more carbon monoxide was produced. These results are supported by direct measurements of oxygen and carbon monoxide concentration which have been made in the reaction zone with gas-sampling techniques [16]. The ventilation a!ected the water mist extinguishing capacity
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Fig. 6. Heat release rate of ethanol samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 7. CO concentration of ethanol sample products before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 8. Oxygen concentration of ethanol samples products before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
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Fig. 9. Smoke temperature of ethanol and pine samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Fig. 10. Fuel mass of the ethanol and pine samples before and after the start of water mist application (discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
signi"cantly in the con"ned space. Under the present experimental conditions, the large scale #ames were more easily suppressed by the water mists than the small ones in the con"ned space with poor ventilation. The radiation heat #ux decreased after the water mist application because of not only the reaction rate decreasing, but also the radiant attenuation of the mists and their vapor [12,20]. The thermocouples located along the #ame centerline also indicated that the temperature decreased in the reaction zone and the thermal plume. The curves of smoke temperature (measured in the duct) vs. time are shown in Fig. 9. Experiments have also been made to compare the behavior of the solid fuel with the liquid during the water mist application period. Here, the rate of fuel mass loss was the proper parameter: the results are shown in Fig. 10. The two samples had the same weight, and were burnt under the same conditions. During the stabilized combustion before the water mist activation, both fuels lost mass at constant rates, but the solid sample lost mass more slowly than the liquid one. When the water mists were injected into the con"ned space, the #ame of the solid sample was suppressed at once.
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Fig. 11. CO concentration and smoke release rate of pine wood sample before and after the start of water mist application (160 to 260 s after ignition, discharge rate is 1.6 ml/s, and air #ow rate is 24 l/s).
Although the radiant conical heater was still working, the solid sample did not appear to lose any more mass. The application of water mist was stopped after 100 s (i.e. at 260 s) for the solid sample which then continued to lose mass under imposed radiant heater but at a lower rate, consistent with a smouldering process rather than #aming. It was found that much more carbon monoxide and smoke were produced, which would be harmful to man and the environment, and that re-ignition could occur. The carbon monoxide concentration and the smoke release rate of the solid sample before and after the application of water mist are both shown in Fig. 11. Initially, because a lot of mass was volatilized from the sample under the radiant heater before the vapors were ignited, the smoke release rate reached a peak and then decreased after ignition. During the period of water mist application, the carbon monoxide concentration increased and the smoke release rate decreased. The carbon monoxide concentration and the smoke release rate after the water mist application both increased with the power of the radiant heater and with the water mist application rate, but only within a certain range. Further work should be conducted to investigate the above phenomena. Because smouldering "res are less dependent on the oxygen concentration, the water mists could not slow down the reaction rate so easily, unless they were injected directly at the fuel. However, the water mists could prevent the transition from smouldering to #aming. The experiments were also made to study in#uences such as ventilation, spray angle, volume #ux, degree of obstruction and disturbance. In the con"ned space, proper spray angle and volume #ux would send the mists into the #ame and displace the oxygen to suppress the "res in a short time and with very little water. Obstructions within the apparatus were observed to a!ect the suppression capacity signi"cantly, especially on small scale "res, because the weaker buoyancy forces could not entrain enough mist into the #ame to suppress the "re. If the interaction of the #ame with mist was kept in a relatively unstable equilibrium, some physical disturbance perhaps could help extinguish the "re. For example, changing the spray angle or the #ow rate could break the balance and extinguish the "re.
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4. Conclusions The interaction of water mists with a di!usion #ame in a con"ned space was studied. In a con"ned space, fuel characteristics and ventilation condition signi"cantly a!ected the water mist suppression capability. If water mist with enough volume #ux was applied to the di!usion #ame, suppression played the dominating role. The lower the ventilation rate to the con"ned space, the more easily the "re could be suppressed and more carbon monoxide was produced. Water mist had a more complex e!ect on the solid fuel than the liquid, and more smoke and toxic gas would probably be produced during or after the water mist application. Future work will consider (1) various nozzle and fuel types, (2) various radiant heat #uxes, and (3) smoke components and movement during and after the water mist application. Acknowledgements The authors appreciate the support of this work by the `211 Project: 103-101a and National Nature Science Foundation of China (59876038) and would like to thank Mr. Fei Ge and Associate Prof. Jun Qin for their useful suggestion, discussion and necessary help. References [1] Rasbash DJ. The extinction of "res by water sprays. Fire Research Abstracts and Reviews 1962;4:24}53. [2] Alpert RL. Incentive for use of misting sprays as a "re suppression #ooding agent. Water Mist Fire Suppression Workshop Proceedings, NISTIR 5207, National Institute of Standards and Technology, Gaithersburg, 1993, p. 31}5. [3] Heskestad G. The role of water in the suppression of "res. J Fire Flammability 1980;12:254}62. [4] National Fire Protection Association, NFPA 750 Standard for Water Mist Fire Suppression Systems, NFPA, Quincy, MA, USA, 1996. [5] Jones A, Nolan PF. Discussions on the use of "ne water sprays or mists for "re suppression. J Loss Prevention Process Industries 1995;8(1):17}22. [6] Mawhinney JR, Richardson JK. A state-of-the-art review of water mist "re suppression research and development } 1996. Internal Report No. 718, National Research Council Canada, 1996. [7] Mawhinney JR. A closer look at the "re extinguishing properties of water mist. Proceedings of the 4th International Symposium on Fire Safety Science, International Association for Fire Safety Science, Boston, MA, 1994, p. 47}60. [8] Mawhinney JR. The role of "re dynamics in design of water mist "re suppression systems. Proceedings of the Seventh International Fire Science and Engineering Conference, 26}28, March, 1996, Cambridge England, p. 415}424. [9] Ames SA. et al. Cabin water sprays for "re suppression: an experimental evaluation. Civil Aviation Authority Paper 93009, Fire Research Station, UK, 1993. [10] Grosshandler WL. et al. Suppression within a simulated computer cabinet using an external water spray. Annual Conference of Fire Research: Abstracts, NISTIR 5499, National Institute of Standards and Technology, Gaithersburg, 1994, p. 75}76. [11] Chow WK, Cheung YL. Simulation of sprinkler-hot layer interaction using a "eld model. Fire and Materials 1994;18:359}79.
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