INTRODUCTION Droplets, I.E

LIMITATIONS OF WATER MIST AS A TOTAL FLOODING AGENT Petra Andersson* and Göran Holmstedt Department of Fire Safety Engineering Lund University Box 118 S-221 00 Lund, Sweden ABSTRACT The limitations of water mist in acting as a total flooding agent are discussed in this paper. The paper focuses on water mist consisting of droplets 1-100 μm in diameter. Water mists are very efficient as an extinguishing media if they are present in the flame. There are, however, several problems associated with introducing the water mist into the flame: the spray reaches only a short distance; small water droplets have a very short lifetime; droplets coagulate into larger droplets; large droplets hit obstacles such as walls and therefore have difficulties in being dispersed throughout the protected volume. INTRODUCTION droplets, i.e. sprays where 90% of the droplets are less than 0.2 mm and 10% less than 0.1 mm. Recent experimental and theoretical work on the Lefebvre’ calls droplets in the region of 70 [Lm a extinction of fires with fine water sprays indi- mist. This paper focuses on water mist consisting cates that water mist may have a wider applica- of droplets 1-100 >m in diameter. tion as an extinguishing agent than previously anticipated. The phase out of Halon necessitates When discussing water mist as a possible total the development of alternative means of active flooding agent, several aspects must be consid- fire protection, and water mist might be a possi- ered. One aspect is, of course, the capability of ble substitute. However, as pointed out by Jones water mist to extinguish a fire, if the mist is and Nolan/ few coherent experimental programs present in the flames. Another aspect is the pro- have been initiated for the direct study of the duction of the mist in the entire protected volume extinguishing effectiveness of sprays of various and its capability of remaining in the air for a properties for given scenarios. Most experimen- certain period of time. Producing mist in the tal programs have been studying a certain mist entire protected volume usually requires several system against specific scenarios. The complex nozzles. To reach all areas in the volume, a high relationships between extinguishing capacity momentum is required and the easiest way to and drop size distribution, spray location, spray produce a high momentum is by the use of mists momentum, room geometry, obstructions in the containing large droplets. Large droplets do not, room and fire induced flows, have not been inves- however, follow the air stream pattern in the tigated thoroughly. It is, therefore, very difficult room and tend to hit obstacles such as walls, etc. to develop general design rules for water mist Keeping the mist in the air for a certain period systems. In this paper, the limitations of water of time is difficult as the droplets evaporate and mist in acting as a total flooding agent are coagulate. In addition, the air-water mist is discussed. lighter, or heavier, than air, depending on the mixture ratio. Radiation from the fire increases An immediate problem when discussing &dquo;water the evaporation rate. All these processes are dis- mist&dquo; is to define a water mist. According to cussed below. Experimental and theoretical NFPA,2 water mist consists of droplets with a studies have been undertaken to throw further diameter less than 1 mm which are separated light on the question, &dquo;is it possible for water into three classes, where Class 1 has the finest mist to act as a total flooding system?&dquo;. *Current address: Swedish National Testing and Research Institute, Box 857, 501 15 Borås, Sweden 31 WATER CONCENTRATION ~ abstraction of heat per unit volume (pure thermal phenomenon) REQUIRED FOR FLAME EXTINGUISHMENT ~ the transport of droplets to all parts of the flame where burning is taking place. Water is very efficient as an extinguishing media per unit weight4-7 when it is present in a flame. Both factors to a on Experimental studies and thermal extinction are, large extent, dependent the spray properties, especially on the drop size theory suggest that water is about 2 times as distribution and the ability to distribute and efficient per unit weight as Halon 1301, i.e., maintain a high water content in the whole vol- water is almost as efficient as dry powders of ume of a fire room. high quality.6 Other phenomena influencing fire extinction with water mist are the blowing effect, Other Extinction Phenomena radiation shielding and surface cooling. Blowing’ is one extinction phenomenon that can Thermal Extinction Theory contribute to the extinction process when using water mist. Another of water mist that Flame extinguishing mechanisms are effectively aspect should be mentioned is its radiation shield- explained by thermal quenching concepts in com- great bination with the flame heat balance .4-8 By ing capacity, which can considerably reduce the rate at which a fire When assuming a limiting adiabatic flame temperature spreads. electromag- netic such as of 1550 K, the concentration of water mist radiation, heat radiation and light, air such as required to extinguish a premixed stoichiometric passes through containing particles, soot, water the are propane-air flame can be calculated to be approx- droplets, etc., photons absorbed and scattered. The and scat- imately 280 glml room volume. Usually there is absorption coefficients are on the a difference of 1.5 to 2 between the demand of tering dependent complex refractive index of the their size and the extinguishing media in diffusion flames as particles, . the of the radiation.10 A esti- compared to premixed flames, i.e., a propane dif- wavelength rough mate of the overall can be obtained fusion flame would require about 140-190 g of absorption the area of the with that water mist per cubic meter room volume. If, by comparing droplets of the radiation which the are instead, water vapor at 100°C was used, the through droplets water demand would be approximately doubled. passing. For instance, a concentration of 300 of 1 will almost The figures given above assume the water mist g/m3 >m droplets completely absorb the incident radiation within a 2 mm is completely evaporated and the vapor is heated thick water mist. In Table similar estimates to the adiabatic flame temperature. 2, are presented for different water concentrations sizes. For water to be as efficient as discussed above, and droplet The radiation shielding by water in the droplets must be small enough to be com- vapor phase is strongly wavelength The is pletely evaporated while they are within the dependent. vapor completely transparent flame. Droplet evaporation is discussed later in at some wavelengths while at others there is this paper. The distance between the droplets strong absorption.&dquo; must also be small enough to stop the flame prop- agating between the droplets. The estimate of the One important extinguishing and fire spread distance required - the quenching distance - is prevention factor is surface cooling. Surface cool- is not usually 1-2 mm. In Table 1, the distances ing effective when the fuel is already gas- between the droplets in water mist of different eous and usually requires somewhat larger drop- to wet the those discussed in droplet sizes and water contents are presented. lets surface than this This shows that, in order to stop a flame propa- paper. gating between the droplets, droplets of less than 100 J.1m are required in mists containing 150-300 g water/m3. EXPERIMENTAL MEASUREMENT OF FLAME EXTINCTION As a summary, the major factors which deter- mine the ability of a water spray to extinguish The most common methodology for measuring a diffusion flame are: the efficiency of different extinguishing agents 32 Table 1. Distances Between Droplets in mm for Different Droplet Sizes and Different Water Contents Table 2. Water Mist Thickness Required to Absorb Almost All Radiation in diffusion flames is the use of a laboratory cup Extinguishing Media Portion (REMP), i.e., the burner. However, the results from cup burner ratio of the mass of agent, to mass of fuel, is an tests depend on many variables and the results indication of effectiveness of the extinguishing are, in most cases, difficult to translate to actual medium. In REMP experiments for water mist,’ fire situations.l2-ls It is also difficult to transfer the tubular burner as shown in Fig. 1 was used. a water mist into a cup burner. It was anticipated The water mist tests were conducted using a noz- that the introduction of the gas burner test would zle, with an orifice of 0.25 mm from Lechler overcome these problems, as the scale can be eas- (denoted 212.085) using water at 40, 60 and ily increased and quantitative results obtained. 80 bar. The method was originally developed by NIST (formerly NBS)1 for studying water spray extinc- According to the test method, the extinguishing tion of large jet flames. The method has been media and the fuel should be mixed before reach- more recently developed to a NORDTEST ing the burner outlet. This is difficult in the case method NTFIRE 044, 18,19 to study the extinction of water mist since the mist is produced in a of buoyant flames. small nozzle. The nozzle was, therefore, placed within the burner and the height of the nozzle There are two extreme possibilities for a water is adjusted so that the water does not hit the rim spray to extinguish a fire: either by direct extinc- of the burner, but as much water as possible is tion of the flame, or by cooling the fuel surface.

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