Suppression Of Diffusion Flames With Water Mist

Abstract

The relation between man and fire is very old. From the early stages of life on the surface of earth, fire has been of great assistance to the mankind. Life would not have remained life, had the element of fire been taken away, but at the same time, if the fire becomes uncontrolled, every thing is brought to naught. Thus, in one way, if fire is the best friend of mankind, at the same it is also the worst enemy of man. For all diffusion flames hydrogen (H) and hydroxyl group (OH) are responsible for flaming. A fire will continue unless radicals are diverted into other reactions by anti-catalytic agents. Flame propagation results from the fast reactions of key species (H and O atoms, OH radicals) with vaporized fuel molecules. These species exist at concentrations far above those expected from thermal equilibrium at flame temperatures. Chemically active agents decompose in the flame to generate the entities that catalytically reduce the radical concentrations toward equilibrium levels. While this catalytic process slows the flame, it does not necessarily extinguish it. Both chemically active and physically active suppressants increase the heat capacity of the fuel/air mixture, reducing the flame temperature and thus, along with the decreased concentrations of radical reactants, decrease the flame reaction rates below the level needed to sustain combustion. These two effects are synergistic. Thus, if weight limitations allow, the effectiveness of a chemical agent can be enhanced by combination with a high heat capacity flame suppressing agent. When the mass of hot gases is surrounded by the cold gases, the hotter and less dense mass will rise upwards due to density difference, or rather, due to buoyancy. This is what happens above burning fuel source, and the buoyant flow, including any flames, is referred as a fire plume. As the hot gases rise, cold air will be entrained into the plume, causing a layer of hot gases to be formed. Flammable liquids tend to burn as pools with uniform horizontal surface. However accidental spills of liquid fuels in industrial process and power plant systems can pose a serious fire hazard. Some liquids are highly volatile at ambient temperatures; they can evaporate and form a flammable mixture with air, leading to a possible explosion in a confined space. Other liquids have high flashpoint and require localized heating to achieve ignition. Once ignited, however, very rapid flame spread will occur over the liquid spill surface. In free burn conditions, the burning rate will quickly reach a constant value, depending on the diameter of the spill. This is a typical case of pool fire. A pool fire is a type of turbulent diffusion flame, which burns above a pool of vaporizing fuel where the fuel vapour has negligible initial momentum. For suppression of such diffusion flames various chemicals such as Halon 1211 (CBrCIF2), Halon 1301 (CBrF3) and Halon 2402 (CBrF2CBrF2) are used but they contribute in ozone depletion (ODP) and global warming (GWP). The two protocols on environment Montreal Protocol and Kyoto Protocol restricted the use of such chemicals for fire suppression applications. Water mist is most suitable alternative technology for suppression of diffusion flames, which has favorable physical properties for very effective fire suppressing agent with zero ODP and GWP. Its high heat capacity (4.2 J/goK) and high latent heat of vaporization (2442 J/g) can absorb a significant quantity of heat from diffusion flames. Water also expands 1700 times when it evaporates to steam, which results in the dilution of the surrounding oxygen and fuel vapors. With the formation of small droplets, the effectiveness of water in fire suppression is further increased due to the significant increase in the surface area of water that is available for heat absorption and evaporation. An internally mixed twin –fluid mist generator (atomizer) can be potentially used in the suppression of diffusion flames inside the enclosed spaces, which provided droplets with sauter mean diagram (SMD) in the 15-85 m range at a low pressure of water and air (2-4 bar). The measured data suggest that such injectors can be employed to control the atomization process by simultaneously varying the liquid and air supply pressures. The minimum extinguishing concentration (MEC) of water mist was experimentally measured in modified cup burner apparatus for suppression of n- heptane diffusion flame. The mist with droplet size of 40 µm was injected into flame chimney and it was observed that MEC increases with increase in the air flow rate and 34% increase in value of MEC was observed. The MEC of water mist was 102 gm /m3 for suppression of n-heptane diffusion flame. It is also observed that on mass to mass basis water mist is 1.7 times more effective then Halon 1301 and 4.4 times more effective then HFC227ea in suppressing a n- heptane diffusion flame as measured in standard cup burner apparatus. The pool fire suppression tests were conducted in large enclosed space (40 m3 chamber) using water mist and results indicate that the water mist suppress the diffusion flame in the enclosed space mainly through the evaporating cooling and oxygen displacement by water vapors resulting in inefficient combustion. The fire suppression time decreases with a decrease in droplet diameter. It is much easier to suppress a larger fire due to faster rates of evaporation of water droplets and therefore, the critical mist concentration decreases with an increase in the fire size. The presence of water vapor in the reaction zone works as a deterrent to the fire by diluting the air and hence reducing the overall concentration of oxygen in the enclosed room. The sharp reduction in the temperature was observed after injection of mist. The evaporation of water droplets into vapor, causing dilution of air and the subsequent slow down of the combustion process, leading to inefficient combustion manifested by an increase in the concentration of CO, is the primary mechanism for fire suppression using water mists in enclosed spaces. Larger the fire size less is the time required to extinguish the fire. This can be attributed to the larger heat available for evaporation of the water mist droplets leading to attaining the inert environment faster by formation of water vapor and causing dilution of oxygen concentration below that required for sustained combustion reaction. Therefore, the critical mist concentration reduces with an increase in heat release rate (HRR) of the fire. The rate of vaporization is enhanced with an increase in heat release rate, thus, reducing the combustion efficiency and impeding the burning of fire. Therefore, the amount of mist required to suppress a pool fire in an enclosure reduces with an increase in the heat release rate. Smaller droplets are more effective in the suppression of fire because of their ease of convection to the flame zone and faster and easier evaporation. Water mist with Sauter mean diameter of 40 µm is observed to be most effective in suppressing the fire. The extinction time increases with an increase in droplet diameter. The critical concentration (MEC) of was measured for suppression of diffusion flames of different HRR and it was observed that critical concentration of mist required to suppress the diffusion flame decreases with increase in HRR of pool fires. For suppression of pool fires of n-heptane having HRR of 800 kW, critical mist concentration (MEC) of 150 gm/m3 was required with droplets having 40 µm SMD. The use of nitrogen as atomizing gas and its effect on diffusion flame suppression performance was evaluated. The critical mist concentration required for suppression of unobstructed & obstructed diffusion flame reduced by 30-50% by use of nitrogen as atomizing gas. The experimentally measured data on critical concentration of mist required for suppression of diffusion flames in large enclosed spaces forms the basis for design of water mist system. The design concentration of mist will be MEC X Factor of Safety (FOS). The FOS will be in the range of 1.5 to 2