Numerical Study On Interactoion Of Water Mist With Pool Fire

With the inevitable phasing-out of the production of Halongen-based agents under theterms of the amended Montreal Protocol, extensive efforts hanve been made over the last fewyears to find alternative means of active fire extinguishment. By far, water mist systems hasbeen proved to be the most common method among other Halon replacement techniques forfire protection because of its economical merits, environmental friendly, low water damage,no toxicity, etc.. Since the concept and mechanisum behind the fine water mist system isquiter different from that of standard sprinklers, considerable attention has been paid toinvestigation the mechanisnm behind fine water spray employed to extinguish and contrl fires.The main mechnisum for different mist properties or fire types has been understood, but thereis insufficient knowledge for a quantitive assessment of which combination of mist and fireenvironment factores will lead to an optimal dasing of water spary system. So this paperfoucuses on providing a quantivitive analysis of the complex interaction occurring betweenwater mist and pool fire under different ventilation conditons in real scale compartment bynumerical method. The main work and conclusion in this paper are as follows:(1) A numerical model based on Large Eddy Simulation (LES) for turbulence isdeveloped to simulate the interaction of water mist and pool fire. The development of theturbulent diffusion flame, originating from the evaporation of liquid methonal pool fire, issimulated using mixture fraction combustion model. As the radiation spectrum of soot iscontinuous, it is possible to assume that the gas behanves as a gray medium, and the spectralindex of referaction data of water mist is described by Mie theory. Flame extinguish iscontrolled by limiting oxygen index and critical flame temperature. The two-phase interactivetransient process is based on the Eulerian-Lagrangian formulations for the gas phase andliquid (droplets) phase, respectively.(2) Parametric studies are performed to determine the effects of droplet diameter,operation pressure, spray cone, horizontal distance between mist nozzle and pool center,vertical distance between mist nozzle and pool centre, heat release rate on water mistperformance under different ventilation condition. The relative contribution of varioussuppression mechanisms such as thermal cooling and oxygen dilution is investigated indifferent regions of fire plume. By introducing critical cool time, the performance of watermist system for the initial fire suppression can be determined. (3) Through the numerical simulation of water mist and pool fire in natural ventilationcompartment, the effect of mist diameter, operation pressure, spray cone, horizontal distancebetween mist nozzle and pool center on water mist performance are analysed. The mosteffective mist diameter is determined and the optimum operation parameters are suggested.(4) Through the numerical simulation of water mist and pool fire in ventilation limittedcompartment, the effect of heat release rate, operationg pressure, vertical distance betweenmist nozzle and pool center on fire extinguish permormance of water mist are analysed. Thetemperature fireld, velocity vector distribution and the gas species concentration are obtained.The optimum nozzle installation is provided. The result shows that water mist addition couldeffectivly prevent flashover and backdraft.