Experimental And Numerical Study On Solid Fires Restrained By Ultra-fine Water Mist In Confined Space

Given the advantages of environmental protection and high efficiency, advanced water mist fire extinguishing technology has become the main alternative technology to halon fire extinguishing agents and has been applied in a wide range of fields. The second generation of water mist fire extinguishing technology---ultra-fine water mist has drawn considerable attention. Because the size of the droplets is less than 50μm, ultra-fine water mist can flow easily and bypass obstacles to reduce the power of the fire source. A high concentration of droplets can suspend for a long period of time in the space, reduce the oxygen concentration in the combustion region, accelerate cooling of flames and smoke. At the same time, ultra-fine water mist has good heat-absorbing properties. Its large surface area can speed up the cooling of smoke and absorb the thermal radiation of flames. In addition, compared with a conventional fine water mist, ultra-fine water mist consumes less water and is much safer in electrically charged spaces.Based on this advanced fire protection technology in this paper, the ultra-fine water mist generation systerm was maken according to the ultrasonic atomizing mechanism. The diameter and the mass fraction of the ultra-fine water mist were measured.The experimental platform for the ultra-fine water mist restaining systerm was built. Two kinds of solid combustibles of typical significance were selected as the subject investigated. The interaction experiment between ultra-fine water mist and these solid combustibles were carried out separately.The experiments of using ultra-fine water mist to restrain wood and PMMA fire are carried out under different pre-ignition time, mist flux and discharging time. The variation trend of smoke layer temperature, the volume fraction of oxygen in the smoke, the volume fraction of carbon dioxide and heat release rate of wood and PMMA fire under different experimental conditions were found. The experimental result showed that heat absorbing and oxygen separating function played an important part in the restraining process.A numerical model based on Large Eddy Simulation (LES) for turbulence was developed to simulate the interaction between the ultra-fine water mist and two combustibles'fire. The process of ulra-fine water mist restraning solid combustibles systerm was analyzed through the experimental phenomenon and simulated outcome. The simulation confirmed the experimental result, and it showed that heat absorbing and oxygen separating function played an important part in the ulra-fine water mist restraining solid combustibles'fire system.