The Study On The Interaction Of Shielded Fire And Water Mist

With the rapid development of China's e-commerce industry,a large number of scenarios such as large warehouses and logistics warehouses have been built to meet the urgent needs of industrial,production and household goods,and the ensuing fire situation has become increasingly serious.In order to optimise the utilisation of space in warehouses,there are often existed phenomenon of obvious shieled fire.In addition,there is a risk of gas leaks to a certain extent in a large number of residential areas with gas network pipes,and the complex spatial structure of the dwellings often makes it easy for the flames obscured by obstacles.Once a fire occurs in such a place,the obstacle will certainly have an impact on the mechanism of the corresponding fire extinguishing technology and its fire extinguishing effect.Therefore,it is of great theoretical significance and practical value to carry out research on the mechanism of fire extinguishing and its influence mechanism in fire shielded scenarios.Water mist as an effective alternative to halon fire extinguisher for better reducing the ozone layer depletion and greenhouse effect,to the 1980s onwards,gradually began to be applied to industrial and production.In recent years,under the consensus of the Paris Agreement,the world's major economies have undergone a technological transition focusing on energy efficiency and emission reduction.In conjunction with our recent goal of working towards carbon neutrality by 2060,water mist as a widely recognized clean,efficient,safe,environmentally friendly fire-fighting technology,is bound to help the fire-fighting industry to achieve its carbon neutral mission.Previous researches into the suppression of shielded fires by water mist has focused on the basis of the variation of thermodynamic parameters under typical cases.However,few research has been studied under a comprehensive consideration of the obstacle shielding conditions(shape and size),distance from the fire source,as well as fire size,burner size,nor has the establishment of critical conditions of fire extingusihment and the fire suppression prediction model.Therefore,an experimental platform for the suppression of premixed laminar flame,non-premixed diffusion flame and methane jet flames by water mist is established with the help of combustion and flow field diagnostic in this study.Experimental and numerical simulations of the interaction between water mist and shielded flames are carried out with the aim of obtaining critical conditions for fire extinguishing,constructing obstacle-obstructing characterisation parameters,developing fire extinguishing prediction models,summarising the combustion chemistry of flame extinguishing and suggesting guidance for water mist system design and installation.In the study of the interaction of water mist with shielded premixed laminar flames,the OH radical concentration distribution characteristics in typical premixed laminar flames at methane/air equivalent ratios from 1.0 to 2.0 were obtained by OH-PLIF.Based on the assumption of a normal distribution of the water mist flux in the radial direction,an effective water flux based relationship equation was constructed to obtain the water flux required for maximum critical quenching under rich and lean combustion.The results indicate that for the laminar flames with equivalent ratios between 1.0 and 2.0,its critical extinguishing effective water flow rate is in the range of 0.00883 L min-1?0.00955 L min-1;for laminar flames with equivalent ratios between 0.78 and 0.9,its critical extinguishing effective water flow rate is in the range of 0.00844 L min-1?0.00883 L min-1.The suppression of laminar flames is influenced by the methane/air equivalent ratio and the water flow rate.The stabilisation of the OH radical concentration in the combustion reaction zone at the base of the flame directly influence the fire suppression ability.Under the same water mist water flow rate,the water mist suppresses the laminar flame of the rich combustion premix more effectively than the lean combustion one.In the study of the interaction between water mist and shielded pool fire,1/6 and 1/2 scaled experimental and simulational cases were carried out.The water mist flow field and the visualisation of the spray-fire interaction was obtained via PIV and OH-PLIF,the results show that the formation of counter-vortex pairs of structures near the obstacle is not conducive to the effective bypassing of the obstacle by the mist droplets into the flame region to participate in flame cooling effect.In contrast,the formation of counter-vortex pairs close to the burner or below the obstacle allows for more efficient mixing of the thermal plume with the droplets,which in turn contributes to the extinguishing of the water mist.In addition,predictive equations for fire suppression based on the geometric projection sheltering ratio and the critical plume to spray thrust ratio were obtained.In practical engineering applications,the equation can be used to determine the minimum requirements for water mist fire suppression.In the study of the interaction between water mist and shielded methane jet flames,a series of experimental and numerical simulations based on OH-PLIF combustion diagnostic measurements were carried out to investigate the effects of different flame Froude numbers,jet flame tilt angles and water mist nozzle installation positions on the flame suppression/extinguishing effect of water mist.Through the construction of gas jet-spray momentum ratio,flame-shielding ratio and effective fire-extinguishing mist ratio and other dimensionless parameters,the critical conditions of water mist extinguishing methane jet fire and the effective protection radius of water mist were obtained.At the same time,based on the OH radical concentration characteristics under critical extinguishing conditions,this experimental study summarises several typical flame extinguishing behaviours based on flame-shielding ratios and flame Froude numbers.By obtaining the quantitative data of OH radical concentration distribution under critical fire extinguishing conditions,it also indirectly illustrates the mechanism by which water mist can be "entrainment" to extinguish fires under fire shielded conditions.