Experimental And Numerical Simulation Study On Fire Extinguishing Efficiency Of Water Mist In Underground Long And Narrow Space

With Chinese rapidly expanding economy,the accommodating service capacity of the existing basic municipal facilities in large cities cannot meet the needs of development,so we will continue to vigorously develop and utilize underground space.Underground space solves the urban land shortage,traffic congestion and other problems,but also brings new problems to fire safety.Due to the particularity of its function,structure,space and layout,some long and narrow underground Spaces will suffer great losses in case of fire.Therefore,it is necessary to study the fire extinguishing efficiency of water mist fire extinguishing system in underground narrow and long space.Water mist fire extinguishing system is a new type fire extinguishing system which combines the advantages of automatic sprinkler system and gas fire extinguishing system.Water mist fire extinguishing system provides no environmental pollution,extinguishes fire fast,consumes less water and damages little to the protected objects.It is an ideal substitute for halon.In this dissertation,the fire extinguishing mechanism and efficiency of water mist fire extinguishing system and pulse water mist fire extinguishing system are studied by the method of full-scale test simulation,theoretical analysis and numerical simulation for underground narrow and long space fire.The research contents of this dissertation include four aspects:the theoretical research on the mechanism of fire suppression by pulsed water mist;Experimental study on fire extinguishing efficiency of continuous water mist in underground narrow and long space;Numerical simulation study on fire extinguishing efficiency of continuous fine water mist in underground narrow and long space;Numerical simulation of fire extinguishing efficiency of pulsed water mist in long and narrow underground space.The research techniques,results and conclusions of this dissertation are:(1)Based on the theory of conservation of mass,energy,the dissertation established the pulse cold water mist fire extinguish physical model,the impulse water mist fire extinguishing the critical conditions.In this dissertation,the characteristics of pulse water mist fire was obtained in the process of pulse water mist extinguishing fire,and further clarified the pulse dominate the extinguishing mechanism of water mist.(2)The dissertation take subway tunnel fire as research object and establish a test model of a long and narrow underground space of fire and smoke control.It is studied that parameters as open time of water mist,system working pressure and nozzle flow coefficient influence temperature on the platform.The experimental results show that,the flue gas is forced settlement on the platform after water mist opening and visibility is reduced significantly.So evacuation should be completed before the water mist is opened.When water mist nozzle is open at about 3 min after ignition,the working pressure is 12MPa and the flow coefficient K=0.5,the temperature control effect is the best.(3)Based on the experimental platform and experimental conditions,the dissertation establishes the corresponding simulation model by FDS software.The numerical simulation results contrast with the experimental results,and further analysis of the effect of water mist on compartment fire.The experiment and numerical simulation validate each other at the same time and proved the validity and reliability of the numerical simulation results.(4)By analysing FDS numerical simulation results of continuous water mist fire extinguishing efficiency,the temperature changes with time in the relatively stable stage emerges a more pronounced oscillation phenomenon.This phenomenon also accords with burning rule.Burning oscillation phenomenon is one of the important research direction of the limited space,usually be considered to be caused by insufficient ventilation and oxygen supply burn shock;Discharge of water mist on the flue gas has the effect of downward traction,this has to do with the buoyancy of flue gas in the opposite direction,at the same time,water mist particles through the smoke layer when arrive at the air layer below,also will strengthen flue gas mixed with air layer,strengthen the flow of energy and matter,water mist can overcome when pressure smoke buoyancy effect,better cooling effect;When the water mist pressure can overcome the flue gas buoyancy,the flow coefficient of the water mist nozzle is small,and the control effect of the platform flue gas temperature is the best.However,when the water mist pressure can not overcome the flue gas buoyancy,the flow coefficient of the water mist nozzle has little influence on the control effect of the platform flue gas temperature.(5)The combustion inhibition and cooling effect of the pulsed water mist were studied by FDS numerical simulation,and it was found that the pulsed water mist could strengthen the disturbance effect on the gas,and it took a certain time for the droplets to evaporate after the water mist was ejected to play the role of combustion inhibition and cooling inhibition.The two acted together,so there was an optimal pulse period.The optimal pulse period is 8s for spray and 8s for pause.The pulsed fine water mist is obviously better than the continuous spray near the fire source,but the efficiency of the pulsed fine water mist is similar to that of the continuous spray at a distance from the fire source.The research results in this dissertation provide theoretical and experimental basis for the design and application of water mist system,and have important practical significance for improving the fire extinguishing efficiency of water mist system,expanding the fire extinguishing theory of water mist and popularizing water mist technology.Study on the pulse cycle of pulse water mist can provides a reliable basis and important reference material for the further research and development of pulse water mist.To control underground long and narrow space fire,to reduce the spread of fire temperature and to provide scientific theoretical basis for fire fighting and rescue work,and then to design long and narrow space fire extinguishing system and personnel evacuation,the research results provide reasonable scientific basis,which is of great practical significance and application prospect.