Simulation Study Of Water Mist Extinguishing System In Underground Pipe Gallery Cabins

With the rapid development of cities in our country,the underground comprehensive pipe gallery has become a hot construction project,and its fire safety issues have also received widespread attention.Due to the narrow structure,complex combustion products,the fire risk is high,and it’s difficult in extinguishing fire.We can only choose to install an automatic fire extinguishing system.However,there are no clearly stated in the current specification which fire extinguishing system installation,so the selection of the fire extinguishing system of the cable cabin of the utility tunnel and the impact of various parameters of the system on its fire extinguishing effect are very important.This paper compares and discusses the characteristics of dry chemical fire extinguishing system,gas fire extinguishing system,aerosol fire extinguishing system,ordinary water fire extinguishing system and high-pressure water mist fire extinguishing system,and analyzes their applicability to cable cabin fire from four aspects:electrical insulation,human safety,space occupancy rate and environmental friendliness,and believes that the high-pressure water mist fire extinguishing system is the best choice due to its higher cost performance,green and pollution-free and better comprehensive fire extinguishing effect.The fire simulation software pyrosim was used to carry out 1:1 modeling with reference to the fire experimental model of a solid pipe gallery,and the feasibility of the simulation was verified by comparing the thermocouple temperature change curve in the same position with the simulated time.Then,pyrosim software was used to model the cable cabin of a pipe gallery in Hefei to study the influence of water mist related parameters on the fire extinguishing effect.Variables considered are nozzle horizontal spacing,nozzle flow coefficient,nozzle flow,nozzle spray direction,and average droplet size.Through the comparative analysis of the simulation results,the following conclusions are drawn:1.After the fire occurred in the cable cabin,the temperature of the fire rose rapidly,and the highest temperature point was near the fire source,followed by the roof.The smoke from combustion spreads rapidly vertically upward at first,is hindered by the roof,gathers in the upper part of the cabin,and then begins to spread to both ends of the cabin,and the height of the smoke layer drops further.By comparing the temperature field and smoke field distribution of the working conditions of the water mist fire extinguishing system at different time points after the fire occurs,it can be seen that the fire extinguishing system has a good fire extinguishing effect,which can quickly reduce the temperature of the fire field,extinguish the fire and have a certain smoke removal effect.2.The spacing between two nozzles is not the smaller the better.Too large spacing will lead to the weakening of fire extinguishing effect because of a large number of uncovered areas,and too small spacing will lead to excessive overlap of adjacent sprinkler nozzles,thus affecting fire extinguishing efficiency.At the same time,too small spacing of nozzles increases the installation cost,so it is suggested that the spacing of sprinkler nozzles should be the distance that makes the coverage of fine water mist uniform,which is 3m in this paper.3.The fire extinguishing effect of fine water mist increases with the increase of sprinkler flow coefficient.However,when the flow coefficient in this paper is 1.5 L/min/（MPa）^1/2 and1.2 L/min/（MPa）^1/2,there is little difference in the overall fire extinguishing effect,and the increase of the flow coefficient will cause certain security risks to the cable cabin.Therefore,this paper suggests that the flow coefficient of the sprinkler should be 1.2 L/min/（MPa）^1/2.4.Sprinkler flow is not the bigger the better.Among the four working conditions selected in this paper,the condition with the best extinguishing effect corresponds to the sprinkler flow of 13 L/min,but it’s overall extinguishing effect is not much different from that of the sprinkler flow of 10 L/min.Considering that the increase of flow rate will lead to the increase of pipe network pressure,which will increase security risks and the corresponding economic input to a certain extent,this paper suggests that the nozzle flow rate of 10 L/min.5.The Angle between the spray direction and the roof determines to a large extent whether the water mist can fully contact with the combustion material,which has a great impact on the ability of the water mist fire extinguishing system to extinguish the fire,cool down and remove smoke.In this paper,when the Angle between the spray direction and the roof is 0°,the fire extinguishing of fine water mist fails.When the Angle between the spray direction and the roof is 30°,60°,90°,the ability of extinguishing and cooling is better.In terms of smoke removal effect,it is necessary to increase the contact area between the water mist and the cabin space as far as possible,and the optimum working condition is 90°.6.The smaller the particle size of water mist drops,the better the smoke removal ability,but in terms of fire extinguishing and cooling effect,the smaller the particle size is not the better.Among the four working conditions in this paper,the one with the best fire extinguishing and cooling effect corresponds to the particle size of fine water mist of 200μm.As there is little difference in smoke removal effect between 100μm and 200μm droplet size,200μm droplet size is comprehensively selected.7.Through variance processing of different variables corresponding to fire extinguishing time data,it can be seen that among the five variables,the spray direction of sprinkler nozzle has the most significant influence on fire extinguishing effect of water mist fire extinguishing system,followed by horizontal spacing of sprinkler nozzle,flow rate of sprinkler nozzle,flow coefficient of sprinkler nozzle and average particle size of fog drops.The first two variables are related to the size of the contact area between the water mist and the combustion substance.Therefore,it can be shown that on the premise of ensuring the installation of the water mist extinguishing system in accordance with the specifications,the water mist sprinkler must be reasonably arranged according to the situation in the cable cabin,so that the water mist sprayed by the sprinkler can fully contact the position of the combustion substance.Figure[47]table[11]reference[53]...