Study On Control Effect Of Mechanical Smoke Exhaust And Water Mist In Underground Mall Fire

With the rapid development of the city, the population continues to increase; many underground constructions emerge and become an important part of urban commercial development. Compared with the ground buildings, underground structures with closed space and dense personnel characteristics are not conducive to safe evacuation and firefighting, personnel casualties and property losses caused by the high number of high temperature and harmful smoke. Therefore, the underground building fire prevention, especially the underground commercial building fire protection, should be smoke control and extraction system and water mist fire extinguishing system according to the laws of the fire smoke flow, which is a scientific and effective control scheme to maximize the protection of personal safety.A small fire model test bench is set up firstly, test and analysis under different working conditions is processed on fire temperature change and the influence of the evolution law of smoke layer under the effect of the mechanical smoke exhaust and water mist. The results show that the single open mechanical smoke exhaust system and smoke exhaust port section average wind speed of 1 m/s smoke control effect is better; If water mist system is started separately, water mist nozzle flow 6.25L/min,9L/min and 12.5L/min extinguishing effect have subtle gap, Due to the applying of water mist, there has been enhanced combustion phenomenon at the early application stage of water mist, and with the increase of the flow rate, the enhanced combustion phenomenon becomes more significant. On the considerations of economic and fire control effects,6.25L/min flow water mist nozzle performance is best. Under the coupling effect of mechanical smoke exhaust and water mist, though the cooling effect on the surface of fuel is poorer, the cooling effect on flame and smoke layer, as well as the increased rate of visibility is significantly improved, which facilitates people to escape.Furthermore, fire dynamics simulation software FDS is applied to build fire model and analyze underground shopping mall fire which has a large space. By analyzing the simulation data of combustible material near the measuring point, the growth characteristics of fire source center temperature, carbon monoxide, smoke layer height, visibility and other parameters over time is analyzed. It is found that in 5 MW fire, smoke extraction volume of 60m3/(h-m2) can achieve better smoke control effect.Then the same model is used to analyze the control effect when the supplying air volume which is respectively 50%,60% and 80% of smoke volume. The simulation result shows that: When mechanical exhaust system is turned on, the amount of makeup air and smoke effects are not proportional. With the increase of the supplying air volume, the fire heat release rate increases, the fire temperature is higher, however, CO concentration is lower, smoke layer is higher, which is relatively favorable for personnel evacuation and health. If water mist is turned on independently 90s after fire is ignited at the working pressure 10 MPa, the flow rate of 15L/ min can achieve better extinguishing effect; Last, the coupling effect of mechanical smoke and water mist is simulated, the interaction analysis shows that, when the supplying air reaches to 50% of the smoke discharge volume,15 L/min water mist flow rate can achieve the optimal control effect.