Studies On Smoke Movement And Ventilation Control Mode In Subway Station Fire

Presently,subway has become a important measure to improve the public transportation situation in large and medium sized cities.While promoting city development,many new problems and challenges to fire safety are presented.In recent years,several disastrous subway fires occurred continuously,with a large number of casualties and the insufficiency of current fire control measures is fully uncovered.Therefore,it is of much significance to study the smoke movement characteristics and ventilation control modes in subway station fires.Statistics showed that the smoke was the most fatal factor in subway fires.To effectively control the fire-induced smoke.it is necessary to get acquainted with the smoke movement and entrainment characteristics along the subway station.In the conventional two-layer zone model,the mass addition of smoke layer is calculated only by a plume entrainment model and the contribution caused by entrainment at other positions are neglected.The length dimension of a subway station is far larger than the width dimension,and evidently the two-layer zone model is not applicable. Based on the different entrainment mechanisms,the smoke movement in a subway station can be divided into 4 stages,i.e.the plume rising stage,the radial spread stage after impinging onto the ceiling,the transitional stage to be one-dimensional horizontal spread and the one-dimensional horizontal spread stage.With theoretical analysis,the models of mass entrainment rate across the smoke layer interface were built for the 2～4th stage.The empirical constants in the models were determinated by the small-scale experimental results.With a series of small-scale mechanical smoke exhaust experiments and numerical simulations,the influence of turbulent entrainment intensity at smoke layer interface,due to different relative locations of exhaust opening to fire source,on mechanical smoke exhaust efficiency were studied.Results showed that when the exhaust openings were operated in the stage of one-dimensional horizontal spread,the turbulent entrainment effect at smoke layer interface would be evidently intensive, leading to poor smoke control effect.It is recommended to operate the exhaust openings in the range of the longitudinal distance to fire source less than 1.33 times of station width.For the fire between one staircase and the end wall of station,the traditional concept that distributing the smoke exhaust openings more evenly and with more openings will lead to better smoke contorl effect is not applicable.It is recommended to operate separately the openings near the end wall,or the openings on both sides of the fire source.And for the fire between two staircases,the openings on both sides of the fire source should be operated symmetrically.During subway fires,the staircase linking two floors is not only the passage of human evacuation,but also smoke spread.Therefore,for the personal safety,it is important to protect this area from smoke.The flow filed of smokeproof air curtain at staircase were divided into 3 sections,i.e.the section of impinging mixture,nonisothermal jet section and impinging floor.In the section of impinging mixture,the angle change of air curtain jet axis by smoke-flow impinging,and based on the hypothesis of uniform mixture,the temperature increment in air curtain jet by energy exchange were analyzed,In the section of non-isothermal jet,combining empirical formula,a model was built for predicting the vertical velocity attenuation of jet axis.In the section of impinging floor,no impinging was taken as the critical condition for complete confinement of fire-induced smoke.Combining two previous submodels,the critical formulas for absolutely preventing smoke from intruding the protected side were obtained.With the numerical solutions of the formulas as the initial and boundary conditions and Fluent,the smoke confinement effect of air curtain at a staircase with a atrium was numerically analyzed.Results showed that the three-section analysis method is reasonable and the simulated results verified the rationality of the theoretical prediction model.With the rapid progress of subway transport,complicated multi-floor subway stations are being built more and more.Aiming at the fire in middle floor of multi-floor station and with the aid of FDS,the influence of the operation mode of air supply system in the lower floor on mechanical smoke exhaust effect in the fire floor was analysed,and a ventilation control mode that the air supply systems in both lower and upper floors should be operated was proposed.