Study On Smoke Transport Law And Longitudinal Ventilation Control For Branched Tunnel Fires

With the development of China’s transportation and the improvement of tunnel construction technology,branched tunnels are more and more used in practice.They bring not only much convenience,but also fire safety problems.In a branched tunnel fire,the hot smoke flow regime is more complex than that in single-tube tunnel,and the smoke control methods,originally proposed for single-tube tunnel,may no longer be applicable for branched tunnels.Therefore,it is necessary to study the hot smoke transportation characteristics and longitudinal ventilation control methods for branched tunnels.Using theoretical analysis,numerical simulation and reduced-scale experiments,this dissertation conducted experimental and numerical studies on the transportation characteristics and longitudinal ventilaiton control of hot smoke during a branched tunnel fire.The main conclusions are as follows:(1)The effects of longitudinal fire location,branched angle and branch slope on smoke transportation characteristics under natural ventilation were studied.The flow field characteristics of hot smoke at one-dimensional propagation stage and at the intersection were analyzed.The mass flux and heat flux entering the branch were quantified,and the longitudinal temperature decay model beneath ceiling was established.The results show that the hot smoke accelerates at the intersection,which increases with increasing branch slope.Longitudinal fire location,branched angle and branch slope have limited influence on smoke transportation.Taking the reduced scale experiments as example,when fire locates at the intersection,the heat flux into the branch accounts for about30%of total heat release rate.Finally,longitudinal temperature profile model under tunnel ceiling under natural ventilation is established and verified by previous experimental data.(2)The effects of branched angle and longitudinal fire location on hot smoke transportation under longitudinal ventilation were studied.The characteristics of velocity field under normal operation were summarized.The temperature profile under tunnel ceiling under typical angle was analyzed,and the mathematical models of critical ventilation velocity correlating with longitudinal fire locations and branched angles were developed.The results show that the influence of branched angle on the local resistance of longitudinal ventilation velocity is not monotonic.When the angle between the ventilation direction and the branch is less than 90~o,the local resistance increases with increasing branch angle,resulting in the gradual decrease of critical ventilation velocity.When this angle is larger than 90~o,the local resistance first decreases and then increases with increasing branched angle,so the critical ventilation velocity first increases and then decreases.A semi-empirical equation for predicting the maximum temperature under the ceiling when the fire source located at the branch under longitudinal ventilation was established.Then,taking the branched angle of 90~o as an example,the mathematical model of the influence of longitudinal fire location on the critical ventilation velocity of branched tunnel was established.(3)Longitudinal synergetic ventilation of fans in the branch and the main tunnel was carried out,and the characteristics of two ventilation modes were put forward:(A)ventilation velocities of branch and the main tunnel are the same;(B)ventilation velocity of the main tunnel is greater than that of the branch.The results show that the control ventilation velocity under synergetic ventilation is related to the critical ventilation velocity v_c when only the main tunnel is ventilated.Under ventilation mode A,when the ventilation velocity is 42%of v_c,the back-layering in the main tunnel vanishes and the smoke cannot flow into the branch.Under ventilation mode B,the main tunnel is ventilated with v_c and the branch tunnel is ventilated with a small velocity(taking the reduced scale tunnel in this dissertation as an example,the ventilation velocity is between 0.10m/s–0.22 m/s).However,under ventilation mode A,the heat release rate could be higher than that of ventilation mode B by 99.4%.Hence,in practice,ventilation mode,under which the ventilation velocitis in branch and the main tunnel are close,should be avoided.(4)The longitudinal ventilation control method for branched tunnels with multiple entrances and exits(i.e.multi-point access tunnel)was proposed.In order to relief the impact scope and reduce the residual time of hot smoke inside the tunnel,Dijkstra algorithm,which was originally developed to search the shortest path,was improved based on ventilation basic laws.This improved algorithm can determine the shortest path in multi-point access tunnel.The complex tunnel is simplified into a weighted directed graph with nodes and lines.Based on the proposed synergetic ventilation strategy,the ventilation velocities and static pressure of fans in different sections of the tunnel can be calculated.In this way,the influence scope of hot smoke can be reduced and the hot smoke can be exhausted quickly.This dissertation studies the hot smoke transportation characteristics in branched tunnel under natural ventilation and longitudinal ventilation,analyzes the influence of branched angles and longitudinal fire source positions on the critical ventilation velocity of branched tunnel,establishes the ceiling maximum temperature model and critical ventilation velocity model,clarifies the law of collaborative longitudinal ventilation in main tunnel and branch tunnel,and puts forward the longitudinal ventilation control strategy for multiple point tunnels.The conclusions can provide a scientific reference for the ventilation control of fire smoke for branched tunnels.There are 123 figures,19 tables and 231 references in this dissertation.