| With the rapid development of urban rail transportation in China over the past decades,more and more extra-long tunnels with a length of more than 3 km have been constructed.Due to the short departure interval of the metro,two or more trains are likely to appear simultaneously appear in the extra-long tunnel.The point extraction ventilation system is an effective alternative to avoid influencing the operation of the following train resulting from longitudinal ventilation.However,the relevant standard and design method are rare.Therefore,this thesis sufficiently investigated the smoke temperature characteristics and smoke extraction characteristics during fire in a subway tunnel equipped with point extraction ventilation based on reduced-scale experiments,numerical simulation,and theoretical analysis.The effect of heat release rate(HRR),exhaust rate,vent interval,and longitudinal fire location on the maximum temperature rise beneath the ceiling and longitudinal ceiling temperature distribution was investigated thoroughly.Experimental results showed that exhaust rate and longitudinal fire location did not affect the maximum temperature rise beneath the ceiling and longitudinal ceiling temperature distribution.Vent interval did not have a considerable effect on the longitudinal ceiling temperature distribution.The relationship between longitudinal ceiling temperature distribution and HRR was revealed based on dimensionless ceiling temperature rise.To be specific,the dimensionless ceiling temperature rise is not related to dimensionless HRR when the smoke flow could be considered as a one-dimensional flow.According to the above results,the theoretical models for the maximum temperature rise beneath the ceiling and longitudinal ceiling temperature distribution was established.Moreover,a simplified model was developed based on experimental data considering engineering applications.The influence of HRR,smoke extraction area,and the width-length ratio of the vent on the plug-holing was discussed experimentally.The results demonstrated that the critical exhaust rate of the occurrence of plug-holing increases with increasing HRR,smoke extraction area,and the width-length ratio of the vent.However,when the full-scale smoke extraction area exceeded 4 m~2,the smoke flow beneath the vent is similar to spill plume,which is different from plug-holing in essence.The impact of HRR,vent interval,smoke extraction area,the width-length ratio of the vent,and longitudinal fire location on critical exhaust rate for complete smoke extraction was analyzed experimentally and numerically.The results showed that the critical exhaust rate increases with increasing HRR and decreases with increasing vent interval,smoke extraction area,and the width-length ratio of the vent.Moreover,it increases with the fire gradually moving away from the tunnel center.According to force analysis beneath the vent,Froude number,namely the ratio of exhaust inertia force to horizontal inertia force,was developed to judge the occurrence of plug-holing and complete smoke extraction.Based on dimensional analysis,a critical criterion for the occurrence of plug-holing and a semi-empirical model for critical exhaust rate of complete smoke extraction were developed.Finally,a design method for the subway tunnel equipped with the point extraction ventilation system during the fire was proposed based on experimental results and theoretical models. |
