Experimental Studies On Entrainment Behavior And Characteristic Parameter Evolution Of Smoke Layer In Tunnel Fire With Ceiling Extraction System

In recent years,the tunnel construction in our country has been developed rapidly,followed by traffic tunnel fire accidents,which occurs frequently in recent years.In the existing smoke exhaust mode of traffic tunnel fire accident,the ceiling extraction system can effectively exhaust smoke out through the centralized smoke extraction outlet,which is helpful to the personnel evacuation in tunnel fire.In this thesis,a series of model tunnel fire experiments were carried out with the effect of ceiling extraction system,the characteristics of air entrainment characteristics,the plug-holing and the distribution of smoke parameters under the coupling effect of longitudinal ventilation and ceiling smoke extraction were studied.The main contents were as follows:In this thesis,the air entrainment coefficient of smoke layer under one dimensional smoke movement of tunnel fire was experimentally studied.Results show that the air entrainment coefficient of smoke layer increased with the increase of the fire heat release rate.When the heat release rate was fixed,the air entrainment coefficient decreased with the increasing ceiling extraction velocities.In the one-dimensional horizontal movement stage,a new model was proposed to describe the entrainment coefficient under the effect of ceiling extraction system.The evolution characteristics of smoke layer thickness and plug-holing in tunnel fires with multi-point ceiling extraction system were studied.The results show that the smoke layer thickness calculated by integral ratio method and within-class variance method was in good agreement with the experimental visual value.For the same heat release rate,the plug-holing rate of smoke layer and the ratio of smoke layer thickness to tunnel height decrease with the number of ceiling extraction port.The critical velocities under the coupling effect between ceiling smoke extraction and longitudinal ventilation were studied.It was found that when the dimensionless heat release rate was fixed,the critical velocities decreased with the increase of ceiling extraction velocities and the critical Froude number increased with the increase of mass flow rate.Finally,a dimensionless model for critical velocity in tunnel fire was established.