| The China Super Ring Positron Collider Project(CEPC)is a large underground cavern complex composed of various tunnels.The ventilation and smoke exhaust of the entire underground cavern complex are completed by only a few shafts,which have strict requirements for the operational ventilation and smoke exhaust design.The bifurcation structure formed by the connection of various tunnels leads to complex and variable airflow in the whole group of chambers,and the description of the local resistance of bifurcated tunnels in the existing tunnel ventilation design code only considers special working conditions,which makes it difficult to meet the needs of actual projects.Therefore,it is necessary to study the local resistance loss and the flow characteristics of fire smoke in bifurcated tunnels.In this thesis,a bifurcated tunnel in a CEPC project is used to study the local resistance loss,critical wind speed,and roof temperature characteristics of the bifurcated tunnel by numerical simulation.(1)By establishing a model of a bifurcated tunnel in merging and diverging forms,the local resistance coefficient is used as the unit of measure to analyze the local resistance loss under different bifurcation angles,tunnel cross-section forms,flow ratios,ventilation wind speed,wall roughness,and other factors.The study shows that:the influence of bifurcation angle and flow ratio on the local resistance coefficient of bifurcated tunnels is larger;the bifurcated tunnels in the form of straight and straight merging and straight bifurcation should preferably choose an equal-area circular cross-section;the bifurcated tunnels in the form of straight and diagonal merging and diagonal bifurcation should choose a horseshoe-shaped cross-section;the influence of ventilation wind speed and wall roughness on the local resistance loss is negligible compared with other factors;and the effect of bifurcation angle and flow ratio on the local resistance coefficient is derived.The local drag coefficient equation,with bifurcation angle and flow ratio as variables,is derived.(2)The FDS was used to establish five different bifurcation angles of the bifurcated tunnel model,and to explore the length of smoke back flow under different fire source heat release rates,fire source height,fire source size,and other factors by varying the longitudinal ventilation air velocity,and then derive the influence law of each of the above factors on the critical air velocity.The results show that the effect of the bifurcation angle on the critical wind speed can be neglected,and the critical wind speed value of the bifurcated tunnel is smaller than that of the prediction model of the single-tube tunnel.The prediction model of Li is modified to predict the critical wind speed of the bifurcated tunnel when the fire source is located in front of the bifurcation node,and the relationship between the fire source height and the critical wind speed shows V_c=-0.02937x~2+0.09503x+2.62156,while the relationship between the critical wind speed and the fire source width shows V_c=2.78605-0.0865x.(3)Based on the study of the critical wind speed of the bifurcated tunnel,the maximum temperature rise and temperature distribution law of the roof under different bifurcation angles,fire source heat release rate,fire source height,fire source size,etc.were analyzed.It is found that the bifurcation angle under the longitudinal ventilation condition has less influence on the maximum temperature rise of the roof,and the fire source height and fire source size under the natural ventilation condition have more influence on the maximum temperature rise of the roof.The prediction model of the maximum temperature rise of the roof is modified on the basis of the existing prediction model of the maximum temperature rise of the roof;the temperature distribution law of the roof under the action of each factor and the prediction model of the temperature decay of the roof considering the bifurcation angle are derived. |
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