| Urban tunnel constitutes an important way to utilize underground space and mitigate traffic congestion.Urban tunnels,typically situated in the downtown area,often face stringent environmental control requirements,which require simultaneous control of the maximum concentration of pollutants in the tunnel and the amount of pollutants discharged at various outlets.At the same time,in order to meet the demands of connecting existing road network or adapting to existing construction layout,the application of unconventional structures such as bifurcation and close-spaced opening is more and more common,which makes the air movement and pollutant control in urban tunnel more complicated.Therefore,a study on ventilation and pollution discharge characteristics of urban tunnels and synergic discharge control technology is undoubtedly of great practical significance.In this paper,by means of multi-disciplinary interdisciplinary approaches such as tunnel engineering,computational fluid dynamics and jet mechanics,the local loss mechanism and ventilation characteristics of dividing flow in bifurcate tunnel,the diffusion mechanism of pollutants near the portals of tunnel group,and the decoupling mechanism between tunnel dynamic parameters and environmental control indexes are studied systematically,through theoretical analysis,numerical simulation and physical experiment.The results are as follows.(1)A 1/20 scale model for ventilation of bifurcated tunnel was set up in accordance with the similarity criterion of Re,Sr and Eu for the first time,to obtain the law of dividing flow local loss varying from the angle and diversion ratio.When the air flux ratio q of downstream mainline tunnel to ramp is equal to their cross-sectional area ratio ?,local loss coefficients are at their minimum;when q>?,loss coefficients decrease with the increase of ?,but loss coefficient for ramp increases while the bifurcation angle rises.When q<?,loss coefficients increase with the increase of ?,but loss coefficient for ramp declines with the increase of bifurcation angle.(2)The separated flow characteristics of tunnel bifurcation was revealed:when the diversion ratio ?(denoted as an air flux ratio of downstream mainline tunnel to ramp)is smaller,the flow is separated on the downstream mainline tunnel sidewall close to bifurcation point and the ramp sidewall away from bifurcation point;when ? is larger,the flow is separated on the downstream mainline sidewall away from bifurcation point and the ramp sidewall close to bifurcation point.Based on the airflow defection angle assumption,a theoretical formula to predict dividing flow local loss coefficient of the bifurcate tunnel is established.Compared to existing formulas,the formula proposed by this paper has a higher precision in prediction.(3)The scale model test for ventilation characteristic of bifurcate tunnel shows that,a linkage coupling effect of air flux exists between the ramp and the main tunnel.In the case that the fan thrust of series ventilation section changes,air flux of all ventilation sections also vary positively;in comparison,when changes occurs in the fan thrust of parallel ventilation section,the air flux of ventilation sections connected to it in series changes positively as well,while that connected to it in parallel changes in a negative manner.Simultaneous adjustment of fan thrust of any two ventilation sections will weaken the coupling effect(4)The principle of synergic emission control of bifurcated tunnel is proposed:for the control over the environment inside tunnel,the traffic volume and speed should be the main disturbance,and the fan in series ventilation section should be the main control variable to meet the requirement of pollutant concentration limit in tunnel;for the control over the environment outside tunnel,the vehicle shunt ratio should be the main disturbance,and the fan in parallel ventilation section should be the main control variable to meet the requirement of amount of pollutants discharged at various outlets.(5)For the closely spaced tunnel with staggered inlet and outlet,both reducing inlet air velocity uin and increasing outlet air velocity uout and lateral distance d can reduce the impact of the negative-pressure zone at the tunnel entrance on the jet flow structure at the tunnel exit,thus weakening the circulating air.When the inlet is placed behind or aligned with the outlet.the circulating air mixing ratio ?c will increase linearly along with the increase of staggered distance Al;when the inlet is placed before the outlet,?c will first increase and then decrease with the increase of ?l.An expression to predict ?c was proposed;it makes up for the defect that the existing formula can only predict ?c of the closely spaced twin tunnels with aligned inlet and outlet.(6)The phenomenon of series flow can be regarded as the flow when the downstream portal is located at different regions of the upstream tunnel pollutant jet flow.The series flow ratio ?s declines with the increase of ?x/D(?x refers to the distance between upstream and downstream tunnel portals;D refers to the hydraulic diameter of tunnel section),but increases with the increase of air velocity ratio udown/uup.Through the innovative introduction of the concept of three-dimensional wall jet feature region,the all-jet flow channeling ratio covering all jet feature areas is constructed,which breaks through the limitation of the portal spacing of the existing formula and extends the applicable scope of the serial flow model to 0<?x/D<28Relevant research results have been successfully applied to Zijingang urban tunnel,so that the environment of about 7 acres of area along the tunnel is free from the impact of the tunnel exhaust emissions,saving more than 21 million yuan of investment and operation cost of the tunnel and 170 million yuan of indirect economic benefits,showing good social,economic and environmental benefits(Zhong Hui Joint Hearing,No.[2017]13 66). |
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