Numerical Study On Pollutants Crossflow Between Road Tunnel Portals

With the rapid development of highway construction in China,the scale of highway tunnel also expands rapidly.A large number of tunnel portals close to each other are constructed in the actual projects,and the problem of pollutant crossflow between portals is also much prominent.Crossflow has an adverse effect on tunnel environment and ventilation operation,therefore,in order to provide reference for engineering construction it’s necessary to study the characteristics and variation rules of crossflow.In this thesis,the basic characteristics and main influencing factors of longitudinal crossflow,transverse crossflow and compound crossflow are analyzed,and the measurement index is refined.The dimensional analysis is used to define both the relatedness between these two sets of variables:inlet velocity ₂v and exhaust velocity ₁v,and between portal longitudinal spacing l（or portal transverse spacing n）and portal equivalent diameter d.In this thesis,a numerical calculation model is established to simulate crossflow,and the accuracy and effectiveness of the numerical calculation method is verified by comparing the theoretical calculation results and the measurement results with the numerical calculation results.The effects of the following factors on longitudinal（or transverse）crossflow are analyzed using the numerical calculation method:inlet velocity and exhaust velocity;portal longitudinal（or transverse）spacing and portal equivalent diameter;transverse wind;hillsides and valleys.The fitting formulas of concentration ratio and crossflow ratio are obtained by data fitting of the calculated results.And division method of different forms of compound crossflow and characteristics of different forms are analyzed.The main conclusions are as follows:（1）In longitudinal crossflow:a)When the longitudinal size ratio l/d is below 22.94,the growth rate of the crossflow ratioλdecreases as the velocity ratio v₂/v₁ increases.When the longitudinal size ratio is above 34.40,the crossflow ratio increases approximately linearly as the velocity ratio increases.The crossflow ratio approximately decreases exponentially as the longitudinal size ratio increases.b)When the longitudinal size ratio is fixed and the speed ratio among the inlet velocity,the exhaust velocity and the transverse wind velocity is fixed,changes in the inlet velocity,the exhaust velocity and the transverse wind velocity have no effect on crossflow.When the inlet velocity and the exhaust velocity are 5m/s and the longitudinal size ratio is 11.47,the crossflow ratio decreases exponentially as the transverse wind velocity increases.Under the same longitudinal size ratio,the crossflow ratio is smaller and decreases faster as the longitudinal size ratio increases when there is a transverse wind.c)The variation of hillside angle or valley depth has little effect on crossflow,and these factors are practically negligible.（2）In transverse crossflow:a)The crossflow ratio increases approximately to the power as the velocity ratio increases.The ratio of channeling decreases exponentially as the transverse size ratio n/d increases.b)When the inlet velocity and the exhaust velocity are5 m/s and the transverse size ratio is 0.46,the crossflow ratio approximately increases linearly as the transverse wind velocity increases under adverse direction transverse wind of 0-5 m/s.c)Transverse crossflow is weak in most velocity ratios and transverse size ratios,and crossflow is further reduced in the presence of favorable direction transverse wind,hillsides or valleys.Transverse crossflow are practically negligible in this case.（3）In compound crossflow:a)The forms of compound crossflow can be divided into separated,critical and intersecting forms according to whether the diffusion range of the pollutants jet of left and right lines intersects.b)When inlet velocities and exhaust velocities are equal,the compound crossflow in critical and separated forms can be approximately regarded as two independent longitudinal crossflow.c)When inlet velocities and exhaust velocities are equal,the pollutants jets discharged from the left line and the right line collide in the intersecting form compound crossflow.The longitudinal crossflow part in the compound and crossflow is weakened.