Movement And Diffusion Mechanism Of Ultrafine Particles In Urban Highway Tunnels And Pollution Control Methods

At present,the number of motor vehicles in China has increased greatly,and the ultra-fine particles in automobile exhaust have become one of the pollution sources in urban life.Because of the semi-closed tunnel structure,the air quality in the tunnel is usually poor.The purpose of this paper is to analyze the unsteady turbulence,dispersion,deposition and coagulation characteristics of ultrafine particles with different particle sizes in the process of vehicle passing through a highway tunnel.We present a realizable k–ε turbulence model based on a combination of Eulerian passive scalar and dynamic mesh models.A 100-m three-dimensional model of a road tunnel containing four vehicles(L×W×H = 4.5 m×1.8 m×1.5 m)in a row and 10-m long outlets was established using STAR-CCM+ computational fluid dynamics software.This paper studied the variation of particulate matter under three driving speeds of 60 km/h,40 km/h and20 km/h respectively,because the vehicle speed in the tunnel is not constant in real life.At the same time,in order to ensure the accuracy of the model calculation,three kinds of time step and grid number were selected for independent experiments.Finally,0.01 s time step and333709 hexahedral grid format were selected for numerical simulation,which not only ensured the calculation accuracy,but also improved the simulation efficiency.By comparing with the measured data of Wehner et al.,the relative error between the simulated results and the experimental data considering both sedimentation and consolidation is less than 8%.The model meets the requirement of calculation precision.At the same time,based on the particle diffusion characteristics in the tunnel analyzed in this paper,a control technology of intelligent fan for urban highway tunnel based on ultra-fine particle concentration was proposed in the last chapter.Based on the measured hourly distribution characteristics of particulate matter in the tunnel,reasonable suggestions for fan layout were put forward.Meanwhile the actual urban tunnel engineering cases are combined with the control method of automatic intelligent fan.According to the measured data of the whole year,three intelligent operation schemes of tunnel fans are put forward,and the annual operation cost,economic analysis and investment payback period are calculated.The results show that: When the study area is close to the exhaust pipe area(located at the0.1m vertical section behind the exhaust pipe),the average particle concentration error is about64% without considering the deposition.Therefore,the deposition effect of particulate matter can not be ignored when the vehicle is driving at low speed.However,no matter how the vehicle speed changes,the concentration distribution of the exhaust gas particles in the breathing zone(regional space 0.5-1.5m above the ground)and the tunnel is not affected by the deposition effect.At the same time,the study found that the coagulation process of particulate matter is very complex,especially when the vehicle is driving at a low speed.When the vehicle speed is20 km/h,if the concentration change of particulate matter near the exhaust pipe(located at the0.1m vertical section behind the exhaust pipe)is not taken into account,a large calculation error will be caused.The relative error of the average particle concentration is as high as 193.51%,while the effect of coagulation in the whole tunnel is not obvious,and the relative error is only2.82%.Therefore,in the study,the influence of coagulation on the particle concentration in the whole tunnel can be ignored when the vehicle is driving at high speed,while the coagulation effect near the vehicle driving at medium and low speed is not recommended to be ignored.It can be seen that both the deposition and the coagulation effect can not be ignored when studying the diffusion characteristics of particulate matter under the condition of vehicle driving at low speed in the tunnel.According to the dynamic characteristics obtained from the analysis and research and the characteristics of diffusion,settlement and coagulation,the intelligent fan control technology is put forward.The maximum estimated annual energy saving cost of adopting the intelligent control system technology for the tunnel is 5.4873 million yuan,and the payback period of investment is only half a year.The research results of this paper are of great significance for the prevention and control of particulate pollution in highway tunnels,the optimization of tunnel ventilation system design and the improvement of air quality in highway tunnels.