Study On Pollutant Transport Mechanism And Ventilation Control Technology During Construction Of Spiral Tunnel

The spiral tunnel could flexibly design the curvature and slope of the tunnel according to the geological structure,avoiding the adverse effects of complex terrain and topography,and had a strong advantage in engineering applications.Different from ordinary straight linear tunnels,spiral tunnels had a circular trend with obvious curvature and slope,which made there were certain differences between construction ventilation of the two types of tunnels,especially in terms of ventilation air flow field and pollutant diffusion.Carrying out research on the ventilation flow field and pollutant transport mechanism during the construction of the spiral tunnel,and proposing ventilation control technology to improve the ventilation effect were of great significance for ensuring the safe construction of the spiral tunnel,creating a civilized construction environment,and maintaining the physical and mental health of the construction personnel.Relying on the actual spiral tunnel project,this paper first tested the key parameters of the ventilation and the concentration of pollutants on the construction site;then used numerical software to simulate the construction ventilation process and studied the flow field and the pollutant diffusion law.Optimized the layout of the air duct based on numerical simulation and proposed pulsating ventilation method to improve the ventilation and drainage effect of the tunnel.The main results achieved were as follows:（1）Carried out on-site testing of spiral tunnel construction to obtain basic parameters required for numerical simulation of construction ventilation.Considering the special shape of the spiral tunnel,the wind speed,the concentration of carbon monoxide after blasting and the concentration of spraying dust along the tunnel were tested at the outer,middle and inner sides of the tunnel centerline.Then summarized the distribution of pollutant concentration during the construction of the spiral tunnel to provide a reference for the numerical calculation results.（2）Using FLUENT to establish a numerical three-dimensional model of spiral tunnel construction ventilation,the pressurized ventilation air flow field during spiral tunnel construction was obtained.The simulation results showed that the flow field had obvious zoning,and the existence of curvature made the wind speed inside the spiral tunnel centerline lower.（3）Simulated the process of carbon monoxide gas diffusion after blasting and spray dust diffusion,then analyzed the pollutant diffusion law.The results showed:after blasting in the tunnel,the carbon monoxide gas moved towards the entrance of the tunnel in the form of"air mass";the range of"air mass"gradually expanded during the movement,but the peak concentration gradually decreased;due to the curvature,the diffusion of carbon monoxide inside the spiral centerline lags behind other regions.During the spraying process,the dust accumulated near the tunnel face under the influence of vortex;when the dust diffused toward the entrance,it was obviously affected by the wind carrying effect and the gravity effect;the larger wind speed outside the spiral centerline made the dust concentration on this side higher;affected by gravity,the closer to the ground,the higher the dust concentration.（4）Determined the time to reach the carbon monoxide concentration after blasting and the dust concentration during the spraying process as indicators to measure the ventilation effect during the construction period.Determined the hanging position of the air duct,the distance from the air duct outlet to the palm face,and the air speed as the main factors affecting the ventilation effect.Each factor arranged five horizontals to design L₂₅（5⁶）orthogonal tables.According to the orthogonal test plan,25 sets of simulation calculations were carried out.Using the range analysis,variance analysis and gray correlation degree to study the influence degree and sensitivity of each influencing factor to the index,then the optimal air duct layout plan sui Tablefor the site was determined.（5）Pulsating ventilation technology was proposed in spiral tunnel and studied its effect through numerical simulation.The result showed:pulsating ventilation can significantly improve the airflow structure of spiral tunnels,especially improve the difficulty of ventilation inside the spiral centerline.The effect of pulsating ventilation was not ideal for the emission of carbon monoxide gas after blasting,but it greatly improved the dust reduction effect of the spraying process,and verified that the pulsating principle is related to the density of pollutants.The pollutant discharge effect had no obvious relationship with the amplitude,but had a close relationship with the frequency.And the higher the frequency,the less the pollutant discharge effect.It was recommended that in the actual application of pulsating ventilation,only a small amplitude and frequency should be set according to the actual supply wind speed.