| With the rapid development of the economic construction,infrastructure construction projects are showing a trend of rapid growth.Harmful gases produced by tunnel construction and blasting seriously endanger the lives and health of construction workers.Reasonable design and solving tunnel ventilation problems have become the key to ensuring the safety of the project.This paper integrated the methods and techniques of theoretical derivation,numerical simulation,mathematical statistics and other interdisciplinary subjects.Based on the forced ventilation method during the construction period,combined with a variety of ventilation methods,the flow field characteristics,the movement of harmful gases,the calculation of the air leakage rate of the duct,the distribution function of harmful gases,and the estimation of the tunnel ventilation time were studied in depth.The main research contents and results are as follows:(1)The distribution law of vortex distribution zone,vortex influence zone,and velocity stability zone in the tunnel longitudinal flow field was revealed.Based on the forced ventilation,the calculation formula of the air leakage rate of the duct was deduced,and the attenuation of the air volume conforms to the change law of the logarithmic function was studied.It was concluded that the migration law of harmful gas from the working face after blasting was mainly manifested in movement and diffusion.(2)The relationship between the flow field characteristics and the migration of harmful gases in the tunnel was clarified by many factors.The sensitivity of tunnel ventilation time affected by different factors was analyzed.Based on the complexity of the flow pattern in the vortex zone near the working face,the influence mechanism of different factors on the migration of harmful gases was revealed.(3)The mathematical relationship between time and space distribution of CO gas in the tunnel after blasting was derived.A calculation method considering the effect of gas convection and diffusion was proposed by numerical experiments.Based on the CO gas distribution function of variables such as fan volume and tunnel cross-sectional area,an estimation function of the ventilation time required for the tunnel to reach the safe standard was constructed.The reliability of the formula was verified by engineering examples.(4)The distribution law of the smoke stagnation zone near the working face under the extracted ventilation and the migration mechanism of harmful gases were revealed.The mechanism of adding a jet fan on accelerating the migration of harmful gases in stagnant areas was clarified.Based on the results of numerical calculations,the time and cost saved by the tunnel under combined ventilation,front-pressing-behind-absorption ventilation and front-absorption-behind-pressing ventilation were estimated.The measure to improve the effective air volume reduction of far-pressing-near-absorption venitlation was proposed.According to the distribution characteristics of the flow field and the migration of harmful gases under various ventilation methods,a new circulation ventilation method was proposed,which provided a reference for the optimization of ventilation time in the project.(5)Based on the characteristics of the flow field and the migration of harmful gases,the relevant opinions on the ventilation time and construction scheme required for different excavation depths of the upstream and downstream main tunnels were put forward.The ventilation time required for tunnels with branches during the construction period under the forced,extracted,combined and circulation ventilation systems was studied.The influence mechanism of the tunnel wall on the air flow under the coupling of temperature and humidity was analyzed,which provided a reference for similar projects. |
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