Study On Parameters And Flow Characteristics Of Coordinated Operation Of Upstream Longitudinal Ventilation And Downstream Natural Smoke Exhaust In Tunnels

With the rapid development of China's economy,the ground transportation infrastructure has been unable to withstand the pressure brought by the sudden increase in the population of the city and the number of cars.Therefore,transfer the traffic volume to underground space has become the main method to alleviate the pressure of traffic congestion.As the main structural form of underground traffic,tunnels have attracted much attention due to the characteristics of high accident rate,narrow and long structure,poor visibility and complex air distribution.Horizontal,vertical and natural smoke extraction are the common smoke extraction methods with their own advantages and disadvantages.Compared with the single smoke exhaust mode,the combined smoke exhaust mode which combine the vertical exhaust upstream and the natural exhaust downstream can play a role in promoting advantages and avoiding disadvantages.In this paper,through the salt water experiment and Matlab image processing technology,the key parameters of the vertical wind speed upstream of the combined smoke exhaust and the size of the downstream exhaust well(height,cross-sectional area of the exhaust pipe)are quantitatively studied.The quantitative smoke data obtained from the experiment,combined with the qualitative flow characteristics of brine near the smoke well,are comprehensively analyze whether the matching of upstream and downstream parameters is reasonable.Further quantify the influence of various factors on the smoke exhausting effect through orthogonal experiments.Firstly,this paper introduces the principle of salt water experiment,demonstrates the feasibility of using brine water experiment to study fire smoke flow,and expounds the similarity principle of brine water experiment and the method of extracting experimental data.The salt water experiment was compared with the small-scale experiment and the full-scale experiment,and the advantages and disadvantages of the salt water experiment were pointed out to conduct a more in-depth and comprehensive study.Then,this paper introduces the quantitative acquisition methods of the important data such as the thickness of the brine layer and the flow rate of the brine layer,and calculates the parameters such as the system smoke exhaust efficiency and the smoke well working efficiency to measure the upstream longitudinal wind speed and downstream size of smoke efficiency to the effect of smoke efficiency.Through the quantitative data combined with the flow characteristics study,it is found that the upstream vertical wind speed excessively large,which will cause the decrease of exhaust efficiency.Allow a certain back-layering can improve the smoke exhaust efficiency;The shaft large size at downstream will cause a decreasing in the working efficiency of the shaft.After that,based on the preliminary conclusions of the previous study,a reasonable level is designed for the four factors of the size(cross-sectional area,height),upstream wind speed,and location of the fire source.Through orthogonal experiment research the primary and secondary of various factors on the efficiency of smoke exhaust,the length of back-layering,the thickness of the smoke layer downstream of the shaft were analyzed.The optimal balance method is used to select the optimal level of each factor,and combine the optimal level to obtain the optimal working condition.The reasonable design range of the upstream wind speed in the optimal working condition is experimentally determined.Finally,the optimal matching parameters of the upstream longitudinal wind speed and the downstream smoke well size obtained by the experiment are compared and verified by FDS numerical simulation.The results show that the numerical simulation is consistent with the experimental data,and the The accuracy of the numerical simulation are verified to experimental results in some extent..The accuracy of the experimental parameters was further verified by comparing the calculation results of the critical wind speed mathematical model proposed by the predecessors with the upstream longitudinal wind speed under the experimental no back-layering condition.For the super-large fire source power that may occur in the experiment,the simulation cannot be performed because the experiment cannot achieve a larger brine water volume.The parameters that are reasonably matched upstream and downstream will be supplemented by numerical simulation.The upstream longitudinal wind speed obtained and the reasonable matching parameters of the downstream exhaust shaft size in this study can be used as reference for practical engineering design.