Discussion On Numerical Simulation Meshing Method For Smoke Flow In Metro Tunnel

The urbanization process is accelerating,and a large number of people move into cities,posing great challenges to the urban environment and ground transportation.Urban rail transit,such as subways and light rails,has greatly eased the pressure on ground traffic.The rapid development of urban rail transit has also brought challenges to its fire safety.There are few entrances and exits in the subway tunnel,narrow passages,long evacuation distance and more people.Therefore,tunnel fires are likely to cause panic.Tunnel fire safety and related research are receiving more and more attention.At present,there are two research methods for the flow characteristics of tunnel fire smoke at home and abroad: experimental research and numerical simulation.Most of the research uses a combination of small-scale model test and CFD software numerical simulation.The commercial CFD software performs numerical simulation of tunnel fire smoke,especially for long-distance tunnels,which requires a large number of grids and a long computer simulation run time.Therefore,the numerical simulation meshing method for the metro tunnel fire is studied to explore a grid with high calculation accuracy and relatively simple engineering application.The division method reduces the calculation time while ensuring the calculation accuracy,thereby simplifying the numerical simulation of the tunnel fire smoke flow to make it practical.The research on meshing method is of great significance for the design and evaluation of tunnel smoke control system.The author summarizes the domestic and international research on the numerical simulation of tunnel fire smoke flow,and summarizes the domestic and international research status of tunnel fire numerical simulation grid division.The paper summarizes the fluid mechanics theory related to the numerical simulation of smoke flow in interval tunnel fires,and the combustion model,turbulence model and thermal radiation model related to smoke flow simulation in CFX software.The CFX software is used to simulate the flue gas flow and the reliability criterion of grid reliability verification.The test results of the 1:10 small-size tunnel model test bench verify the independence parameters of CFX software and the reliability of CFX research tunnel fire smoke flow.The length dimension of the tunnel is much larger than the tunnel width and height,and it is different from the geometrical characteristics of the general building space.According to the characteristics of tunnel smoke flow,the concept of partitioning and segmentation grid is proposed,that is,the fine grid is used in the vicinity of the fire source,and the coarse grid is used in other areas.On this basis,the effects of tunnel fire heat release rate and longitudinal wind speed on the length of fine mesh region and the relationship between coarse mesh size and fine mesh size are studied.It is verified in the simulation of the long interval tunnel that the meshing method can greatly reduce the numerical simulation time.The meshing method is applied to the engineering case,and the simulation results are compared with the temperature measurement results of the full-scale test tunnel ceiling.The application of the meshing method proposed in the paper is discussed.The paper proposes the CFX software to simulate the smoke flow of the interval tunnel and the criterion of grid independence verification,and the grid size can be determined according to the fire source characteristic diameter D*.When the heat release rate of the interval tunnel fire source is less than 7.5 MW,the ratio of the coarse and fine mesh sizes may be 3:1,that is,the coarse mesh size of the far fire source region may be between 1-3 times of the fine mesh size.The heat release rate of different fire sources,the length of the fine mesh region are different,the heat release rate of the fire source is larger,and the length of the fine mesh region is larger.The fine mesh areas of the fire source heat release rates of 2.5 MW,5 MW,and 7.5 MW are 100 m,150 m,and 200 m,respectively.The meshing method proposed in the paper is applied to practical engineering cases,and the credibility of the meshing method in practical engineering applications is verified.