## 2 Numerical Model Settings
### Gas Leak Model
#### 2.1.3 Simulation Setup of Leakage  
(1) Mesh Setting  
ANSYS19.2 Mesh software was used to mesh the model and generate unstructured mesh. The local mesh quality will determine the reliability of the simulation results, and the location of the leakage holes and air inlet and outlet compared to the fire interval length of 200 m is a large difference in magnitude, so the leakage holes and air inlet and outlet needed a local mesh refinement process. The total number of grids delineated is 50,138, with a grid quality of approximately 0.7. (2) Grid independency test  
Under the condition that the gas cabin has no ventilation and no baffle and the leakage speed is 50 m/s, the gas concentration at a monitoring point 7.5 m downstream of the leak and 2.7 m high is taken as the basis. The influence of mesh sizes of 0.5 m, 0.3 m, 0.15 m, and 0.1 m on the calculation results were tested, respectively. The result is shown in Figure 2. When the mesh size is 0.15 m, the mesh size is further reduced, and the change in the calculation result is not obvious. When the mesh size is 0.15 m, grid division will not affect the calculation result. (3) Solver Setup  
Fluent calculations are based on a pressure solver, which considers the convective diffusion of the leaking gas component in the cabin air in a turbulent state. k-\(\varepsilon\) models are the most commonly used turbulence models, which have a wide range of applications and high accuracy. The standard k-\(\varepsilon\) turbulence model is the most commonly used turbulence model with a wide range of applications and high accuracy. The results of the standard k-\(\varepsilon\) turbulence model meet the engineering accuracy requirements and are less expensive to calculate. Therefore, the standard k-\(\varepsilon\) model is used to simulate turbulent flow. The SIMPLE solution method is used. The unsteady calculation is adopted, and the time step is 0.5 s. The Species Transport model is also used to simulate the diffusion of gas leaks in the atmosphere, defining the component as methane-air, taking into account the effect of full buoyancy. (4) Boundary Conditions  
In gas pipeline leaks, the probability of a small bore leak is higher than that of a large bore leak as well as a pipe break [29]. Small-hole leaks are more likely to cause gas accumulation because of their small flow rates and low concentrations. This model uses a small-hole leak model with a 10 mm leak opening and makes the following assumptions: 1 the pressure inside the pipe is neglected by the influence of the leak, while the frictional influence of the gas inside the pipe is not considered; 2 the leak source is a continuous leak source, and the leak rate remains constant throughout the process, i.e., equal to the starting maximum leak rate. The simulation conditions are set as follows. 1. Initial conditions: methane normally accounts for more than 90% of the gas, and the leak component is chosen to be simulated as CH\({}_{4}\). At a time of \(t=0\), the leak has not started, the gas chamber is full of air, the CH\({}_{4}\) concentration is 0, and the pressure is 0.1 MPa at one standard atmosphere. 2. Leak source conditions: the CH\({}_{4}\) and air components involved in the simulation process do not occur during the chemical reaction, only the diffusion of components. The small hole leakage aperture was selected to be 10 mm, and the CH\({}_{4}\) component at the leak source was set to 1. The leak port was set to be a velocity inlet, and the leak volume was adjusted by changing the leak velocity, which was set to 50 m/s and 100 m/s. 3. Inlet conditions: assume that the wind direction and wind speed do not change with the vertical height of the gas leak diffusion space, that is, the wind speed involved is parallel to the ground and constant, and the component is air. According to the needs of different working conditions, the air inlet is set to speed inlet, and the ventilation wind speed is set to 0 m/s, 1.38 m/s. 4.