## 3 Modeling and Simulation
### Meshing and Method of Solution  
The mathematical model described above was realized in CFD software Fluent using the control volume method.  
To obtain reasonable and accurate calculation results more efficiently with limited computing resources, this paper first uses three sets of grids for pre-calculation and unstructured grid division of the artificial climate chamber model. Due to the large difference between the geometric dimensions of the artificial climate chamber model and the oxygen supply port, local grid encryption is performed near the oxygen supply port. The pre-calculation sets the same boundary conditions and only differs in the number of grid cells. The verification result of grid independence is shown in Figure 3. The number of grids is changed from 42 million to 53 million and the change in axial velocity is very small, which satisfies the requirement of independence. Therefore, considering the accuracy and computational efficiency, selecting a model with a mesh number of 42 million can meet the requirements.  
The computational grid of the model is shown in Figure 4. CFD simulations were performed using a double-precision unsteady-state implicit solver, with the coupled algorithm to solve the pressure-velocity coupling problem in the momentum equations. A  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Air Supply Format** & \multicolumn{2}{c}{**Velocity (m-s-1)**} \\ \hline Up supply down return on the same side & \(V_{\text{In}}=0.85\) & \(V_{\text{in}}=1\) & \(V_{\text{in}}=1.4\) \\ Up supply down return on the opposite side & \(V_{\text{In}}=0.85\) & \(V_{\text{in}}=1\) & \(V_{\text{in}}=1.4\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Numerical simulation conditions and contents under air-conditioning.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number of Oxygen** & **Oxygen** & **Symbol** & **Schematic Diagram** \\
**Supply Vents** & **Supply Method** & **Symbol** & **Schematic Diagram** \\ \hline Single vent & vertical & 1\# & \\ Double vents & vertical & 2\# & \\ \hline Double vents & relative 45\({}^{\circ}\) & 3\# & \\ Double vents & opposite 45\({}^{\circ}\) & 4\# & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Initial conditions of the oxygen supply vent methods.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Oxygen Supply Methods** & \begin{tabular}{c} **Oxygen Supply Vent** \\ **Diameter (m)** \\ \end{tabular} & \
\begin{tabular}{c} **Oxygen Supply** \\ **Flow Rate (m-3-h-1)** \\ \end{tabular} &
\begin{tabular}{c} **Oxygen Supply** \\ **Velocity (m-s-1)** \\ \end{tabular} \\ \hline Single vent (1\#) & \(\text{D}_{1}=0.006\) & 0,2, 0.5, 1, 1.5, 2 & 1.96, 4.91, 9.83, 14.74, 19.66 \\ Single vent (1\#) & \(\text{D}_{2}=0.01\) & 0,2, 0.5, 1, 1.5, 2 & 0.71, 1.77, 3.54, 5.31, 7.08 \\ Double vents (2\(\cdot\)3\(\cdot\)4\(\cdot\)4) & \(\text{D}_{1}=0.006\) & 0,2, 0.5, 1, 1.5, 2 & 0.98, 2.46, 4.91, 7.37, 9.83 \\ Double vents (2\(\cdot\)3\(\cdot\)4\(\cdot\)4) & \(\text{D}_{2}=0.01\) & 0,2, 0.5, 1, 1.5, 2 & 0.36, 0.89, 1.77, 2.66, 3.54 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Initial conditions of the oxygen supply parameters.  
SIMPLE discretization scheme was used for the pressure and velocity. To reduce numerical diffusion, a second-order upwind scheme was selected for the discretization of the momentum and energy equations. The solution was assumed to converge when the different residuals had reached sufficiently low steady values of \(10^{-4}\). The time step was 0.01 s, and the overall calculation time was 1200 s.