## 2 Materials and Methods
### Numerical Simulation Models and Methods
#### 2.3.3 Mesh Model and Parameter Settings  
To ensure comparability with the experimental results, the scale of the CFD model and the experimental model is 1:1, as shown in Figure 3. A tetrahedral mesh partitioning scheme was adopted after optimizing the geometric model of the silo. The total grid number was 680,000, and the transient calculation time step (i.e., time interval) was set as 6 s.  
The inlet of the model of numerical simulation was set as the mass flow inlet with the value of 0.51 kg/s. The outlet of the numerical model was set as the pressure outlet. Considering that the walls of the silo were all insulated during the experiment, the wall of the silo for the numerical simulation was set as a non-slip adiabatic wall. To maintain consistency with the experiment, the initial temperature of the wheat pile in the numerical simulation was defined based on the average temperature of each layer measured experimentally before conducting low-temperature ventilation drying. The initial temperature of the first layer was 33.7 \({}^{\circ}\)C, the initial second layer was 34 \({}^{\circ}\)C, the initial temperature of the third layer was 28.4 \({}^{\circ}\)C, and the initial temperature of the fourth layer was 25 \({}^{\circ}\)C. The density of wheat was set as 800 kg/m\({}^{3}\), and the initial dry basis moisture content was 12.5%. The temperature of drying air was set as 15.7 \({}^{\circ}\)C, and its relative humidity was 60%. In addition, the internal pressure caused by self-weight was relatively large due to the 3 m height of the wheat pile, resulting in a non-uniform distribution of porosity inside the grain pile. Therefore, the porous medium model in simulation adopted a non-uniform porosity distribution to maintain consistency with the experiment, and the effect of the uniform porosity condition on numerical simulation results was specially studied here. Considering both simulation accuracy and time, the convergence residuals of the continuity equation, momentum equation, and air moisture transfer equation were set as 1.0 \(\times\) 10\({}^{-3}\), and the value for the energy equation was set as 1.0 \(\times\) 10\({}^{-6}\). The pressure-velocity decoupling method utilizes the SIMPLEC algorithm. The relaxation factor of energy and UDS was set as 0.8, and the transient equation scheme was set as first-order implicit. Moreover, the viscous resistance coefficient and inertial resistance coefficient were calculated using the Ergun equation [35] with values of 2.21 \(\times\) 10\({}^{7}\) m\({}^{-2}\) and 1.15 \(\times\) 10\({}^{3}\) m\({}^{-1}\), respectively. The non-uniform porosity distribution formula of wheat grain piles in silos in this study is as follows [19], with the variation range of non-uniform porosity with a depth of 0.37-0.42 being obtained via Equation (10), and the average value of 0.40 being taken as the uniform porosity:  
\[\varepsilon=\varepsilon_{0}-(\varepsilon_{0}-\varepsilon_{\text{min}})\cdot( 1+0.016H_{b}+0.002t-1.013) \tag{12}\]  
where \(H_{b}\) is the depth of the grain unit body; \(\varepsilon\) is the porosity of the grain pile at the corresponding depth \(H_{b}\); \(\varepsilon_{0}\) is the initial porosity; \(\varepsilon_{\text{min}}\) is the minimum porosity inside the grain pile, which can be considered as the porosity of the bottom surface of the grain pile; and \(t\) is the storage time of wheat, in the numerical simulation of this paper, \(t\leq\) 720 h.  
Before the ventilation test was conducted, the wheat in the silo was subjected to static treatment to achieve an equilibrium state between the temperature of the wheat pile and the temperature of the air inside the pores. Hence, the initial temperature of the wheat piles and the initial temperature of the air inside the wheat piles were set to be the same. Then, the cold air was supplied to the wheat piles through the ventilation cage, and the cold air  
Figure 3: Geometric mesh of the numerical model. entering the inside of the wheat pile would undergo heat exchange with the grain pile to achieve the purpose of drying.