## 4 Numerical Simulation Results and Analysis
### Change in Area of High-Temperature Region  
In the process of long-term grain storage, due to grain and microbial respiration, the heat in the grain pile accumulates, leading to the rise in the local temperature of the grain pile. If the grain is exposed to the high-temperature region for a long time, mildew is formed, resulting in the decline of seed vigor, the decrease of germination rate, and even the loss of seed value. The temperature of the test chamber is 25 \({}^{\circ}\)C, and the region with the temperature higher than 25 \({}^{\circ}\)C is defined as the high-temperature region. The temperature value of each point in the bulk grain pile was obtained by numerical simulation, and the area of the high-temperature region in the grain pile was calculated.  
Figure 9 shows the change in the area of the high-temperature region with increase in storage duration under different pressures. The results show that the proportion of the area of the high-temperature region decreases linearly with the storage time under different pressures. It is expressed as:  
\[\eta_{T}=\eta_{0}-aT \tag{5}\]  
where \(\eta_{T}\) is the proportion of the area of the high-temperature region (%), \(T\) is the storage duration (h), \(\eta_{0}\) is the proportion of the area of the high-temperature region of the grain pile initially under various pressures (%), and \(a\) is the reduction coefficient of the area of the high-temperature region of the grain pile with the storage duration. For parameter \(a\), when the vertical pressure is 50 kPa, \(a\) is the minimum with a value of 0.0003 (R\({}^{2}\) = 0.99). With the increase in the vertical pressure, the value of \(a\) increases. When the vertical pressure is 100 kPa, \(a\) is 0.0005 (R\({}^{2}\) = 0.98), and at 200 kPa, it is 0.0007 (R\({}^{2}\) = 0.98). When R\({}^{2}\) is not less than 0.95, it is indicative that the mathematical model selected is suitable for the fitting of the area of the high-temperature region of the bulk grain pile under the simulated conditions when the vertical pressure range is approximately 50-200 kPa.  
It can be seen from Figure 9 that under different vertical pressures, the change trend of the areas of the high-temperature region is essentially the same, and there are differences  
Figure 8: Temperature distribution of the middle plane of grain pile under different pressures: (a) 50 kPa; (b) 100 kPa; (c) 200 kPa.  
in the of the areas of the high-temperature region under different pressures calculated at the same time instant. When the grain storage for 72 h under a vertical pressure of 50 kPa was simulated, the proportion of the areas of the high-temperature region decreased from 50.77% to 48.21%, with a change rate of 5.04%. When the vertical pressure increased to 200 kPa, the proportion of the area of the high-temperature region decreased from 49.21% to 43.41% and the change rate was 11.79%. With the increase in the vertical pressure, the proportion of the area of the high-temperature region decreases. As the pressure changes, the porosity of the bulk grain pile and subsequently affects the heat transfer rate, and with the change in the storage time, the change in the area of the high-temperature region is significantly affected by the pressure.