## 3 Experimental Results and Analysis
### Effect of Vertical Pressure on Heat Transfer  
Figure 3 shows the change in average temperature at the measurement points (a4, b4, and c4) in the middle plane of the bulk grain pile under different vertical pressures. The change trend of the average temperature at the measurement point is basically the same under different vertical pressures; however, the temperature measured at the same instant is different. When the high-temperature boundary wall is set at 35 \({}^{\circ}\)C and the low-temperature \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Parameter** & **Value** & **Unit** \\ \hline \multirow{4}{*}{Bulk grain pile} & Density & 599 & kg/m\({}^{3}\) \\  & Thermal conductivity & 0.1128 & W/(m-K) \\  & Specific heat capacity & 1670.39 & J/(kg-K) \\  & Porosity & 0.49–0.0015P\({}^{0.7287}\) & – \\ \hline \multirow{4}{*}{Air} & Density & 1.225 & kg/m\({}^{3}\) \\  & Thermal conductivity & 0.0242 & W/(m-K) \\  & Specific heat capacity & 1006.43 & J/(kg-K) \\ \cline{1-1}  & Dynamic viscosity & \(1.79\times 10^{-5}\) & Pa\(\cdot\)s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of bulk grain pile and air. Figure 2: Temperature change trend of grain bulk under different pressures: (**a**) 50 kPa; (**b**) 100 kPa; (**c**) 200 kPa. **Note: The points a2–a6, b2–b6, and c2–c6 are shown in Figure 1**. boundary wall at 10 \({}^{\circ}\)C, with the initial bulk grain temperature at 25 \({}^{\circ}\)C, the average grain temperature exhibited an increasing trend for 0-5 h that gradually decreased after 5 h. When the vertical pressure was 50 kPa, the average grain temperature was the highest. After 72 h of static storage, the grain temperature decreased by 0.6 \({}^{\circ}\)C. At 100 kPa, the grain temperature decreased by 1.2 \({}^{\circ}\)C, as compared to its initial temperature. When the vertical pressure was 200 kPa, the grain temperature was the lowest. After 72 h of static storage, the grain temperature decreased by 2.7 \({}^{\circ}\)C. With the increase in vertical pressure, the porosity of the grain pile decreases, the contact area between the grain particles increases, and the thermal conductivity of the grain particles increases. The thermal conductivity of the bulk paddy pile is 0.1128 w/(m\(\cdot\)K), which is 4.7 times the thermal conductivity of air. Heat transfer through micro-airflow between the pores is reduced. When there is a temperature difference between the low-temperature boundary wall and bulk grain pile, heat transfer from the grain pile to the nearby low-temperature boundary wall is accelerated, and the average grain temperature difference at the low-temperature boundary decreases. Temperature difference is the main factor affecting the heat transfer in grain piles. The variation trend of the temperature difference between the middle plane of the grain pile and the measurement point close to the low-temperature boundary wall was studied under different vertical pressures. As the measurement point is close to the low-temperature boundary wall, the temperature dropped rapidly, and the temperature difference (between the middle plane and the measurement point near the low-temperature boundary) increased gradually with the increase in storage duration. The variation trend of the temperature difference between the two sections is shown in Figure 4. The variation of temperature with time under different pressures is essentially the same. There are differences in temperature observed between the middle plane of the bulk grain pile under different pressures and the measurement points near the low-temperature boundary wall at the same instant. When the bulk grain pile was stored for 72 h, the temperature difference was the largest at 50 kPa, which was 9.4 \({}^{\circ}\)C. When the pressure was 100 kPa, the temperature difference was 8.8 \({}^{\circ}\)C and when the pressure was 200 kPa, the temperature difference was 7.7 \({}^{\circ}\)C. As the vertical pressure increased from 50 kPa to 200 kPa, the temperature difference rate reached 18%.