## 4 Research Progress of Air-Fertilizer Mixing Devices  
The air-fertilizer mixing device is a core component of the pneumatic centralized fertilizer discharge system, using high-speed airflow to mix the air-solid two-phase flow evenly. The air-fertil mixing device uses the Venturi effect to achieve the uniform mixing of the airflow and fertilizer. It is a key component in achieving continuous fertilizer supply. Scholars at home and abroad have conducted extensive research on the internal structure and key parameters of air-fertil mixing devices, achieving good results. [PERSON] et al. [12] designed an air-fertilizer mixing device for the problem of fertilizer falling into the fertilizer inlet of the fertilizer discharge system, as shown in Figure 9. The commercial compound fertilizer was selected, with an equivalent diameter of 3.26 mm and a density of 1448 kg/m\({}^{3}\). Through a theoretical analysis, the optimal parameters for fully mixing the fertilizer and airflow field in the air-fertil mixing device were determined. Addressing the issue of the uneven distribution of fertilizers during the fertilization process in paddy fields, [PERSON] et al. (2019) designed a Venturi tube mixing cavity and analyzed the flow field. According to the angle between the fertilizer inlet and the air inlet, mixing chambers can be divided into the right angle type, the acute angle type and the obtuse angle type, as shown in Table 1. This study showed that when the diameter of the mixing chamber was 15 mm, and the fertilizer inlet and the air inlet were acutely angled, the air-fertil mixing uniformity was better, there was no backflow at the fertilizer outlet, and fertilizer transportation was faster and smoother. [PERSON] et al. (2019) used the discrete element method and computational fluid dynamics to carry out air-solid coupling numerical research on the air-fertilizer mixing device of a rice fertilizer applicator. The movement law of airflow and fertilizer particles was analyzed by studying the influence of the internal structure parameters, air inlet pressure and fertilizer discharge rate of the air-fertilizer mixing device. In order to improve the uniformity of the Venturi feeding tube, [PERSON] et al. (2019) analyzed the variation in the internal flow field under different contraction angles of the nozzle port by using the coupling simulation method. It was concluded that when the contraction angle was 70\({}^{\circ}\), the pneumatic conveying effect was the best, and the coupling field did not appear. The phenomenon of blockage and accumulation, in addition to transportation, was continuous and smooth. The test showed that the main factors affecting the uniformity of seed supply were displacement and wind speed. The structure of the seed supply tube is shown in Figure 10. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Structural Style** & **Structural Diagram** & **Characteristic** \\ \hline Right Angle Type & & There is turbulence at the fertilizer inlet, and the flow of fertilizer particles is not smooth, which is not conducive to fertilizer discharge. \\ \hline Acute Angle Type & & The high wind speed at the fertilizer inlet is conducive to the fertilizer entering the mixing chamber, and the mixing uniformity of air and fertilizer is good. \\ \hline Obtuse Angle Type & & There is turbulent flow at the inlet of the fertilizer, which is not conducive to the uniform mixing of air-solid two-phase flow. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structure analysis of air–fertil mixing devices. Figure 9: Structural schematic diagram of an air–fertilizer mixing device. 1. Air–fertil mixing chamber. 2. Transition room. 3. Fertilizer receiving room. 4. Fertilizer inlet. 5. Nozzle. To increase the uniformity of fertilizer discharge, [PERSON] et al. [59] designed a mixed fertilization device. Through simulation experiments, the movement law of the fertilizer particles in the fertilizer mixer was analyzed, and the optimal structural parameters of the fertilizer mixer were determined. [PERSON] et al.