3.5.1. Radial Distribution of Droplet Diameter under Different Pressures

The commonly used methods for calculating the droplet diameter at home and abroad include the number-weighted method, the water-weighted average method, and the median diameter method [31,32]. In this study, the water-weighted average method was used to calculate the average droplet diameter at each measuring point. The Exp2PMod1 exponential fitting model was used, and its equation is as follows:

$$\mathbf{cd} = \mathbf{a} \mathbf{e}^{bl} \,, \tag{6}$$

where a and *b* are fitting coefficients, d is the droplet diameter, and *l* is the distance from the nozzle position.

After fitting the measured data, it was found that the fitting correlation coefficient R2 of each nozzle shape under each working pressure was between [0.92, 0.99], which was larger than 0.9, indicating that the fitting accuracy of this exponential function was high. The results for each nozzle are presented in Figure 12. As it can be observed in Figure 12, in all cases, the higher the pressure, the smaller the slope of the exponential function curve and the smaller the average droplet diameter at the same measuring point. This indicated that the increasing trend of the droplet diameter along the distance from the nozzle decreased with increasing pressure and the jet break-up was more severe. Among them, the slope of the radial profile of the C2 nozzle decreased the most. As shown in Table 5, among these three nozzles under the same pressure, the slope of the radial profile of the diamond nozzle was the largest and that of the circular nozzle was the smallest. It indicated that the droplet diameter of the diamond nozzle increased the most along the distance from the nozzle position.

**Figure 12.** Relationship between average droplet diameter and distance from the nozzle. (C2 refers to the circular nozzle with a diameter of 5 mm; D2 refers to the diamond nozzle with an aspect ratio of 1.32; E1 refers to the elliptic nozzle with an aspect ratio of 1.43).

In addition, when the droplet diameter was 3 mm, the distance from the C2 nozzle at 100 kPa, 150 kPa, and 200 kPa was 7.9 m, 11 m, and 12.4 m, respectively; the distance from the D2 nozzle at 100 kPa, 150 kPa, 200 kPa, and 250 kPa was 7.6 m, 10 m, 11.7 m, and 12.5 m, respectively; and the distance from the E1 nozzle at 100 kPa, 150 kPa, 200 kPa, and 250 kPa was 8.5 m, 9.5 m, 10.7 m, and 12.3 m, respectively. At 300 kPa, the droplet diameters of the diamond and elliptical nozzles were less than 3 mm. In general, droplets with a smaller diameter tend to drift and have evaporation losses, while larger-diameter droplets can cause greater damage to the soil surface, which is not conducive to water and soil conservation and crop growth. Thus, the droplet diameter range suitable for spraying is within 1~3 mm [15]. Consequently, the E1 nozzle had the optimal wetted radius except for the 100 kPa case, where the C2 nozzle had the optimal wetted radius. When the distance was larger than 8 m, the pressure had a significant effect on droplet diameter.
