3.5.2. Radial Distribution of Droplet Diameter under Different Aspect Ratios

Figure 13 depicts the relationship between mean droplet diameter and distance from the nozzle for different *L/Ds* at different working pressure levels. For the nozzles with the same *L/D*, the slope of the exponential curve of the droplet diameter decreased with increasing pressure, i.e., the radial increase trend of the droplet diameter decreases with increasing pressure. Under the same pressure, shape, inlet cone angle, outlet diameter, and different aspect ratios, the droplet diameter at each measuring point was almost the same within 6 m from the nozzle position. The relationship between average droplet diameter distance from the nozzle generally followed the following order of E3 > E2 > E1, i.e., the larger the *L/D*, the larger the average droplet diameter. The E3 nozzle (largest *L/D*) had the shortest wetted radius and the largest overall droplet diameter. The E1 nozzle (smallest *L/D*) had the smallest droplet diameter along the radial direction and a relatively large wetted radius. Therefore, nozzles with a small *L/D* should be selected in sprinkler spraying.

**Figure 13.** Relationship between mean droplet diameter and distance from the nozzle for different aspect ratios at different working pressure levels. (E1, E2, and E3 refer to the elliptic nozzles with aspect ratios of 1.43, 2, and 2.58, respectively).

#### *3.6. Droplet Velocity Distribution*

3.6.1. Droplet Velocity Distribution of Nozzles with Different Shapes

The velocity of spraying droplets is an important factor to determine the kinetic energy of striking droplets. 2DVD was used to measure the velocity of droplets with different diameters sprayed from different nozzles.

There is a logarithmic function that can express the relationship between droplet diameter and droplet velocity. The relationship is as follows [18]:

$$\mathbf{v} = \mathbf{a} - \mathbf{b} \ln(\mathbf{d} + \mathbf{c}) \mathbf{a},\tag{7}$$

where a, b, and c are fitting coefficients, v is the droplet velocity, and d is the droplet diameter.

Figure 14 shows the results where the fitting correlation coefficient R2 was between [0.93, 0.97]. As it can be observed, for the nozzles with the same inlet cone angle and outlet diameter, the velocity curve slope of the circular nozzle was the largest and that of the elliptical nozzle was the smallest, i.e., with the increase of the droplet diameter, the increase rate of the velocity of the elliptical nozzle droplets was the smallest. The droplet velocity increased with increasing droplet diameter, and the increasing trend gradually decreased, indicating that droplet diameter is an important factor affecting droplet velocity. In general, the fitted curves of the three nozzles with equal flow rate were almost identical. It shows that, when the flow rate, inlet cone angle, and outlet diameter are the same, the nozzle outlet shape has little effect on the relationship between average droplet diameter and droplet velocity.

**Figure 14.** Relationship between average droplet diameter and velocity sprayed from nozzles with different shapes. (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).

3.6.2. Droplet Velocity Distribution under Different Aspect Ratios

The relationship between the diameter and velocity of droplets sprayed from elliptical nozzles with different aspect ratios is shown in Figure 15. The fitting correlation coefficient R2 was between [0.86, 0.98]. As it can be observed in Figure 15, for elliptical nozzles with the same inlet cone angle and outlet diameter, and different aspect ratios, the velocity curve slope followed the following order: E3 > E2 > E1, i.e., the velocity curve slope increased with increasing *L/D*. In addition, the magnitude of droplet velocity increased with increasing droplet diameter. At the same droplet diameter, the droplet velocity of the E2 nozzle was always the maximum. When the droplet diameter was less than 2.5 mm, the droplet velocity of the elliptical nozzle followed the E1 > E3 order. When the droplet diameter was greater than 2.5 mm, the droplet velocity followed the opposite order (E3 > E1). This indicated that there was an aspect ratio between the maximum and minimum aspect ratios. At this aspect ratio, the velocity of droplets with the same diameter was the highest, and therefore, the kinetic energy of the striking droplets was the highest.

**Figure 15.** Relationship between average droplet diameter and velocity sprayed from elliptical nozzles with different aspect ratios. (E1, E2, and E3 refer to the elliptic nozzles with aspect ratios of 1.43, 2, and 2.58, respectively).

The relationship between the diameter and velocity of droplets sprayed from diamond nozzles with different aspect ratios is demonstrated in Figure 16. The fitting correlation coefficient R2 was between [0.93, 0.96]. As it can be seen in Figure 16, for nozzles with the same shape and inlet cone angle, the velocity curve slope followed the following order: D3 > D2 > D1, i.e., the velocity curve slope decreased with increasing *L/D*. In general, the smaller the *L/D*, the higher the droplet velocity, which increased with increasing droplet diameter. When the droplet diameter was less than 5 mm, the D1 > D2 > D3 order was followed, indicating that, when the droplet diameter is the same, the smaller the *L/D*, the lower the droplet velocity.
