Experimental research on the jet-breaking characteristics and hydraulic performance of a novel automatic rotating sprinkler

Sprinkler irrigation equipment comprises water pumps, piping systems, monitoring systems, and sprinklers (Johar et al. 2018). It can save water, avoid damage to the soil structure, save time and effort, and adjust the field microclimate. Sprinklers are a critical component of sprinkler irrigation equipment, and their performance directly determines the quality of sprinkler irrigation. With the continuous development of science and technology, the sprinklers in sprinkler irrigation equipment gradually develop toward low-pressure operation conditions; thus, the entire sprinkler irrigation system has an energy-saving effect (Hui et al. 2021). The sprinkler, according to the working pressure, can be divided into low-pressure, medium-pressure, and high-pressure sprinklers, wherein low pressure is less than 200 kPa; medium pressure is between 200 and 500 kPa; high pressure is greater than 500 kPa (Gilley & Watts 1977). However, the lower working pressure of impact sprinklers reduces the degree of a jet breakup, worsening the sprinkler irrigation quality, i.e., leading to a shorter radius of throw, lower uniformity of combined sprinkler irrigation, and higher kinetic energy of water droplets, which can affect the yield and quality of crops (Issaka et al. 2018). Therefore, the hot and complex point of current research is to ensure better sprinkler irrigation quality under low-pressure conditions.

To improve the irrigation performance of impact sprinklers under low working pressure, current research is mainly focused on special-shaped nozzles (Chen et al. 2022), sprinkler aeration (Xiang et al. 2021), and water dispersion devices (Xu et al. 2012). Regarding special-shaped nozzles, researchers have exploited the asymmetric structural characteristics of nozzles with non-circular cross-sections to transform the axisymmetric flow of the water jet into non-axisymmetric, which increases the instability of the water flow and, in turn, causes the water jet to breakup and thus improve the sprinkler irrigation performance of impact sprinklers under low-pressure conditions. At present, several studies mainly concern the jet-breakup law of special-shaped nozzles under low working pressure and their hydraulic performance. For example, Sharma & Fang (2014) analyzed the breakup length, width, axis conversion phenomenon, and atomization degree of the jet ejected through special-shaped nozzles, and concluded that the jet fragmentation length of the non-circular nozzle was shorter than that of the circular one. Hua et al. (2019) simulated the jet-breakup characteristics of nozzles with different outlet shapes using the large-eddy simulation (LES) and volume-of-fluid (VOF) methods and revealed that, for non-circular nozzles, the jet breakup was more sufficient in the near-field region. Moreover, they achieved the best water distribution. Wang & Fang (2015) used Rayleigh's vibration theory to rationally explain the breakup mechanism of special-shaped nozzles. It was proved that this theory can better explain the liquid jet-breakup phenomenon. Jiang et al. (2020a, 2020b) investigated the surface wave shape, breakup process, and flow characteristics of the jet produced by special-shaped nozzles, and found that the special-shaped nozzle could not only promote the breakup of the jet but also improve the hydraulic performance of the sprinklers. Chen et al. (2011) found that the irrigation uniformity of special-shaped nozzles is higher than that of circular nozzles. Yuan et al. (2010) reported that special-shaped nozzles can reduce the intensity of raindrops and improve sprinkler irrigation uniformity. Hua et al. (2018) studied the effect of special-shaped nozzles on the hydraulic performance of impact sprinklers and concluded that non-circular nozzles can improve the water distribution uniformity of low-pressure sprinklers. Although special-shaped nozzles can improve sprinkler irrigation uniformity, this method can shorten the sprinkler pattern radius by about 5–20% (Zhou et al. 2017). In combined sprinkler irrigation, the number of sprinkler heads increases for the same sprinkler irrigation area, leading to an increase in investment at the early stage of developing the sprinkler irrigation system.

In aerated sprinklers, the gas disturbs the water flow by increasing the turbulence and causing the water jet to breakup; thus, the hydraulic performance of the sprinkler under low working pressure is improved. Currently, the research is mainly focused on the droplets of the jet breakup and the hydraulic performance of aerated sprinklers. For instance, Wang et al. (1989) found that the degree of atomization in multi-hole aeration is higher than that of single-hole aeration under low-pressure conditions. Lin et al. (2020) explored the effect of jet aeration on droplet structure and morphology and found that the droplet size decreases with the increase of the aerated mass ratio. Xiang et al. (2021) analyzed the droplet diameter distribution of aerated jets and concluded that the average water droplet diameter of aerated sprinklers is higher than that of unaerated ones. Jiang et al. (2020a, 2020b) found that the aeration of impact sprinklers can improve the radial water distribution as well as the overlapping uniformity. Although adding air to the sprinkler can improve its hydraulic performance, this method is not fully mature in sprinkler irrigation systems. This is mainly due to that if the aeration volume cannot be accurately controlled, the axial velocity of the jet will be attenuated, thereby affecting the hydraulic performance of the sprinkler (Wei et al. 2013).

In impact sprinklers equipped with a fixed water dispersion device, researchers in related fields have mainly used water dispersion devices to forcefully break the liquid column of the jet and improve sprinkler irrigation performance under low-pressure conditions. Since the method is relatively simple to implement, it has been widely promoted in the market of impact sprinklers. Li et al. (2019) found that fixed water dispersion devices can increase the jet diffusion angle, improve radial water distribution, and improve the uniformity of sprinkler irrigation. Jiang et al. (2019a, 2019b) investigated the initial breakup length of the jet under a fixed water dispersion device and derived an empirical equation for the sprinkler pattern radius related to the water dispersion device. Issaka et al. (2020) found that a fixed water dispersion device can improve the radial water distribution of an impact sprinkler, reduce the kinetic energy of water droplets, and improve sprinkler irrigation uniformity. Nevertheless, this method comes with certain limitations since, when the depth of the fixed water dispersion device inserted into the jet liquid column is shallow, the jet fragmentation degree is weak, and the effect of improving the hydraulic performance of the nozzle cannot be achieved. When the fixed water dispersion device penetrates deeper into the water, it can increase the fragmentation of the water jet and enhance the uniformity of the combined sprinkler irrigation; however, it will further reduce the radius of the throw. Therefore, in practical applications, it is difficult to precisely control the depth of the fixed water dispersion device inserted into the water jet liquid column and achieve optimal sprinkler irrigation performance.

Based on the above literature review and the present situation, it can be deduced that the methods employed to improve sprinkler irrigation performance under low working pressure have some flaws. To make the impact sprinkler have better hydraulic performance under low pressure, that is essential to transform the existing impact sprinkler and implement a new sprinkler with a novel water dispersion device. In this paper, three new types of automatic rotating sprinklers are designed, each of which is equipped with a novel water dispersion device. To test the performance of the new sprinkler heads, high-speed photographic experiments are conducted to compare and discuss the jet-breakup morphology of the new sprinklers and the prototype impact sprinkler (model: PY 15). Moreover, hydraulic performance experiments are performed to compare and discuss the hydraulic performance of the different sprinklers in detail. This work aims to provide a reference for the design and optimization of impact sprinklers under lower pressure.

To ensure that the structural parameters of the various fixed water dispersion devices are consistent with the parameters on the design drawings, 3D printing technology (Shahrubudin et al. 2019) was used to construct the fixed water dispersion devices.

Due to the jet-flow entraining the air, the cross-section of the jet-flow will become more prominent; thus, a ring-shaped water dispersion device (RSWDD) with a circular and square inner hole section was designed separately. When the water jet with a larger cross-section hits the RSWDD, the outer wall will cut the jet surface. Moreover, the inner holes of the RSWDD will eject the fluid of the jet core area. The fluid ejected from the outer wall will replenish the water at the near and middle positions of the radius of throw, while that ejected from the inner hole will replenish the water at the far end and the end of the radius of throw. The fluids on the jet surface and the core area complement each other to improve the hydraulic performance of the sprinkler.

A unified standard was established to assess the jet breakup characteristics and hydraulic performance of sprinklers with RSWDDs and NSWDDs. The RSWDD was first installed at a position 50 mm away from the nozzle outlet, then, the swingarm of the impact sprinkler was removed, and the impact sprinkler was set to not rotate under the working pressure of 100 kPa. By adjusting the structure size of the RSWDD inner hole, it was ensured that the spray distance of the jet was equal to that under the NSWDD. This way, the structural dimensions of the RSWDD were determined, as shown in Table 1.

In order to hinder the effect of the varying rotating speed on the hydraulic performance of the sprinkler, an automatic rotating device was designed, which included a gear retarding device, a motor device, and a speed control device. Through the cooperation of these three devices, it is effortless to maintain the speed of the automatic rotating device consistent with the average speed of the impact sprinkler under different working pressures during regular operation.

The advantage of utilizing the combined uniformity of sprinkler irrigation in the experiments is that the amount of calculations is of small-scale and only requires the input of the measurement results into Equation (3). However, due to the limitations related to the sprinkler's working pressure, wind speed, and topography, the reproducibility of the experimental data becomes demanding, and the results of the CU value can be diverse. To address the above-mentioned restrictions, the cubic spline interpolation model (Liu et al. 2021) was employed to simulate the water overlap of the sprinklers in the square combination arrangement. The conversion method between radial water volume and grid nodes refers to the method reported by Zhu et al. (2015).

Table 2 shows the sprinkler pattern radii of different sprinklers under different working pressures. Table 3 shows the loss percentage of the pattern radius of different sprinklers after the working pressure has been reduced.

According to Table 2, the sprinkler pattern radius of the automatic rotating sprinklers and the original impact sprinklers with different water dispersion devices increased gradually with increasing working pressure. When the working pressure was 200 kPa, the pattern radii reached the maximum value. The sprinkler pattern radius of the original impact sprinkler was the farthest under the same working pressure. This is attributed to that, when the liquid column is not affected by the swingarm, it behaves like a free jet, and it is affected only by the air resistance, air entrainment, and gravity. The automatic rotating sprinkler with the water dispersion device C had the second-farthest pattern radius under the same working pressure, which is because the jet is not only affected by the above parameters, but also by the circular inner hole and the outer wall of the RSWDD. The automatic rotating sprinkler with the water dispersion device B had a shorter range than that with the water dispersion device C under the same working pressure. The reason is that the shape of the cross-section of the inner hole of the water dispersion device C was similar to that of the jet and had little effect on the jet column. On the other hand, the cross-section of the inner hole of the water dispersion device B was square, which is quite different from the shape of the jet liquid column and had a more significant impact on the jet surface. The automatic rotating sprinkler with the water dispersion device A had the smallest range under the same working pressure since the NSWDD destroyed the core area of the jet.

According to Table 3, when the working pressure was reduced from 200 to 150 kPa, the pattern radii of the original impact sprinkler and the automatic rotating sprinklers were reduced to varying degrees. The pattern radius of the automatic rotating sprinkler with the water dispersion device A was reduced the most, reaching 8.94%. Then, the reduction degree of the original impact sprinkler reached 8.28%, followed by that of the sprinkler with the water dispersion device B, which was reduced by 7.69%, while that of the sprinkler with the water dispersion device C underwent a minor reduction of 5.07%. When the working pressure was reduced from 150 to 100 kPa, the pattern radius of the original impact sprinkler and automatic rotating sprinklers with water dispersing devices A, B, and C decreased further, i.e., 9.77, 15.18, 16.67, and 16.79%, respectively. This indicates that, when the working pressure is not significantly decreased from intermediate pressure to low pressure, the effect of the RSWDD on the pattern radius is weaker than that of the original impact sprinkler, while that of the NSWDD is stronger than that of the original impact sprinkler. When the low working pressure is further decreased, the effect of the water dispersion devices on the pattern radius is stronger than that of the original impact sprinkler.

In addition, the radial water distribution curve of the automatic rotating sprinkler equipped with the water dispersion device A exhibited a valley peak shape, and the peak value gradually decreased with increasing working pressure. The peak is mainly attributed to the incomplete breakage of the jet by the NSWDD, which causes the water distribution to be concentrated at the end of the pattern radius, thereby promoting a higher water application rate.

Combined uniformity is a crucial index used to evaluate the hydraulic performance of sprinklers. Table 4 lists the combined uniformity of the square layout of the sprinklers equipped with different water dispersion devices at different combination spacings. It can be observed that, under the same working pressure and combined spacing, the CU of the automatic rotating sprinkler was higher than that of the original impact sprinkler, mainly because the jet of the automatic rotating sprinkler was broken under the action of the water dispersion device. Nevertheless, the original impact sprinkler only breaks the jet when the swingarm acts on it. Under the same working pressure and combined spacing, the sprinklers arranged according to the CU from high to low were the automatic rotating sprinklers with water dispersion devices B, C, A, and the original impact sprinkler. This phenomenon is mainly related to the jet fragmentation degree caused by the geometric structure of the water dispersion device. By comparing the CU of the automatic rotating sprinkler with the water dispersing device B under a working pressure of 100 kPa and that of the original impact sprinkler under a working pressure of 200 kPa, it can be found that the maximum CU of the former under a combined spacing of 1.2 R was 81.16%, while that of the latter under a combined spacing of 1.0 R was 74.51%. This indicates that the automatic rotating sprinkler with the water dispersion device B can significantly improve sprinkler irrigation uniformity and maximize economic benefits under low-pressure conditions.

This paper compares and analyzes of the jet fragmentation characteristics and hydraulic performance of automatic rotating sprinklers and a prototype impact sprinkler. The study finds that different types of water dispersion devices can cause variations in the jet diffusion angle under low pressure. Among them, water dispersion device B causes the largest jet diffusion angle, resulting in significant jet fragmentation and a large number of water droplets uniformly spraying onto crops and the ground, ultimately improving the combination uniformity of the sprinkler. For instance, the combination uniformity of the automatic rotating sprinkler with the water dispersion device B can reach 81.16% at a working pressure of 100 kPa, which is 6.65% higher than the combination uniformity of the prototype impact sprinkler at a working pressure of 200 kPa. This phenomenon implies that water dispersion devices with square inner holes can enhance the spray uniformity of sprinklers while saving energy, which is significant. However, due to the unique structure of the water dispersion device, it can only be installed on complete fluidic sprinklers rather than on impact sprinklers because they have a swingarm structure. Therefore, further improvement of the structure of the existing water dispersion device B is necessary.

In addition, this study proposes a new type of automatic rotating sprinkler suitable for low working pressure conditions. Due to the existence of an automatic control component in this automatic rotating sprinkler, the rotational speed can be freely controlled by the component without being affected by the working pressure, making it more suitable for low-pressure intelligent irrigation systems. However, the current power supply for the automatic rotating sprinkler is a battery, which can only work for about 6 h and cannot provide a stable power source for a long time. Therefore, the power system of automatic rotating sprinklers is also an urgent issue that needs to be addressed.

• The jet diffusion angle of the water dispersion device increases with the increase in the working pressure, but the jet diffusion angle under the influence of the water dispersion device B with a square inner hole is the largest among all water dispersion devices (when the working pressure increases from 100 to 200 kPa, the jet diffusion angle increases from 69.32° to 91°), resulting in the optimal water distribution and combination uniformity of this type of sprinkler. This result indicates that a larger jet diffusion angle can help improve the hydraulic performance of the sprinkler.

• Under the square combination irrigation with a combination spacing of 1.2 R, the irrigation depth of the automatic rotating sprinkler with the water dispersion device B increased from 3.6 to 13 mm/h as the working pressure increased from 100 to 200 kPa, and this irrigation depth was optimal among all tested sprinklers. This finding suggests that a well-designed water dispersion device can significantly enhance the combined irrigation performance of sprinklers while achieving energy efficiency, and can also prevent excessive or insufficient irrigation in the sprayed area.

This work was supported by the National Natural Science Foundation of China (51939005), Jiangsu Province and Education Ministry Co-sponsored Synergistic Innovation Center of Modern Agricultural Equipment (XTCX2018), Changzhou Key Research and Development Program (No. CE20222024) and the Youth Talent Development Program of Jiangsu University.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.