Characteristics of Spot Spraying and Continuous Spraying Systems

Abstract

This paper studied the atomization characteristics of different spray nozzles under the spot spraying method and designed a test system for the atomization characteristics. First, the effective spray height range was determined based on the effective droplet size of 106–403 μm, the spray height of 200–500 mm, the operating speed of 0.5–1 m/s, and the droplet size requirements. The effective height ranges of the HVV25-02, HVV40-02, and HVV50-02 nozzles are 277–500 mm, 200–426 mm, and 200–266 mm, respectively. Second, the influences of pressure, the opening time of the solenoid valve, and the nozzle aperture on the atomization characteristics were studied through experiment. The experiment was repeated three times, with 10,000 points monitored each time. The test results show that the droplet size of spot spraying decreases with the increase in pressure, while the droplet velocity and droplet distribution relative span have no correlation with pressure. With the increase in the opening time of the solenoid valve, the droplet size does not change regularly, the droplet velocity generally shows an upward trend, and the droplet distribution relative span (RS) value decreases gradually. With the increase in the nozzle aperture, both droplet size and droplet velocity increase, and the distribution span shows a trend of first increasing and then decreasing. The droplet velocity of spot spraying is 4.1 m/s lower than that of continuous spraying, on average, and the droplet distribution relative span value is 2.2 higher than that of continuous spraying. This research can provide a basis and reference for the selection of appropriate spot spraying operation parameters.

1. Introduction

Chemical pesticide application remains the primary method for pest and disease control [1,2]. There are a number of drawbacks to the conventional method of continuous spraying, including pesticide waste, low utilization rates, and excessive pesticide residue. Through integrated target identification, and more precisely, targeted application (spot spraying) of the chemical, the cost efficiency of pesticide application can be improved [3]; in particular, by adjusting the width of the array nozzle, the spray pressure, the spray time, the spray height, and the nozzle orifice size, the atomization characteristics, such as droplet size, droplet speed, and droplet uniformity, are directly affected [4,5,6], which further influences pesticide droplet adhesion, sliding, and drifting [7,8].

In studies worldwide, researchers have investigated spray atomization characteristics, both in theory and in practice. Calvert et al. [9] introduced a novel robotic spot spraying solution, namely, AutoWeed, and reported the methodology and results of an in situ field trial. The experimental results showed that in low-, medium-, and high-density pest infestations, the pest control rate of AutoWeed spot sprayers used in a cactus crop was equivalent to conventional spraying methods. Allmendinger et al. [10] introduced the combination of high-resolution drone-based weed mapping with targeted spraying of corn to measure and analyze weed and crop density, weed control effectiveness, herbicide savings, and corn yield. The test results showed that the corn yield in plots with targeted spraying was the same as that of plots under conventional spraying. These two results suggest that there is no significant difference between targeted (spot) spraying and conventional spraying in terms of pest control effectiveness and crop yield. However, the above studies did not consider the influences of point spray time, spray height, and nozzle aperture on the accuracy of point spray deposition.

In terms of continuous spraying, Li et al. [11] concluded that factors such as spray pressure (150–350 kPa) and nozzle aperture (0.8–1.5 mm) have significant influence on the atomization characteristics of the nozzle. With the increase in spray pressure, the droplet size decreases and the droplet velocity at the nozzle outlet increases. When the nozzle aperture increases, the droplet size and droplet velocity increase accordingly. Dai Qiufang et al. [12] studied and concluded that the greater the pressure (700–1400 kPa), the smaller the pore size (1.0–1.8 mm), and the finer and more uniform the droplets. Zhang Pengjiu et al. [13] found that the droplet size decreased with the decrease in the pore diameter (0.7–1.4 mm) and the increase in the pressure (500–3500 kPa). Yuan Feixiang et al. [14] used a 360° rotatable adjustable hollow cone nozzle to study atomization characteristics under different pressure conditions. Based on changes in the spatial droplet atomization characteristics and spatial distribution in the spray field under various pressure conditions, their study examined variations in droplet size, velocity, and droplet spectrum relative span at different positions. As the nozzle orifice distance increased, the hollow effect in the spray field gradually decreased, resulting in an increase in relative span. Concerning the PWM intermittent spraying aspect, Longlong et al. [15] used three types of standard fan spray nozzles (ST110-02, ST110-03, and ST120-04) produced by Lechler, Metzingen, Germany, to measure the nozzle flow rates at frequencies ranging from 12 to 37 Hz and duty cycles from 20% to 100%, and they also measured the droplet size of these three nozzles. Changyuan et al. [16] designed a pulse width modulation (PWM) control system and conducted a quadratic regression orthogonal combination design, obtaining the nozzle flow model from experiments with frequencies ranging from 2 to 10 Hz and duty cycles from 30% to 90%. Huanyu et al. [6] designed a dynamic PWM variable spraying experimental platform, setting the required frequencies from 3 to 9 Hz and duty cycles from 20% to 80% for single-factor experiments, thus evaluating the uniformity of single-nozzle dynamic spray distribution. The solenoid valve opening time was set differently in each of these studies: 5–83 ms [15]; 30–450 ms [16]; and 22–266 ms [6]. Compared to PWM spraying, spot spraying requires a longer solenoid valve opening time; however, PWM spraying does not consider the target size, whereas spot spraying can adapt to the target size by adjusting the spray angle and height. The above research has clarified the influences of spray pressure, the intermittent opening time of the solenoid valve, nozzle diameter, droplet size, droplet speed, and droplet distribution relative span on continuous or intermittent spraying. However, the atomization laws with respect to variables such as spray pressure, solenoid valve opening time, and nozzle diameter in spot spraying are not clear.

From previous studies, it is evident that the current research mainly focuses on PWM variable spraying applications, where PWM sprays in continuous intervals over short periods, emphasizing the effect of different frequencies and duty cycles on atomization characteristics. In such scenarios, the solenoid valve spraying time is generally within 20–100 ms. Different from the PWM variable spray scenario, the spot spraying apparatus typically ejects the spray in a point impact manner at the target, requiring a quick solenoid valve response and switching, as well as a longer opening time. The duration of the nozzle opening, spray volume, and instantaneous impact atomization characteristics of droplets in the spot spraying method have not been sufficiently studied.

Spot spraying is different from PWM spraying and continuous spraying. It is precisely controlled based on the position and size of the target. After the control system issues control instructions to the target, it sprays in a point-to-point manner. However, in the point-to-point spraying mode, the liquid volume, nozzle aperture, spray height, and point-to-point spraying time have a direct impact on indicators such as droplet size, droplet speed, and droplet uniformity [4,5,6]. Selecting a nozzle that is suitable for the target size and meets the required atomization characteristics is a key problem in further refinements of spot spraying technology.

Therefore, the goal of this study is to optimize operating parameters in spot spraying and to improve spraying efficiency and effectiveness. Field-grown, early-to-mid-stage cabbage was selected as the target for spraying, based on actual field planting and operational conditions. The spray atomization characteristics of spray produced from nozzles differing in size, angle, and height were investigated. The influence pattern of spray times, spray pressures, and spray heights on droplet size distribution and droplet velocity were also assessed, with the goal of achieving the most efficient droplet deposition on the experimental crop.

2. Materials and Methods

2.1. Spot Spraying Atomization Characteristics Test System Design and Working Principle

In this paper, a spot spraying characteristic test system is constructed. The spot spraying characteristic test system is mainly composed of a spot spraying droplet characteristic control system and a droplet size measurement system. The composition of the spot spraying characteristic control system is shown in Figure 1. This system consists of four parts: the drug supply and pressure stabilization module, the spot spraying control module, the droplet size detection module, and other modules.

The spot spraying characteristic test system mainly consists of a computer, a controller (HSC37, Suzhou Hesheng Microelectronics Technology Co., Ltd., Suzhou, China), a pressure sensor (Type 131-B, pressure range: 0–2500 kPa, Beijing, China), a solenoid valve (2W-050-08ES-V type, Ningbo, China), a nozzle, a pressure gauge, a pressure tank, a pressure regulating valve (IR2020-02 type, set operating pressure: 10–800 kPa, Wuxi, China), and an air compressor (WX-1.5HP-type maximum working pressure; it is composed of a 1000 kPa pump, a self-priming pump, Shenzhen, China), and a droplet size measuring instrument, etc., as shown in Figure 1. During the test, the liquid medicine was used as the spray reagent. The self-priming pump injected the liquid medicine into the pressure tank through the ball valve, and then the DC motor drove the air pump to provide air pressure for the pressure tank. The gas pressure is set through the pressure-regulating valve, and the gas pressure data are read through the digital pressure gauge connected to the pressure regulating valve. The upper part of the pressure tank is filled with gas, while the lower part holds the liquid medicine. The liquid medicine flows out from the bottom of the pressure tank through pressurized gas. A digital pressure gauge is installed on the outlet branch pipeline to read the pressure data of the liquid medicine. The C37 controller monitors the real-time pressure in the pipeline through the pressure sensor and controls the opening and closing of the solenoid valve through the switch signal. When the solenoid valve opens, the nozzle begins to spray. When the solenoid valve is closed, the spray stops. The nozzle parameters are shown in

Table 1. Nozzle Parameters.

Nozzle Number	Nozzle Size (Diameter, mm)	Flow Rate (L/min)
HVV 01	0.66	0.39
HVV 02	0.91	0.79
HVV 03	1.1	1.2
HVV 04	1.3	1.6

. When in operation, we turned on the air compressor and adjusted the pressure regulating valve to stabilize the air pressure at the regulated level and ensure a stable output. When the solenoid valve is open, and the nozzle will spray out the liquid medicine. The droplet size measuring instrument (VisiSize P15) was turned on for monitoring. The solenoid valve was opened to start the spot spraying. One group of tests was divided into three groups. Each group of tests monitored 10,000 points. Finally, the average value was taken, and the records were saved. We calculated the standard error of each group of data to evaluate the accuracy of the average value. The test results are presented in the form of charts. All data are reported as mean ± standard error to show the central tendency and distribution range of each group of experimental data.

2.2. The Effective Spraying Height Range of Different Nozzles Is Determinednt

As stipulated in the ASAE S572.3 standard [17], the droplet size of most agricultural chemicals can range from fine, medium, or coarse sprays (106–403 μm). In this study, the effective droplet diameter range was within this range. The droplet diameter is measured using the Oxford Lasers VisiSize portable particle image velocimeter and Particle/Droplet Image Analysis (PDPI) (Oxford Lasers, Didcot, UK). The PDPI is fixed on a lifting platform, and the height is determined by adjusting the height of the platform. The measurement height is the distance from the lowest point of the nozzle to the center of the droplet size measurement area, with the measurement height adjusted to four different heights: 200, 300, 400, and 500 mm. During spraying, the nozzle remains stationary, directly above the center of the measurement area. The atomization characteristics of different nozzles in the spot spraying mode under the following conditions: temperature, 28 °C; humidity, 37.5%; and no wind, as shown in Figure 2.

Following the guidelines of the American Society of Agricultural and Biological Engineers (ASABE) and the American National Standards Institute (ANSI) 572.1 standard [18], the droplet spectrum distribution of agricultural spray nozzles is evaluated based on the relative span (RS). A smaller RS indicates a narrower droplet size distribution, higher consistency in atomized droplet size, and a more concentrated and uniform droplet diameter distribution.

$$\mathrm{R}\mathrm{S}={\displaystyle \frac{D_{V90}-D_{V10}}{D_{V50}}}$$

where D_V90 is the droplet diameter (μm) at which the cumulative volume is 90% of the total droplet volume; D_V10 is the droplet diameter at which the cumulative volume is 10% of the total droplet volume; and D_V50 is the droplet diameter at which the cumulative volume is 50% of the total droplet volume. A smaller RS value indicates a more concentrated and uniform droplet diameter distribution.

Taking as the study subject and open-field cabbage crop in northern China, the gaps between the individual plant canopies were found to be most suitable for spot spraying in the 7 to 35 days after planting. The cabbage canopy sizes during this period range from 123 to 310 mm. As shown in Figure 2, when the nozzle angle is α, the minimum and maximum effective heights are H0 and HH, respectively. The effective height range is from H0 to HH, and the solenoid valve opening time is determined by both the target size and spray height. Assuming the target diameter is T and the spray height is H, the spray height is calculated as follows.

$$\mathrm{H}={\displaystyle \frac{\mathrm{T}}{2\tan \frac{\alpha }{2}}}$$

The forward speed of the target sprayer is 0.5 to 1 m/s, the opening time of the solenoid valve is t, and the forward speed of the target applicator is V. The opening time of the solenoid is calculated as follows.

$$\mathrm{t}={\displaystyle \frac{\mathrm{T}}{\mathrm{V}}}$$

Then, the opening time of the electromagnetic valve is from 123 to 620 ms. The midpoint of this range, 372 ms, was chosen to test the effective height of the nozzles at different angles, as shown in Figure 3.

During the experiment, the pressure is set to 300 kPa, and the three nozzles listed in

Table 2. Effective Height Range and Target Range.

Nozzle Type	Droplet Size Range (μm)	Effective Height Range (mm)	Target Range (mm)
HVV25-02	127.8~172.4	277~500	123~221
HVV40-02	161.0~384.2	200~426	146~310
HVV50-02	153.7~231.2	200~266	186~248

are used for spraying. The nozzle height range is determined by the nozzle angle and target size combination. The droplet diameter is measured within the nozzle height range, and the effective height and target range are determined based on droplet size, as shown in Table 2.

After determining the nozzle effective height range, further experiments were conducted to analyze the relationship between pressure, solenoid valve opening time, nozzle aperture, and atomization characteristics within the effective spray height range.

2.3. Experimental Design on the Influence of Pressure on Atomization Characteristics

Based on the data shown in Table 2, the height values within the effective target range were rounded, and the target size corresponding to each nozzle model (HVV series fan-shaped nozzles, Shenzhen Chengyuanda Spray Purification Technology Co., Ltd., Shenzhen, China) was calculated. The solenoid valve opening time range is determined based on the target spraying operation with a travel speed of 0.5–1 m/s. The middle value of the opening time range is used as the spot spraying control time. The test parameters of droplet diameter, droplet velocity, and droplet distribution relative span were measured under different pressure conditions, as shown in

Table 3. Test Parameters at Different Pressures.

Nozzle Type	Height (mm)	Target Size (mm)	Solenoid Valve Opening Time Range (ms)	Opening Time Mean Value (ms)
HVV25-02	300	133	133~266	200
	400	177	177~354	266
	500	222	222–444	333
HVV40-02	200	146	146–292	219
	300	218	218–436	327
	400	291	291–582	437
HVV50-02	200	186	186–327	279

. Each group of tests was repeated three times, and 10,000 points were tested in each test. The working pressure was set at 200, 300, 400, and 500 kPa.

2.4. Experimental Design on the Influence of Solenoid Valve Opening Time on Atomization Characteristics

Based on the target spraying operation with the sprayer traveling at a speed of 0.5–1 m/s and the pressure set at 300 kPa, the solenoid valve opening time was divided into five segments. The test parameters of droplet diameter, droplet velocity, and droplet distribution relative span under different solenoid valve opening times were analyzed, as shown in

Table 4. Test Parameters at Different Solenoid Valve Opening Times.

Nozzle Type	Height (mm)	Target Size (mm)	Solenoid Valve Opening Time (ms)	Solenoid Valve Opening Times (ms)
HVV25-02	300	133	133~266	133, 167, 200, 233, 266
	400	177	177~354	177, 222, 266, 310, 354
	500	222	222~444	222, 278, 333, 389, 444
HVV40-02	200	146	146~292	146, 183, 219, 256, 292
	300	218	218~436	218, 273, 327, 382, 436
	400	291	291~582	291, 364, 437, 510, 582
HVV50-02	200	186	186~327	186, 233, 279, 326, 372

. Each group of tests was repeated three times, and 10,000 points were tested in each test.

To further analyze the effect of the solenoid valve opening time on atomization characteristics, under a pressure of 300 kPa, the maximum opening time of the electromagnetic valve for three different nozzles was measured, and the droplet volume median diameter, droplet velocity, and droplet distribution relative span (RS) for continuous spraying were also measured. A comparative test of droplet diameter, droplet velocity, and droplet distribution span under the maximum opening time for spot spraying and continuous spraying is shown in

Table 5. Experimental Design on the Influence of Nozzle Aperture on atomization Characteristics.

Nozzle Type	Spraying Height (mm)	Spraying Mode	Solenoid Valve Opening Time (ms)
HVV25-02	300	Point	266
		Continuous	Fully open
	400	Point	354
		Continuous	Fully open
	500	Point	444
		Continuous	Fully open
HVV40-02	200	Point	292
		Continuous	Fully open
	300	Point	436
		Continuous	Fully open
	400	Point	582
		Continuous	Fully open
HVV50-02	200	Point	372
		Continuous	Fully open

. Each group of tests was repeated three times, with 10,000 points tested in each test.

2.5. Experimental Design on the Influence of Nozzle Aperture on Atomization Characteristics

With the sprayer traveling at a speed of 0.5–1 m/s, the experiment was conducted with nozzle apertures of 0.66, 0.91, 1.1, and 1.3 mm, as shown in

Table 6. Test Parameters at Different Nozzle Apertures.

Nozzle Type	Nozzle Apertures (mm)	Height (mm)	Solenoid Valve Opening Time Mean Value (ms)
HVV25	0.66, 0.91, 1.1, 1.3	300	200
		400	266
		500	333
HVV40	0.66, 0.91, 1.1, 1.3	200	219
		300	327
		400	437
HVV50	0.66, 0.91, 1.1, 1.3	200	279

. Droplet diameter, droplet velocity, and droplet distribution relative span were tested under different nozzle aperture conditions. Each group of tests was repeated three times, and 10,000 points were tested in each test.

3. Results

3.1. Test on the Influence of Pressure on Atomization Characteristics

3.1.1. Test on the Influence of Pressure on Droplet Size Distribution

As shown in Figure 4, the droplet size decreased gradually as the spraying pressure increased within the range of 200 to 500 kPa. The increase in spray pressure increases the flow velocity of the liquid inside the nozzle. As the velocity field increases, molecular collisions become more intense, promoting the atomization process. Therefore, increasing pressure is likely to generate smaller droplets. Ronghua et al. [19] found that as spraying pressure increases, droplet size tends to decrease, whether the spraying mode is continuous or spot spraying. The nozzle angle and spray height together influence droplet size. With a 40-degree nozzle at a height of 200 mm, the droplet size exceeded the maximum effective droplet size of 403 μm, which indicates that the 40-degree nozzle, at 200 mm, cannot meet the spraying requirements. For the same nozzle, as the height increases, the droplet size decreases. For any given nozzle, as the height increases, the droplets are suspended in the air for a longer time, which allows for further breakage and reforming of the droplets.

The overall average droplet size of nozzle HVV25-02 is 175.0 μm. The overall average droplet particle size of nozzle HVV40-02 is 295.6 μm. The average overall droplet size of the HVV50-02 nozzle was 201.2 μm. The standard errors of the average droplet size of the three types of nozzles were 6.3 μm, 8.0 μm, and 5.9 μm, respectively.

In this paper, the stepwise function in MATLAB R2019a is used to perform the stepwise regression of the regression model by the backward method, with a p-value of 0.05. The spray pressure P, spray Angle A, and spray height H are very important for the droplet size D_V50, and the droplet size is changed by three factors. The D_V50 significance test has a very significant impact on the entire model; the coefficient of determination $R^2$ = 0.9141. The regression model is as follows:

$$D_{V0.5}=-0.4164\mathrm{P}-2.4487\times {10}^{-8}{·\mathrm{A}}^6+9.9624\mathrm{A}+2.6345\times {10}^{-14}{·\mathrm{H}}^6-5.1830\times {10}^{-6}{·\mathrm{H}}^3+271.175$$

Among them, D_V50 is the droplet size; P (kPa) is the spray pressure; A (°) represents the spray Angle; and H (mm) represents the spray height.

3.1.2. Test on the Influence of Pressure on Droplet Velocity

The results of droplet velocity tests at different pressures in spot spraying mode are shown in Figure 5, showing that the droplet velocity does not show a regular change with pressure. This suggests that there is no clear relationship between droplet velocity and pressure in spot spraying mode. However, different from continuous spraying conditions, Li et al. [20] found that droplet velocity generally increases with pressure. The main reason for the difference is that in the spot spraying mode, the nozzle opens and closes intermittently, and droplet generation is instantaneous. Although pressure increases, the spray time is too short to allow the droplets to gain significant momentum.

The overall average droplet velocity of nozzle HVV25-02 is 3.2 m/s, the overall average droplet velocity of nozzle HVV40-02 is 4.3 m/s, the overall average droplet velocity of nozzle HVV50-02 is 5.2 m/s, and the standard errors of the average droplet velocities of the three nozzles are 0.1 m/s, 0.1 m/s, and 0.3 m/s, respectively.

3.1.3. Test on the Influence of Pressure on the Distribution Span of Droplets

The test results for droplet distribution relative span under different pressures in the spot spraying mode are shown in Figure 6, indicating that as pressure increases, the droplet distribution relative span does not show a consistent trend. This result suggests that pressure does not have a clear impact on the droplet distribution relative span in the spot spraying mode. In contrast, in continuous spraying, the increase in pressure will lead to a decrease in the droplet distribution relative span [21]. In the spot spraying mode, the droplet size span of a 25-degree nozzle shows significant fluctuations, indicating that it is more affected by pressure variations. At the same time, in contrast to continuous spraying, the rapid opening and closing of the solenoid valve in intermittent spraying causes instantaneous pressure fluctuations. During the spraying process, the acceleration and deceleration stages of the liquid appear alternately, resulting in no clear relationship between pressure and droplet distribution relative span.

The overall average droplet distribution relative span of nozzle HVV25-02 is 5.0, the overall average droplet distribution relative span of nozzle HVV40-02 is 3.1, the overall average droplet distribution relative span of nozzle HVV50-02 is 5.5, and the standard errors of the average droplet distribution relative spans of the three nozzles are 0.4, 0.2, and 0.7, respectively.

3.2. Test on the Influence of Solenoid Valve Opening Time on Atomization Characteristics

3.2.1. Test on the Influence of Solenoid Valve Opening Time on Droplet Size

The test results for the effect of solenoid valve opening time on droplet size in spot spraying mode are shown in Figure 7. Under a pressure of 300 kPa, the droplet size does not change regularly with the increase in solenoid valve opening time since the electromagnetic opening time valve does not affect the liquid flow rate and pressure, which are the critical factors influencing droplet size. In a previous study, it was reported that at a frequency of 10 Hz, the volume median diameter (VMD) of droplets and the duty cycle for three different types of nozzles are negatively correlated, and after the duty cycle reaches 70% (70 ms), the droplet size changes are minimal [22], and this is consistent with the spot spraying time range of 123 to 620 ms in the present study. Therefore, it can be concluded that within the range of spot spraying time, there is no correlation between droplet size and solenoid valve opening time.

The overall average droplet particle size of nozzle HVV25-02 is 176.2 μm. The overall average droplet particle size of nozzle HVV40-02 is 314.3 μm, the overall average droplet size of nozzle HVV50-02 was 226.4 μm, and the standard errors of the average droplet size of the three nozzles were 6.6 μm, 5.4 μm, and 5.1 μm, respectively.

3.2.2. Test on the Influence of Solenoid Valve Opening Time on Droplet Velocity

The effect of solenoid valve opening time on droplet velocity in the spot spraying mode is shown in Figure 8. As the solenoid valve opening time increases, the droplet velocity tends to increase. The lengthening of the solenoid valve opening time allows for more liquid to flow through the nozzle, which imparts greater kinetic energy, resulting in an increase in droplet velocity. This is consistent with the pattern observed in PWM spraying, where it was found that droplet velocity increases with the duty cycle [23]. The average increase in droplet velocity with solenoid valve opening time was 0.53 m/s for a 20-degree nozzle, 1.27 m/s for a 40-degree nozzle, and 1.1 m/s for a 50-degree nozzle. The effect of solenoid valve opening time on droplet velocity is smaller under conditions of small-angle nozzles.

The overall average droplet velocity of the HVV25-02 nozzle is 3.3 m/s, the overall average droplet velocity of the HVV40-02 nozzle is 4.4 m/s, the overall average velocity of nozzle HVV50-02 is 5.1 m/s, and the standard errors of the average droplet velocities of the three nozzles are 0.1 m/s, 0.1 m/s, and 0.1 m/s, respectively.

3.2.3. Effect of Solenoid Valve Opening Time on Droplet Distribution Relative Span (RS) Test

The droplet distribution relative span (RS) values under different solenoid valve opening times are shown in Figure 9. Under a pressure of 0.3 MPa, RS gradually decreases as the solenoid valve opening time increases. This is mainly because the sprayed liquid becomes more stable as the solenoid valve opening time increases, and the droplet size tends to become more uniform, which results in a smaller RS value. This finding is consistent with the results reported by Li Longlong et al. [24], who found that a higher PWM duty cycle (60–80%) can effectively reduce the droplet size distribution span. The effect of solenoid valve opening time on the droplet distribution relative span is more pronounced for the 25-degree nozzle, indicating that the shorter the solenoid valve opening time at smaller angles, the more unfavorable it is to the formation of evenly sized droplets.

The overall average droplet distribution relative span of nozzle HVV25-02 is 5.7, the overall average droplet distribution relative span of nozzle HVV40-02 is 2.8, the overall average droplet distribution relative span of nozzle HVV50-02 is 3.5, and the standard errors of the average droplet distribution relative spans of the three nozzles are 0.1, 0.1, and 0.3, respectively.

3.2.4. Comparative Test of the Maximum Opening Time of Point Spray and the Atomization Characteristics of Continuous Spray

The relationship between droplet volume median diameter, droplet velocity, and droplet distribution relative span (RS) in spot spraying, with maximum solenoid valve opening time, and continuous spraying is shown in

Table 7. Comparison between spot Spraying and Continuous Spraying.

Nozzle Type	Spraying Height (mm)	Spraying Mode	Solenoid Valve Opening Time (ms)	Droplet Size (μm)	Droplet Velocity (m/s)	RS Value
HVV25-02	300	Point	266	258.0	3.5	4.0
		Continuous	Fully Open	261.1	8.9	1.1
	400	Point	354	158.8	2.9	6.3
		Continuous	Fully Open	174.1	6.9	1.5
	500	Point	444	131.1	3.6	1.9
		Continuous	Fully Open	135.8	5.2	1.6
HVV40-02	200	Point	292	370.9	5.3	2.0
		Continuous	Fully Open	347.9	14.7	0.8
	300	Point	436	338.7	5.0	2.3
		Continuous	Fully Open	314.8	8.3	1.1
	400	Point	582	235.1	4.7	3.5
		Continuous	Fully Open	214.8	6.0	1.6
HVV50-02	200	Point	372	218.2	5.5	4.0
		Continuous	Fully Open	265.2	9.2	0.9

. Each group of tests was repeated three times, and 10,000 points were tested in each test.

As can be seen from the results shown in Table 6, neither spot spraying nor continuous spraying has a clear effect on droplet size, which is consistent with the finding that there is no correlation between spot spraying time and the droplet size in the spot spraying mode. The average droplet speed for spot spraying is 4.36 m/s, whereas for continuous spraying, it is 8.46 m/s, with spot spraying droplets being 4.1 m/s slower than droplets in continuous spraying. The average RS value for spot spraying is 3.43, whereas for continuous spraying it is 1.23. The droplet distribution relative span in spot spraying is 2.2 higher than that of continuous spraying. In spot spraying, droplet formation is achieved through intermittent opening and closing of the electromagnetic valve; by necessity, the opening time is short, which leads to unstable fluid dynamics and, consequently, uneven droplet distribution [25].

In both point spray and continuous spray, the droplet size decreases as the pressure increases. In continuous spray, the droplet velocity gradually increases as the pressure rises. However, in point spray, the droplet velocity does not change regularly as the pressure increases. An increase in point spray pressure will raise the flow rate of the liquid inside the nozzle. As the velocity field increases, the more intense the mutual collisions between liquid molecules are, the more it promotes the atomization process, which is conducive to the generation of smaller-sized droplets. The spot spraying is intermittently turned on and off, and the formation of droplets is instantaneous. Despite the increase in pressure, the spray time is relatively short, and the droplets cannot fully obtain greater kinetic energy [12,13].

In PWM spraying, at higher frequencies, the average droplet size is less affected by the duty cycle, with a decreasing trend in droplet size as the duty cycle increases [15]. Longer interval times may make the spray more dispersed. Wei et al. [26] found that the droplet size of spray from the TeeJet XR8001 and XR8002 nozzles did not change significantly across different duty cycle ranges, and the differences in droplet size distribution between different duty cycles may be due to hydraulic impacts that occur when the PWM solenoid valve is closed during each 0.10 s switching cycle. A shorter valve opening time during the working cycle may lead to a more unstable spray pattern below the nozzle orifice. In contrast, spot spraying generally has a longer valve opening time than PWM spraying. As the spot spraying time increases, no regular change in droplet size is observed, mainly because the increased solenoid valve opening time may cause a change in the flow state within the nozzle; the transition from laminar to turbulent flow would be expected to increase flow instability. Therefore, the droplet size does not change regularly with the solenoid valve opening time [27,28]. In continuous spraying, when the liquid accelerates inside the nozzle under a stable pressure, a conical liquid film is formed and continuously emitted. This liquid film is affected by air resistance at the nozzle outlet; the resulting surface tension causes the liquid to break into smaller droplets. The atomization effect is improved in this mode, leading to a more uniform spray.

3.3. Test on the Influence of Nozzle Aperture on Atomization Characteristics

3.3.1. Test on the Influence of Nozzle Aperture on Droplet Size

As shown in Figure 10, when the pressure is 300 kPa, the droplet size increases with the increase in nozzle aperture in the spot spraying system. This is most likely due to the increase in liquid flow rate through the larger aperture. Consequently, the speed at which the liquid forms droplets at the nozzle also increases, and the higher flow rate may also lead to the formation of larger droplets, which is consistent with the conclusion that droplet size increases with nozzle aperture size in continuous spraying [29]. In the present study, when the nozzle aperture was 1.3 mm, the droplet size exceeded the maximum allowed droplet size of 403 µm. Therefore, in the spot spraying mode, a nozzle with a 1.3 mm aperture should not be used, and the spraying height for a 25-degree nozzle should not be less than 300 mm; and in the case of the 40-degree nozzle, the spraying height should not be less than 200 mm.

The overall average droplet particle size of the HVV25-02 nozzle is 301.1 μm, the overall average droplet size of the HVV40-02 nozzle is 344.6 μm, the overall average droplet size of nozzle HVV50-02 is 310.5 μm, and the standard errors of the average droplet size of the three nozzles are 7.9 μm, 7.8 μm, and 5.6 μm, respectively.

3.3.2. Test on the Influence of Nozzle Aperture on Droplet Velocity

As shown in Figure 11, the droplet velocity increases with the increase in nozzle aperture. The primary reason for this is that an increase in the nozzle aperture will lead to more liquid passing through the nozzle; the increase in the kinetic energy of the liquid leads to an increase in droplet velocity and greater aerosolization. The situation is not the same in continuous spraying, where, as the nozzle aperture increases, the droplet velocity first increases and then decreases [30].

Droplet velocity is an important factor affecting droplet settling and the degree to which the droplets adhere to the target crops [31]. Under certain surface tension and contact angle conditions, liquid droplets adhering to the leaf surface should follow the critical curve of droplet splashing in hydrophilic leaf surfaces. Droplet adhesion is directly related to the angle at which the droplet contacts the leaf surface. The smaller the contact angle, the less likely it is that the droplet will bounce off the surface, thus allowing for larger droplets sprayed at a higher velocity. The nozzle angle used in spot spraying is relatively small (110-degree). In spot spraying, the droplets hit the leaf surface with a smaller contact angle compared to that in continuous spraying, which aids in droplet adhesion to the leaves [32].

The overall average droplet velocity of the HVV25-02 nozzle is 4m/s, the overall average droplet velocity of the HVV40-02 nozzle is 4.8 m/s, the overall average velocity of nozzle HVV50-02 is 5.1 m/s, and the standard errors of the average droplet velocities of the three nozzles are 0.2 m/s, 0.1m/s, and 0.1 m/s, respectively.

3.3.3. Test on the Influence of Nozzle Aperture on Droplet Distribution Relative Span (RS)

As shown in Figure 12, with the increase in nozzle aperture, the droplet size distribution span of the three nozzles first increases and then decreases. Considering that the stability of the flow rate is crucial for the uniformity of droplet size, a stable flow rate helps produce a more uniform droplet distribution [33]. In our analysis, we found that when the aperture is small (0.66 mm), the sprayed liquid does not initially form a uniform liquid film; however, this relatively smaller amount of liquid interfaces with the surrounding air in a more uniform manner. As the aperture increases (0.91 mm), allowing more liquid to pass out of the nozzle, a uniform liquid film is not produced and neither is there complete interfacing with the air; thus, a larger droplet distribution relative span is produced. As the aperture continues to increase, a uniform liquid film does form and interfaces sufficiently with the air, but because of the larger overall liquid volume, the break-up or atomization effect is insufficient, leading to larger overall droplet sizes and a smaller droplet distribution relative span. Thus, it appears that the optimal nozzle size to produce the desired droplet size and distribution is the smaller nozzle (aperture 0.66 mm).

The overall average droplet distribution relative span of nozzle HVV25-02 is 3.5, the overall average droplet distribution relative span of nozzle HVV40-02 is 2.6, the overall average droplet distribution relative span of nozzle HVV50-02 is 2.8, and the standard errors of the average droplet distribution relative spans of the three nozzles are 0.2, 0.2, and 0.2, respectively.

4. Discussion

In spot spraying mode, under pressure ranging from 200 to 500 kPa, there was no correlation between droplet speed, droplet distribution relative span, and pressure; this was different from continuous spray. This difference is primarily attributed to the intermittent characteristics of spot spraying. During spot spraying, the rapid opening and closing of the solenoid valve leads to instantaneous pressure fluctuations, causing the alternate appearance of liquid in the acceleration and deceleration stages, which, in turn, affects the droplet formation process and motion characteristics. Furthermore, the type of nozzle clearly influenced the atomization characteristics. Under a pressure range of 200–500 kPa, the variation coefficient of the droplet distribution relative span of the spray from the 2502 and 5002 nozzles was very similar, i.e., 46.4% and 46.7%, respectively. The 4002 nozzle shows better stability in the effective height range of 200–426 mm, with a variation coefficient of 41.9% (somewhat lower than that of the 2502 and 5002 nozzles). This suggests that the nozzle angle not only affects the spraying width but also influences atomization quality by altering the liquid flow state. This finding is consistent with the research by Wei et al. [26], who observed that nozzle geometric parameters have a significant effect on the atomization process.

In both point spray and continuous spray, the droplet size decreases as the pressure increases. In continuous spray, the droplet velocity gradually increases as the pressure rises. However, in point spray, the droplet velocity does not change regularly as the pressure increases. An increase in point spray pressure will raise the flow rate of the liquid inside the nozzle. As the velocity of the liquid inside the nozzle increases, the collisions between liquid molecules become more intense, thereby promoting the atomization process and facilitating the generation of smaller sized droplets. The spot spraying spray is opened and closed intermittently, and the generation of droplets is instantaneous. Although the pressure increases, the spray time is short, and the droplets cannot fully obtain greater kinetic energy [12,13].

In the spot spraying time range of 123–620 ms, the solenoid valve opening time had no significant effect on droplet size, but it had a clear impact on droplet speed and droplet distribution relative span. When the solenoid valve opening time was short, the liquid flow did not fully develop, leading to inadequate atomization. As the opening time increased, the liquid flow gradually stabilized, which facilitated the formation of more uniform droplets. This is similar to the pattern found by Longlong et al. [15] in their study on PWM spraying; higher duty cycles helped reduce the droplet distribution relative span. The study reported that the droplet distribution relative span value was 2.2 higher in spot spraying compared to continuous spraying, reflecting the adverse effect of intermittent spraying on atomization uniformity due to the non-steady-state characteristics of the liquid flow during spot spraying. This suggests that measures need to be taken in practical applications to improve droplet uniformity. However, the droplet speed in spot spraying was 4.1 m/s lower than in continuous spraying on average; this characteristic can help in the droplet attachment to plant leaves.

Based on the regular patterns observed in the atomization characteristics of spot spraying, the following conclusions can be drawn: Droplet size gradually decreases with the increase in spray pressure, and droplet velocity generally increases with the increase in solenoid valve opening time. The RS value decreases gradually as the solenoid valve opening time increases. Both droplet size and droplet speed increase as the electromagnetic aperture increases. Different atomization characteristics have different deposition behaviors: larger droplets tend to settle more easily and are less likely to evaporate, disperse, or drift with the wind; while smaller droplets, being lighter, are more easily affected by airflow and prone to drifting. Finer droplets provide better coverage density and uniformity on crop leaf surfaces compared to larger droplets, with better attachment ability and less likely runoff [34]. The greater the droplet velocity, the more likely it is to bounce off the surface at a certain surface tension. However, droplet loss is also due to uneven droplet size distribution and poor attachment to the plant leaf, resulting in bouncing and rolling of the droplets. Based on the deposition performance of spot spraying atomization characteristics, and considering that the crop targets in spot spraying are typically single plants (which do not require droplet penetration), the smaller droplet speeds in spot spraying can meet the operational requirements, reduce splashing and drifting during spraying, and facilitate droplet deposition and attachment on the target. Therefore, spot spraying is considered beneficial for droplet deposition.

For a canopy range of 123–146 mm, the recommended nozzle type is 2502. For a range of 146–186 mm, although both 2502 and 4002 nozzles can be used, the 4002 nozzle is preferred considering its lower average droplet speed and better droplet uniformity. The selection of the optimal nozzle in spot spraying systems will increase the effectiveness of the pesticide application, reduce pesticide waste (and, consequently, material costs), and generally reduce the overall volume of pesticides applied to croplands.

5. Conclusions

We developed a spot spraying droplet characteristic testing system, with which we tested small-angle nozzles under a working pressure of 300 kPa, with the aim of improving pesticide application onto crops. The effective height range and target range were determined based on the effective droplet size and nozzle angle. Further experiments were conducted to analyze the relationship between pressure, solenoid valve opening time, and nozzle aperture on atomization characteristics within the effective height range.

In the pressure range of 200–500 kPa, spot spraying has the same performance as continuous spraying in terms of droplet size; that is, droplet size decreases with increasing pressure; that is, the droplet size decreased as the pressure increased. In the spot spraying system, the droplet velocity and droplet distribution relative span showed no correlation with pressure, which was not the case in the continuous spraying system.

In both the point and continuous spraying systems, the droplet size did not show regular changes with the increase in solenoid valve opening time. As the solenoid valve opening time increased, the droplet speed generally showed an increasing trend. The RS value gradually decreased with the increase in solenoid valve opening time. Spot spraying showed no clear effect on droplet size compared to continuous spraying, which aligns with the pattern of spot spraying time not being correlated with droplet size. The average droplet speed in spot spraying was 4.1 m/s lower than in continuous spraying, and the droplet distribution relative span value was 2.2 higher in spot spraying than in continuous spraying.

Both droplet size and droplet speed increased as the nozzle aperture increased. With the increase in nozzle aperture, the droplet distribution relative span for the three nozzles first increased and then decreased. However, no significant correlation was found with spot spraying time.