| The use of conventional air-assisted sprayer to apply pesticides has ensured production of high-quality fruits and ornamental nursery crops and enhanced our living standard. However, conventional air-assisted sprayers discharge constant air flows that are independent of tree size and foliage density. The air velocity profiles of these sprayers cannot be altered to uniformly deliver droplets to different plant canopies. In many cases, the air velocities are either too high or too tow and consequently, crops are either over sprayed or under sprayed. Another problem with the fixed air velocity patterns are off-target losses to the air and ground.Variable-rate air-assisted sprayers that match spray outputs to target needs may be the solution to those problems associated with conventional sprayers. Development of future variable-rate sprayers should include a variable capacity control of both liquid and air flows to match tree canopy structures.The cross section and foliage density of a tree vary throughout its entirety during the growing season. To accommodate this variability and achieve spray delivery, spray outputs should be tailored to each targeted section. An automatic air-assisted sprayer that implements five-port air-assisted nozzles to perform variable-rate spray outputs may have the potential to achieve this objective. This nozzle was developed to improve spray penetration and air jet velocity distribution inside dense nursery crops by dividing conventional large air jets into five small jets. The USDA-ARS-ATRU has developed the experimental air-assisted five-port sprayer to achieve liquid variable rates on tree occurrence, structure and foliage density. However, the mechanics to achieve variable air rates for this type of sprayer have not been developed. Also, to elucidate the mechanistic principles underlying air-assisted five-port nozzles in variable-rate applications, the magnitude of influence of the spray parameters on droplet size distributions first must be determined. Therefore, the objectives of this study were to investigate possible methods to achieve variable-rate air flow rates for this sprayer and to determine the effect of spray parameters and variable-rate air velocities on droplet size distributions. These objectives were used to form a basis for the future development of an automatic device to control air flow rates for variable-rate sprayers.Design of the PWM-controlled, air-assisted, five-port nozzle with consistent droplet size distributions is the first step for the development of intelligent sprayers that have automatic variable spray output functions to match the variations in plant canopy structures. Parameters that influence droplet sizes from the nozzle include solenoid valve modulation rate, liquid pressure, air velocity discharged from the nozzle, and spray solution physical property. The effect of modulation rate, spray solution, air velocity, and liquid pressure on droplet size distributions produced from an air-assisted, five-port nozzle coupled with PWM solenoid valves were investigated. Droplet diameter consistencies varied with air velocity, liquid pressure, modulation rate, and spray solution physical properties. Among these four variables, droplet size was most affected by liquid pressure, followed by spray solution and then air velocity. Droplet diameters did not vary with modulation rates at20%～100%, but they were more variable at the10%modulation rate. The optimal conditions that minimize droplet diameter variations for variable-rate spray applications with the air-assisted five-port nozzle were to maintain a constant liquid pressure and to use modulation rate of20%～100%. However, droplet diameters also varied with air velocity, and this variation was accepted for the use of the air-assisted five-port nozzle in future automatic variable-rate sprayer development.Unimpeded air jet velocities from the air assisted, five-port sprayer in an open field were measured at four heights above ground, seven distances up to3m from the sprayer outlets, and five sprayer travel speeds from0to8.0km/h. Air jet velocities were adjusted by changing the sprayer fan inlet diameter. Air velocities were measured with a constant temperature anemometer system coupled with hot-wire sensors. The air jets expanded at a50°angle and intersected with adjacent air jets at0.027m from the five-port nozzle. At a sprayer travel speed of0km/h, axial air velocities from nozzle outlets increased as fan inlet diameters increased and decreased as a hyperbola function as the distance increased. Variations in the peak air velocities and dynamic airflow pressures with the travel speeds of3.2to8.0km/h and heights of0.2to2.0m were insignificant. When the sprayer was in motion, own to air entrainment and air jet disturbance, the peak air velocities decreased and dynamic airflow pressures increased slightly as the distances from nozzle outlets increased. For all the parameters tested, the peak air velocities and dynamic airflow pressure decreased as the fan inlet diameters increased, demonstrating that changing fan inlet diameters achieved variable air flow rates with uniform air profiles.Based on the conclusions of the air velocity distributions in an open field, this research was continued to determine whether variable air velocity distributions inside different sizes of tree canopies could be achieved by varying fan inlet diameters. Air jet velocities impeded by plant canopies were measured at various locations inside canopies of three different tree sizes and foliage densities. Tree heights were1.65,2.35and3.0m, and leaf area indexes were13.4,2.5, and1.5, respectively. Air jet velocities were adjusted by changing the sprayer fan inlet diameters and measured with a constant temperature anemometer coupled with hot-wire sensors. Peak air velocity and dynamic airflow pressure decreased with foliage density and canopy depth. For the0.34m fan inlet diameter, dynamic airflow pressure ratio of front portion to back portion of the canopies was8.0,1.5and2.6for Tsuga canadensis, Ficus benjamina and Acer rubrum, respectively. Similarly, the front to back peak air velocity ratio was8.55,1.59, and1.89times for T. canadensis, F. benjamina and A. rubrum, respectively. Variations were significant for peak air velocities and dynamic airflow pressures among the three different tree volumes and foliage densities. Increased fan inlet diameters from0.13to0.34m, increased average airflow pressure from0.4to0.8N/m2,0.8to1.5N/m2, and0.5to0.9N/m2inside canopies of T. canadensis, F. benjamina and A. rubrum, respectively. Therefore, alterations of fan inlet diameters for the five-port air assisted sprayer achieved variable air flow rates for different canopy sizes and foliage densities. Hence, the new sprayer was able to provide uniform air distributions along the tree heights inside canopies with different fan inlet diameters. A dimensionless parameter, which was the ratio of leaf area index and specific canopy depth to maximum canopy depth, was correlated with peak air velocity and dynamic airflow pressure.In addition to the investigation of droplet sizes and air velocities for the new variable-rate sprayer, effects of modulation rate, fan inlet diameter and travel speed on spray coverage and deposition inside canopies were also determined. For a given condition, spray coverage increased as the modulation rate and fan inlet diameter increased. At a constant travel speed, variations in deposits inside canopies with the fan inlet diameter and modulation rate were insignificant. The coverage inside canopies increased as the travel speed decreased. The spray coverage inside canopies decreased as the canopy depth increased due to slow air disbursed inside dense canopies. The spray deposition and coverage inside canopies increased with the peak air velocity and dynamic airflow pressure as logarithmic functions increased. |
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