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Characterization of air profiles impeded by plant canopies for a variable-rate air-assisted sprayer

J. Gu, H. Zhu, W. Ding, X. Wang
Transactions of the ASABE 2014 v.57 no.5 pp. 1307-1315
Acer rubrum, Ficus benjamina, Tsuga canadensis, air, air flow, air-assisted sprayers, canopy, droplets, fruit trees, leaf area index, leaves, orchards, pesticide application, pesticide use reduction, temperature, variable rate application
The preferential design for variable-rate orchard and nursery sprayers relies on tree structure to control liquid and air flow rates. Demand for this advanced feature has been incremental as the public demand on reduction of pesticide use. A variable-rate, air assisted, five-port sprayer had been in development to achieve variable discharge rates of both liquid and air. To verify the variable air rate capability by changing the fan inlet diameter of the sprayer, air jet velocities impeded by plant canopies were measured at various locations inside canopies of three different tree sizes and foliage densities. Air jet velocities were adjusted by changing the sprayer fan inlet diameters with an airflow regulator and measured with a constant temperature anemometer coupled with hot-wire probes. Peak air velocity and airflow pressure decreased with foliage density and canopy depth. For the 0.34 m fan inlet diameter, airflow pressure ratio of front portion to back portion of the canopies was 2.45 for a 1.65 m tall and 13.4 leaf area index (LAI) Tsuga canadensis, 1.43 for a 2.35 m tall and 2.5 LAI Ficus benjamina, and 1.64 for a 3.0 m tall and 1.5 LAI Acer rubrum. Similarly, the front to back peak air velocity ratio was 8.55, 1.59, and 1.89 times for T. canadensis, F. benjamina and A. rubrum, respectively. Variations were significant for peak air velocities and airflow pressures among the three different tree volumes and foliage densities. Increasing the fan inlet diameter from 0.13 to 0.34 m increased average airflow pressure from 2.84 to 4.01, 3.88 to 5.82 kg m-2, and 2.46 to 3.75 kg m-2 inside canopies of T. canadensis, F. benjamina and A. rubrum, respectively, while it also increased average peak air velocity from 2.6 to 4.5 m s-1, 5.5 to 9.1 m s-1, and 3.0 to 5.2 m s-1 inside these three tree canopies. Therefore, the new sprayer design with the airflow regulator to alter the fan inlet diameter was able to provide variable air flows for different canopy sizes and foliage densities, and offered a potentially effective approach to discharge uniform airflow profiles to carry droplets with efficient spray penetration into plant canopies for efficient pesticide applications.