Electrostatic atomization of soybean oil for pesticide spraying

John Blake Wilkerson, Purdue University

Abstract

The use of chemical pesticides in agriculture is costly and can be a threat to the environment when improper application techniques are used. Increased interest in low-volume spray applications of pesticides has created a need for a spray nozzle which produces smaller droplet diameters with a narrow droplet spectrum. The need for smaller droplets creates an additional practical problem in that as the droplet size is reduced, the gravitational force resulting from the mass of the droplet also decreases. This results in smaller droplets which drift off target due to the wind. The use of electrostatic spraying systems is a possible method for increasing spray deposition on the desired target while reducing drift. The electrostatic forces associated with a charged droplet can provide an additional force which aids in increasing the mobility of a charged droplet toward an electrically conducting target. These additional forces have been shown to increase spray deposition on plants while reducing unwanted drift. A prototype electrostatic nozzle designed and constructed to atomize soybean oil; a potential carrier for some pesticide formulations. The performance of the atomizer was evaluated based on the nozzle and fluid parameters. The fluid parameters considered were viscosity, surface tension. and volumetric electrical resistivity. The system parameters evaluated were related to the nozzle's geometry, flow rate of the fluid through the atomizer, and the voltage applied to a charging electrode located within the nozzle. Prediction models were developed for droplet size and charge-to-mass ratio based on the fluid's resistivity and surface tension, fluid flow rate, and electrode voltage. All of the variables mentioned were significant ($\alpha$ = 0.05) in determining droplet and charge-to-mass ratio. The VMD (Volume Mean Diameter) for the oil droplets could be varied from 40 to 500 microns and the charge-to-mass ranged from.1 to 6.0 mC/kg. The spray was charged to 21% of the theoretical limit for a liquid droplet as defined by Rayleigh's limit. Results suggest that the droplet size increased with increasing viscosity, surface tension, and flow rate while decreasing with an increase in electrode voltage. The charge-to-mass ratio decreased with an increase in droplet size as shown experimentally and theoretically. The nozzle shows great promise for low-volume atomization of vegetable oils. However, fluids which are water-based or water-oil suspensions were not successfully atomized. Therefore, for use as a pesticide spraying system, the pesticide and carrier must both be oil soluble. The resistivity of the fluid mixture should be below 10$\sp{10}$ $\Omega$-m for adequate atomization. Fluid combinations which meet this criteria should work well in applying pesticides where a low-volume application is desirable.

Degree

Ph.D.

Advisors

Gaultney, Purdue University.

Subject Area

Agricultural engineering

COinS