Planar Laser Induced Fluorescence (PLIF) Flow Visualization applied to Agricultural Spray Nozzles with Sheet Disintegration; Influence of an Oil -in-Water Emulsion

Abstract

A typical agricultural spray process involves atomizing a liquid st ream of diluted pesticide solution through hydraulic spray nozzles that inherently produce a wide spectrum of spray droplet sizes. Finer droplets have higher potential for off-target movement or drift, which is of concern due to t heir potential impacts on neighboring crops and livestock, sensitive ecological resources, and human health. A fa ctor that has been found to reduce fines production is the addition of an oil phase in the form of an oil-in-water emulsion. The mechanism of the effect is not fully understood. The flow visualization technique Planar Laser Induced Fluorescence ( PLIF) is commonly employed for characterization of scalar mixing. PLIF has also been used to characterize sprays. For this study, Rhodamine WT fluorescent dye was mixed into the solution to be sprayed. The l aser sheet, aligned vertically, is passed through the exit area of the nozzle causing the droplets to fluoresce. T he fluoresced pattern is imaged. For sprays, image pairs are taken on the order of 100 µs apart, thus the displacement and hence velocity vectors of drops can be computed throughout the spray pattern. Additionally, the images are of suffici ent quality to study liquid sheet breakup physics. For sprays with sheet breakup, such as many fan-type agricultural sprays, PLIF is effective at measuring the velocity within the spray before, during, and after the sheet disintegration. The motion of sur face features on the sheet before disintegration allow the velocity to be calculated in a similar fashion to discrete particles. Traditional Particle Image Velocimetry (PIV) can in principle work as well, however the image quality from light scattered off of the unbroken sheet in PIV was found to be inferior to images from fluoresced light generate d within the sheet by PLIF. The method was applied to fan sprays (hydraulic and air-induced) in common use in agricultural applications. Measurements were performed with and without an emulsifiable methylated soybean oil concentrate phase. Velocity profiles were shown to be consistent with patternation dat a taken via a mechanical collection method. It was shown that the oil phase has a large impact on velocity profile . The effect of the emulsion on liquid sheet breakup physics was elucidated from the PLIF imaging.