Design of a hydraulic nozzle with a narrow droplet size distribution

Nowadays, pesticides use is necessary to satisfy the growing demand for agricultural products. During pesticide spray application, the agricultural mixture containing the active ingredient is fragmented to a cloud of droplets of various sizes and speeds. The droplet size distribution within the spray affects the efficiency of the treatment. According to the target, a specific droplet size should be used. However, hydraulic nozzles usually used in field application produce sprays with a wide droplet size distribution containing an important proportion of small or too large droplets. This results in non-optimal application and therefore losses of product into the environment. This statement conducted the research as early as in the 60’s to create the rotary atomizer. This device is able to produce narrow droplet size distribution using Plateau-Rayleigh break up mechanism. The characteristics of the produced spray by the rotary atomiser can be tuned by adjusting the rotational speed and the volumetric flow rate. Although offering agronomic and environmental qualities this has not met success for the application of plant protection products in field crops because of their cost, size and complexity.
The aim of the thesis was the design of a hydraulic nozzle with the rotary atomizer qualities and without the constraints of rotating parts. Unlike the rotary atomizer which has two control variables, the developed hydraulic nozzle will have a narrower operating range, thus a specific geometry has to be design for each kind of treatment. The new hydraulic nozzle design is composed by an inlet pipe ending perpendiculary on a plate. There is a thin opening at the junction between the pipe and the plate. The edge of the plate is constituted of channels formed by structures. These channels aim to divide the liquid sheet in multiple jets. The breakup of these jets into droplets generate a narrow droplet size distribution. The thesis can been seen as a roadmap providing design tools at each step starting from the determination of an optimal droplet size according to the kind of treatment and ending with a nozzle geometry. The prediction of the optimal droplet size according to the treatment was done using integrated modelling approach of the spray transport and retention by the plant. As each spray requires a specific nozzle geometry, an analytical model of the flow on the nozzle was developed. This model allows the determination of geometry according to the desired spray. Finally, a prototype of nozzle has been built. The measurement of the spray characteristics of the prototype was realized using a developed high-speed imaging technique providing the droplet size and speed. The prototype showed results in term of narrowing of the droplet size distribution.