Spray atomisation in multiphase flows with reference to tank mixes of agricultural products

Abstract

First-principle models describing the breakup of liquid sheets into droplets are used to screen spray adjuvants and agricultural tank mix partners to eliminate combinations that might negatively impact droplet size, reducing the need for extensive experimental testing. An agent-based model is developed to predict droplet spectra following spray atomisation from a flat-fan nozzle for a continuous oil-in-water liquid sheet that comprises the mechanisms of hole nucleation, hole growth, hole coalescence, ligament formation and rupture into atomised drops. Hole nucleation is driven by Marangoni spreading of oil globules at the interface that initiates sheet breakup, followed ultimately by fluid ligament rupture into droplets of sizes conforming to a gamma distribution. This mechanistic model is used to predict the effect of various relevant parameters on the volume fraction of driftable fines (DFV, fine spray that can easily become entrained in wind, typically <150 μm diameter) resulting from spray atomisation. An increase in the concentration of the emulsion immiscible phase increases the rate of hole nucleation in the spray sheet and decreases the DFV. In addition, increasing the surface tension of the emulsion aqueous phase is also found to substantially decrease the DFV. A small decrease in DFV is observed with increase in the mean oil globule diameter, and negligible change is observed when varying the viscosity by 10× over water. Experimental droplet spectra from commercially available formulations mixed together are used to validate this numerical approach for mixture performance with excellent agreement.