Diameter-Resolved Velocity Correction for Laser Diffraction Calibration and Wind Tunnel Measurement of Sprays

Abstract

In agricultural sprays, droplet size influences both the efficacy and drift potential for pesticide applications. The droplet size spectra of these sprays are commonly evaluated via laser diffraction (LD) measurements within a spray-classification wind tunnel. A recurring issue when performing these measurements is the validation of instrument performance. For performing calibration verification on the instrument, a polydisperse standard with a known distribution is preferred over a monodisperse standard as a proxy for the spray distributions to better represent the spray and therefore challenge the instrument transfer function. However, this can introduce dispersion-related bias error if not all diameter droplets are moving at the same velocity. Herein a method is developed that allows for calculating diameter-resolved correction factors from first principles, enabling the correction of the bias error associated with the dispersion of polydisperse standards. The correction factor is found from the diameter-resolved residence time within the measurement volume, which is calculated by simulating the acceleration of the droplet from the point of generation. Herein this method is applied to LD calibration with gravity-dispersed National Institute of Standards and Technology traceable glass microbeads, allowing for the successful recovery of the known distribution. This method enables LD users to easily verify LD measurements without relying on the instrument manufacturer or in cases in which the instrument manufacturer is unwilling to perform verification, such as through glass windows rather than open air. Additionally, the present method is expanded to predict diameter-resolved bias in LD measurements performed in spray-classification wind tunnels consisting of spray in a coflowing carrier gas in which the spray is not in velocity equilibrium with the gas flow.