What Determines the Drop Size in Sprays?

Abstract

In many instances, sprays are formed from the breakup of liquid jets or sheets. We investigate the different parameters that determine the characteristic drop size in the breakup of sheets. We vary both the spraying parameters, such as the pressure and geometry of the nozzle, and the fluid parameters, such as viscosity and surface tension. The combined results show that the drop size is determined by a competition between fluid inertia and surface tension, which allows for the prediction of the drop size from the Weber number and geometry of the nozzle. Once rescaled with the average drop size, the size distribution is found to be described by a compound gamma distribution with two parameters,nandm, with the former setting the ligament corrugation and the latter the width of the ligament size distribution. Fit values formindicate that nozzles of a conical type produce ligaments of almost equal size, while the flat fan nozzles produce broader distributed ligament sizes. Values fornshow that, for all nozzles, ligaments are very corrugated, which is not unexpected for such spray formation processes. By using high-speed photography of the sprays, the parametersmandncan be directly measured and, indeed, govern the drop-size distribution.

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Plain Language Summary

Spraying is important in a wide range of contexts such as agriculture, drug administration, printing, and firefighting. Controlling the size and distribution of droplets is critical for effective spray application. While much work has been done on optimizing drop size in sprays, most research to date has dealt only with a few specific aspects of droplet formation. Here, we experimentally investigate all parameters that affect drop size and show how to predict drop size and distribution from first principles.

We study the breakup of flat or conical liquid sheets formed with standard spraying nozzles used in many applications, and we systematically vary parameters such as nozzle type, spraying pressure, and fluid properties such as viscosity and surface tension. We determine the droplet size distribution with laser diffraction, which allows for the measurement of droplet sizes in specific locations of the spray. The drop size results from a competition between fluid inertia and surface tension and can be described with knowledge of the nozzle geometry and the Weber number, a dimensionless parameter that characterizes fluid flow in the presence of interfaces.

Our results should lead to smarter designs for nozzles that produce monodispersed droplets. Future work could look at non-Newtonian fluids and a wider range of nozzle types and spray formation mechanisms.