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, n and m, with the former setting the ligament corrugation and the latter the width of the ligament size distribution. Fit values for m indicate that nozzles of a conical type produce ligaments of almost equal size, while the flat fan nozzles produce broader distributed ligament sizes. Values for n show 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 parameters m and n can be directly measured and, indeed, govern the drop-size distribution.
Highlights
Spraying is one of the most common processes in everyday life; it is important for agriculture, drug administration, printing, firefighting, spray painting, etc. [1,2]
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
The drop size and drop-size distribution in sprays are of paramount importance for effective spray application; it is important in agriculture pesticide spraying, for instance, that the drop sizes are small for a good deposition and coverage [3], but not too small because of the environmental hazards of airborne spray drift [4,5,6]
Summary
Spraying is one of the most common processes in everyday life; it is important for agriculture, drug administration, printing, firefighting, spray painting, etc. [1,2]. The formation of droplets in sprays results from the breakup of liquid ligaments, which are often themselves transitorily formed during the destabilization of jets or sheets [10]. It was understood that one of the main breakup mechanisms for nozzles is due to waves on the surface of the sheet that are produced by friction with the surrounding air These waves, which were first described by Squire [14], grow in amplitude, causing thickness modulations of the sheet. The modulations will cause the sheet to thin to such an extent that it will rupture, creating sheet fragments of a well-defined size, the Squire wavelength These fragments will contract to form ligaments, which subsequently break up into droplets. The pump pressure was varied between 1.0 and 5.0 bar, and five different standard spray nozzles were used; see Appendix A for details about the nozzles
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