Control of spray drop size by rheology

Atomization experiments were conducted using flat fan nozzles on a variety of fluids. Dilute polyethylene oxide (PEO) and guar polymer solutions as well as constant viscosity elastic “Boger fluids” were studied. The concentrations of PEO and guar ranged from 25ppm to 625ppm (w/w). “Boger fluids” (i.e. fluids with constant Newtonian viscosities but with varying levels of elasticity) were made by adjusting the shear viscosity with 2 × 104 Mw polyethylene glycol (5–14.5% w/w) and the level of elasticity with 25–125ppm PEO of 9 × 105 to 8 × 106 Mw. Dilute emulsions and carrageenan gel microbeads were also investigated. The emulsion concentrations ranged from 0.01% to 1% (w/w) and the carrageenan concentration ranged from 1% to 3% (w/w). The solutions were atomized at liquid pressures of 40–60psi and the droplet size distributions were measured by laser light diffraction.; The solvent viscosity was found to play an important role in the spray process by enhancing the effects of extensional thickening polymers. Through the use of “Boger Fluids” an empirical correlation was obtained between extensional viscosity, shear viscosity, and surface tension. This correlation used the Capillary number, ratio of viscous to surface tension forces, to determine the critical strain rate for the Atomization process. However, a universal critical strain rate was not found to exist for breakup from flat fan nozzles. The critical strain rate depends upon the nozzle geometry and the liquid pressure.; The breadth of the drop size distribution was found to vary depending upon the fluid type being sprayed. Extensional thickening polymers broadened the droplet spectra when compared to water only. In contrast, emulsions and structured fluids (gel microbeads) narrowed the drop size distribution. The gel microbeads yielded an extremely narrow drop size distribution and therefore offer the most promising solution for controlling the drop size distribution. The breadth of the distribution was also found to be related to the breakup mechanism of the fluids. Fluids that breakup by wave instability (water and extensional thickening polymers) produced broader drop size spectra than fluids that breakup either by perforation (emulsions) or by a 2-D stretching instability (microbead dispersions) similar to ductile failure in fiber spinning.; Breakup of individual droplets in high speed air flow (secondary atomization) was investigated for various complex fluid solutions. Viscoelastic solutions, emulsions, and elastic gel microbead dispersions were analyzed. Weber numbers ranged from 3 to 45 and wind speeds ranged from 45 to 120mph. The Weber number was found to accurately predict conditions under which bag formation and secondary atomization occurred for Newtonian and extensional thickening solution droplets in high velocity airstreams. The extensional viscosity of the droplets was not sufficient to inhibit bag formation. A stable drop occurred for We ≤ 8 and droplets breakup via bag formation for We ≥ 8. Both Newtonian and extensional thickening systems produce a broad drop size distribution due to the breakup of the bag. However, the emulsion and microbead systems were found to breakup by filament stretching. The resulting drop size distribution appears to be narrower than that of Newtonian and viscoelastic solutions.