Spray characteristics of shear-thinning viscoelastic liquids downstream of a counterflow atomizer

This study presents droplet diameter distributions downstream of a Counterflow nozzle for atomization of water and four non-Newtonian liquids, with a Flow-Blurring nozzle used as a reference. The test liquids were aqueous solutions of sodium carboxymethylcellulose (0.5%, 1%, 1.5% and 2% by weight) and had zero shear viscosity values in the range 13.8 – 1280 mPas. Rheological measurements show significant shear-thinning behavior accompanied by viscoelasticity, as quantified by a characteristic time scale for capillary thinning, as well as increasing levels of storage and loss moduli with polymer concentration. Droplet diameters were measured using a Particle Digital Image Analysis technique combined with diffuse back-lit shadowgraphy. High-speed imaging suggests that at the higher weight concentrations, the near-field structure of the spray has a tree-like structure similar to that observed in effervescent atomization. Long filaments with droplets at their end are visible, attesting to the extensional stresses in the fluid. At lower concentrations, the spray emerges as a cloud of ligaments and droplets that rapidly disintegrate, accompanied by a gas core. Further downstream, beads-on-a-string structures are seen, corresponding to capillary instabilities. An unusual non-monotonic behavior of the Sauter Mean Diameter(SMD) is observed in the downstream direction, which may be the result of an experimental pitfall associated with the assumption of spherical geometry, along with the motion of beads-on-a-string structures from regions of high to low shear. The spatial variation of the SMD indicates that, compared to Newtonian liquids atomized using this nozzle geometry, secondary atomization plays a more dominant role in determining the far-field droplet diameter distribution. The SMD is expressed as a weak function of the ratio of viscoelastic relaxation time to the visco-capillary scale characterizing ligament thinning.