Effect of shear-thinning on pressure-swirl atomization

Abstract

Atomization of non-Newtonian liquids is an underexplored topic despite their numerous spray applications. Key spray characteristics in such processes are mean droplet size and size distribution. Several studies demonstrate that non-Newtonian shear-thinning liquids can narrow the droplet size distribution compared to Newtonian liquids, reducing the number of excessively small or large droplets. Various spray applications benefit from minimized occurrence of droplets outside the desired size range. This applies to atomizers in spray towers or agricultural sprays where too-small droplets are blown away while those too large are ineffectively used.

In this study, several non-Newtonian dilute aqueous solutions with different degrees of shear-thinning were prepared by mixing Xanthan Gum or Sodium carboxymethyl cellulose with deionized water. Their performance was compared with Newtonian sprays (water and water−glycerol solution) of comparable shear viscosity at defined shear rates. A common pressure-swirl atomizer was used, and a range of operational pressures along with varying viscosities allowed for examining the spraying process across a wide spectrum of Reynolds and Weber numbers.

Velocity and size of droplets in the spray were measured simultaneously using a 1D phase Doppler anemometer. High-speed visualization was employed to track spray morphology and the breakup process. Calculations of the flow parameters inside the atomizer complemented these outcomes.

Results show that varying viscosity and shear-thinning behaviour influence the flow dynamics from the liquid entry into the atomizer to the fully developed spray. Viscoelasticity complicates these processes further. The discharge occurs near the infinite-shear rate viscosity plateau, and its character depends, primarily on the flow conditions near the exit orifice. The shear-thinning and elasticity slightly affects liquid breakup, with production of more frequent and longer-lasting ligaments. Droplet size reduces with increasing pressure as expected, and this effect is more pronounced for non-Newtonians, the impact on the Relative span factor is inconsistent. Downstream droplet size increases for all liquids due to coalescive droplet collisions, with the secondary breakup and evaporation being ineffective.