Atomizing high-viscosity non-Newtonian fluids with the ACLR nozzle: Correlation between internal flow and external spray instabilities

Abstract

Spray drying is a widely used method for producing food powders in large quantities, but it also has a high energy demand. To address this, one can increase the solid content of liquid feeds, although this, in turn, poses the challenge of atomizing high-viscosity liquids into fine droplets. The Air-Core-Liquid-Ring (ACLR) nozzle offers a potential solution for the atomization by inducing air and creating an annular flow inside the nozzle. Nevertheless, a challenge of this design is that it tends to present internal instabilities. This study investigates how feed viscosities up to 3 Pa·s, and feed dry-matter contents up to 57 % wt., influence the internal flow conditions and the resulting spray performance, i.e., the droplet size distribution and the spray angle. The results show that, while internal instabilities increment with increasing viscosities, the ACLR can seemingly achieve atomization with viscosities as high as 3 Pa·s, even at, compared to pressure swirl nozzles, low pressures (7 bar) and low air-to-liquid mass ratios (0.8). Nonetheless, a fraction of droplets over 500 µm remains, which needs to be addressed through higher ALRs or a geometrical optimization of the nozzle, before the nozzle can be considered for industrial applications. Additionally, we showed that the internal flow and the external spray instabilities can be correlated with each other. This confirms that any future studies attempting to increase the spray stability of the ACLR nozzle can focus on only one of these factors, and reasonably expect that the others will also improve.