Characterization of droplet size and their distribution using low operating conditions in a rotary bell cup spray-field via shadowgraph

This study characterises droplet size and distribution across axial (x/D) and radial (r/D) locations in the spray field of a rotary bell-cup atomizer under low operating conditions. Shadowgraph was used to measure droplet size and distribution at 16 dimensionless locations in both directions. Two low rotational-speed ranges, 3–3.5 krpm (case I) and 8–8.5 krpm (case II), were achieved using shaping-air of 30–100 L/min under turning-air of 50 and 60 L/min, respectively, at a constant flow rate of 0.5 L/min. The Sauter mean droplet diameter ranged from ∼ 45–98 µm in case I and narrowed to ∼ 45–85 µm in case II due to stronger centrifugal forces at the higher rotational speed. The modal droplet diameter remained below 50 µm under all operating conditions, typically within 42–45 µm. Among the two cases, the largest droplet sizes occurred in case I at shaping-air inputs of 60–80 L/min owing to a balance between competing aerodynamic and centrifugal forces. In contrast, shaping-air of 60 L/min and above tended to increase droplet size in case II. The droplet-size distribution in case I was broad across all planes (centerline, front, left, and right), spanning 20–150 µm, with a volume fraction of 20–25% and a droplet count of approximately 8,000–10,000. In case II, the higher rotational speed compressed the distribution to 20–60 µm across all planes, with a volume fraction of 15–25% and a droplet count of approximately 4,000–9,000. These results demonstrate the potential of low operating parameters to produce droplets below 100 µm suitable for many applications.