Characteristics of ultra-strong swirl flow field and its effects on liquid jet breakup and atomization

This study investigated the influence of swirl number (Sn = 0–3.9) on the flow characteristics and atomization behavior of coaxial liquid jets. Velocity and vorticity fields, jet breakup processes, and droplet dynamic characteristics were analyzed within a wide range of swirl numbers. Results showed that geometric swirl numbers agreed well with experimental and numerical swirl numbers under weak swirl conditions, but were significantly higher at ultra-strong swirl conditions due to flow separation and angular momentum loss. The central toroidal recirculation zone (CTRZ) was small at weak swirl condition, but expanded significantly with increasing swirl numbers due to enhanced centrifugal force. The flow field evolved from a single-vortex feature (at weak swirl) to dual- and multi-vortex features (at strong and ultra-strong swirl), resulting in more homogeneously distributed vorticity. After introducing the liquid jet, flow stability decreased significantly as the swirl number increased. Under weak swirl condition, the CTRZ remained suppressed and the flow structure was relatively stable. Under ultra-strong swirl condition, the CTRZ expanded dramatically and flow instability intensified. Furthermore, as the swirl number increased, both the normalized breakup location and timing of the jet were delayed. However, atomization was improved with increasing swirl number, and the droplet dynamic characteristics transitioned from jet-dominated to swirl-dominated.