Characteristics and instabilities of two-phase swirling jet flow generated by swirling microbubble generators

The gas-liquid two-phase flow characteristics in a swirling microbubble generator dominate the bubble breakup process. This study integrates experiments and simulations to investigate the effects of gas-phase Reynolds number (ReG), liquid-phase Reynolds number (ReL), and swirl number (S) on the swirling jet evolution. Three distinct jet types are identified: bubble vortex breakdown jet flow (BVBJF), spiral vortex breakdown jet flow (SVBJF), and Coanda jet flow (CJF). As S or ReL increases or as ReG decreases, the flow transitions occur as follows: BVBJF → SVBJF → CJF. The dynamics of different types of swirling jets are characterized based on the proper orthogonal decomposition (POD) and spectral POD (SPOD). BVBJF exhibits periodic air core pulsations. SVBJF demonstrates dynamic competition between single- and double-helix structures, accompanied by flapping instability and wavy breakup. Despite the failure of POD (SPOD) analysis due to a dense bubble cloud, numerical simulations confirm the presence of a dual vortex core structure in CJF.