Particle Image Velocimetry Experimental Investigation on the Gas Flow Hydrodynamics of the Coaxial Jet Mixer with Inner Swirl

The coaxial jet mixer with an inner swirl (CJM-IS) has been widely used in rapid gas-phase chemical reactions due to its highly efficient mixing performance. However, its complex gas flow hydrodynamics remain insufficiently understood. In this study, particle image velocimetry (PIV) was employed to systematically investigate the flow evolution in the CJM-IS under varying swirl intensities (S = 0–1.1) and momentum ratios (Mr = 0.25–4). A flow regime map was constructed based on time-averaged velocity fields to identify the critical conditions for flow regime transitions. Two empirical correlations were developed to predict the dimensionless lengths of the central recirculation zone (CRZ) and vortex breakdown recirculation zone (VBRZ) as functions of S and Mr. Furthermore, the effects of flow pattern transitions on mixing mechanisms were elucidated through the analysis of vorticity, turbulent kinetic energy (TKE), Reynolds stress, and entrainment efficiency. Results indicate that the outer straight jet suppresses vortex breakdown, leading to an increase in the critical swirl number (SCR) for vortex breakdown at low Mr values. Under nonswirling and low-swirl conditions (S ≤ 0.3), shear layer instabilities dominated the flow development, with vorticity and TKE concentrated along the shear layers between inner and outer jets. Under moderate swirl conditions (S = 0.5), the flow field exhibited distinctive transitional features. Under high swirl conditions (S ≥ 0.7), the formation of VBRZ changed the energy transport mechanisms, significantly increasing the number of small-scale downstream vortices and the peak TKE, thereby enhancing entrainment efficiency. This study reveals the multiscale dynamic behavior of gas flow in the CJM-IS, providing a theoretical basis for its structural optimization and flow control.