Flow dynamics of shear-coaxial cryogenic nitrogen jets under supercritical conditions with and without acoustic excitations

Abstract

The three-dimensional flow structures and dynamics of shear-coaxial nitrogen jets under supercritical conditions are comprehensively investigated using the large-eddy simulation technique. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Cryogenic liquid nitrogen (132 K) is delivered through the inner tube of a shear-coaxial nozzle, and gaseous nitrogen (191 K) is injected through the outer annulus, into a high-pressure environment at 233 K. Particular attention is paid to the influence of operating parameters on the flow evolution of the coaxial jets. Two supercritical pressure conditions, 4.94 and 10.0 MPa, and three outer-to-inner velocity ratios in the range of 2.03–3.75, are considered. Results reveal that the flowfield downstream of the coaxial jets is characterized by two potential cores and three shear layers. Two counter-rotating recirculation zones are formed next to the central post-tip. The characteristic frequencies of flow recirculation and shear-layer instability are analyzed using power spectral analysis. Increasing the pressure and the velocity ratio enhances turbulent mixing, promotes the entrainment of the outer stream into the inner region, and advances the pinching point of vortical motion to the centerline, thereby leading to a decreased length of the inner dark core. The predicted dark core length and outer spreading angle show good agreement with experimental results. The introduction of transverse acoustic excitation induces large sinusoidal oscillations of the inner dense-fluid stream along the direction of acoustic motion and promotes the mixing of the inner and outer streams.