Numerical study of the mixing dynamics of trans- And supercritical coaxial jets

Abstract

Characterization of the transcritical coaxial injectors, accounting for the geometrical features and thermodynamics nonlinearities, is of both practical and fundamental importance. In the present study, the interactions and effects of turbulent mixing and pseudo-boiling phenomena are investigated. To do this, the mixing dynamics of bi-shear jets injected under trans- and supercritical conditions has been investigated numerically using the large-eddy simulation technique. The numerical framework provides real-gas thermodynamics and transport properties, using the Peng-Robinson equation-of-state and Chung's models, respectively. The obtained flow quantities are in good agreement with the pervious experimental and numerical results. Investigations show that formation of the highly stratified mixing layer and thermal-shield, a layer of large isobaric specific heat encompassing the transcritical jet, influences the transcritical mixing layer evolution. However, significant interactions of turbulent coherent structures and real-gas thermodynamics nonlinearities, referred to as the Turbulence-Thermodynamics Interaction (TTI), can locally destruct the thermal-shield and make alternate regions of gas-like and liquid-like fluids, appearing as finger-like structures on the jet interface. Therefore, it can be speculated that the initial stage of the mixing of a transcritical jet is controlled by the pseudo-boiling phenomenon that can produce strong TTI. Furthermore, local distortions of the thermal-shield provide opportunities for prominent activation of the baroclinic-torque and volume-dilatation, which, in turn, can facilitate the hydrodynamic instabilities. The growth of these instabilities disintegrates the transcritical jet's core and results in shedding of dense lumps from the core. In addition, results indicate that nearly throughout the flow field, the baroclinic-torque to a great extent affects the turbulent-kinetic-energy.