Characterization of mixing dynamics of the transcritical bi-swirl jets

Investigation of the transcritical coaxial swirl injectors, considering their geometrical features along with the thermodynamics nonidealities, is of central importance in science and technology. Toward this aim, the mixing dynamics of transcritical bi-swirling jets has been investigated numerically. The scale-adaptive simulation method which allows formation of a turbulent spectrum is employed to close the governing equations. The Soave-Redlich-Kwong equation-of-state along with a required portion of standard database of the national institute of standards and technology are applied to the numerical framework to estimate the real-gas thermodynamic and transport properties, respectively. Results reveal that the present numerical framework accommodating a rather low-cost turbulence approach is capable of predicting the main characteristics of transcritical coaxial injectors, including central vortex core, central recirculation zone, Kelvin-Helmholtz instabilities, and vortex interaction. Analyzing oscillatory flowfield dynamics via the power-spectral-density and proper-orthogonal-decomposition techniques highlights the prominent influences of shedding, pairing, and merging of the vortices as well as the hydrodynamic Kelvin-Helmholtz instabilities on nearfield mixing dynamics. Effects of transcritical injector back-pressure on the characteristics flow features have been studied for the first time within a wide pressure range. Numerical simulations indicate that mixing quality, in terms of characteristics time- and length-scales, increases with the ambient pressure.