A large-eddy simulation study of the transcritical mixing process in coaxial jet flow under supercritical condition

Supercritical water gasification (SCWG) is a promising clean energy technology for utilizing fossil fuels or organic solid waste with the challenge of delivering and dispersing liquid feedstock into the reactor. This paper investigates the transcritical mixing process in coaxial injections using large-eddy simulation. The results reveal that large temperature and density gradients exist between the inner injected water and the ambient fluid in the near field of transcritical water injection. Structure comparison demonstrates that the cross-section of the inner potential core shrinks more quickly in the non-premixing coaxial injection than the single injection, which can facilitate the dispersion and dilution of injected water during the heating process due to the shear mixing with the coaxial high-velocity outer injection. In addition, in the premixing coaxial injections, the substantial radial velocity fluctuation induced by the contracting cross-section leads to a much shorter inner potential core and more intensive heat and mass transfer. Swirling inlets can significantly strengthen the radial velocity fluctuation, thereby accelerating the temperature increase and species blending. Coaxial injections can effectively be used for dispersing feedstock in the SCWG process, particularly for high-viscosity materials.