Numerical simulations and local entropy generation in a two-phase transcritical carbon dioxide Ranque-Hilsch vortex tube

The Ranque-Hilsch vortex tube arouses a renewed interest as a robust expansion device able to produce thermal energy separation. However, its two-phase transcritical CO2 operation remains unclear. In this work, 3D computational fluid dynamics simulations are performed using the open-source SU2 solver which key features are the implicit time integration and the real gas density-based formulation. The homogeneous equilibrium two-phase model is coupled with both the k−ω shear stress transport and the standard k−ϵ turbulence closures while a fast tabulated version of the Span and Wagner equation of state enables to compute the CO2 properties. First, the validation is performed on pressure measurements, condensation onset positions and visualizations in a transcritical CO2 nozzle available in the literature. Then, the vortex tube transcritical CO2 temperature, energy and phase separations are thoroughly investigated as well as the integral and local entropy generations due to viscous and heat dissipations. The results reveal that the temperature separation is produced below the inlet temperature at both outlets due to real gas effects. It is also shown that the energy separation is reversed as well as the available vortex tube outlet powers for heating and cooling purposes. Furthermore, higher irreversibility due to fluctuating components is unveiled near the vortex tube sharp angles at high cold mass fraction and high inlet operating pressure. Throughout this analysis, compelling arguments in favor of the friction theory for the energy separation phenomenon are also disclosed. Overall, the present work gives useful operation guidelines and sheds light on the complex phenomena at play in a Ranque-Hilsch vortex tube for transcritical CO2 heat pumps.