Abstract The method of entropy generation analysis has been implemented with the aim to investigate a double-circuit vortex tube thermodynamics. The value of the enthalpy growth rate was utilized as an indicator of the useful work done by the system. Post-processing of the numerical simulation results was carried out in order to determine the irreversible energy transformations within the computational domain. Particular attention was paid to comparative analysis of the standard k-e and SAS-SST turbulence models from thermodynamic point of view. The near main nozzle area was mentioned as a source of irreversible energy losses because of active production of turbulence dissipation due to high local values of mean rate of strain tensor modulus. Existence of the negative work area, designated as a parasitic zone, was found in the center of the vortex tube in the close proximity to the additional flow inlet. Its formation can be explained by insufficient turbulent interactions between central and peripheral flows. It was shown that this area contributes significantly to the irreversibility and should be eliminated. The integral characteristics were developed on the ground of the three dimensional distribution of both entropy generation and enthalpy growth rate. The given characteristics are suggested to be used as objectives for the optimization studies. The vortex tube diameter, main nozzle geometry and the turbulization of the additional flow were suggested to be optimized.