Abstract: The flow field of ideal CO₂ in a vortex tube was empirically approximated,mathematically formulated with standard k-ε turbulence model and numerically simulated.The influence of the hot-end tube diameter on the energy separation,distributions of velocity,temperature and pressure was investigated under the conditions: an inlet temperature of 298.15 K,an inlet pressure of 6.5 MPa and a cold flow-rate of 0.1.The simulated results show that the hot-end tube diameter has a major impact.To be specific,as the diameter increases from 4.02 o 6.0 mm,the radial and axial spaces of the internal swirl increase,the tangential velocity decreases,the axial distance of the cold-hot flow boundary increases and the radial distribution of pressure decreases,resulting in an optimized cold-hot balance at a diameter of 5.0 mm.We suggest that the simulated results be of some technological interest for design optimization of vortex tubes.