Abstract: The flow-field in a vortex tube, with ideal CO₂ gas medium, was mathematically formulated with a standard k-ε turbulence model and numerically simulated.The influence of the hot end tube length on the flow-field distributions and energy separation was investigated under the conditions:298.15 K and 6.5 MPa at the inlet, and a cold flow-rate of 0.1.The simulated results show that the hot end tube length has a major impact.To be specific, as the length increases in 100~200 mm range, the axial velocity increases, accompanied by a decreasing tangential velocity;the interface, separating the distributions of internal/external swirling flows, changes in an increase-decrease manner;both the refrigeration and heating effects change in an increase-decrease mode.The optimized refrigeration (ΔT_c=21.12 K) can be obtained with an aspect ratio of 25, that is, a length of 125 mm.