Abstract: The flow field of CO₂ gas in a vortex tube was mathematically formulated with standard k-ε turbulence model, theoretically analyzed and numerically simulated with software CFD. The impact of the known variables, including the temperature and pressure at the input, cold mass ratio and cold orifice diameter, on the cooling/heating of the vortex tube was investigated. The simulated results show that a continuous CO₂ heat/mass exchange between the inner forced-vortex and outer free-vortex zones explains the energy separation of the vortex tube; and that the cold orifice diameter significantly affects the cooling/heating. For example, as the cold orifice diameter increases from 1. 7 to 2. 62 mm, the temperature difference increases because of the cooling/heating. With a diameter of 2. 62 mm and a cold mass ratio of 0. 9, the temperature difference of the vortex tube is 36. 83 K.