Abstract: The influence of the inlet CO₂ pressure and cold fraction on the energy separation of vortex tube was experimentally investigated with the lab-built test platform,comprising a six-channel nozzle,for the operation conditions optimization.The results show that the inlet CO₂-pressure and cold fraction all had a major impact.To be specific,as the inlet pressure increased,the cooling/heating effect of the vortex tube changed in an increase-decrease mode,the best cooling occurred at 0.4 MPa and at a cooling fraction of 0.4,and the best heating happened at a cold fraction of 0.9.As the cold fraction increased from 0.2 to 0.5,the hot end wall temperature slowly increased;however,as the cold fraction went on increasing from 0.6 to 0.9,the peak temperature there started decreasing at an increasing decline-rate.Possible mechanism was also tentatively discussed in a thought provoking way.