A vortex tube is a thermal device that splits a pressurized gas stream towards two lower pressure exits: cold gas on one side and hot gas on the other. Due to its small size and simple design with no moving parts, it is widely used for varying industrial cooling applications. The tangential injection of compressed gas into the tube generates a swirl flow movement. However, the physical phenomenon producing the temperature separation in the tube is still not fully understood. The current research work presents a CFD study of vortex tubes with Ansys Fluent. 3D simulation of a selected tube vortex design has been realized in order to identify and investigate the influence of input parameters on the thermal performances of vortex tubes. A geometric optimisation is conducted in order to improve the performance of such device. An analysis of the simulation results raises the question of Mach number and supersonic outlet velocities, which would be of interest for further study on vortex tube.