Title Heat and Mass Transfer Between Cold and Hot Vortex Cores inside Ranque-Hilsch Vortex Tube-Optimization of Hot Tube Length Type of Research Article Keywords Vortex tube air separator, Optimization, Separation process, Main length, Numerical simulation Abstract This research describes numerical investigations on the impacts of length of main (hot) tube (95 to 125 mm) and type of the boundary condition on the thermal capability (heating and cooling) of the vortex tube and the heat and mass transfer between the cold and hot vortex cores inside a Ranque-Hilsch vortex tube applying the 3D CFD models. The cooling and heating capabilities increase severely with an increase in the main tube length upto 115mm (optimum value) and beyond L=115mm the thermal effectiveness of vortex tube decreases. The results have been compared to the experimental values with good agreement (less than 7 %). Researchers Seyed Ehsan Rafiee (First Researcher)، mohammad bagher mohammad sadeghi azad (Second Researcher)