This work is focused on setting numerical simulations for testing and validate cooling channels effectiveness in rotor blade of a heavy industrial gas turbine. Many numerical simulations have been set in order to calculate most important physical parameters of internal air flow, as well as heat transfer coefficient in many sections of the cooling path. Objects of these simulations are the old cooling scheme ("radial holes") and currently adopted one (ceramic core insert): geometry modification was necessary in order to improve casting procedure effectiveness. Comparison between the two solutions is also part of the present work.