In this work, the cooled TG20 first rotor blade is numerically simulated using the commercial software StarCCM+. In the selected case study, the coolant enters the root of the blade in the radial direction through ellipse-shaped ducts characterized by a low aspect ratio. The flow enters a large plenum and then is fed to radial cylindrical channels, providing cooling to the blade. Eventually, the coolant is discharged into the main flow at the blade tip, contributing to the formation of the tip leakage vortex. Steady and unsteady RANS simulations of the rotor blade, along with hot gas and coolant flow were performed to describe the internal flow evolution and its interaction with the main flow, with particular focus on the internal and external heat transfer.