Nowadays, Gas Turbines in both industrial and aeronautical applications have high pressure components that work at very high temperatures because they must achieve maximum performance in terms of specific work and cycle efficiency. Therefore, it is necessary to cool down the hot gas path in order to maintain durability. Film cooling is the most studied system given its high efficiency in vane and blade cooling. The research topic of this Master Thesis concerns external film cooling in a transonic fully cooled High Pressure Turbine Nozzle Guide Vane. The NGV is a component which is positioned immediately downstream of the combustor, therefore it is affected by the highest temperature of the Joule-Brayton thermodynamic cycle but also by non-uniform inlet conditions in terms of hot-spot and swirl.