An experimental investigation on the aerodynamic behavior of cooling channels for the leading edge of gas turbine blades

The purpose of this dissertation is to provide a strong background knowledge about the flow field behavior inside leading edge internal cooling systems while addressing the importance of aerodynamic data for cooling channels design. This research is mainly focused on the characterization of rotation effects inside a triangular channel since, in the open literature, the only channel geometry exhaustively studied had been the rectangular one. In fact, in recent years, the interest of the scientific community had been shifted to the direct analysis of the thermal field, which is indeed the final figure of evaluation of the cooling system. However, the thermal field characterization alone (i.e. Nusselt number distribution), is not able to provide enough information in order to allow a complete and deep understanding of the heat transfer problem because it describes an effect giving too little clues about its cause, which is the aerodynamic field indeed. Even in simple geometries, the aerodynamic behavior usually is too much complex to be inferred just exploiting the thermal data. This is a major problem if an optimization process of the cooling system is considered: with the thermal data only a trial-and-error kind of iteration is possible. On the contrary, the knowledge of the flow field allow to operate in a more thoughtful way. For the reasons explained above, the first part of the research has been dedicated to the complete aerodynamic characterization of the simplest geometry suitable to represent a leading edge cooling system: a straight channel with triangular equilateral cross section, without bleeding holes, and with squared turbulence promoters perpendicular to the bulk velocity and placed on both leading and trailing sides. This campaign is the following of a previous work, where a smooth channel had been studied and the rotation effect was deeply characterized with both experimental and numerical analysis. In this work, the effect of centrifugal buoyancy forces is also taken into account and characterized. The data gathered with the PIV investigation had also been used to validate a numerical model and therefore extend the analysis, providing a complete characterization of the flow field evolution along the channel. The second part of the research project involves the investigation and complete characterization of the flow field inside an advanced leading edge cooling system, with impingement cooling and film cooling extraction holes. The aim was firstly to be able to correctly set the experiment boundary conditions and constraints, such as equal repartition of mass flow rate between each film cooling hole and maintain the same distribution in both static and rotating tests, in order to avoid a wrong experiment conditioning. Flow field measurements have been conducted mostly in order to evaluate if rotation effect in the jet's alimentation channel could affect its aerodynamics and the interaction between the jet and the coolant extraction holes.