Innovative cooling systems for gas turbines applications through additive manufacturing

Modern gas turbines operating temperatures are expected to reach 2000K, which is far beyond the maximum allowable temperature of the commonly employed super-alloys for such applications. For this reason, to cool down the turbine components becomes mandatory and requires the design of advanced cooling systems to prevent structural failure due to excessive thermal loads. In particular, one of the most critical areas to manage efficiently is the trailing edge of a turbine blade, where the surrounding environment can lead to rapid deterioration if not properly cooled. Furthermore, the difficulty to integrate a cooling system into such tiny space leads to a combination of film cooling and internal cooling by means of turbulating structures in the trailing edge region. Moving on to manufacturing techniques, the majority of the turbine components are realized through investment casting which is inherently bounded by the mold shapes and prevent many interesting geometrical concept from being produced. In this context, Additive Manufacturing has been proving a very interesting potential to produce objects having both tiny dimensions and complex and twisted shapes. For this reason, in the last years, many efforts have been done to employ dedicated AM techniques in the gas turbine cooling field. The purpose of this thesis, developed in collaboration with Baker Hughes, is to assess the potential of Additive Manufacturing when applied to innovative cooling systems for the trailing edge of a turbine blade. In particular, both film cooling and internal cooling have been explored within the Design-for-Additive-Manufacturing framework. Concerning the film cooling, an optimization procedure based on both numerical and experimental tools has been proposed in order to develop a novel hole geometry. The methodology starts with a numerical optimization driven by fluid dynamics goals and constraints which generates a promising shape which, however, could not be printed. For this reason, some changes have been added to the original design according to experimental findings and optical-aided observations in order to overcome the issues of the printing process. This procedure emphasizes the need to include manufacturing constraints to properly account for the printing process limits into the geometry optimization, especially when the characteristic dimensions involved are very small — less than 1mm. In such cases, a compensation strategy is crucial to avoid or at least reduce the geometry collapse. Furthermore, a statistical analysis has been proposed to assess the repeatability of the printing process based on measurement retrieved from different samples. Moving on to the internal cooling, different cooling schemes have been investigated ranging from traditional ones to more complex and innovative, like matrix/lattice structures, whose manufacturability is strictly linked to additive processes. Even if many lattice structure are available in the literature, the elementary Kagome cell has been selected in this work. First, many layouts are proposed to be tested in a low TRL test rig which allowed to directly compare their performances — both in terms of heat transfer and pressure drop — with other designs in a range of flow Reynolds number which was not explored in the literature. Then, due to their competitive overall features, a specific arrangement has been designed to be housed in the trailing edge of a real turbine vane as the assessment of its cooling characteristics compared to the well-established pin-fins can provide a benchmark for the employment of additive-based internal cooling systems.