Design tools and innovative concepts for gas turbine cooling applications

The improvement in thermal effciency and specific power output in gas turbine engines depends on the possibility of increasing Turbine Inlet Temperature (TIT). This has become during the last years one of the main target in engine development, and nowadays TIT can overtake higher values than those allowed by material's resistance. Despite the constant development concerning materials, it is possible to raise engine operating temperature only by cooling gas turbine components, ensuring them an adequate lifetime. Since cooling system use air bled from the compressor, it has a cost on the performance of thermodynamic cycle: a progressive reduction of the global efficiency inevitably occurs increasing the amount of coolant. Gas turbine designers have the goal to maximize cooling system effectiveness, or in other terms ensure required component lifespan using a minimal amount of coolant flow. To this purpose, it is necessary to develop tools able to accurately and quickly estimate thermal loads and predict metal temperature on gas turbine components. Three inherently linked heat transfer problems have to be solved for estimating metal temperature in a GT component: external convection performed by hot gas coming from the combustion chamber, internal convection achieved from coolant flow, and conduction within the metal component. It is here proposed a numerical procedure, developed in collaboration with GE Oil&Gas Nuovo Pignone, aimed to perform Conjugate Heat Transfer (CHT) calculations of cooled vanes and blades, where the internal cooling system is modeled by a one dimensional thermofluid network solver mainly based on heat transfer and pressure losses correlations coming from the open literature. Thermal loads and pressure distributions on the external profile are obtained by 3D CFD analysis, while heat conduction in the solid is estimated by a 3D FEM solver, which uses data computed by 3D CFD and fluid network solver as boundary conditions. The great advantage of this procedure is represented by the possibility to speed up the design phase respect to a complete CHT calculation, since CFD and FEM calculations are performed without discretization of cooling holes, using correlative and lower order methodologies, and since the solution of internal cooling system is performed by a fluid network solver, decoupled from other domains. This allows to decrease computational time and permits a faster analysis without renouncing to an adequate overall accuracy, assuring feasibility in both preliminary and detailed design phases. In order to validate the proposed methodology, the procedure was applied to two different test cases, which represent two gas turbine blades with different cooling configuration. Numerical tools involved in the procedure are then used for the design of two different innovative gas turbine cooling systems: one based on impingement and the other on film cooling. These concepts are designed by thinking of a production via additive manufacturing, where limits imposed by casting and chip removal machining can be overcome. CFD analyses and experimental activities are carried out in order to improve the innovative concepts' performance and validate the numerical methodology, recognizing its capabilities as a fast design tool for both existent and innovative cooling systems.