Assessment of Advanced Numerical Methods for the Aero-Thermal Investigation of Combustor-Turbine Interactions

The individual components of modern gas turbines are optimized to such a level that any noticeable improvement of the global performance can only be the result of a significant technological effort. In this sense, one of the main strategies to increase the efficiency of new generation engines lies in the investigation of more complex but more accurate design methodologies. In particular, the different modules of a gas turbine are historically designed separately, despite the fact that a turbomachine is a fully integrated system where all components interact with each other. In the turbomachinery community there is a growing interest in including the interfaces between different components into the design process, as it would enable to design compressors, combustors and turbines in an integrated manner by taking into account the real operating conditions of the machine. The interface between combustion chamber and high pressure turbine is considered as the most critical one, as it directly affects the maximum temperature reached by the thermodynamic cycle. The flow field at the combustor-turbine interface is characterized by very high turbulence levels, swirl and temperature distortions, and the CFD methods currently used to design high pressure turbine (HPT) blades lack of validation for such an aggressive environment. The aim of the present work is to develop new numerical methodologies and to analyze the accuracy of the existing ones when applied to multi-component simulations in turbomachinery. The attention is mainly focused on the interaction between combustor and turbine, as it represents the most critical interface for a modern gas turbines. To take into account the mutual interaction between combustion chamber and HPT, in the first part of this thesis three CFD methodologies are developed to solve the flow field of the two components in an integrated framework. The procedures are validated on test cases of increasing complexity and successfully applied to a configuration representing a modern combustion chamber coupled to a nozzle guide vane. In the second part, the advantages and limitations of RANS and LES applied to the study of the hot streak migration in HPTs are discussed. In this sense, a LES tool for the external aerodynamics of turbine blades is developed and validated, with the aim to be applied in the future to the investigation of the combustor-turbine interaction. The accuracy of LES is then exploited to validate less time-consuming RANS models in predicting the hot streak migration in a turbine stage. The current investigations indicate that integrated multi-component studiesare necessary to reproduce the actual operating conditions of the different components, as the complex interaction between hot streaks, swirl, turbulence and potential effect of the NGV cannot be reproduced without resolving the combustor and turbine at the same time. Moreover, the extreme turbulence level at the combustor-turbine interface must be modeled with care, since it plays a major role in the migration and diffusion of the hot streak in turbine.