Combustor-Turbine interaction in modern gas turbines: analysis and modelling of aerothermal phenomena

Combustion–turbine interaction phenomena are attracting ever-growing interest in recent years. In fact, a complex and unsteady flow field, characterized by a high level of turbulence and temperature distortions, usually can be found at the interface plane between the two hot gas path components, especially with the implementation of lean-burn combustors that allow the reduction of NOx emissions, but at the cost of requiring highly swirled and turbulent flows to stabilize the flame. Such severe conditions at the inlet of the first stage nozzle can have a potential impact on the performance of the component, resulting into a perturbation of the heat transfer, the aerodynamics and the effectiveness of the cooling system. For this reason, a better understanding of the physical processes related to the combustor-turbine interaction nozzle becomes a mandatory step for a reliable the aerothermal design. The main objective of this work is to study the behaviour of film cooling system and the heat loads on the first stage nozzle in lab-scaled and engine-like conditions. Firstly, an annular sector rig, made by a non-reactive, trisector combustor simulator and nozzle cascade, where both adiabatic effectiveness and HTC measurements had been carried out, is investigated by performing a systematic computational study by using RANS and SBES. The comparison between numerical predictions and the available experimental results is exploited to assess the capability of advanced scale-resolving methods in the characterization of the mutual combustor-turbine interaction, along with the heat transfer coefficient and film-cooling system behavior. This evaluation aims thus to assess if such more advanced and more timeconsuming methods than RANS, including also turbulence models able to capture the transition, can be more reliably used for a proper prediction of the vane thermal loads. Then, a fully integrated combustor-nozzle configuration is investigated by using SBES, along with a realistic turbine nozzle cooling system, under realistic engine-like operating conditions. Decoupled RANS/SBES simulations of the stand-alone NGV are also reported to highlight the risks and uncertainties associated with carrying out decoupled simulations. The aim is to evaluate the influence of the combustor’s presence under realistic operating conditions and a realistic annular geometry. Then, the focus is shifted to the generation and application of highly representative and reliable boundary conditions at the inlet of the first-stage nozzle in order to enable the separate study of the two components by conducting decoupled simulations. To do so, several SBES decoupled simulations of the stand-alone NGV are carried out by applying two-dimensional unsteady boundary conditions extracted from the fully integrated combustor-NGV SBES simulation. Additionally, the POD technique is applied, considering three different numbers of POD modes, corresponding to a descending level of energy content relative to the total energy of the flow. In this way, the proper coherent structures of turbulent flows are identified and the complexity of the dynamics of a system is reduced, just taking into account the most important modes. Finally, the research focuses on the prediction of heat loads in engine-like conditions by simulating a single set of uncooled vanes. The main objective of these analyses is to obtain a scaling criterion for the HTC obtained in lab-scaled conditions in order to obtain representative HTC values in engine-like conditions. In fact, when the engine operates in realistic conditions at full speed and full load, direct experimental measurements are not available and therefore the introduction of a scaling criterion becomes necessary. The present study demonstrates the importance of accurately capture the mutual combustor-turbine interaction, by employing advanced scale resolving methods such as SBES calculations, for a reliable aerothermal design of the first stage nozzle also under representative and realistic engine-like operating conditions. To the author’s knowledge, a very limited number of studies have been carried out with this kind of EXP-CFD benchmarking, due to the very limited amount of experimental data under representative combustor outflow conditions. In this context, to author’s knowledge, this research represents also one of the few study present in literature in studying the impact of the combustor-turbine interaction phenomena on the aerodynamics and thermal fields, including a realistic turbine nozzle cooling system, with realistic inlet conditions representative of the engine marching at full speed and full load.