A coupled CFD approach for combustor-turbine interaction

The current approach in the industry to numerically investigate the flow in a gas turbine considers each component, such as combustor and turbine, as a stand-alone part, involving no or very minor interactions with other parts, mainly applied through static boundary conditions. Efficient and very specialised CFD codes have been developed in the past to address the different flow characteristic occurring in the different regions of the engine. In order to meet the future requirements in terms of fuel consumption and pollutants emissions, an integrated approach capable of capturing all the possible interactions between different components is necessary. An efficient and accurate way to achieve integrated simulations is to couple already existing specialised codes in a zonal type of coupling. In this Thesis work a methodology to couple an incompressible/low-Mach number pressure-based combustion code with a compressible density-based turbomachinery code for industrial application has been developed. In particular two different couplings have been implemented: the first, based on the exchange of existing boundary conditions through files, comes as a completely separated tools from the original codes, of which no modifications are required, and it is applied to steady state simulations; the second instead, based on the exchange of boundary conditions and body forces through message passing, requires some modifications of the source codes and it is applied to both steady and unsteady cases. A simple analysis shows that not all the primitive variables can be made continuous at the coupling interface between the two codes and a compromise was found that allows minor discontinuity in some of the variables while achieving mass flow conservation and continuity of the temperature profiles. The coupling methodology has been applied to a simplified but realistic industrial case, consisting of a RQL (Rich Burn - Quick quench - Lean burn) combustor coupled with the first stage of the HP turbine. The analysis of the steady case has shown that the combustor field is affected as far as 150% axial chord lengths upstream of the blades leading edge, affecting RTDF and OTDF at the interfaces. In the turbine stage significant differences in both efficiency and degree of reaction were found in the coupled cases with respect to standard standalone simulations using radial inlet profiles. The analysis of the unsteady simulation has instead shown the hot streaks behaviour across the turbine, that are only partially mitigated by the stator blades and, due to segregation effect of hot and cold gases, migrate towards the pressure side of the rotor blades