Loss Predictions in the High-Pressure Film-Cooled Turbine Blade Cascade T120D by Mean of Wall-Resolved Large Eddy Simulation

Film cooling is commonly used to protect turbine vanes and blades from the hot gases produced in the combustion chamber. The design and optimization of these systems can only be achieved if a precise prediction of the fluid mechanics and film efficiency is guaranteed.  This results in the need to numerically identify and assess the various origins of the losses taking place in such systems.  In this context, the present study addresses the issue of loss assessment based on a wall-resolved Large Eddy Simulation (LES) of the film-cooled high-pressure turbine blade cascade T120D from the European project AITEB II.  The main objectives are: first, to evaluate the capacity of LES to predict adiabatic film cooling effectiveness in a well mastered academic case; as well as, to investigate the loss generation mechanisms in such aerothermal configuration.  Results produce instantaneous and mean flow structures of the coolant jets that are coherent with literature findings and common knowledge on these flows.  The jets detach from the wall when injected into the main passage to then reattach on the wall producing a cooling film along the pressure side of the blade.  Satisfactory agreements are numerically retrieved for the pressure load prediction and adiabatic film effectiveness when compared to experimental data. Based on these predictions and to gain understanding on the film cooling effectiveness evolution along the pressure side, the mixing process between the hot stream and the coolant flow is specifically investigated. From such an analysis, turbulent mixing of the coolant mass fraction show that the majority of the mixing originates from the region where jets detach from the wall. For this specific configuration, the mixing is indeed mainly induced by the unsteady features occurring in the cooling pipes and the interaction of the initial jet with the main stream.  Loss generation mechanisms are then investigated by probing total enthalpy and entropy balance equations showing that all total quantities (i.e. total temperature and total pressure) are impacted along the passage in agreement with theory. Further analyses of loss maps making use of the Second Law Analysis method show that aerodynamics losses dominate the regions of highly sheared flow while mixing losses are mainly located in the coolant film.  As the temperature difference between the hot and coolant flows is low in the experimental conditions, mixing losses remain largely inferior to aerodynamic losses in the film region. In terms of overall contributions, unsteady effects are found to be the main contributor to the losses, the mean steady field only dominating in the boundary layer regions. If looking specifically at the loss generation in the coolant film, losses are observed to be mainly generated in the detached jet region. In the region where the coolant film is attached to the wall, losses are produced at wall and are linked to the flow wall shear stress and mixing process between the hot and coolant streams. As for the mixing loss, it tends to zero close to the blade surface due to the adiabatic wall condition.