Effect of self-sustained pulsation of coolant flow on adiabatic effectiveness and net heat flux reduction on a flat plate

Advanced film-cooling systems are necessary to guarantee safe working conditions of high-pressure turbine stages. A fair prediction of the inherent unsteady interaction between the mainflow and the jet of cooling air allows for correctly describing the complex flow structures arising close to the cooled region. This proves to be crucial for the design of high-performance cooling systems. Results obtained by means of an experimental campaign performed at the University of Karlsruhe are shown along with unsteady numerical data obtained for the corresponding working conditions. The experimental rig consists of an instrumented plate where the hot flow reaches Mach = 0.6 close to the coolant jet exit section. The numerical campaign models the unsteady film cooling characteristics using a third-order accurate method. The ANSYS®FLUENT®software is used along with a mesh refinement procedure that allows for accurately modelling the flow field. Turbulence is modelled using the k-w SST model. Timeaveraged and time-resolved distributions of adiabatic effectiveness and Net Heat Flux Reduction are analysed to determine to what extent deterministic unsteadiness plays a role in cooling systems. It is found that coolant pulsates due to fluctuations generated by flow separation at the inlet section of the cooling channel. Visualizations of the fluctuating flow field demonstrate that coolant penetration depends on the phase of the pulsation, thus leading to periodically reduced shielding. Eventually, unsteadi-ness occurring at integral length scales does not provide enough mixing to match the experiments, thus hinting that the dominant phenomena occur at inertial length scales.