In order to improve the efficiency of modern gas turbine combustors, efficient cooling techniques are required to allow for increased turbine inlet temperatures. Film cooling is already a widely used technique to reduce heat loads on mechanical components such as liner and turbine blades. New hole geometries and sophisticated flow concepts to further increase the cooling effectiveness are subject of present studies. However, although flow fields in real combustion chambers show a significant level of turbulence, most literature studies are carried out at low turbulence level in the main flow.An active turbulence generator is installed in our wind tunnel facility to study film cooling efficiency at flow conditions that are more representative for real gas turbines. In previous work, hot wire and high-speed PIV measurements were carried out to characterize the turbulent flow field in the test section when operating the wind tunnel with such a turbulence generator. Turbulence intensity (TI) and turbulent length scale (TL) were determined at several axial stations.In the present study, we compare results from 3D Large Eddy Simulations (LES) and 1D hot-wire data at locations downstream of the active turbulence generator. The turbulence generator is a rotating plate which is supposed to generate large vortices in the wind tunnel. In comparison to more complex turbulence generators this simple geometry allows for structured meshing. The LES with wall-modeling and featuring a moving mesh insert was computed with second order accuracy in space and time. Computational point probes sampled the 3 velocity components at the selected locations. The LES probe signals were transformed into numerical hot-wire signals by determining the velocity magnitude consisting of the two velocity components which are responsible for the cooling of the 1D hot-wire probe in the measurement. The comparison of these numerical hot-wire data with the measured signal shows good agreement in absolute velocity level, turbulence intensity and turbulent length scale. Hence, it can be assumed that the LES is able to predict the full three-dimensional flow field generated by the turbulence generator.From the CFD data we are able to determine TI and TL in each direction of the flow field. In our previous study the TL was determined from the velocity signal by using an auto-correlation function. This function requires validity of the Taylor hypothesis, which, however, is not always valid in a highly intermittent developing turbulent flow field. Therefore, the TL here will be determined with a cross-correlation approach, which does not depend on the Taylor hypothesis. The resulting TL data will then be compared with results from the auto-correlation function to demonstrate the advantage of the new post processing approach.Furthermore, the velocity signal downstream of a turbulence generator contains harmonic frequencies since the rotating plate is periodically blocking the channel cross section. Therefore, the velocity field is influenced by the forced periodic oscillation of the flow field. We want to filter the harmonic frequencies from the turbulent velocity signal to evaluate the blocking effect on the turbulence intensity and turbulent length scale. Finally, it is planned to apply the Q-criterium to the instantaneous LES flow field. The sizes of the scales can be compared to the determined TL which should be of the same order of magnitude. This may be used to further validate our new approach.The overall goal is to deepen our understanding of the interaction between realistic main flow turbulence and cooling film. Such an understanding is essential for the development and characterization of present and future film cooling configurations.

Analysis of LES and 1D Hot-Wire Data to Determine Main Flow Turbulence in a Film Cooling Test Rig

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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