Experimental Aero Study on Turbine Rear Structures at Engine-Realistic Flow Conditions

Licentiate thesis, 2021

The aviation industry has made great progress in continuous improvements that reduce the key pollutants associated with aero engines. However, high demands on the aviation industry drive manufacturers and designers to develop even more efficient aero engines. The introduction of a geared turbofan engine (GTF) was a breakthrough, although it created new challenges for suppliers and designers of individual components. The subject of this thesis is a study of the novel turbine rear structure (TRS) and the effects of its geometric features, such as the polygonal shape of the shroud endwall, thickening of the outlet guide vanes (OGV), and implementation of bumps, with an overall analysis of TRS aerodynamics in terms of possible flow separations, corresponding pressure losses, and flow turning performance.

This work summarizes the results obtained from experimental studies of the engine-realistic TRS for two design configurations: baseline (annular) and state-of-the-art (polygonal) designs equipped with OGVs of different geometries. Each of these two concepts was assembled and aerodynamically investigated for the on-design and off-design conditions in a modern 1.5 stage facility (Chalmers OGV-LPT rig) which provides realistic boundary conditions for the TRS. The aerodynamic study was performed by traverse pressure measurements for the inlet and outlet planes by multi-hole pressure probes, oil-film visualization, and static pressure measurements with embedded pressure taps. Steady state RANS simulations of the TRS were also done to estimate the prediction capabilities of a commercial CFD tool.

OGVs with increased thickness introduce slight increase of pressure losses, although the bump vane with add-on does influence the flow substantially. Apart from downstream influence with additional vorticity, generated from bump itself, and corresponding increased pressure losses, it also has upstream influence indirectly affecting inlet conditions into the TRS. For baseline configuration, the higher inlet swirl angles in the hub region result in increased vane loading and pressure losses. For state-of-the art configuration, blade- and bump-loading analysis show clear mutual influence of the bump and vane pressure distributions and, therefore, the need to design the vane and bump combination as one aerodynamic unit. Moreover, the present work gives a thorough comparison between experimental and numerical data showing that using the CFD tool captures secondary flow structures well, although CFD predictions are conservative.

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