Steady and Unsteady Performance of a Transonic Compressor Stage with Non-axisymmetric End Walls

The development of axial aircraft compressors has led to extremely high stage loadings and therewith reductions in entire engine size and weight. This trend also promotes several disadvantages such as the risks of flow separation and higher secondary flows that are associated with increased stage loading. The application of non-axisymmetric end walls is one approach to reduce blade loading in the end wall regions and to control end wall flow with the main objectives of increasing the component efficiency and the total pressure ratio. The emphasis of this work is to analyze the steady and unsteady performance of a transonic compressor stage with non-axisymmetric end walls where the profiled end wall serves to control the addressed flow inhomogeneities. The axisymmetric layout of Configuration I of the Darmstadt Transonic Compressor serves as the datum design. As a tool to find the optimum non-axisymmetric end wall shape, a fully-automated multi-objective optimizer connected to a steady 3D-RANS flow solver is used. The goal is to analyze how effective such a design tool can work on such a challenging task and to derive first design rules and compare the differences and features in common to the experience made by turbine researchers. In this context, the stator and rotor row of the stage are subsequently and individually optimized. As a further step, the obtained optimized geometries are investigated in unsteady mode. The study demonstrates the potential of profiled end walls to suppress separation phenomena in a stator row with 2D blading involving an enormous increase in efficiency. The exit flow field's whirl angle distribution is made more uniform which is generally desirable for the inlet conditions of a downstream stage. The profiled end wall design proofs to control the static pressure distribution which results in the migration of the loss cores from the end wall onto to the airfoil suction surface. This is identified as the main mechanism responsible for the obtained improvements. For the non-axisymmetric end wall design of the rotor row, a mechanism similar to the principle of lean is observed. The optimized rotor produces blockage and reduces the loss production in the outer regions (>5% channel height) at the expense of an increase in near hub end wall losses. This highlights the different effects of contoured end walls in the rotor and the stator rows and also provides the potential to serve as an alternative to design techniques such as 3D-blading. The unsteady analyses of the different geometries confirm the impact of the rotor wake on the separation phenomena in the downstream stator which cannot be captured by the steady approach. However, they also demonstrate that unsteady phenomena can be neglected during the optimization process of the profiled end walls for this compressor configuration