AUTOMATED OPTIMIZATION OF THE NON-AXISYMMETRIC HUB ENDWALL OF THE ROTOR OF AN AXIAL COMPRESSOR

Kurzfassung

In this study an axial compressor consisting of four stages is studied. The endwall of the hub of the last rotor is modified in an automated optimization process in order to reduce the losses and gain surge margin. The endwall is allowed to take non-axisymmetric shapes and is parametrized in such a way, that a groove along the suction side and a fillet are the basic forms. Additionally the lower part of the blade is allowed to vary, while the upper part remains fixed. In order to save calculation time only a set up consisting of stator 3, rotor 4 and stator 4 is calculated during the optimization process. The boundary conditions of the calculations are taken from CFD calculations of the complete four stage rig. For CFD calculations the DLR in-house TRACE-code is used. The optimization tool is AutoOpti, a DLR developed tool, which is based on a genetic algorithm speeded up by surrogate models. In the optimization two operating points are taken into account. One is the aerodynamic design point (ADP), the other one is an operating point near the surge limit (OPSL). The basic forms which evolved during the optimization are a large fillet at the leading edge and the long stretched groove along the suction side of the blade. The optimization shows that a significant reduction of turbulent eddy viscosity and corner stall can be achieved. As the reduction of total pressure losses is limited to the hub region, thus is the overall gain in isentropic efficiency, which rises about 0.2 % in the ADP, for the OPSL an efficiency gain of about 0.5% can be reached. Especially the OPSL shows a strong reduction in secondary flow and correspondingly in corner stall. As only the hub region of the rotor is changed the overall total pressure characteristic of the stage remains basically the same. Using non-axisymmetric endwall shaping can contribute to enhancing axial stages with already highly efficient blading.