Optimization of the Endwall Contour of a Multistage Compressor

Kurzfassung

In the domain of turbomachinery, the designs have achieved a high level of development. The potential for improvement in the undisturbed blade flow is nearly exhausted. Particularly in compressors, a big source of losses are secondary air flows, causing losses like in corner stall in the hub region. This project is realized on the rig 250, which is a 4 stage rig. Especially the rotor 4 is further analyzed in this study. In this works the influence of the stator outflow and its hub clearance on the following rotor is detailed. The results show a high importance of the hub clearance of the stator which strongly modifies the distribution of the outflow angle of the stator. This distribution flows to the following rotor and consequently has a large influence on the total pressure losses in the hub region. An influence on the secondary air flow is realized by the use of non-axisymmetric endwalls. Additionally, modified blades in the hub region and variable fillets around the blade are used. In order to distinguish the effects of each parametrization, several optimizations are realized separately and a final one combines all the parametrizations. The parametrization of the endwall was developed and is a compromise between the number of parameters and the detail of description of the geometry. As described in the work of Dorfner [9], a separation edge followed by a groove along the suction side showed a good reduction of the corner stall. That is why the parametrization developed enables such a description at a reduced number of parameters. As at the beginning of the project, no fillet generator existed, a parametrization approximating a fillet, and permitting a variation of the fillet form and size in positive and negative direction was developed. On the blade, two profiles at the hub and at 10 % of the blade are let free for optimization. The mesh is a wall function mesh at the endwalls, and a low Reynolds mesh on the blades. The CFD calculation is realized by the CFD in-house solver TRACE. U-RANS equations are used for the calculation together with the k − ! turbulence model. For the optimization process the in-house optimizer AutoOpti is used at two operating points: the Aerodynamic Design Point (ADP) and the Operating Point near Surge Line (OPSL). In the first optimizations the objective functions are the isentropic efficiency for each operating point. An optimization of the fillet showed that a large fillet is positive for the OPSL and unprofitable for the ADP. Then, a separate optimization of the groove showed an evolution of the geometry along the Pareto front. The better the geometry is in the ADP, the deeper the groove is on the suction side, and for a geometry better in OPSL, the groove moves progressively to the pressure side. After understanding of these phenomena, an optimization combining both parametrizations and also the parametrization of two profiles of the blade and of the global contour was carried out. The focus is put on the losses in the hub region by the objective function of the optimization which minimizes the integral of the losses in the first 20 % of the blade height. In the analysis of the members of the optimization, a significant reduction of the corner stall is shown in the OPSL as well as in the ADP. The form consists of a large fillet at the leading edge, a groove on the suction side, and surprisingly a negative fillet on the pressure side. Finally an analysis of the geometry is done, and the reduction of the secondary air flow is shown. This numerical analysis showed the influence of the fillets on the flow and the benefits of contoured endwalls, which significantly reduce the losses in the hub region, particularly the corner stall. One of the optimized geometries will be constructed and measured on the test bench of DLR to confirm the numerical results, and the benefits of contoured endwalls.