CFD Optimization of a Transonic Compressor Stage with a Large Tip Gap

Other conference contribution, 2015

Large tip gaps can be found in the rear stages of transonic compressors. If the size of the tip gap is large in relation to the blade height it will likely affect the flow in the rotor passage significantly. Therefore, including a large tip gap in the optimization process of a compressor stage can already be of importance in the early design phase. In the present study, a transonic compressor stage is optimized with and without a large tip gap (2.5% of the rotor leading edge span) with respect to part speed stability and polytropic efficiency at a design point. The performance is evaluated using 3D CFD calculations. The two approaches are compared to determine the importance of including the tip gap in the optimization. It is shown that, when the compressor stage is optimized with a tip gap, redistribution of mass flow in the rotor passage affects the design at mid span and near the hub. A lower stagger angle is preferred away from the tip region to allow for a higher mass flow at lower spanwise positions. The k-epsilon turbulence model with wall functions is used to evaluate the performance of the stages during optimization. To support the use of wall functions, Chien’s low-Reynolds model with a more dense mesh is used to evaluate the polytropic efficiency of a number of stages with a tip gap. The results show that the ranking of designs using a low-Reynolds model show the same trend as using wall functions.