Simulation and Optimization of an Axial Compressor Considering Tip Clearance Flow

Licentiate thesis, 2016

With ever-increasing demands for high efficiency in axial compressors, it has become important to consider more geometrical features of the manufactured component in the design phase. Many possibilities open if the level of fidelity of the computational model can be increased. A higher level of detail leads to enhanced performance of components, as the need to be conservative in the design phase is reduced. Better performance is important for reducing fuel consumption and the weight of the component and, consequently, decreasing the environmental impact. An axial compressor consists of rotating and stationary blade rows, where a distance between the rotating blades and the inner casing (shroud) is called the tip gap or tip clearance and is required to avoid contact of the blades with the shroud during engine operation. Large tip gaps in relation to the blade height can typically 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 can affect the flow in the rotor passage significantly. Including a tip gap in the optimization process of a compressor stage can therefore be of importance even in the early design phase to find geometries that will reach the design point at the design rotational speed. In this thesis, different turbulence models and wall modeling approaches are used to calculate the flow in a transonic compressor stage with a large tip clearance. The benefits of including the tip clearance in the optimization process of a transonic compressor are shown and discussed. It is shown that considering the tip clearance in the optimization process is important to be able to reach the specified design point. Furthermore, from the optimization results it is shown that redistribution of the flow as a result of blockage in the tip region impacts the design variables over the entire span.