AERODYNAMIC INVESTIGATION OF THE PERFORMANCE OF A TWO STAGE AXIAL TURBINE AT DESIGN AND OFF-DESIGN CONDITIONS

This paper describes numerical aerodynamic investigations of a two-stage, high pressure axial turbine at design and off-design operating conditions. The flow field in a high pressure turbine is highly complex due to unsteadiness of the flow and the various effects of blade row interaction. Blade loss mechanisms generally include primary and secondary loss mechanisms. Examples of primary loss mechanisms include boundary layer losses, shock losses and mixing losses, whereas examples of secondary losses include tip leakage losses and end wall losses which both create secondary flow characteristics. Although modern numerical analysis techniques have provided good understanding of the flow field, it is still difficult to accurately predict impact due to the aforementioned loss effects. This is generally due to errors predicting in boundary layers, transition as well as false entropy generation due to numerical dissipation. When a turbine is operated at off-design conditions the primary and secondary loss effects are further increased and create further reductions in engine efficiency. In this study a numerical model of the two-stage axial turbine was constructed and run under boundary conditions designed to mimic the operating conditions applied during engine operation. The shear stress transport (SST) turbulence model was selected for its versatility in turbomachinery applications. A comparison was made between both experimentally measured efficiencies and numerically predicted efficiencies.