Mitigating secondary flows in a 1½ stage axial turbine by a guide groove casing

The interaction of secondary flows with the main passage flow in turbomachines generates entropy and aerodynamic loss. This loss source is most relevant to low aspect ratio blades. One approach for reducing this loss is by end-wall contouring. However, limited work has been reported on using non-axisymmetric end-walls at the stator casing and on its interaction with the tip leakage flow. In this paper, a non-axisymmetric end-wall design for the stator casing is achieved through a novel surface definition, towards mitigating secondary flow losses. The casing features a groove that follows the natural path of selected secondary flow structures. This design is tested on a three-dimensional axial turbine RANS model built in OpenFOAM Extend 3.2, with − SST turbulence closure. The stage isentropic efficiency is predicted to increase by 0.59% at the turbine design point and by 0.63% off-design, by using the new casing. This work also used an automated workflow process, linking surface definition in MATLAB, meshing in ICEM CFD, flow solving in OpenFOAM, and post-processing in Tecplot. This has generated a casing contouring design tool with a good portability to industry, to design and optimize new turbine blade passages. Stages designed with this new non-axisymmetric casing are likely to reduce the CO2 emissions from power generation, towards achieving the UNFCCC emissions goals.