Reshaping the Rotor Hub of a 1.5-stage Axial Turbine to Reduce Pressure Losses by a Parametric Groove

The interaction among the vortices that develop over an axial turbine passage hub leads to pressure losses and, consequently, to a decrease in the stage isentropic efficiency. The turbine performs better if flow separation and secondary flows are reduced. To achieve this, this paper explores by computational fluid dynamics the application of rotor hub contouring to a one-and-a-half-stage axial turbine, the "Aachen Turbine." The pressure side arm of the rotor horseshoe vortex is guided by a groove in the end-wall rotor hub surface, which is defined parametrically using non-uniform rational B-splines (NURBS). This novel rotor hub groove runs from the leading edge of the rotor blade to the trilling edge of the rotor blade. A three-dimensional steady Reynolds Averaged Navier-Stokes (RANS) k-omega-SST model of the one-and-half-stage turbine with axisymmetric end-walls is validated against reference experimental measurement from the Institute of Jet Propulsion and Turbomachinery at RWTH Aachen in Germany. By contouring the hub of the upstream stator and of the rotor, the overall pressure loss coefficient predicted by openFOAM computational fluid dynamics is reduced by 5.2%, using Kriging optimized groove shape parameters.