Observations on the selection of objective function for the optimisation of turbine endwalls using computational fluid dynamics

Abstract

This study examines the results of a CFD and rotating experiment, comparing an annular and a generic end wall design for a model turbine rotor in a 1½ stage turbine at the CSIR, in an attempt to draw some conclusions regarding the proper selection of objective functions during the optimisation process. The CSIR has a rich body of experimental and computational data from a rotating test rig with both profiled and planar end walls, which provides an ideal opportunity to examine the validity of various objective functions. A 1½ stage, low speed, turbine test rig, originally of the University of Natal, where it was used for tip loss measurements, has been installed and refurbished at the CSIR. In Figure 3 the general layout and instrumentation of the test rig is indicated. The test rig has a hub and tip radius of 0.142 m and 0.203 m respectively and is designed for a maximum wheel speed of 3000 RPM. A radial fan draws atmospheric air through the rig, inducing a pressure drop of 4.8 kPa for a mass-flow of 3.39 kg/s, corresponding to an inlet velocity of about 30 m/s. The radial fan is driven by an electric motor with variable speed control. The turbine power is absorbed by a hydraulic motor, giving independent rotor speed control. Inlet turbulence intensity was measured to be less than 1%.