Mach Number Effects on Secondary Flow Development Downstream of a Turbine Cascade

The results of an investigation of the three-dimensional flow downstream of a transonic turbine cascade are presented. The investigation was carried out for a wide range of Mach numbers, extending from M_2is = 0.2 up to 1.55. Measurements were made in five planes at different axial locations downstream of the trailing edge (covering more than one chord length), by using a miniaturized five hole probe especially designed for transonic flows. The results are presented in terms of local loss coefficient, vorticity and secondary velocity plots; these plots give a detailed picture of the secondary flow development downstream of the cascade and show how flow compressibility influences the vortex configuration. As Mach number increases, the passage vortex is found to migrate towards the endwall and secondary flow effects are more confined in the endwall region. The pitchwise mass averaged loss and flow angle distributions along the blade height appear to be affected by the expansion ratio; at high Mach number both underturning and overturning angles are found to be smaller than in low velocity flows. Overall losses, vorticity and secondary kinetic energy versus Mach number are also presented and discussed.