The Effects of Unsteadiness and Compressibility on the Interaction Between Hub Leakage and Mainstream Flows in HP Turbines

Abstract

This paper investigates the effects of compressibility and unsteadiness due to the relative blade row motion and their importance in the interaction between hub leakage (purge) and mainstream flows. First, the challenges associated with the blade redesign for low-speed testing are described. The effects of Mach number are then addressed by analyzing the experiments in the low-speed linear cascade equipped with the secondary airflow system and computations performed on the low- and high-speed blade profiles. These results indicate that the compressibility does not significantly affect the interaction between the leakage and mainstream flows despite a number of compromises made during the design of the low-speed blade. This was due to the fact that the leakage-mainstream interaction takes place upstream of the blade throat where the local Mach numbers are still relatively low. The analysis is then extended to the equivalent full-stage unsteady computations. The periodic unsteadiness resulting from the relative motion of the upstream vanes appreciably affected the way in which the leakage flow is injected and the rotor flowfield in general. However, the time-average flowfield was still found to be dominated by the rotor blade’s potential field. For the present test arrangement, the unsteady effects were not very detrimental, and caused less than a 10% increase in the losses due to the leakage injection relative to the steady calculations.