The Influence of the Clearance of Shrouded Rotor Blades on the Development of the Flowfield and Losses in the Subsequent Stator

Abstract

Although turbines are commonly noted for their high efficiency this efficiency can be improved further. The importance of the leakage flow for the losses of turbine stages makes this flow a promising candidate to be examined for loss reduction: first it decreases the workflow through the rotor and second, the suction side incidence of the re-entering leakage flow in the subsequent stator causes additional losses. As an essential parameter for leakage flow the clearance and its influence on the losses is investigated by experimental and numerical approach as well. The measurements of the experimental part were carried out on a 1.5 stage axial model turbine of an enlarged scale. The rotor of the turbine consists of shrouded blades with two teeth on the shroud. The clearance was varied from s/D = 0.0007 up to 0.004. Linear traverse measurements with a pneumatic 5-hole probe were taken in front of and behind the second stator. The re-entering leakage flow extremely influences the flow properties in radial direction (up to 50% of the span) at the inlet of the following stator. So the flow angle α1 deviates up to 90° of the flow angle in the mid-span, depending on the clearance. Along with mixing losses, this suction side incidence leads to an increase of the losses within the stator. In addition numerical investigations of the flow field were done with the commercial flow solver CFX-TASCflow. The interaction between the leakage flow and the secondary losses of the following stator is shown, and a comparison with available loss correlations is carried out in this paper.