NUMERICAL INVESTIGATION OF NEAR-TIP MODIFICATIONS FOR A HIGHLY-LOADED LOW-SPEED ROTOR UNDER THE INFLUENCE OF DOUBLE LEAKAGE

This paper numerically investigates the flow field in the tip region of a highly-loaded axial low-speed compressor rotor. The rotor in focus is of a hybrid blade configuration, featuring a tandem profile in the mid-span region and single blade profiles near the endwalls. The simulated 1.5-stage configuration also consists of a single row IGV and a tandem stator. Due to high loading coefficients (Ψ= 0.58) in combination with moderate flow coefficients (φ= 0.58), double leakage of the tip leakage vortex plays a major role at all operating conditions. As already shown in the past, double leakage is a stability influencing dominant flow phenomenon in compressor rotors and influencing that can be the key to an efficient working range enhancement on compressor stages. This work introduces a simple and powerful method of enhancing the working range for low-speed rotors under the influence of double leakage. It is shown, that modifications to the pressure side shape of rotor tip profiles can influence the interaction of the tip leakage vortex with the adjacent blade, comparable to the effects of casing treatments. These unconventional blade shapes result from the transformations from tandem blade configurations to hybrid blades near the endwalls, as Eckel et al. have shown [1]. The resulting single blade profile close to the tip features a convex profile element (an increase in local thickness, called the -”belly”), which significantly influences the local static pressure gradients. This unusual pressure-side profiling can change mass flow over the tip and, by correct positioning, may reduce the amount of double leakage and, consequently, increase the rotors efficient working range. This work presents the geometry definitions for the hybrid blade single-segments, a detailed analysis of the impact on design and off-design operating conditions together with their underlying flow phenomena, and finally proposes design guidelines for the blade-tip geometries of highly-loaded rotors under the influence of double leakage.