NUMERICAL INVESTIGATION OF LEAKAGE FLOW IN AXIAL FLOW TURBINES: EFFECT OF TIP CLEARANCE AND RELATIVE MOTION

Performance of a turbomachinery is strongly related to the flow structure in the tip gap. Flow in the tip gap is 3-D and highly complex. Pressure driven flow between the pressure side and suction side of the blade is a source of aerodynamic loss. Thus, in order to improve the performance flow structure in this small gap should be clarified. Studies indicate that interaction of tip leakage flow with main flow inside the turbomachinery results in a considerable loss. However, structure of leakage flow has not been understood completely. Small values of tip gap height creates difficulties for experimental measurements. In this study, flow structure through the tip gap will be investigated comprehensively by implementing Computational Fluid Dynamic (CFD) methods. Effects of tip clearance height (% 0.7, % 1, and % 1.5) and relative motion between blade and casing on formation of leakage flow and tip leakage vortex have been investigated. CFD results are consistent with studies in literature. It was observed that k-omega SST turbulence model was better to predict flow structures. Increasing tip clearance height results in higher aerodynamic losses and leakage flow. It was obtained that relative motion between blades and casing reduces the aerodynamic loss by weakening the tip leakage vortex.