EFFECT OF THE TIP CLEARANCE ON THE CHARACTERISTICS OF A 1.5 COMPRESSOR STAGE WITH REGARD TO THE INDEXATION

Axial compressors have inherently unsteady flow fields because of relative motion between rotor and stator airfoils. This motion leads to viscous and potential interactions between blade rows. As the number of stages increases in turbomachines, the build up of convected wakes can lead to progressively more complex interactions. Variation in the relative circumferential position between stators or between rotors can change these interactions, leading to different unsteady forcing functions on airfoils and to different compressor efficiencies. Previous quasi-three-dimensional Navier-Stokes analysis showed that airfoil clocking can produce significant performance variations at the Mach numbers associated with an engine-operating environment. The potential benefit of airfoil clocking is an increase in stage efficiency. Depending on the turbulence model used, the variation of efficiency was on the order of 0.7 % and 1.5 % in the stage [ref. 3, ref. 1]. The losses in the second-stage stator passage appear to vary most as the airfoils are clocked. The 1.5 stage compressor used is made of an inlet guide vane (IGV), a rotor, and a stator. The indexing is performed between the IGV and the stator. In the absolute frame of reference downstream of the rotor, a bubble of loss issued from the interaction between the IGV-wake and the rotor tip clearance was measured in the tip region. In the hub region, the presence of the IGV-wake was found to have an influence on the extend of the stator hub clearance flow. Hence the indexing between the IGV and the stator will not only imply the positions of the IGV-wake at stator inlet, but also the here mentioned secondary flows