| Abstract: |
Tip clearance between the rotor blade and casing is a major source of performance deterioration for axial turbomachines. Compressor tip clearance causes a leakage of air across the blade tip from the pressure side to the suction side. This tip leakage interacts with the primary air stream and wall boundary layer. In fact, tip leakage flow dominates the aerothermodynamic behavior of the end-wall flow and the blade-to-blade flow in the tip regions, as it has a strong impact on pressure ratio, efficiency and stability of compressors.In this study a set of three dimensional, unsteady, viscous, compressible flow governing equations representing the real flow field is solved using commercial code Fluent 6.3.26., where RNG turbulence model is employed.In the present work, the effect of tip clearance on compressor performance is numerically investigated. This is carried out by changing the clearance ratio (the ratio between the clearance height and the mean rotor chord) from 0 to 3%. It is found that the compressor performance parameters (total pressure ratio, efficiency and surge margin) are deteriorated as the clearance ratio increases.Two approaches for eliminating the effect of tip clearance are examined; namely, passive approach and active approach. Casing treatment which is considered as passive approach is carried out by cutting circumferential grooves in the casing opposite to rotor blade tip. A parametric investigation for the shape, inclination angle and number of circumferential grooves is developed in the present study. Five radial and inclined grooves in the casing have total pressure ratio ,surge margin and efficiency higher than the other number of grooves (two, three and four grooves) and the constant clearance gap=1.5% of rotor chord. Concerning inclination angle, five grooves with 1350 inclination angle has higher total pressure ratio, surge margin and compressor efficiency than other angles (450, 600, 900 and 1200) and the untreated casing. Moreover, five grooves with trapezoidal shape have higher total pressure ratio, surge margin than both radial grooves and constant clearance gap. Regarding to the efficiency, the trapezoidal shape has an efficiency close to the radial shape.
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