The impact of second-harmonic tip clearance distributions caused by casing Ovalization and blade height variation in a large-scale compressor stage

Abstract

Affected by extreme operating conditions, external loading and casing stiffness, the casing ovalization will potentially occur for the axial compressor, resulting in a second harmonic distribution of rotor tip clearances over the full annulus. The same clearance layout can be reproduced by varying the blade height over the circumference with a circular casing. In this paper, both of these non-axisymmetric configurations are respectively modelled with full-annulus simulations in the first 1.5 stage of the large-scale research compressor at Shanghai Jiao Tong University (SJTU-LSRC), with the blade adjustment item further complemented by experimental measurements.The impacts are evaluated and compared in terms of compressor performance and flow mechanisms. Ovacasing case leads to a 32.57 % stall margin (SM) benefit compared to the prototype, while ovablade experiences a 31.75 % SM extension. Both configurations cause a 4 % efficiency decrease at the design conditions. The second-harmonic clearance distribution results in a 15 lag in ovablade and a 30 lag in ovacasing for the total pressure ratio at the rotor exit, due to local mass flow adaptation related to circumferential tip clearances. Ovacasing configuration exhibits substantial reverse flow and a low-velocity region in the tip of large clearance sector, while ovablade shows reduced reverse flow and secondary leakage flow. This underscores the impact of blade tip relationships and height differences from adjacent rotors on leakage flow and its secondary behavior, even with identical clearances. There exists a critical clearance (2.57 % span), below which adjusting blade heights effectively simulates the performance impact and flow field redistribution caused by casing ovalization and rotor eccentricity.