Delaying breakdown of the flow in the tip region of a tip-critical compressor rotor as long as possible, i.e. improving the surge margin, is of great interest to the turbomachinery community and is the focus of this study. The surge margin of ten compressor rotors is evaluated numerically, each with different blade loading and geometry at the tip. Previous work in the field has shown the dependence of an interface in the tip region of a compressor rotor between the incoming flow and the tip clearance flow with the passage flow coefficient ϕ. Previous work in the field has also shown that a higher incoming meridional momentum in the tip region can be beneficial to the surge margin of a tip-critical rotor. The present study generalizes these findings by taking into account the local blade loading of the rotor tip section and the level of loss in the tip region. The surge margin is found to improve if the blade loading of the rotor tip section is increased, which acts to increase the incoming mass flow rate and improve the surge margin provided that an increase in loss, mainly related to the strength and direction of the tip clearance flow, does not negate the effect as the compressor is throttled. Two quantities are proposed as objective functions to be used for optimization to achieve a compressor rotor with high surge margin based on the flow field at the design point. Finally, an optimization and analysis of the results is made to demonstrate the proposed objective functions in practise.

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