Minimizing the gap between the blade tips and the casing, while ensuring the safe operation of the compressor, is one of the effective measures to reduce leakage area. Correctly selecting the minimum safe clearance is a complex issue. It is closely related to the manufacturing precision of the parts, the rigidity of the rotor and casing, and the coordination of thermal expansion of the parts during operation. Currently, in some engines, in addition to using a double-layer casing to maintain the roundness of the inner casing, the following measures are also taken:
(1) For example, in JT9D74R and PW4000, after the high-pressure compressor rotor is assembled, the rotor blade tips are ground once using a special grinding machine. This improves the rotor's balance and ensures a uniform outer diameter.
(2) In some engines, an easy-wear coating is applied to the inner wall of the casing, and wear-resistant material is applied to the rotor blade tips. During assembly, the rotor is used as a grinding wheel to grind a concentric circle inside the casing, maintaining a uniform blade tip gap.
(3) Casing materials with a low coefficient of linear expansion can also be selected.
