Abstract
The fan systems of typical high bypass civil engines encounter strong flow distortions originating in the intake and the bypass duct. These flow distortions cause the fan stage operation point to vary from its design intent, thus reducing the fan stage performance and increasing low engine-order fan blade forcing. A cyclic pattern design for the fan Outlet Guide Vanes (OGV) can be effectively used to recover the fan stage performance and to control its system-level aeromechanical behavior. This paper presents the development of an OGV pattern design philosophy using the numerical experimentation technique. Multiple fan-intake unsteady computational fluid dynamic computations are conducted by clocking the circumferential pressure profile at the fan exit. The study revealed that a mild, low-harmonic fan back pressure profile with a suitable clocking position is able to improve the fan rotor efficiency and reduce the first engine order (1EO) fan forcing simultaneously. Such a profile can be generated by designing a cyclic OGV pattern that allows the bifurcation potential fields of controlled intensity and phase to pass through the OGV blade row, thus termed the translucent design philosophy. Further, a sensitivity study is performed to assess the effects of simultaneous distortions upstream and downstream of the fan. The study showed that a correctly clocked intra-stage static pressure profile can consistently improve both the performance and aeromechanical behavior of fan systems having different intake lengths and at different flight conditions. The implementation of the proposed translucent design philosophy in a new OGV pattern design tool is discussed.
