Development of Translucent Design Philosophy for the Cyclic Pattern Design of Fan Outlet Guide Vanes

Abstract

The fan systems of typical high bypass civil engines encounter strong flow distortions originating in the intake and in 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 behaviour. This paper presents the development of a OGV pattern design philosophy using numerical experimentation technique. Multiple fan-intake unsteady CFD computations are conducted by clocking the circumferential pressure profile at 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 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 as 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 behaviour of fan system 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.