Abstract
The fan systems of typical high bypass civil engines encounter strong flow distortions originating in the intake and in the bypass ducts and its bifurcations. 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 to understand its effect on fan rotor efficiency and 1EO fan forced response. The study revealed that a mild, low-harmonic fan back pressure field with a suitable clocking position is able to improve the fan efficiency and reduce the fan forcing simultaneously. Such a pattern 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. The notion of deliberately generating a clocked 1EO intra-stage static pressure field by means of cyclic OGV pattern design is 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 in two flight conditions. The study showed that a correctly clocked intra-stage static pressure field can effectively counter the fan upstream flow distortions and at the same time improve the fan rotor performance and fan system aeromechanical behaviour. The implementation of the proposed translucent design philosophy in a new OGV pattern design tool is discussed.
