HARDWALL FAN CASE WITH STRUCTURED BUMPER

摘要

The invention provides a novel hardwall fan case for encasing the radial periphery of a forward fan in a gas turbine engine. The fan case includes a rigid annular fan case shell spaced a selected radial distance from the tips of the fan blades, thus defining an annular internal air path surface of the fan case. The shell has a rigid hardwall fore section generally parallel to the blade tips and coated with a fore layer of abradable material. The fore section serves as a hardwall to limit the radial movement of fan blades deflecting under bird strike conditions and thereby to control the erosion of fan case linings. Limiting the radial blade deflection thus maintains the resulting fan tip clearance within acceptable limits. Uncontrolled or excessive erosion of fan case linings during bird strike conditions has in the past led to potentially catastrophic engine surge conditions where highly aerodynamically loaded fans are used that are very sensitive to enlarged tip clearances. The aft section of the rigid shell is radially spaced from the fore section thus defining a recess between the aft section of the rigid shell and the air path surface. The recess houses compressible material that absorbs the impact of the broken blade fragment propelled radially, and can retain the fragment in certain conditions. However, the rigid shell includes a novel rigid bumper between the fore and aft sections. The bumper has a rigid rear edge disposed an offset distance forwardly of the fan blade centers of gravity. When a broken blade fragment is propelled radially under centrifugal force, the fragment strikes the bumper edge. The blade fragment is rotated about the bumper edge under a force moment equal to the centrifugal force multiplied by the offset distance. As a result, the blade fragment is redirected from a radial trajectory and is rotated rearwardly to be ejected axially through the gaspath, or alternatively is retained within the compressible material housed in the rigid shell. Both the rigid hardwall fore section and the aft compressible material are preferably covered with a relatively thin layer of abradable material that allows the rotating fan blades on initial operation to achieve close tip tolerance with the hardwall fan case.