Fan or compressor casing

Abstract

The surge margin of a fan or compressor stage may be increased by radial, circular grooves in the casing ( 2 ) encircling the rotor stage. The present invention provides improved aerodynamic performance by replacing circumferential grooves by multiple helical grooves 18 . Preferably the grooves ( 18 ) have multiple start and end positions ( 20,22 ) spaced apart around the casing ( 2 ) and at least the downstream ends ( 22 ) of the grooves ( 18 ) are tapered to aid expulsion of particles in the grooves ( 18 ).

1 . A gas turbine engine driven fan or compressor stage casing comprising a generally cylindrical casing portion ( 2 ) encircling a fan or compressor rotor having a multiplicity of blades ( 8 ) spaced apart around the rotor, the casing portion ( 2 ) having formed in its inner surface over the tips ( 10 ) of the blades ( 8 ) at least one helical groove ( 18 ) characterised in that the at least one helical groove ( 18 ) has a gradually reducing depth in the downstream direction. 
   
   
       2 . A casing for a fan or compressor stage as claimed in  claim 1  characterised in that the at least one helical groove ( 18 ) extends for a plurality of turns around the inner surface of the casing ( 2 ). 
   
   
       3 . A casing for a fan or compressor stage as claimed in  claim 1  characterised in that the casing ( 2 ) and the at least one helical groove ( 18 ) extends downstream of the trailing edge ( 26 ) of the tips of the blades ( 8 ). 
   
   
       4 . A casing for a fan or compressor stage as claimed in  claim 3  characterised in that the at least one helical groove ( 18 ) has a gradually reducing depth in the upstream direction. 
   
   
       5 . A casing for a fan or compressor stage as claimed in  claim 4  characterised in that the at least one helical groove ( 18 ) extends upstream of the leading edge ( 24 ) of the blades ( 8 ). 
   
   
       6 . A casing for a fan or compressor stage as claimed in  claim 2  characterised in that the casing ( 2 ) and the at least one helical groove ( 18 ) extends downstream of the trailing edge ( 26 ) of the tips of the blades ( 8 ).