Patent Publication Number: US-6905305-B2

Title: Engine casing with slots and abradable lining

Description:
The present invention relates to an engine casing provided with slots and an abradable lining. The casing is particularly suitable for use in the compressor section of a gas turbine engine. 
   The aerodynamic design of an aero-engine is optimised for a particular working line, typically the cruise condition. During starting or other manoeuvres the aerodynamics can become unstable. To improve the stability of the aerodynamics away from the working line casing treatments are used. 
   Various treatments are available and include the provision of slots of varying depths and forms in the inner surface of the casing. The slots are put in the casing above the blade tips to allow recirculation of the air. 
   A problem with slotted casings is the inclusion of an abradable rotor path lining. Abradable linings are used on rotor casings to provide the tightest tip clearance whilst accommodating radial growth of the blades. Abradable linings are however easily damaged when slotted and difficulties occur in applying them to a slotted casing. Abradable linings are therefore rarely incorporated onto slotted casings and so an increase in the tip clearance is then required to compensate. 
   The present invention seeks to provide an abradable lining on a slotted casing, which overcomes the aforementioned problems. 
   According to the present invention an engine casing encloses a rotor, the casing comprises a wall having an inner surface adjacent the rotor, at least a portion of the inner surface of the wall has at least one slot therein, an abradable lining is attached to the inner surface of the wall, the abradable lining is fluid permeable and extends across the slot. 
   The casing may be provided with a plurality of slots equi-spaced circumferentially in the inner surface of the wall. The slots may be radially inclined and the radial depth of the slots may vary. 
   Preferably the abradable lining is a cellular structure and is attached to the slotted casing by adhesive. Regions of the cellular structure between the slots may be blocked to prevent the passage of the fluid therethrough. The regions of the cellular structure between the slots may be blocked by adhesive. 

   
     The present invention will now be described with reference to the accompanying figures in which; 
       FIG. 1  is a partially sectioned side view of a gas turbine engine having a casing in accordance with the present invention. 
       FIG. 2  is a partially sectioned view of part of the compressor shown in FIG.  1 . 
   

   Referring to  FIG. 1 , a gas turbine engine generally indicated at  10  comprises in axial flow series a compressor  11 , combustion equipment  12 , a turbine  13  drivingly connected to the compressor  11  and an exhaust nozzle  14 . The engine functions in conventional manner, that is a fluid, such as air, enters the compressor  11  and is compressed by alternate rows of rotor blades  15  and stator vanes (not shown). The compressed air is mixed with fuel and combusted in the combustor  12 . The combustion products drive the turbine  13  before being exhausted to atmosphere through the exhaust nozzle  14 . 
   To improve the aerodynamic performance of the compressor  11 , an abradable lining  19  is provided on the inner wall  17  of the compressor casing  16  adjacent the tips of the rotor blades  15 . The lining  19  reduces the clearance between the tips of the rotor blades  15  and the wall  17  and is abradable to accommodate radial growth of the blades  15 . 
   The lining  19  is fluid permeable and extends across a plurality of discrete angled slots  20  which are machined into the inner wall  17  of the compressor casing  16 . The angled slots  20  are equi-spaced around the circumference of the inner wall  17  and have a uniform radial depth. Whilst a number of discrete slots  20  are shown it will be appreciated that a single circumferential slot could be used. The radial depth of the slots  20  could also be varied. 
   The lining  19  is attached to the inner wall  17  of the casing  16  by adhesive  18 . The lining  19  has a cellular construction, which allows the passage of air therethrough. In the regions where the lining  19  extends across the slots  20 , air passes through the cells into the slot  20  where it recirculates. In the regions between the slots  20  air passes through the cells and is blocked by the inner wall  17  of the casing  16 . These cells become pressurised preventing little recirculation or turbulence. 
   In the regions between the slots  20  adhesive  18  blocks some of the cells in the lining  19 . The blocked cells further reduce the recirculation or turbulence in the lined regions between the slots  20 . 
   The use of a fluid permeable lining  19  allows the slots  20  in the casing  16  to be exposed to the air stream. The air recirculates within the slots  20  as usual. 
   As the lining  19  is fluid permeable there is no need to machine further slots into the lining  19  and the integrity of the lining  19  is maintained. 
   During repair and overhaul the entire lining  19  is removed and replaced. As the lining  19  extends over the slots  20 , the difficulties that have previously been encountered in applying the abradable lining  19  only to those regions between the slots  20  are avoided.