Patent Publication Number: US-7210905-B2

Title: Compressor having casing treatment slots

Description:
FIELD OF THE INVENTION 
   The present invention relates to a compressor having casing treatment slots and in particular to a turbofan gas turbine engine compressor having casing treatment slots. 
   BACKGROUND OF THE INVENTION 
   Conventionally casing treatment slots are provided on the inner surface of a compressor casing, or a fan casing, around the tips of the compressor blades, or fan blades, to extend the stable flow range over which the compressor, or fan may operate. 
   Typically casing treatment slots are provided around the first stage of compressor blades or around the fan blades. 
   Our UK patent GB1518293 discloses a compressor casing treatment comprising a plurality of circumferentially spaced slots in the inner surface of the compressor casing and around a stage of compressor blades. The slots are arranged at an angle to the axis of rotation of the compressor blades. 
   Our UK patent GB2245312B discloses a compressor casing treatment comprising a plurality of circumferentially spaced slots in the inner surface of the compressor casing and around a stage of compressor blades. The slots are arranged at an angle to the axis of rotation of the compressor blades. The slots are also arranged at an angle to the radial direction. The upstream edges of the slots are upstream of the leading edges of the compressor blades and the trailing edges of the slots are upstream of the trailing edges of the slots. 
   In each of these arrangements the slots are identical, they have the same length, the same depth, the same width, the same angle of inclination to the axis of rotation and the same angle of inclination to the radial direction etc. 
   However, the compressor casing suffers from cracking of the webs, the pieces of compressor casing circumferentially between the compressor casing treatment slots. It is believed that the cracking of the webs occurs due to the unsteady pressure acting on them due to the periodic passing of the compressor blades. All the slots are identical and have the same geometry and thus they experience the same unsteady pressure variations but with a time lag related to the passing frequency of the compressor blades. In adjacent slots there will be a time lag between the pressure variations and thus the pressure variations are out of phase and this results in a pressure difference across the webs. Additionally because all the compressor blades are substantially identical and all the casing treatment slots are substantially identical, the presence of an incoming distortion may substantially increase the unsteady forces on the rotor creating further phase differences within the casing treatment slots and thence unsteady forces on the webs. 
   SUMMARY OF THE INVENTION 
   Accordingly the present invention seeks to provide a compressor having a novel arrangement of casing treatment slots. 
   Accordingly the present invention provides a compressor comprising a rotor having a plurality of circumferentially spaced radially outwardly extending rotor blades, a casing surrounding the rotor and rotor blades, the casing having an inner surface, a plurality of circumferentially spaced slots in the inner surface of the casing, each slot having a length, a depth, a width, an angle of inclination relative to the radial direction, an axial position relative to the rotor blades and a circumferential position relative to an adjacent slot, wherein the slots are arranged such that at least one of the length, depth, width, angle of inclination relative to the radial direction, axial position relative to the rotor blades and circumferential position relative to an adjacent slot varies circumferentially around the casing. 
   Preferably the depth of the slots varies circumferentially around the casing. 
   Preferably inserts having different depths are provided in the slots to vary the depth of the slots circumferentially around the casing. 
   The slots may be arranged such that at least two of the length, depth, width angle of inclination relative to the radial direction, axial position relative to the rotor blades and circumferential position relative to an adjacent slot varies circumferentially around the casing. 
   Each of the depth, width, angle of inclination relative to the radial direction and circumferential position relative to an adjacent slot may be varied circumferentially around the casing. 
   Each of the length, depth, width, angle of inclination relative to the radial direction, axial position relative to the rotor blades and circumferential position relative to an adjacent slot varies circumferentially around the casing. 
   Preferably the leading edges of the slots are arranged upstream of the leading edges of the rotor blades. 
   The radially outer ends of the slots may be connected to an annular chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully described by way of example with reference to the accompanying drawings in which: 
       FIG. 1  is a partially cut away view of a turbofan gas turbine engine having a compressor according to the method of the present invention. 
       FIG. 2  is an enlarged cross-section view of through the compressor casing shown in  FIG. 1 . 
       FIG. 3  is an enlarged cross-section view in the direction of arrows A—A in  FIG. 2 . 
       FIG. 4  is an alternative enlarged cross-section view in the direction of arrows A—A in  FIG. 2 . 
       FIG. 5  is a further alternative enlarged cross-section view in the direction of arrows A—A in  FIG. 2 . 
       FIG. 6  is another alternative enlarged cross-section view in the direction of arrows A—A in  FIG. 2 . 
       FIG. 7  is an additional alternative enlarged cross-section view in the direction of arrows A—A in  FIG. 2 . 
       FIG. 8  is a view in the direction of arrow B in  FIG. 2 . 
       FIG. 9  is an alternative view in the direction of arrow B in  FIG. 2 . 
       FIG. 10  is a cross sectional view of a portion of a compressor casing according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A turbofan gas turbine engine  10 , as shown in  FIG. 1 , comprises in axial flow series an intake  12 , a fan section  14 , a compressor section  16 , a combustion section  18 , a turbine section  20  and an exhaust  22 . 
   The turbine section  20  comprises one or more turbines (not shown) arranged to drive a fan rotor  24  via a shaft (not shown) and one or more turbines (not shown) arranged to drive one or more compressor rotors  40  via one or more shafts (not shown). 
   The fan section  14  comprises the fan rotor  24  and a plurality of circumferentially spaced radially outwardly extending fan blades  26  are carried on the fan rotor  24 . The fan rotor  24  and fan blades  26  are surrounded by a fan casing  28 , which is arranged coaxially with the fan rotor  24 . The fan casing  28  partially defines a fan duct  30  and the fan duct  30  has an outlet  32  at its downstream end. The fan casing  28  is secured to a core engine casing  34  by a plurality of circumferentially spaced radially extending fan outlet guide vanes  36 . 
   The compressor section  16  comprises a compressor rotor  40 , which carries a plurality of stages of compressor blades  42  and each stage of compressor blades  42  comprises a plurality of circumferentially spaced radially outwardly extending compressor blades  42 . The compressor rotor  40  and compressor blades  42  are surrounded by a compressor casing  44  which is arranged coaxially around the compressor rotor  40  and compressor blades  42 . The compressor casing  44  also supports a plurality of stages of compressor vanes  46  and each stage of compressor vanes  46  comprises a plurality of circumferentially spaced radially inwardly extending compressor vanes  46 . The stages of compressor vanes  46  and the stages of compressor blades  42  are arranged alternately through the compressor section  16 . 
   The turbofan gas turbine engine  10  operates conventionally and its operation will not be discussed further. 
   The compressor casing  44 , as shown more clearly in  FIG. 2  has an inner surface  48  and that portion of the inner surface  48  immediately around one of the stages of compressor blades  42  has a plurality of circumferentially spaced slots  50 . Each slot  50  has a leading edge wall  52 , a trailing edge wall  54 , a first side wall  56 , a second side wall  58  and an end wall  60 . The compressor casing comprises webs  51  between the slots  50 . The slots  50  may be around one or more stages of compressor blades  42  in a low-pressure compressor, e.g. the fan, an intermediate-pressure compressor and/or a high-pressure compressor. 
   Each slot  50  has an axial length L between the leading edge wall  52  and the trailing edge wall  54 , a radial depth D between the inner surface  54   48  and the end wall  60 , a width W between the first side wall  56  and the second side wall  58 , an angle θ of inclination relative to the radial direction, an axial position AP relative to the rotor blades  42  and a circumferential position CP relative to an adjacent slot  50 . 
   The axial position AP of the slots  50  is measured between the arc scribed by the leading edges  41  of the compressor blades  42  and any suitable position of the slot  50 , for example the leading edge  52  or the trailing edge  54  or the axial position AP is measured between the arc scribed by the trailing edges  43  of the compressor blades  42  and any suitable position of the slot  50  as mentioned above. The circumferential position CP of the slots  50  is measured between the circumferential mid positions of the adjacent slots  50  or is measured between first sides  56 , or second sides  57 , of the slots  50 . 
   As shown in  FIG. 3  the depth D of the slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying depths D 1 , D 2 , D 3  etc and in this case D 3 &gt;D 2 &gt;D 1 . It is easier to manufacture casing treatments with identical slots  50  and to aid manufacture the slots  50  are preferably machined to the same depth and a set of inserts  63  of varying depth are then inserted and secured into the slots  50 . The inserts  63  have the same length L and width W as the slots  50 . The slots  50  all have the same length L, width W, angle θ of inclination, circumferential position CP and axial position AP. 
   As shown in  FIG. 4  the angles θ of inclination relative to the radial direction of the slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying angles θ 1 , angles θ 2  angles θ 3  and in this case θ 1 &gt;θ 2 &gt;θ 3 . The slots  50  all have the same length L, width W, depth D, circumferential position CP and axial position AP. 
   As shown in  FIG. 5  the width W of the slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying width W 1 , W 2 , W 3  etc and in this case W 3 &gt;W 2 &gt;W 1 . The slots  50  all have the same length L, depth D, angle θ of inclination, circumferential position CP and axial position AP. 
   As shown in  FIG. 6  the circumferential position CP of the slots  50  relative to adjacent slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying circumferential positions CP 1 , CP 2  etc where CP 2 &gt;CP 1 . The slots  50  all have the same length L, depth D, width W, angle θ of inclination and axial position AP. 
   As shown in  FIG. 7  the depth D of the slots  50 , the angles θ of inclination relative to the radial direction of the slots  50 , the width W of the slots  50  and the circumferential position CP of the slots  50  relative to adjacent slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have depths D 1 , D 2  and D 3 , widths W 1 , W 2  and W 3  angles θ 1 , θ 2  and θ 3  and circumferential positions CP 1 , CP 2 . The slots  50  have the same lengths L and axial positions AP. 
   As shown in  FIG. 8  the axial position AP of the slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying axial positions AP 1 , AP 2  and AP 3  relative to the arc scribed by the leading edges  41  of the compressor blades  42 , where AP 1 &gt;AP 2 &gt;AP 3 . The slots  50  all have the same depth D, length L, width W, angle θ of inclination and circumferential position CP. 
   As shown in  FIG. 9  the axial length L of the slots  50  varies circumferentially around the compressor casing  44 . The slots  50  have varying lengths L 1 , L 2  and L 3 , where L 3 &gt;L 2 &gt;L 1 . The slots  50  all have the same depth D, width W, angle θ of inclination, circumferential position CP and axial position AP. 
   The main advantage of the present invention is that the non-uniform slot geometry modifies the unsteady pressures in adjacent slots so that there is no longer a simple phase lag relationship between them. The unsteady pressure is modified in adjacent slots to reduce the peak cyclic force and hence reduce the likelihood that the webs will crack. 
   An additional advantage of the present invention is that the non-uniform slot geometry may be used to counteract the effect of non-uniformities in the incoming airflow into the compressor. In the case of distortion of the air flow into the compressor the compressor rotor and compressor blades experience an unsteady force and the compressor rotor performance, shock pattern, changes as it experiences the inlet distortion. If the compressor rotor shock pattern variation due to inlet distortion is known, it may be possible to arrange the casing treatment slots to improve the compressor rotor performance. The inlet distortion may also lead to high unsteady forces on some of the webs, which may be counteracted by the present invention. 
   Although the present invention has been described with reference to a compressor casing treatment it is equally applicable to a fan casing treatment. 
   Although the present invention has been described with reference to slots arranged at an angle to the radial direction it is equally possible to apply the present invention to casing treatment slots, which are not arranged at an angle of 0° to the radial direction. Similarly the casing treatment slots may extend purely with an axial component or may be helical, arranged at an angle to the axial direction. Additionally, although the present invention has been described with reference to the leading edge of the slots being arranged upstream of the leading edge of the rotor blades it may be possible for the leading edge of the slots to be arranged substantially in the same plane as the leading edge of the slots or perhaps downstream of the leading edge of the slots. 
   The slots may be any suitable shape in axial cross-section, for example rectangular as shown in the figures, or they may be curved. 
   The present invention is also applicable to casing treatments as disclosed in published European patent application EP0688400A, in which the radially outer ends of the casing treatment slots are connected to an annular chamber. The annular chamber may be uniform circumferentially around the casing. 
   Alternatively the radially outer wall  72  of the annular chamber  70 , as shown in  FIG. 10 , may have axially extending corrugations  74  circumferentially spaced around the casing and the corrugations  74  comprise super-positioned sine waves such that the radial depth of the annular chamber  70  varies circumferentially in a non-uniform manner, as illustrated by the changes in length of X 1 , X 2 , X 3  and X 4 , and/or the circumferentially spacing between the corrugations  74  varies circumferentially around the casing. 
   Alternatively the outer wall of the annular chamber may have circumferentially extending corrugations axially spaced along the casing and the corrugations comprise super-positioned sine waves such that the radial depth of the annular chamber varies circumferentially in a non-uniform manner and/or the axial spacing between the corrugations varies circumferentially around the casing. 
   Additionally it may be possible to provide a casing treatment in which each slot has a length, a depth, a width, an angle of inclination relative to the radial direction, an axial position relative to the rotor blades and a circumferential position relative to an adjacent slot and these are the substantially the same for all the slots. 
   Preferably, the radially outer wall of the annular chamber has axially extending corrugations circumferentially spaced around the casing and the corrugations comprise super-positioned sine waves such that the radial depth of the annular chamber varies circumferentially in a non-uniform manner. Thus the radial depth of the corrugations and/or the circumferential spacing between the corrugations varies circumferentially around the casing. 
   Alternatively the outer wall of the annular chamber may have circumferentially extending corrugations axially spaced along the casing and the corrugations comprise super-positioned sine waves such that the radial depth of the annular chamber varies circumferentially in a non-uniform manner and/or the axial spacing between the corrugations varies circumferentially around the casing.