Abstract:
A gas turbine engine has a compressor assembly and a turbine assembly rotationally mounted on a shaft, the turbine assembly being driven by hot gases discharged from a combustion chamber disposed between the compressor and turbine assemblies and an engine case encasing a portion of the engine, the case having an oblong bleed hole having a major axis parallel to a gaspath direction and a minor axis perpendicular to the gaspath direction.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to gas turbine engines and, more particularly, to bleed-off holes for gas turbine engines. 
       BACKGROUND OF THE ART 
       [0002]    Gas turbine engines such as those used as aircraft turbojets or turbofans typically comprise a rotating fan, a low-pressure compressor and a high-pressure compressor as well as high-pressure and low-pressure turbines that are axially mounted to separate coaxial shafts for rotation about a central axis of the engine. The compressor and turbine assemblies are enshrouded within a turbofan case conventionally manufactured by joining together a number of flanged cases such as, for example, the fan case to the intermediate case, the gas generator case to the combustion chamber case, the combustion chamber case to the low-pressure turbine case, the low-pressure turbine case to the turbine exhaust case. One or more of these cases may have bleed-off holes for drawing off pressurized air into one or more bleed air systems or for exhausting air into the bypass duct. In certain engine designs, the bleed holes are prone to recirculation of bypass flow which cause resonance and acoustic noise. Furthermore, the bleed holes can sometimes give rise to excessive losses in the bypass. 
         [0003]    Accordingly, there is a need to provide improved bleed-off holes that address one or more of these deficiencies. 
       SUMMARY OF THE INVENTION 
       [0004]    It is therefore an object of this invention to provide an improved bleed-off hole that is oriented, shaped an structured such that it reduces the likelihood of resonance and acoustic noise and/or the susceptibility to losses in the bypass. 
         [0005]    In one aspect, the present invention provides a gas turbine engine having a compressor assembly and a turbine assembly rotationally mounted on a shaft, the turbine assembly being driven by hot gases discharged from a combustion chamber disposed between the compressor and turbine assemblies. The gas turbine engine also includes an engine case encasing a portion of the engine, the case having an oblong bleed hole having a major axis parallel to a gaspath direction and a minor axis perpendicular to the gaspath direction. 
         [0006]    In another aspect, the present invention provides a monocase assembly for a gas turbine engine, the monocase assembly including a fan case portion for housing a fan rotor assembly and an intermediate portion connected to the fan case portion downstream of the fan case portion and connected to a gas generator portion upstream of the gas generator portion. The monocase assembly also includes a splitter mounted within the intermediate portion for splitting airflow between core flow and bypass flow, the splitter comprising a plurality of oblong bleed holes, each bleed hole having a major axis parallel to a gaspath direction and a minor axis perpendicular to the gaspath direction. 
         [0007]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figure depicting aspects of the present invention, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a turbofan as an example of a gas turbine engine that could incorporate embodiments of the present invention; 
           [0010]      FIG. 2  is an exploded isometric view of a turbofan case having oblong bleed holes in accordance with an embodiment of the present invention; 
           [0011]      FIG. 3A  is an enlarged isometric view of oblong bleed holes in accordance with an embodiment of the present invention; and 
           [0012]      FIG. 3B  is a plan view of an oblong bleed hole showing major and minor axes. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to  FIG. 1 , a turbofan gas turbine engine incorporating an embodiment of the present invention is presented as an example of the application of the present invention, and includes a housing  10 , a core casing  13 , a low pressure spool assembly seen generally at  12  which includes a shaft  15  interconnecting a fan assembly  14 , a low pressure compressor  16  and a low pressure turbine assembly  18 , and a high pressure spool assembly seen generally at  20  which includes a shaft at  25  interconnecting a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies  12  and  20  in order to define a main fluid path (not indicated) therethrough. In the main fluid path there are provided a combustion section  26  having a combustor  28  therein. Pressurized air provided by the high pressure compressor assembly  22  through a diffuser  30  enters the combustion section  26  for combustion taking place in the combustor  28 . 
         [0014]      FIG. 2  illustrates, in an exploded view, a turbofan case  32  having a plurality of spaced-apart oblong bleed holes  100  in accordance with an embodiment of the present invention. The turbofan case  32  which, in this particular embodiment, is a monocase assembly includes an inlet  34 , a fan case portion  44 , which houses the fan rotor assembly  13 , an intermediate portion  46  downstream of the fan case portion  44  and a gas generator portion  52  downstream of intermediate portion  46 . The intermediate portion  46  includes a compressor shroud  48  which encircles the blade tips of the compressor assembly  16  as well as a splitter  42  for splitting the air flow into the core flow and the bypass flow. The gas generator portion  52  has a plurality of mounting points  54  to which other engine components can be mounted such as fuel injecting means (not shown). 
         [0015]    As shown in  FIG. 2 , the intermediate portion  46  of case  32  also includes an inner hub  76 . A flanged outer ring  60  is coaxial to the inner hub  76 . A plurality of casing struts  40 , which are circumferentially spaced apart as shown in this figure, extend radially outwardly and rearwardly from the inner hub  76  to the outer ring  60 . A plurality of circumferentially spaced-apart slots  90  extend from the front face of the splitter rearward into the splitter  42  for receiving the respective casing struts  40 . 
         [0016]    As illustrated in  FIG. 2  and in the enlarged view of  FIG. 3A , the bleed holes  100  are disposed in the splitter  42 . As shown in  FIG. 3B , the bleed holes  100  define an oblong opening having a major axis  100   a  parallel to a gaspath direction  101  and a minor axis  100   b  perpendicular to the gaspath direction  101 . The gaspath direction means the predominant direction of air flow at that location in the engine. Oblong, for the purposes of this specification, means that the hole has a length that is greater than a width. Preferably, as shown in the figures, the oblong hole has ends  100   c  that are rounded to ensure smooth air flow. In the embodiment illustrated, the sides  100   d  of the oblong hole are parallel for most of the hole length although, in another embodiment, the oblong bleed holes can be elliptical, again having a major axis that is parallel to a gaspath direction and a minor axis perpendicular to the gaspath direction but in which the sides are curved to form an ellipse. 
         [0017]    In the particular embodiment shown in  FIG. 3A , the case  32  has a flared portion  102  at least partially surrounding a periphery of the bleed hole  100  to inhibit recirculation of bypass air flow. Without these flarings, air in the bypass is prone to recirculate via a cavity in the splitter (i.e. the air travels downstream, enters the splitter through a downstream hole, travels forward through the splitter and then recirculates into the bypass through an upstream hole, thus defining a recirculation path.) The flared portion effectively curtails this unwanted recirculation effect which can lead to resonance and acoustic noise. 
         [0018]    In the embodiment illustrated in  FIGS. 2 and 3A , each of the oblong bleed holes  100  is located in the splitter  42  immediately downstream of a respective casing strut  40 . As noted above, each casing strut  40  extends through a forward portion of the splitter  42 . In the particular embodiment shown in  FIGS. 2 and 3A , the engine case  32  is a monocase assembly. Accordingly, each casing strut  40  extends radially from the inner hub  76  to the outer ring  60  of an intermediate portion of the monocase, with the flared and oblong (or elliptical) bleed holes  100  located behind each of the casing struts  40 . 
         [0019]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the impeller baffle can be used not only for turbofans or turbojets, but also for turboprops, turboshafts or any other gas turbine engine. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.