Abstract:
A gas turbine engine includes a high-pressure compressor having an impeller and an impeller baffle having a generally annular body, the impeller baffle spaced apart from a back face of an impeller to create an airspace therebetween, the airspace communicating between an impeller exit and an apparatus to be pressurized, the impeller baffle including a plurality of generally radially-disposed ribs extending into the airspace for diminishing the velocity of air swirling in the airspace.

Description:
TECHNICAL FIELD 
   The invention relates generally to gas turbine engines and, more particularly, to impeller baffles of gas turbine engines. 
   BACKGROUND OF THE ART 
   A gas turbine high-pressure compressor may include a centrifugal impeller. At the compressor exit and immediately behind (downstream of) the centrigual impeller the compressed air, also known as the “impeller backface air” may be used to pressurize an engine air system, referred to herein as the P3 Bleed Air System, as well as pressurizing nearby air-oil interfaces, such as those sealing adjacent bearing cavities. To drive such a system, sufficient pressures must be maintained in the impeller back face air. 
   An impeller baffle can be disposed downstream of the impeller backface can act as a diffuser, slowing the impeller backface air to locally increase the air pressure. However, despite the presence of an impeller baffle, the pressure at the impeller backface (and particularly at the tip of the impeller baffle) can still sometimes be too low. 
   Accordingly, there is a need to provide an improved design to address this potential susceptibility. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide an improved impeller baffle. 
   In one aspect, the present invention provides a gas turbine engine comprising a low-pressure compressor and a low-pressure turbine rotationally mounted on a first axial shaft, and a high-pressure compressor and a high-pressure turbine together defining a high-pressure spool rotationally mounted on a second shaft coaxial with the first shaft, the high-pressure compressor having an impeller for pressurizing air in the engine, the high-pressure compressor and the high-pressure turbine being rotationally supported by a high-pressure spool bearing disposed between the impeller and the high-pressure turbine. The gas turbine engine also includes a carbon seal disposed at the front of a bearing housing that houses the bearing to prevent oil from leaking from the bearing housing. The gas turbine engine further includes an impeller baffle having a generally annular body, the impeller baffle including a front face having a plurality of ribs for diminishing the velocity of air swirling behind an impeller backface to thereby increase air pressure to improve the efficacy of the carbon seal to thus prevent bearing oil leak. 
   In another aspect, the present invention provides an impeller baffle for deswirling air downstream of an impeller in a gas turbine engine. The impeller baffle includes a generally annular body and a plurality of ribs formed on a front face of the annular body of the baffle, the ribs adapted to deswirl the air behind an impeller backface to thereby increase air pressure to improve the efficacy of the carbon seal to thus prevent bearing oil leak. 
   In another aspect, the present invention provides a method of installing an impeller baffle in a gas turbine engine. The method includes steps of press fitting an impeller baffle to a bearing housing of a bearing that rotationally supports a high-pressure spool of the engine, the impeller baffle having a plurality of ribs on a front face of the baffle for deswirling the air downstream of an impeller to thereby increase air pressure to improve the efficacy of the carbon seal to thus prevent bearing oil leak. The method also includes a step of affixing a retaining ring to further secure the impeller baffle to the bearing housing. 
   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 
     Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
       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; 
       FIG. 2  is an enlarged cross-sectional view of the bearing housing and surrounding structure, showing the airflow through the bearing housing due to the restriction caused by the impeller baffle; 
       FIG. 3  is a front isometric perspective view of the impeller baffle in accordance with an embodiment of the present invention; and 
       FIG. 4  is a rear isometric perspective view of the impeller baffle shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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 . 
     FIG. 2  shows, in cross section, an upper portion of the high-pressure spool assembly  20  of a turbofan such as the one shown in  FIG. 1 . The high-pressure spool assembly  20  (also referred to herein as simply the “high-pressure spool”) includes the high-pressure compressor and the high-pressure turbine section which are both rotationally mounted on bearings to a common shaft. The high-pressure compressor includes multiple axial stage rotors followed by a centrifugal impeller  40  shown in  FIG. 2 , having a coverplate  41 . The compressor supplies pressurized air to the combustor  28 , the hot gases from which drive the high-pressure turbine assembly  24  as well as the low-pressure turbine assembly (not shown in this figure). 
   As shown in  FIG. 2 , the impeller has a backface  42  behind which the air swirls at high velocity due to the rotation of the impeller. This swirling, high-velocity air tends to lower the pressure of the “backface air”, i.e. the air immediately behind (i.e. downstream of) the impeller backface  42 . 
   As further shown in  FIG. 2 , within a bearing housing  44  is a bearing  46 , i.e., a high-pressure spool bearing, which rotationally supports the high-pressure spool (that is, the high-pressure compressor and the high-pressure turbine). The high-pressure spool bearing  46  is lubricated and cooled with oil circulated by a lubrication system within the gas turbine engine. A front carbon seal  48  (as well as a rear carbon seal  49 ) are mounted to the bearing housing  44  to ensure that oil is sealed within the bearing housing. The front carbon seal  48 , which is a controlled-gap seal, is pressurized to prevent oil from leaking out of the bearing housing. 
   To improve pressurization, an impeller baffle is mounted to the coverplate  41 /bearing housing  44  for deswirling the air (i.e., slowing the air). Reducing the velocity of the swirling air increases the air pressure at the backface of the impeller, which thus increases the pressurization of carbon seal  48  immediately in front of the bearing  46 . Thus, the front carbon seal  48  is further aided in sealing the bearing  46 . 
   In accordance with an embodiment of the present invention, an improved impeller baffle  50 , which is illustrated in  FIGS. 2-4 , has deswirling elements that deswirl the air (i.e., interfere with the swirling of the air to slow the air and thus increase its pressure to thereby pressurize he front carbon seal  48 ). The improved impeller baffle  50  is mounted to a forward-facing portion of the bearing housing  44  as shown in  FIG. 2 . 
   As shown in  FIGS. 3-4 , the improved impeller baffle  50  has a generally annular body (not numbered) having a central opening  54  therethrough. As shown in  FIG. 3 , the impeller baffle  50  has a front face  55  upon which are formed a plurality of ribs  56  (also known as “deswirling ribs” or “standoff ribs”) which interfere with the swirling of the air to diminish the air velocity behind an impeller backface to thereby increase air pressure to enhance the sealing efficacy of the front carbon seal  48  to thus guard oil within the bearing housing  44 . 
   In one embodiment, the ribs  56  extend radially from the central opening  54  of the baffle toward an outer periphery of the baffle, The ribs preferably have a curved profile matching a contour of the impeller backface as best shown in  FIG. 2 . 
   In the particular embodiment shown in  FIG. 3 , there are 8 radial ribs formed on the front face of the baffle and these are equidistantly spaced so that each rib and its immediate neighbour form an angle of 45 degrees. However, the number of ribs and their exact arrangement on the front face of the baffle can be varied. Likewise, the profile of the ribs can be varied from what is shown in the figures. 
   As further illustrated in  FIG. 3 , the impeller baffle  50  can include puller grooves  58  formed in a lateral (or peripheral) surface of the baffle to facilitate removal of the impeller baffle (after it has been installed into a tightly fitting position against the bearing housing). Installation and removal of the impeller baffle will be described in greater detail below, with reference to  FIG. 2 . 
     FIG. 4  shows a rear isometric perspective view of the impeller baffle  50  of  FIG. 3 . In this embodiment, the impeller baffle  50  includes a plurality of air slots  60  on a rear face  57  of the baffle which further deswirl the air and thus further increase the pressure of the air at the carbon seal. In the particular embodiment shown in  FIG. 4 , there are 12 slots although the number of air slots can of course be varied. Likewise,  FIG. 4  shows that, in this particular embodiment, the air slots  60  are formed by a plurality of small wedges or lobes  61  that extend radially from the central opening  54  and are each spaced 22.5 degrees from the immediately neighboring wedge-shaped lobe. The number, shape, and configuration of these air slots can be varied to suit the desired airflow characteristics behind the baffle. 
     FIG. 2  shows how the impeller baffle  50  is mounted to the bearing housing  44 . The impeller baffle  50  has a lateral (peripheral) surface that defines a lip  52  that forms a tight fit (or press fit) with the bearing housing  44  which secures the impeller baffle in place. The impeller baffle is further secured to the bearing housing  44  by a retaining ring  64 . 
   As further illustrated in  FIG. 2 , the inner surface  59  of the annular body of the baffle extends radially inwardly into close proximity with an outer surface of a runner  70  supporting the carbon seal  48  to thus define a narrow air gap  72  that functions as a restrictor to restrict airflow over the rear face of the baffle. The airflow path is shown in  FIG. 2  by the dashed line. 
   A method of installing the impeller baffle in a gas turbine engine includes steps of press fitting the impeller baffle to the bearing housing and then affixing the retaining ring to further secure the impeller baffle to the bearing housing. To disassemble or remove the impeller baffle, the retaining ring is first removed and then the baffle is extracted from its tight fit by hooking a tool into the puller grooves and then pulling the baffle off the housing. When installing the baffle, the puller grooves should be aligned to provide maximum access to facilitate disassembly. 
   The impeller baffle can be machined from stainless steel bar stock although other metals having equivalent or similar mechanical and thermal properties could be substituted. The impeller baffle should have a thickness that is sufficiently large so that the natural frequency of the baffle is outside of any running range. 
   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. In the described embodiment, improved pressurization of impeller back face air for sealing air-oil interfaces is desired, though the apparatus of the present invention may be used for any suitable purpose. 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.