Abstract:
Gas flow is redirected by a feature disposed on a trailing edge of at least one segment of a peripheral gas path defining surface to improve alignment with a downstream portion of the gas path.

Description:
TECHNICAL FIELD 
   The invention relates to gas turbine engine design and, in particular, reducing gas path pressure losses in a gas turbine engine. 
   BACKGROUND OF THE ART 
   Without question, the design of an efficient gas turbine engine is an exercise in compromise. Gas paths are designed to maximize work output, minimize losses, extend component life, and operate reliably. To maximize the work obtained from the flow, aerodynamics typically prevail through the provision of an expanding and curving gas path through the turbine section. This curvature inevitably results in pressure losses, however the penalty is necessary to optimize efficiency. There is room for improvement, however, as it is desirable to reduce losses while still maximizing the work done by the turbine. Often however, the designer is limited in what he or she can do, without disrupting the complex optimization of the turbine design. 
   SUMMARY OF THE INVENTION 
   In one aspect the invention provides a component for a gas turbine engine, the engine defining a primary gas path including at least two adjacent sections, a first of said sections channelling gases in a first general direction and a second of said sections channelling gases in a second general direction, the second section disposed downstream of the first, the first and second general directions different from one another, the component comprising a primary gas path defining surface, the surface being a circumferential portion of an annular surface of revolution, the surface providing a portion of said first section and generally aligned in the first general direction, the surface co-operating with at least a pair of spaced-apart airfoils to define an aerodynamic throat therebetween, the surface including a lip portion located downstream of the throat, the lip portion generally aligned with the second general direction. 
   In a second aspect the invention provides a component for a gas turbine engine, the engine defining a primary gas path including at least two adjacent sections, a first of said sections channelling gases in a first general direction and a second of said sections channelling gases in a second general direction, the second section disposed downstream of the first, the first and second general directions different from one another, the component comprising a primary gas path defining surface, the surface being a circumferential portion of an annular surface of revolution, the surface providing a portion of said first section and generally aligned in the first general direction, the surface co-operating with at least a pair of spaced-apart airfoils to define an aerodynamic throat therebetween, the surface including means for redirecting gas flow thereover to the second direction, said means located downstream of the throat. 
   Further details of the invention and its advantages will be apparent from the detailed description included below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be readily understood, examples of the invention are illustrated in the accompanying drawings, in which: 
       FIG. 1  is an axial cross-section through a turbofan gas turbine engine employing the invention; 
       FIG. 2  is a axial sectional view through the turbine section of an engine according to the present invention; 
       FIG. 3  is a schematic side view of a vane according to the present invention, followed by a downstream blade; 
       FIG. 4  is a schematic side view of a blade according to the present invention, followed by a downstream vane; 
       FIGS. 5 and 6  are enlarged views or portions of  FIGS. 3 and 4 , respectively; and 
       FIG. 7  is a view similar to  FIGS. 3 and 4 , showing a further embodiment incorporated in a static shroud, followed by a downstream vane. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an axial cross-section through a turbofan gas turbine engine  10 . It will be understood however that the invention may also be applied to any type of airborne or land-based gas turbine engine. Air intake into the engine passes over fan blades  12  is split into an outer annular flow through the bypass duct  14  and an inner flow through a compressor  16  to a combustor  18 , where it is combusted and the resulting hot gases are expelled through the turbine section  20 , which includes vanes  22  and turbine blades  24 , before exiting the engine. 
   Referring to  FIG. 2 , the turbine section has a gas path  26  defined therethrough which is generally annular and extends axially from the engine inlet to the exhaust (neither indicated). The gas path  26  is defined by an inner wall  28  and an outer wall  30  which each comprise a surface of revolution about the longitudinal engine axis  32  (reference  FIG. 1 ). As best seen in  FIG. 2 , the gas path wall  28  and  30  are not continuous, although they are generally designed for optimal aerodynamic properties. Thus, the gas path  26  typically comprises a plurality of successive sections  34 , wherein the direction and/or relative expansion or compression of the gas path changes relative to upstream and/or downstream sections  34 . Successive sections  34 , therefore, have general directions (i.e. the major direction in which the section is aligned, ignoring any local deviations) which are typically disposed at angles relative to the adjacent upstream and downstream sections  34 . These direction changes, and relative expansion or contraction of the gas path shape, is typically provided to maximize work extracted from the turbine cycle, for example, or in the case of a compressor, maximize compression efficiency, etc. 
   The gas path walls  28  and  30  of sections  34  are defined by successive gas turbine components such as rotor blade platforms  36 , blade tip shrouds  38 , static shrouds  40 , and vane platforms  42  and  44 . The platforms  36 ,  42 , and  44  and static shrouds  40  thus provide gas path defining surfaces  48 , which direct air/combustion gases through the primary gas path. The general angle relative to the engine centreline  14  of the gas path as defined by each gas path defining surface  48  defines the overall shape of gas path  26 . The blades and vanes each have airfoils  46  which have trailing edges  50 . Together with airfoils  46 , and in particular trialing edges  50 , platforms  36 ,  42 , and  44  and static shrouds  40  also respectively define a plurality of aerodynamic throats  52 . The platforms  36 ,  42 , and  44  and static shrouds  40  also have trailing edges  54 , which are downstream of trailing edges  50  and thus throats  52 . 
   According to the present invention, the gas path defining surfaces  48  provided by platforms  36 ,  42 ,  44  and shrouds  40  and  38  may be provided with an integrally angled lip or gas flow redirector  56  adjacent a trailing edge thereof, downstream of an exit of aerodynamic throat  52 . Referring to  FIG. 3 , vane platform  42  is shown with a downwardly angled lip  56 . Referring to  FIG. 4 , blade platform  36  is provided with an upwardly angled lip  56 . As indicated in  FIGS. 3 and 4  with angle α, the lip  56  deviates from the general direction or shape “A” of the platform in a manner so as to redirect the airflow passing gas path defining surface  48  into better alignment with a general direction or shape “B” of a downstream platform  58  of downstream article  60  (in this case, a blade and vane, respectively), and thereby reduce losses associated with turbulence caused by airflow disruptions. Line “A” therefore represents the general direction of the upstream section  34 , while line “B” represents the general direction of the downstream section  34 , as it relates to the gas path wall  28 ,  30  of interest (i.e. the inner and outer walls  28 ,  30  may not have the same general direction). Referring again to  FIG. 3 , is can be seen that the general direction of the downstream section  34  (i.e. line B) is not necessarily the same as the local direction of the downstream section  34  immediately downstream of lip  56 . Rather, lip  56  may redirect air past such local inconsistencies in direction, and towards the more global general direction provided in the downstream section  34 . 
   It has been found that redirection of gas in advance of a change in general direction of the walls  28 ,  30  of the gas path reduces pressure losses and thereby helps to better optimize engine efficiency. As mentioned, the lip  56  is downstream of the aerodynamic throat  52 , to thereby minimize any aerodynamic effects experienced at the throat (e.g. choking, etc.) and the present invention thereby interferes minimally, if at all, with the aerodynamic design of the gas path vis-à-vis maximizing work output from the combustion gases. Losses may therefore be reduced without affecting any macro design aspects of the gas turbine engine. 
   As mentioned, the gas flow redirector lip  56  can be located at various and multiple positions in the engine. In the embodiments shown, the redirector lip  56  is shown on a radially inner surface of the gas path, however it will be appreciated that redirector lip  56  can also be used on an outer gas path surface in the turbine, such as the static shroud embodiment depicted in  FIG. 7  or on a turbine blade shroud  38  (embodiment not depicted) and, likewise, the invention may be employed in a compressor or other areas of the gas turbine gas path, as well. The exact shape and angle of the lip  56  can be to the designer&#39;s preference. Referring to  FIGS. 5 and 6 , the active or redirecting surface of lip  56  may be a linear surface of revolution about the engine axis (i.e. appears “flat” in  FIG. 5 ) or may be curved in the axial and/or circumferential directions on a suitable constant or variable radius r (i.e. appears “curved” in  FIG. 6 ) as desired. It will be understood that the relative proportions of the lips  56  shown in the Figures have been exaggerated for illustration purposes, and that in fact the lip may only be a few thousandths of an inch in height. It will also be understood that a “lip” may protrude from the primary gas path defining surface  48 , or may recess therefrom. Although the “A” direction is shown in each example as horizontal for ease of illustration, the skilled reader will appreciate that the invention may be applied to any relative “A” and “B” directions within the gas path. 
   The direction or angle provided to lip  56  preferably includes a slight over- or under-correction (as the case may be) so that gases are directed smoothly over the boundary layer region of the downstream section of the gas path, and preferably avoids any local obstacles or direction changes located between the lip  56  and the general direction provided by the downstream section. 
   Still other modifications will be apparent to the skilled reader which do not depart from the invention. Therefore, although the above description relates to a specific preferred embodiments as presently contemplated by the inventor, it will be understood that the scope of the present invention described herein is intended to be limited only by the appended claims.