Abstract:
An arrangement for reducing vorticity downstream of a turbine nozzle guide vane  34  in a gas turbine engine. The arrangement including directing cooling air through the guide vane  34  and out through outlets  40  in directions to counteract the wave vorticity produced downstream of the guide vane  34.

Description:
FIELD OF THE INVENTION 
   This invention relates to an arrangement for reducing vorticity in a gas turbine engine, a component for a gas turbine engine, and a turbine for a gas turbine engine. 
   BACKGROUND OF THE INVENTION 
   In gas turbine engines a wake is produced downstream of components past which gases flow, and especially components which turn the direction of the gas flow. This wake may roll up into a discrete vortex of intense vorticity that can result in significant flow distortions as well as mixing and interference losses downstream thereof. Such components include turbine aerofoils, and the wake produced downstream thereof can have an effect upon subsequent blade rows where it results in flow distortions, incidence and other loss mechanisms. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided an arrangement for reducing vorticity downstream of a component in the gas flow of a gas turbine engine, the arrangement including means for introducing a gas into the gas flow in a direction or directions to counteract the wake vorticity produced downstream of the component. 
   Means are preferably provided for introducing the introduced gas through the component. 
   The arrangement may be configured such that the introduced gas is air, and may be cooling air which has already passed through the component. 
   Guide means may be provided which are arranged to direct the introduced gas at an inclination relative to the gas flow. The guide means may include slots or openings, and openings may be directed onto guide formations to provide the required inclination. 
   The guide means may be arranged to directly introduce gas in different directions from different parts of the component. The guide means may be arranged to direct the introduced gas in different directions from different sides of the component. 
   The guide means may be arranged to swirl the introduced gas in a manner which results in vorticity with an opposite sign to that of the wake vorticity, and the guide means may include passages with vanes, grooves, fences, rifling or other formations on the walls thereof to cause swirling. 
   The component may include an aerofoil, which component may form part of a compressor or turbine, and in particular the component may be a turbine nozzle guide vane. 
   The invention also provides a component for a gas turbine engine, the component being according to any of the preceding seven paragraphs. 
   The component may be a turbine nozzle guide vane. 
   The invention yet further provides a turbine for a gas turbine engine, the turbine including a plurality of components according to any of the preceding nine paragraphs. 
   The invention still further provides a method of reducing wake vorticity downstream of a component in the gas flow in a gas turbine engine, the method including introducing a gas in a direction or directions to counteract the wake vorticity produced downstream of a component. 
   The gas may be introduced through the component. 
   The introduced gas may be directed in different directions from different parts of the component. 
   The introduced gas may be swirled to result in a vorticity with an opposite sign to that of the wake vorticity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic cross sectional view through a conventional turbine nozzle guide vane of a gas turbine engine; 
       FIG. 2  is a diagrammatic axial section view through part of a conventional turbine looking upstream; 
       FIG. 3  is a similar view to  FIG. 2  but with other indications provided; 
       FIG. 4  is a diagrammatic axial section view through part of a first gas turbine according to the invention looking upstream; 
       FIG. 5  is a similar view to  FIG. 4  of a second gas turbine according to the invention; 
       FIG. 6  is a similar view to  FIG. 5  but with other markings thereon; 
       FIG. 7  is a diagrammatic cross sectional view through part of a component of the second gas turbine; 
       FIG. 8  is a similar view to  FIG. 1  but of a turbine nozzle guide vane according to a third embodiment of the invention; and 
       FIGS. 9 and 10  are respectively views in the direction of the lines A—A and B—B illustrated in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a conventional turbine nozzle guide vane  10  with a leading edge  12  and a trailing edge  14 . The inlet gas flow direction is shown by the arrow  16  and this gas flow is turned by the guide vane  10  to the direction shown by the arrows  18 . The shape of the guide vane  10  produces a pressure side  20  and a suction side  22 . As is conventional air cooling is provided through the nozzle guide vane  10  as illustrated by the arrows  24 , and some cooling air may exit the guide vane  10  at the trailing edge  14 . 
     FIGS. 2 and 3  diagrammatically show two such nozzle guide vanes  10  looking upstream with their suction sides  22  and pressure sides  20 . Arrows  26  in  FIG. 2  illustrate how the gas flow may tend to pitch down on the suction side  22  whilst pitching up on the pressure side  20 .  FIG. 3  indicates by arrows  28  an example of the induced negative streamwise vorticity. In  FIGS. 2 and 3  the guide vanes  10  are seen extending between the casing  30  and the hub  32 . 
     FIG. 4  shows a first arrangement according to the invention with a nozzle guide vane  34  extending between the casing  30  and hub  32 . Again the view is upstream so the trailing edge  36  is visible with the pressure side  20  and suction side  22 . Arrows  38  illustrate the main flow pitching down on the suction side  22  and pitching up on the pressure side  20 . The trailing edge  36  is shown schematically to have four outlets  40  for cooling air passing through the nozzle guide vane  34 . The lower two outlets  42  are directed downwardly as shown by the arrows illustrated therein whilst the upper two outlets  44  are directed upwardly again as illustrated by the arrows shown therein. The inclination and arrangement of the outlets  40  is chosen to, result in reduced streamwise vorticity in the wake of the guide vane  34 . 
     FIGS. 5 and 6  show a second embodiment with a turbine nozzle guide vane  46  again extending between the casing  30  and hub  32 . In this instance seven outlets  48  are schematically shown. Four upwardly inclined outlets  50  are provided on the suction side  22 , whilst three offset downwardly inclined outlets  52  are provided on the pressure side  20 .  FIG. 6  illustrates the main stream induced negative vorticity by the arrow  54 , whilst the arrow  56  illustrates the positive vorticity induced by the cooling air through the outlets  48 . 
   Whilst in this example the outlets  50  and  52  are offset, it is not always necessary to provide an offset configuration. 
     FIG. 7  illustrates the trailing edge  58  of the nozzle guide vane  46 . Coolant supply chambers  59 , which may be shared or separate, for the cooling air passing through the nozzle guide vane  46  are shown leading to outlets  50 ,  52 , with the outlets  50  angled down and the outlets  52  angled up. The freestream flow pitching up is shown by the arrow  60  on the pressure side  20 , with the freestream flow pitching down illustrated by the arrow  62  on the suction side  22 . The flow of cooling air is illustrated by the arrows  64 . 
     FIGS. 8 to 10  illustrate a further turbine nozzle guide vane  66  which again ejects cooling air through its trailing edge  68 . A plurality of passages  70  are provided in the trailing edge  68  for ejection of cooling air. Helical fences  72  are provided in the passages  70  to swirl cooling air passing therethrough as illustrated by the arrows  74  which show a positive induced vorticity to counteract the main flow pitching down on the suction side  22  and pitching up on the pressure side  20 . Whilst helical fences  72  have been described, the swirling of the cooling air can be produced by a number of other means such as rifling, grooves or vanes. 
   There are thus described various arrangements for counteracting the wake vorticity produced behind turbine nozzle guide vanes. The reduction of this vorticity provides for a number of advantages. These include increased performance, due to the reduction of efficiency losses such as mixing, flow distortions and downstream incidence effects. In addition, aerodynamic unsteady forcing contributing to fatigue failure of downstream components is also reduced. 
   Various other modifications may be made without departing from the scope of the invention. For instance, the ejection of the cooling air could be in a number of different directions, and could extend at least to some degree laterally to counteract the vorticity. Obviously any required combination of inclinations and number of cooling air ejectors can be chosen. These ejectors can be nozzles or could for example be openings directed onto a profiled member. Whilst the use of cooling air has only been described, it may be possible for air other than cooling air or other gases to be used to counteract the vorticity. This would obviously be of particular relevance to uncooled components. 
   Whilst the invention has been described in terms of nozzle guide vanes for turbines, the invention could be applicable to a wide range of products located in the gas stream, and particularly products which turn the gas stream. Such products include for instance compressor guide vanes. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.