Abstract:
A cooling arrangement ( 56 ) having: a duct ( 30 ) configured to receive hot gases ( 16 ) from a combustor; and a flow sleeve ( 50 ) surrounding the duct and defining a cooling plenum ( 52 ) there between, wherein the flow sleeve is configured to form impingement cooling jets ( 70 ) emanating from dimples ( 82 ) in the flow sleeve effective to predominately cool the duct in an impingement cooling zone ( 60 ), and wherein the flow sleeve defines a convection cooling zone ( 64 ) effective to cool the duct solely via a cross-flow ( 76 ), the cross-flow comprising cooling fluid ( 72 ) exhausting from the impingement cooling zone. In the impingement cooling zone an undimpled portion ( 84 ) of the flow sleeve tapers away from the duct as the undimpled portion nears the convection cooling zone. The flow sleeve is configured to effect a greater velocity of the cross-flow in the convection cooling zone than in the impingement cooling zone.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED DEVELOPMENT 
     Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a cooling arrangement for a hot gas duct having significantly varying cooling requirements along its length. 
     BACKGROUND OF THE INVENTION 
     Conventional gas turbine engines utilizing a can-annular combustion arrangement include a transition duct that receives hot combustion gases from a combustor can and guides the combustion gases toward a turbine inlet. Typically a guide vane between the downstream end of the transition duct and the turbine rotor inlet orients the hot gases for delivery onto the first row of turbine blades. The hot gases exhausting from the combustor outlet typically flow below 0.2 mach. The hot gases accelerate slightly as they travel within the transition duct, but most of the acceleration occurs as the hot gases flow through the guide vanes, where the hot gases are accelerated to approximately 0.7-0.9 mach. 
     Cooling requirements for the transition duct are influenced by the speed of the hot gases flowing through the transition duct. Since the speed of the hot gases flowing through conventional transition ducts remains reasonably constant along the length of the transition duct, conventional transition duct cooling arrangements have been designed to remove heat at relatively constant rates along the length of the transition duct. 
     In contrast to the conventional combustion arrangements, an emerging can-annular combustion arrangement reorients the combustors and directs the hot gases along a straight flow path toward the turbine inlet annulus. The associated transition duct technology uses the transition duct itself to accelerate the hot gases, thereby eliminating the guide vanes conventionally placed between the transition duct and the turbine rotor inlet. Accelerating the combustion gases within the transition duct increases the amount of heat transferred to the transition duct in those regions where the hot gases flow faster. Consequently, there remains room in the art for improved cooling arrangements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic, longitudinal cross section of a cooling arrangement disclosed herein. 
         FIG. 2  is a schematic cross-section of the flow sleeve taken along line  2 - 2  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have devised a unique cooling arrangement adapted to the unique cooling requirements for transition ducts associated with certain emerging can-annular combustion arrangements. In these combustion arrangements the combustors are oriented in a manner that permits delivery of the hot gases along a straight flow path and directly on to a first row of turbine blades via transition ducts that accelerate the hot gases and thereby eliminate the need for the conventional guide vanes immediately upstream of the turbine rotor inlet. The cooling arrangement forms various zones capable of meeting the cooling requirements or different regions of the transition duct by varying the type of cooling provided. Types of cooling provided including impingement cooling, convection cooling, and combination impingement and convection cooling. 
       FIG. 1  shows a downstream end  10  of a combustor can  12  having an outlet  14  from which hot gases  16  exhaust while flowing along a straight flow axis  18 . The hot gas ducting includes a transition cone  30  having an upstream end  32  that receives the downstream end  10  of the combustor can  12  and defines a passageway for the hot gases. A diameter of the transition cone  30  transitions from an inlet diameter  34  to a smaller, outlet diameter  36  at a downstream end  38 . This diameter change decreases a flow area for the hot gases  16  which accelerate in response to the decreasing diameter. This convergence occurs over a cone converging length  40  that spans from the inlet diameter  34  to the outlet diameter  36 . 
     Surrounding the transition cone  30  is a flow sleeve  50  which defines a cooling plenum  52  there between. Surrounding the flow sleeve  50  is a casing plenum  54  that contains compressed air received from the compressor and used as the cooling fluid. The cooling arrangement  56  may include an impingement cooling zone  60 , an optional blended cooling zone  62 , and a convection cooling zone  64 . These zones represent zones of varying rates of heat transfer from the hot gases  16  to the transition cone  30 . Both the impingement cooling zone  60  and the blended cooling zone  62  form a zone having impingement cooling. 
     In the convection cooling zone  64  hot gases  16  may be flowing at a speed below mach 0.2 and therefore transfer a relatively low amount of heat to the transition cone  30  in this zone. In the blended cooling zone  62  the diameter of the transition cone  30  decreases. This accelerates the hot gases  16  and this increased flow velocity increases the amount of heat transferred from the hot gases  16  to the transition cone  30  (i.e. the heat flux) in the blended cooling zone  62  when compared to the convection cooling zone  64 . In the impingement cooling zone the diameter of the transition cone  30  continues to decrease. This continues to accelerate the hot gases  16  resulting in an even greater rate of heat transfer from the hot gases  16  to the transition cone  30  in the impingement zone  60  when compared to the blended cooling zone  62 . 
     Readily available types of cooling include impingement cooling and convection cooling, both of which are used in the cooling arrangement  56 . Impingement cooling is used in the impingement cooling zone  60  because it is extremely effective and therefore a good match for the extremely high cooling requirements of the narrowest portion of the transition cone  30  where hot gases may flow above approximately 0.5 mach. In an exemplary embodiment, in the impingement cooling zone  60  impingement cooling may be responsible for the majority of the heat removal from the transition cone  30 , and convective cooling may be responsible for a minority of the heat removal. Here fast moving jets  70  of cooling fluid  72  are directed onto an outer surface  74  of the transition cone  30  to be cooled. Once spent, (i.e. post-impingement), the cooling fluid  72  becomes a cross-flow  76  of cooling fluid  72 . The cross-flow  76  flows along and convectively cools the outer surface  74 . However, as the cross-flow  76  flows along the outer surface  74  a volume of the cross-flow increases because more impingement jets  70  are feeding cooling fluid  72  into the cross-flow  76 . This can interfere with the flow of the impingement jets  70 , reducing the penetration of the impingement jets  70  to a point where the impingement cooling effect is reduced. 
     To reduce this interference the inventors have developed an innovative dimpled arrangement  80  where individual dimples  82  extend radially inward from an undimpled portion  84  of the flow sleeve  50 , such as a sheet. Each dimple  82  includes an outlet  86  from which a respective impingement jet  70  emanates. The dimples  82  can be configured such that all outlets  86  are at any distance  88  desired from the outer surface  74 . In one exemplary embodiment all of the outlets  86  are at a same distance from the outer surface  74 . In an exemplary embodiment the ratio of distance  88  to diameter of the outlet  86  in the impingement cooling zone  60  may be set at 3-5. The closer the outlets  86  are to the outer surface  74 , the less pressure necessary to form an effective impingement jet  70 . Thus, this dimple arrangement can be used more effectively in areas where the driving pressure difference is relatively small. The dimples  82  may be aligned with each other and in a direction of the cross-flow  76  so that the cross-flow  76  is guided around the impingement jets  70  by the dimples  82  and is free to flow in the rows between the dimples. In this manner the cross-flow  76  does not interfere with the impingement jets  70 . 
     In between the dimples  82 , the undimpled portion  84  forming the cross-flow channels may be characterized by a diameter  90  having a rate of taper  92 . This rate of taper  92  may be tailored with respect to a rate of taper  94  of the outer surface  74  so a cross sectional area of the cooling plenum  52  is increased, or optionally, maintained or even reduced. By increasing the cross sectional area of the cooling plenum  52 , the cooling plenum  52  can be configured to maintain a same flow velocity of the cross-flow  76  along a length of the cooling plenum  52  despite the addition of cooling fluid  72  with each impingement jet  70  in a direction  96  of flow of the cross-flow  76 . Having a slower flow velocity reduces an interference between the cross-flow  76  and the impingement jets  70 . Alternately, the flow velocity of the cross-flow  76  could be decreased or increased based on other design considerations. This unique arrangement allows for individual tailoring of the flow velocity of the cross-flow  76  and the number of impingement jets  70  and their distance  88  from the outer surface  74 . By controlling the flow velocity of the cross-flow  76  one can also control the amount of convective cooling that is achieved via the cross-flow  76 . Together, the impingement cooling and the convection cooling are effective to meet the cooling requirements of the transition cone  30  in this zone that might not be met by convection cooling along. 
     The blended cooling zone  62  is similar to the impingement cooling zone  60  in that both impingement cooling jets  70  and cross-flow  76  convective cooling may be used, but in this zone and in an exemplary embodiment the convective cooling effects of the cross-flow  76  may be predominant, and the impingement jets  70  are responsible for a minority of the heat transfer from the transition cone  30 . This blended cooling is sufficient to meet the needs of the transition cone  30  in this zone where hot gases  16  may flow at rates between approximately 0.5 mach and 0.2 mach. In an exemplary embodiment the ratio of distance  88  to diameter of the outlet  86  in the blended cooling zone  62  may be set at 3-5. 
     In the convective cooling zone  64  all cooling is accomplished by convection. While the cooling requirements are lowest in this zone, the cross-flow  76  must still be accelerated so it can transfer enough heat from the transition cone  30 . Consequently, in this zone the flow velocity of the cross-flow  76  is greater than the flow velocity of the cross-flow  76  in the impingement cooling zone  60  and in the blended cooling zone  64 . The acceleration of the cross-flow  76  can be accomplished in at least two ways. In a first configuration a cross sectional area of the cooling plenum  52  may be reduced in the convection cooling zone  60  and this will accelerate the cross-flow  76  to the desired flow velocity. This may be accomplished in an exemplary embodiment by having a diameter  100  at an upstream end  102  of the convection cooling zone  64  be less than a diameter  104  of the undimpled portion  84  immediately upstream of the upstream end  102  of the convection cooling zone  64  with respect to a direction of flow of the cross-flow  76 . 
     Alternately, or in addition, a flow sleeve opening  106  may be positioned to allow cooling fluid  72  into the convection zone  64 . The increased volume of cooling fluid will cause the cross-flow velocity to increase. The increase can be tailored as necessary by sizing the size of the flow sleeve opening  106  alone or together with the diameter  100  at the upstream end  102  of the convection cooling zone  64  or anywhere else in the convection cooling zone  64  as desired. Alternately, or in addition, the flow sleeve opening  106  may be angled as shown so that a momentum of the cooling fuild  72  traveling through the flow sleeve opening  106  and entering the cross-flow  76  may contribute to an acceleration of the cross-flow  76 . 
     In a transition region  110  between the blended cooling zone  62  and the convection cooling zone  64  the flow sleeve  50  may be configured to take advantage of the changing diameters of the flow sleeve  50 . For example, a ramp  112  may be formed that directs circumferential portions of all of the converging cross-flow  76  toward the transition cone  30  as indicated by arrow  114 . This ramp  112  can be configured at any angle desired or may undulate circumferentially, resulting in regions of greater and lesser impact on the transition cone  30  circumferentially. Such circumferential undulation may be a natural result of the last circumferential ring  116  of dimples  82 . 
     Cooling fluid  72  exhausting from an outlet  118  of the convection cooling zone  64  may exhaust into an inlet of the combustor and used for further cooling and/or combustion. 
       FIG. 2  shows a cross section of the flow sleeve  50  alone, looking downstream along the flow axis  18 . Visible are the dimples  82 , outlets  86 , and undimpled portions  84  of the flow sleeve  50 . In this view it is apparent that the dimples  82  may align with the direction  96  of flow of the cross-flow  76  to form rows  130  of dimples, leaving cross-flow channels  132  there between in which the cross-flow  76  can flow and avoid the impingement jets  70 . The cross-flow channels  132  are open and allow for the cross-flow  76  to flow unimpeded. This reduces a pressure drop in the flow sleeve which, in turn, increases engine efficiency. Alternately, the dimples may be spaced in alternating rows for more effective and uniform impingement cooling. Cross flow effects on the impingement jets can be minimized by increasing further the spacing of the undimpled portion of the flow sleeve. 
     From the foregoing it is apparent that the inventors have devised an innovative solution to new cooling requirements created by a new combustion arrangement. The cooling arrangement is responsive to the much greater variation in cooling requirements of different regions of the duct than exists in prior art combustion arrangements. Consequently, the cooling arrangement is able to satisfy the varying cooling needs of these regions, but does so using cooling fluid in a much more efficient manner than would be possible if the prior art cooling arrangements were applied. Thus, the cooling arrangement represents an improvement in the art. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.