Abstract:
A compressor diffuser for a gas turbine includes: a compressor diffuser having an upstream end and a downstream end, the downstream end defined by a peripheral annular edge, the annular edge formed with a plurality of substantially axially-oriented slots extending from an opening at the downstream edge in an upstream direction.

Description:
This invention relates generally to gas turbine combustion technology and, more specifically, to modifications in the compressor diffuser to reduce aerodynamic loss associated with the compressor discharge casing of some industrial gas turbines. 
     BACKGROUND OF THE INVENTION 
     An aerodynamic loss has been identified with the compressor discharge casing of some industrial gas turbines. The loss is produced by reacceleration of compressor discharge flow in narrowed areas or “pinch points” just downstream of the compressor diffuser, and it causes increased fuel consumption and reduced cooling of some combustion parts. Generally, newer turbine designs with multi-passage radial discharge diffusers or with redesigned flow sleeves, liners, etc. are not feasible for existing gas turbines because of high development and installation costs. 
     There remains a need, therefore, for a relatively low-cost solution suitable for field modification. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with an exemplary but non-limiting implementation of this invention, there is provided a compressor diffuser for a gas turbine comprising an upstream end and a downstream end, the downstream end defined by a peripheral annular edge, the annular edge formed with a plurality of substantially axially-oriented slots extending from an opening at the annular edge in an upstream direction. 
     In another exemplary but non-limiting implementation, the invention relates to a gas turbine comprising a compressor, an annular array of combustor cans arranged to supply combustion gases to a first stage of the turbine in a first direction, wherein the compressor includes a diffuser shaped to direct compressor discharge air in a second opposite direction to an aft end of the combustor cans for use in combustion; the diffuser having an upstream end and a downstream end formed with a plurality of substantially axially-oriented slots. 
     In yet another exemplary but non-limiting implementation, the invention relates to a gas turbine comprising a compressor, an annular array of combustor cans arranged to supply combustion gases to a first stage of the turbine in a first direction, wherein the compressor includes a diffuser shaped to direct compressor discharge air in a second opposite direction to an aft end of the combustor cans for use in combustion; the diffuser having an upstream end and a downstream end; and means located at the downstream end for enhancing reversal of compressor discharge air from the first direction to the second direction. 
     In still another exemplary implementation, the invention relates to a method for enhancing air flow reversal in a gas turbine combustion system where compressor discharge air is reverse-flowed to a combustor comprising: forming a compressor diffuser with a plurality of substantially axially-oriented slots extending from a downstream end of the diffuser in an upstream direction; and associating at least one flow direction vane with one or more of the substantially axially-oriented slots. 
     The invention will now be described in connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross section of a conventional gas turbine compressor and combustor; 
         FIG. 2  is a partial perspective view of a modified compressor diffuser in accordance with the first exemplary embodiment of the invention; 
         FIG. 3  is an enlarged detail in perspective taken from  FIG. 2 , and with a turning vane added to a slot; 
         FIG. 4  is a partial perspective view of a compressor diffuser as in  FIG. 3  but from below the diffuser wall; 
         FIG. 5  is a partial perspective view of a third exemplary embodiment of the invention; 
         FIG. 6  is a partial perspective view taken from the underside of the diffuser shown in of  FIG. 5 ; 
         FIG. 7  is a partial perspective view of a fourth exemplary embodiment of the invention; and 
         FIG. 8  is a partial perspective view illustrating how vanes can be added to the compressor discharge casing struts. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a can-annular reverse-flow combustor  10  is illustrated. The combustor  10 , along with several other similar combustors (or combustor cans), are arranged in an annular array about the turbine rotor, and generate the gases needed to drive the turbine wheels in the various turbine stages. In operation, discharge air from compressor  12 , indicated by flow arrow A, flows through the diffuser  28  and reverses direction as it passes over the outside of the combustor  10  and then reverses direction again as it enters the forward ends of the combustors. Combustion air and fuel are burned in the combustion chambers  14  (one shown), producing high-temperature gases that flow through a transition duct  16  to the first turbine stage indicated at  18 . 
     On its way to the combustor  10 , the compressor discharge air flows through a flow sleeve  20  which forms an annular gap or passage  22  radially between the flow sleeve  20  and the combustor liner  24 . A similar flow sleeve  26  surrounds the transition duct  16  and joins with the flow sleeve  20  at the interface between the liner  24  and the transition duct  16 . It will be understood that discharge air flows into the gap  22  by way of arrays of holes in the flow sleeves (not shown). To this point, the turbine combustor arrangement is of conventional design. 
     Turning to  FIG. 2 , in a first exemplary but nonlimiting embodiment, a plurality of substantially axially-oriented slots  30  are formed in the aft end of the compressor casing  28  (typically referred to as the compressor diffuser), circumferentially about the diffuser, and between a series of compressor casing support struts  32 . These slots enhance the reversal of flow direction of the compressor discharge air. 
     In this exemplary but nonlimiting embodiment, two slots  30  are provided for each combustor “can”, occupying the space between pairs of radially-oriented struts  32 . The slots  30  extend from openings at the downstream end or edge  31  of the diffuser in an upstream direction, thus providing additional flow path areas and an earlier radial turn for the compressor discharge air to reverse flow toward the combustors, at least in part avoiding the pinch points. By providing increased flow path area at an otherwise narrowed flow path location where the reverse flow occurs, the pressure drop at this location is reduced. It will be appreciated that other slot configurations could be employed, e.g., with one or more than two slots per can. In a variation of this slot configuration, the downstream edge of the diffuser could be made continuous, such that slots  30 . 
     A further air flow turning enhancement can be realized by adding a deflector vane  34  in each slot  30 . This arrangement is shown in  FIG. 3 , where a single vane  34  is installed within the slot  30  and oriented to aid in turning the air flowing into the slot. i.e., with its concave side facing the flow. The vane  34  extends on both sides of the slot (see  FIG. 4 ) so as to be impinged upon by air flowing through the diffuser, while continuing to have a turning effect as the air passes through the diffuser wall. Note that in  FIG. 3 , the compressor orientation is reversed, so that air flow is reversed relative to  FIGS. 1 and 2 . Variations in the number of vanes per slot are also possible. For example,  FIGS. 5 and 6  show an arrangement where three similarly oriented turning vanes  36  are installed in each slot  38 . 
       FIG. 7  illustrates a further alternative arrangement where one slot  40  is provided per can, and a turning or deflector vane  42  is installed on the nearest adjacent strut  44 , downstream of the slot.  FIG. 8  illustrates one example of how a pair of turning or deflector vanes  46  can be attached to opposite sides of a strut  48 . Specifically, each vane  46  is provided with a mounting base  50  with a strut engaging face  52  having a surface profile matching the strut. The vanes may be attached using screw fasteners  54  or other suitable means, such as rivets or the like. As indicated above, the number of slots per can, as well as the number and location of the turning vanes may vary as needed. Presently, the preferred arrangement is to have two slots  30  per can, with one turning vane  34  per slot, either in the slot or mounted on the nearest adjacent strut  32 . 
     In a variation of  FIG. 7 , the deflector vanes  46  could be utilized alone, without the slots  40 . While less effective than the combination of slots and vanes, the vanes alone would nevertheless provide some enhancement of air flow reversal. 
     The diffuser modifications described herein can be performed in the field on existing turbine engines, or in the factory, providing performance improvement to both services customers and new unit customers. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.