Abstract:
A gas turbine transition into an emission reduction catalyst is improved by adding properly curved surfaces so as to induce the Coanda effect. Such a surface allows for a reduction in pressure drop, shorter duct lengths, and elimination of some or all of traditionally used flow re-distribution devices.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the distribution of gas as it transitions through a turbine exhaust duct from a gas turbine exhaust to a larger area necessary to accommodate emissions catalysts. More particularly, the invention relates to a transition section in the turbine exhaust duct for improving distribution of the turbine exhaust gas. 
       BACKGROUND OF THE INVENTION 
       [0002]    Catalytic reduction systems are used to remove pollutants such as carbon monoxide (CO) and nitrogen oxides (NOx) from combustion products of gas turbines used in power generation. The catalysts used in such catalytic reduction systems are designed to be used within a specific range of air flow velocities. The catalyst is typically presented in a large vertical porous structure located in an exhaust duct or conduit. The porous structure allows exhaust gases to pass through in proximity to catalyst elements. Other designs of catalyst trays may also be used. To accommodate the catalyst, a significant expansion of duct cross-sectional area is required as compared to the cross sectional area of the turbine exhaust. Symmetric or asymmetric transition ducts may be required to accommodate the large catalysts, depending on available space, equipment orientation, and other factors associated with a gas turbine unit. 
         [0003]    A conventional prior art gas turbine and gas turbine exhaust duct, as shown in  FIGS. 1 and 2 , uses a straight wall transition duct with flow redistribution devices, such as perforated plates, that redistribute gas turbine exhaust gas flow by creating local obstructions or areas of higher pressure drop. This method can be expensive as it requires long duct lengths and high system pressure drops to achieve the needed redistribution. 
         [0004]    Example prior art gas turbine unit  10  is disclosed in greater detail as follows. 
         [0005]    Referring now to  FIGS. 1-3 , shown is a prior art gas turbine unit designated generally  10  ( FIG. 1 ). Example gas turbine unit  10  is a simple cycle SCR unit. However, the invention described herein may be used with other types of gas turbine units, including emission reduction systems, units with heat recovery steam generation systems or other types of gas turbine units. Gas turbine unit  10  includes inlet air filtration system  12  which feeds air to gas turbine  14 . Gas turbine exhaust exits from gas turbine  14  through gas turbine exhaust outlet  16 . Gas turbine exhaust flows into inlet  18  of gas turbine exhaust duct  20  ( FIGS. 1 and 2 ), whereupon gas turbine exhaust is directed to exhaust stack  22 . 
         [0006]    As shown in  FIG. 2 , gas turbine exhaust gas  24  can be seen entering inlet  18  of gas turbine exhaust duct  20 . In exemplary gas turbine exhaust duct  20  of  FIGS. 1 and 2 , gas turbine exhaust duct  20  supports and encloses vertical CO catalyst  26  ( FIGS. 2 ,  3 ), vertical ammonia injection grid  28  ( FIG. 2 ) and vertical SCR (selective catalytic reduction) catalyst  30  ( FIG. 2 ). Gas turbine exhaust duct  20  is made up of a transition section  32  ( FIGS. 1-3 ) which transitions from a relatively small inlet  18  to a relatively larger area, i.e., expanded area  33  ( FIGS. 1 ,  2 ) that accommodates catalysts  26  and/or  28  and/or  30  or other suitable catalysts. 
         [0007]    Referring now primarily to  FIG. 3 , shown is an enlarged isometric view of prior art transition section  32 . Transition section  32  is made up of top wall  34 , bottom wall  36 , first side wall  38 , and second side wall  40 . It can be seen that walls  34 - 40  converge to form inlet  18  on a first end and expand outwardly to define an outlet end  42 . Perforated plate redistributive device for housing catalyst  26  is visible within transition section  32 . 
         [0008]    In the prior art design of  FIG. 3 , turbine exhaust gas must be forced by some means into the expanded area. This often requires large pressure drops and long duct lengths as the gas flow tends to form eddies and does not naturally follow the angle of the duct walls. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention relates to an exhaust duct designed to better distribute flow of exhaust gas from a gas turbine. The turbine exhaust gas expands within the exhaust duct to flow through an emissions reduction catalyst. 
         [0010]    A curved surface inserted into a flow stream tends to induce a flow of gas to follow the surface. This phenomenon is often referred to as the Coanda effect. The current invention introduces at least one curved exhaust duct wall in a transition section between a turbine and a catalyst, thereby allowing a reduction in either or both duct length and/or redistributive devices as well as an immediate reduction of pressure drop. 
         [0011]    By providing a curved surface for at least one duct wall that is shaped to optimally draw the gas from a high speed exhaust stream into an expanded area of a duct, an improved distribution effect may be achieved. The use of curved surfaces on other duct walls may also be used to achieve a desired distribution effect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an isometric view of a prior art simple cycle SCR gas turbine unit of the type that may be fitted with the duct transition section of the invention; 
           [0013]      FIG. 2  is an enlarged elevation view of a prior art turbine exhaust duct of  FIG. 1  having a transition section; 
           [0014]      FIG. 3  is an enlarged isometric view of a prior art exhaust conduit transition section of  FIGS. 1 and 2  having perforated plates and straight angled walls; 
           [0015]      FIG. 4  is an isometric view of a curved wall transition section of the invention depicting one wall, i.e., a top wall, of the transition section curved to induce a Coanda effect; 
           [0016]      FIG. 5  is a plan view of another embodiment of a curved wall transition section depicting curved sidewalls of a transition section of an exhaust conduit to induce a Coanda effect; 
           [0017]      FIG. 6  is a plan view of another embodiment of a curved wall transition section depicting one curved sidewall of a transition section of an exhaust conduit to induce a Coanda effect; 
           [0018]      FIG. 7  is an elevation view of another embodiment of a curved wall transition section depicting an additional embodiment of a curved upper wall of a transition section of an exhaust conduit to induce a Coanda effect; 
           [0019]      FIG. 8  is an elevation view of another embodiment of a curved walled transition section depicting curved upper and lower walls of a transition section of an exhaust conduit to induce a Coanda effect; 
           [0020]      FIG. 9  is an elevation view of another embodiment of a curved wall transition section depicting an additional embodiment of a curved upper wall of a transition section of an exhaust conduit that curves into the gas flow before curving away to induce a Coanda effect. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The invention relates to an inventive transition section of a gas turbine exhaust duct that better distributes flow of exhaust gas from a gas turbine. The transition section embodiments discussed below may be used to replace prior art transition section  32  of  FIGS. 1-3 , or may be used as transition sections in other gas turbine units to achieve improved gas distribution. 
         [0022]    Referring now to  FIG. 4 , duct transition section  432  of the invention includes a gas turbine transition duct  420  having an upper curved wall  434 . Upper curved wall  434  is curved in a non-linear manner to follow a path that assists in expanding turbine exhaust gas into larger duct area  433 . Gas turbine exhaust duct  420  has inlet  418 , and transition section  432 . Transition section  432  has curved top wall  434 , bottom wall  436 , first side wall  438 , and second side wall  440 . Transitional section  432  additionally has an outlet end  442 . In one embodiment, upper curved wall  434  increases in slope over a first distance, then levels off to interface with expanded area  433 . In one embodiment, the curve followed by top wall  434  may be described by a third degree polynomial equation. 
         [0023]    Referring now to  FIG. 5 , shown is another embodiment of a gas turbine transition section  532  of a gas turbine exhaust duct. Gas turbine transition section  532  has an inlet  518  and an outlet  542 . In this embodiment, transition section  532  expands laterally to accommodate a duct having a width greater than the width of inlet  518  (not shown). Therefore, top wall  534  and bottom wall  536  may be straight and flat, while first side wall  538  and second side wall  540  curve outwardly. In one embodiment, the curves followed by side walls  538 ,  540  increase in slope with regard to a center line of transition section  532  over a length of transition section  532 . In one embodiment, the curve followed by side walls  538  and  540  may be described by a second degree polynomial equation. 
         [0024]    Referring now to  FIG. 6 , shown is another embodiment of a transition section  632  of a gas turbine exhaust duct  620 . Gas turbine transition section  632  has inlet  618 , a curved top wall  634 , a bottom wall  636 , a first side wall (not shown), and a second side wall  640 . Transitional section  632  additionally has an outlet end  642 . In one embodiment, the curve followed by top wall  634  increases in slope over a length of transition section  632 . In one embodiment, the curve followed by top wall  634  may be described by a second degree polynomial equation. 
         [0025]    Referring now to  FIG. 7 , shown is transition section  732  of a gas turbine exhaust duct  720 . Gas turbine transition section  732  has inlet  718 , curved top wall  734 , bottom wall  736 , first side wall (not shown), and second side wall  740 . Transition section  732  additionally has an outlet end  742 . In one embodiment, the curve followed by top wall  734  increases in slope over a length of transition section  732 . In one embodiment, the curve followed by top wall  734  may be described by a third degree polynomial equation. 
         [0026]    Referring now to  FIG. 8 , shown is an elevational view of transitional section  832  of gas turbine exhaust duct  820 . Transition section  832  has an inlet  818 , curved top wall  834  and curved bottom wall  836 . First side wall (not shown) and second side wall  840  may be straight. Transitional section  832  additionally has an outlet end  842 . In one embodiment, the curve followed by curved walls  834  and  836  has a slope that increases in magnitude with regard to a centerline of transitional section  832  over a first distance, then levels off to an interface with an expanded area (not shown). In one embodiment, curves followed by walls  834  and  836  may be described by a third degree polynomial equation. 
         [0027]    Referring now to  FIG. 9 , shown is gas turbine transition section  932  of a turbine exhaust duct  920 . Gas turbine transition section  932  has inlet  918 , a curved top wall  934 , a bottom wall  936 , a first side wall (not shown), and a second side wall  940 . Transition section  932  additionally has an outlet end  942 . In one embodiment, curved top wall  934  has a straight portion adjacent to inlet  918 , a portion where top wall  934  follows a curve with decreasing slope over a length of transition section  932 , which results in a narrowing of transition section  932 , then a portion of increasing slope. In one embodiment, the curve followed by top wall  934  may be described by a second degree polynomial equation. 
         [0028]    Turbine transition ducts  432 ,  532 ,  632 ,  732 ,  832 , and  932  may be used with gas turbine exhaust ducts of simple cycle units, units with emission reductions systems, or units with heat recovery steam generation systems or other turbine units. The curved transition ducts  432 ,  532 ,  632 ,  732 ,  832 , and  932  are equally appropriate for expansion or contraction of gas streams. 
         [0029]    In the present invention, duct walls nearest the turbine exhaust preferably begin with a straight surface parallel to the turbine exhaust gas stream flowing along it. In some applications, this wall may actually be slightly curved toward the exhaust stream (see, e.g., upper wall  934  in  FIG. 9 ) to capture a greater percentage of the gas flow. In a preferred embodiment, after capturing the gas flow with the straight or convex surface, the subsequent duct surface of a duct wall, e.g., walls  434 ,  538 ,  540 ,  634 ,  734 ,  834 ,  836 , begins to curve away from the flow stream with an angle that begins small and that increases in magnitude for a length as the wall progresses. The turbine exhaust gas that was in contact with the straight duct wall continues to follow the curved wall as the gas turns away from the rest of the flow stream. Walls that follow a well designed curve will immediately reduce the pressure drop of the system while allowing for a shorter transition duct.