Patent Publication Number: US-10787918-B2

Title: Gas turbine transition duct

Description:
PRIORITY CLAIM 
     This application claims priority from European Patent Application No. 16185198.5 filed on Aug. 22, 2016, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a gas turbine transition duct. 
     BACKGROUND 
     As is known, a can-combustor gas turbine assembly comprises a compressor section, a combustor assembly, that includes a plurality of can combustors arranged circumferentially about a main axis, and a turbine section, where exhaust hot gases produced in the combustor assembly are expanded to convert thermal and kinetic energy of the hot gas flow into mechanical energy. The can combustors are fluidly coupled to the turbine section through respective transition ducts, which are generally provided as tubular bodies having an upstream end at an outlet of the respective combustor can and a downstream end at an inlet of the turbine section. The transition ducts are configured to guide the hot gas flow from the combustor outlet to the turbine inlet and are thus exposed to extremely high temperatures. In some kinds of gas turbines, for example, the temperature of hot gas may reach 1800 K. As other components of gas turbines, also transition ducts therefore require cooling to avoid damages caused by overheating and to increase lifetime. 
     For the purpose of cooling the transition ducts, a relatively fresh airflow is usually taken from the compressor delivery air in a plenum around the combustors assembly. This cooling airflow is directed substantially perpendicularly to an outer surface of the transition duct in the form of impinging air jets that provide for cooling. Impinging air jets may be obtained by feeding the cooling airflow through cooling holes in a wall surrounding the transition duct at a distance. 
     Impingement cooling, however, is not free from limitations. In the first place, creating impinging jets by feeding air through the surrounding wall and against the outer surface of the transition duct causes a substantial pressure drop across the impingement cooling holes, on account of the fluidic resistance of the holes and turbulence created by impingement. The pressure drop thus results in a loss of efficiency of the gas turbine, because quite a large airflow needs to be subtracted from the overall airflow processed by the compressor in order to provide a given cooling effect. 
     Another limitation of impingement cooling resides in that it may be difficult to adequately cool the downstream portion of the transition duct, at the connection with the turbine section of the gas turbine engine. On the one side, the residual amount of air at the downstream end of the transition duct may be rather small, as most of the available airflow flows through the impingement cooling holes before reaching the connection with the turbine section. On the other side, the geometry of the downstream section and of the coupling members for connection with the turbine may be quite complex, so directing impinging air jets may be hardly feasible. 
     In addition, air supply is required all around the transition ducts, to provide impinging air jets also on the sides thereof. Therefore, adjacent transition ducts need to be circumferentially spaced apart from one another to provide sufficient air passage between respective surrounding walls. This leads to quite bulky combustor assemblies and, even worse, to uneven hot gas flow into the turbine section between adjacent transition ducts. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention is to provide a gas turbine transition duct which allows to overcome or at lease attenuate the limitations described. 
     According to the present invention, there is provided a gas turbine transition duct comprising: 
     an inner tubular body, defining a transition channel and having a first upstream end and a first downstream end for coupling to a can combustor and to a turbine section of a gas turbine assembly, respectively; 
     an outer tubular body, arranged around the inner tubular body and having a second upstream end at the first upstream end of the inner tubular body and a second downstream end at the first downstream end of the inner tubular body; 
     wherein a convective cooling channel is defined between the inner tubular body and the outer tubular body, the convective cooling channel having an inlet between the first downstream end and the second downstream end; 
     and wherein the outer tubular body is continuous between the second upstream end and the second downstream end. 
     The convective cooling flow may be smoothly guided into the cooling channel that is defined between the inner and outer tubular bodies avoiding corners and without creating turbulence and intersections of flow lines. Accordingly, substantial pressure drops are also avoided and efficiency of the gas turbine assembly is increased. 
     The cooling flow inlet is located at the downstream ends of the inner tubular body and outer tubular body and no cooling feed is required through the side wall of the outer tubular body. Therefore, there is no need to leave space between adjacent transition ducts than in excess to space required to accommodate thermal expansion of the outer bodies. This is not only beneficial to the overall dimension of the combustor assembly, but also allows to supply uniform and evenly distributed hot gas flow to the gas turbine. In fact, the transition ducts may be provided with broader outlets and the spacing between the outlets of adjacent transition ducts is reduced, thereby comparatively reducing flow changes at the border. 
     Another advantage of the invention resides in that also the downstream end of the inner tubular body and outer tubular body can be uniformly and effectively cooled, thus substantially reducing the risk of overheating of the components and increasing lifetime. 
     The cross-section of the cooling channel and the geometry of the inlet can be accurately controlled. 
     According to an aspect of the invention, the cooling channel is configured to convey a cooling flow along an outer surface of the inner tubular body from the first downstream end and second downstream end towards the first upstream end and second upstream end. 
     In use, hot gas produced by burning an air-fuel mixture in the combustor flow through the transition channel from the upstream end to the downstream end of the inner tubular body, while a cooling medium flows is in the opposite direction along the outer surface of the inner tubular body. Accordingly, a counter-flow heat exchange results, which is very efficient in cooling and in reducing losses. 
     According to an aspect of the invention, the cooling channel extends all around the inner tubular body, the inlet defines an inlet flow direction transverse to the outer surface of the inner tubular body at all sides of the inner tubular body and the inlet is configured to guide the cooling flow along the outer surface of the inner tubular body. 
     Smoothly guiding a fluid flow through an angle (from transverse to parallel to the outer surface of the inner tubular body) can be quite easily accomplished. Accordingly, the transverse inlet at all sides of the inner tubular body provides for sufficient supply of cooling fluid for the cooling channel without causing any major loss of pressure. 
     According to an aspect of the invention, the inner tubular body has a first frame at the first downstream end; the outer tubular body has a second frame at the second downstream end, the second frame being arranged parallel to and at a distance from the first frame; and the inlet of the convective cooling channel is defined between the first frame and the second frame. 
     The inlet between frames at the downstream ends of the inner tubular body and outer tubular body, which connect to the turbine section, favours effective cooling even of the terminal portion of the transition duct. 
     According to an aspect of the invention, the transition duct comprises a splitter in the inlet of the convective cooling channel. 
     The splitter helps guiding the cooling fluid, further reducing loss of pressure. 
     According to an aspect of the invention, the splitter comprises a third frame, arranged between the first frame and the second frame. 
     According to an aspect of the invention, the splitter has an edge bent to guide the cooling flow along the outer surface of the inner tubular body. 
     A simple and effective structure of the splitter is thus provided. More than one splitter may be provided, in accordance with the needs. 
     According to an aspect of the invention, the transition duct according comprises spacers between the first frame and the second frame, the spacers being set across the inlet of the convective cooling channel. 
     The spacers allow to accurately define the relative position of the first frame and of the second frame and, accordingly, also the geometry of the inlet of the cooling channel. Since the spacers are set across the cooling channel, they also benefit from the cooling action and are protected from overheating. 
     According to an aspect of the invention, the second frame is secured to the first frame by fasteners that extend through respective spacers. 
     In this manner, also fasteners are effectively cooled. 
     According to another aspect of the invention, there is provided a gas turbine assembly comprising: 
     a compressor section, extending along a main axis; 
     a plurality of can combustors circumferentially arranged about the main axis; 
     a turbine section; 
     at least one transition duct as above defined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings, which show some non-limitative embodiment thereof, in which: 
         FIG. 1  is a side view, cut along an axial, longitudinal plane, of a gas turbine assembly; 
         FIG. 2  is a perspective view of a transition duct in accordance with an embodiment of the present invention; 
         FIG. 3  is a perspective view of a detail of the transition duct of  FIG. 2  on an enlarged scale; 
         FIG. 4  is a side view, cut along an axial, longitudinal plane, of the transition duct of  FIG. 2 ; 
         FIG. 5  is a front sectional view of the transition duct of  FIG. 2 , taken along the plane V-V of  FIG. 4 ; and 
         FIG. 6  is a side view, cut along an axial, longitudinal plane, of a transition duct according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a simplified view of a gas turbine assembly, designated as whole with numeral  1 . The gas turbine assembly  1  comprises a compressor section  2 , a combustor assembly  3  and a turbine section  5 . The compressor section  2  and the turbine section  5  extend along a main axis A. The combustor assembly  3  may be a single-stage combustor assembly or a sequential combustor assembly. In one embodiment, the combustor assembly  3  comprises a plurality of single-stage can combustors  7 , circumferentially arranged about the main axis A. 
     The compressor section  3  of the gas turbine assembly  1  provides a compressed airflow, which is added with fuel and burned in the can combustors  7 . The airflow delivered by the compressor section  2  is supplied to the combustor assembly  3  and to the turbine section  5  for the purpose of cooling. 
     In the combustor assembly  3 , the can combustors  7  are coupled to the turbine section  5  through respective transition ducts  8 . 
     As illustrated in  FIGS. 2-5 , a transition duct  8  according to one embodiment of the present invention comprises an inner tubular body  10  and an outer tubular body  11 . 
     The inner tubular body  10  defines a transition channel that convey a hot gas flow from the respective can combustor  7  (upstream) to the turbine section  5  (downstream). More precisely, the inner tubular body  10  has an upstream end  10   a  coupled to the respective can combustor  7  and a downstream end  10   b  coupled to the turbine section  5  of the gas turbine assembly  1 . 
     The outer tubular body  11  is arranged around the inner tubular body  10  and has an upstream end  11   a  in the vicinity of the upstream end  10   a  of the inner tubular body  10  and a second downstream end  11   b  in the vicinity the downstream end  10   b  of the inner tubular body  10 . The outer tubular body  11  surrounds the inner tubular body  10  at a distance therefrom. 
     In one embodiment, the outer tubular body  11  is defined by a continuous wall that extends from the upstream end  11   a  to the downstream end  11   b  and passage of air across the continuous wall is prevented. 
     A convective cooling channel  13  thus is defined between the inner tubular body  10  and the outer tubular body  11 . The convective cooling channel  13  has an inlet  13   a  between the downstream end  10   b  of the inner tubular  10  and the downstream end  11   b  of the outer tubular body  11 , while an outlet is defined at the opposite upstream ends  10   a ,  11   a . The convective cooling channel  13  extends all around the inner tubular body  10  and is configured to convey a cooling flow along an outer surface  10   c  of the inner tubular body from the downstream ends  10   b ,  11   b  towards the first upstream ends  10   a ,  11   a  of the inner tubular body  10  and outer tubular body  11  (see dashed arrow CF in  FIG. 1 , that shows, in a simplified manner, the cooling flow delivered by the compressor section  2  of the gas turbine assembly  1 ). 
     The inner tubular body  10  has a first frame  15  at its downstream end  10   b  for mechanical coupling to the turbine section  5 . The first frame  15  is substantially perpendicular to the downstream end  10   b  of the inner tubular body  10  and is provided with lugs  17  for receiving fasteners  18 , such as, but not limited to, nuts and bolts (see  FIG. 4 ). In one embodiment, the first frame  15  is a separate component fitted to the downstream end  10   b  of the inner tubular body  10 , with the interposition of a seal  20 . The first frame  15  is set against a rib  21  that extends around the outer surface  10   c  of the inner tubular body  10 . The rib  21  has a flat face against the first frame  15  and curved concave face, opposite to the flat face (see  FIG. 4 ). 
     The outer tubular body  11  has a second frame  22  at its downstream end  11   b . More precisely, the second frame  22  is integral to the outer tubular body  11  and is arranged parallel to and at a distance from the first frame  15 . In one embodiment, spacers  23  are set between the first frame and the second frame  22  to accurately define the distance therebetween. The spacers  23  may be integral with the inner tubular body  10 . The second frame  22  is secured to the first frame  15  by the fasteners  18  that extend through respective spacers  23  and engage respective lugs  17  of the first frame  15 . 
     The inlet  13   a  of the convective cooling channel  13  is defined between the first frame  15  and the second frame  22 . Therefore, the spacers  23  and the fasteners  18  are set across the inlet  13   a  of the convective cooling channel  13  and are immersed in the cooling flow CF entering the cooling channel  13 . 
     As illustrated in  FIG. 4 , the inlet  13   a  is defined all around the transition duct  8 , so that a sufficient cooling flow CF be supplied to the convective cooling channel  13  from all sides. No cooling flow supply is required through the continuous wall defining the outer tubular body  11 , in particular from sides facing adjacent transition ducts  8 . Accordingly, adjacent transition ducts  8  may be arranged at a very close spacing from one another. 
     The inlet  13   a  of the cooling channel  13  defines, for the cooling flow CF, an inlet flow direction transverse to the outer surface  10   c  and is configured to steer the cooling flow CF along the outer surface  10   c  of the inner tubular body  10 . Smooth steering of the cooling flow CF is favoured by the curved concave face of the rib  21 , that delimits a portion of the inlet  13   a  of the cooling channel  13  and forms a turn without corners. 
     In one embodiment, the transition duct  1  comprises a splitter  27  in the inlet  13   a  of the convective cooling channel  13 . The splitter  27  may be in the form of a third frame, arranged between the first frame  15  and the second frame  22  and has an inner edge  27   a  bent to steer the cooling flow CF along the outer surface  10   c  of the inner tubular body  10  from the inlet direction. 
     In some embodiments, not shown, the transition duct may comprise two or more splitters in the inlet of the cooling channel. 
     The splitter however may not be present, as in the embodiment of  FIG. 6 . In this case, a transition duct  108  comprises an inner tubular body  110  and an outer tubular body  111 , that is arranged around the inner tubular body  110  at a distances therefrom to form a convection cooling channel  113 . An inlet  113   a  of the convection cooling channel  113  is defined at a downstream end of the transition duct  108  between a first frame  115 , fitted to a downstream end of the inner tubular body  110 , and a second frame  122 , integral with the outer tubular body  111  and arranged parallel to and at a distance from the first frame  115 . In this case, the inlet  113   a  of the is free, apart from spacers  123  and fasteners  118  which set and secure in their respective positions the first frame  115  and the second frame  122 . 
     Finally, it is evident that the described transition duct may be subject to modifications and variations, without departing from the scope of the present invention, as defined in the appended claims.