GB2087066A - Transition duct for combustion turbine - Google Patents

Abstract

The duct (22) extends between the combustion chamber and the turbine and its critical mouth portion (42) is provided with an outer tubular shell member (50) and an inner skin member having coolant channels (56) arranged to face the shell member so as to direct coolant generally longitudinally of the duct portion. The shell member has entry means (57, 52) for directing compressor discharge air from the duct exterior to and through the skin coolant channels (56). The coolant is discharged from the channels (56) to the duct gas flow from at least one location longitudinally spaced from the entry means. The entry means comprises at least one substantially transverse coolant channel (52) disposed about said tubular shell member and a plurality of openings (57) extending through the shell wall to the duct exterior. In a modification the inner skin may be of porous material. <IMAGE>

Description

SPECIFICATION Transition duct for combustion turbine The present invention relates to large combustion turbines for industrial process and electric power generation usage and more particularly to transition ducts employed therein to direct the hot driving gases from the turbine combustors to the turbine blades.

As distinguished from aircraft engines in which the combustors are compactly arranged about the engine axis, the large plant combustion turbine structural design requires that combustors be peripherally arranged about the turbine longitudinal axis at a compratively outward radial location. Typically, each combustor is coupled to a transition duct which is structured at its inlet end with a circular cross-section like that of the combustor and its downstream outlet end with a cross-section corresponding to a section of the annular space through which the turbine blades rotate. Further, the transition duct normally extends radially inwardly along its length so that the combustor gases are directed from the radially outward combustor location to the more inward radial location of the downstream turbine blades.

Even with the use of improved stainless steel or nickel based alloys such as Hastalloy or Inco 617 (Registered Trade Marks), it has become increasingly difficult to provide adequate cooling of transition ducts to assure long duct life and, to some extent, long blade and vane life. Thus, gas operating temperatures have been increasing through the years to obtain higher power generation more efficiently. At present, the temperature of operating gases is typically 23000F while the temperature operating limit for special metals used for transition ducts is about 15000 F. It is expected that turbine gas operating temperatures will be increased even further in the future.

Cooling of transition ducts is presently provided by compressor discharge air which is circulated about the outer duct surface prior to intake into the combustor baskets where it supports combustion of the liquid or gaseous fuel. Such external duct cooling has generally been adequate.

However, it is becoming less so with increasing turbine gas operating temperatures for the reason already indicated.

Moreover, external duct cooling has in certain respects been inadequate in the past as well.

Thus, even at current and lower turbine gas operating temperatures, some transition ducts have failed in operation because of thermal stress fatique especially at bends. Duct bends and similar structural features would have even more reduced creep life with increased operating termperatures.

Further, failures have occured in the duct portion having an annulus section at the duct downstream end since it is not as structurally effective as the duct portion having the circular cross-section at the duct upstream end at withstanding thermal stesses and pressure differentials from the inside of the duct to the outside of the duct.

To provide additional duct wall cooling, coolant air can be introduced along the inside surfaces of the duct walls to provide internal boundary layer cooling. However, this approach distorts the temperature profile across the gas stream which causes a shortening of turbine blade and vane life.

Further, such use of coolant air reduces the machine efficiency.

It is an object of this invention to provide an improved transition duct for a combustion turbine with a view to overcoming the deficiencies of the prior art.

The invention resides in a transition duct for a combustion turbine comprising an elongated outer tubular wall member arranged to be coupled with a combusion chamber at its upstream end and to supported at its downstream end to direct the hot driving gases through the annular turbine blade space, characterized in that said tubular wall has at least a portion thereof structured for internal wall cooling including an outer tubular shell member, a skin member having a continuous innner tubular surface and an outer surface secured peripherally about and against an inward facing tubular surface of said shell member, first coolant channel means arranged peripherally about and in one of said shell member and skin member to face the other member and to direct coolant generally longitudinally of said duct portion between said shell and skin members, said shell member having entry means for directing compressor discharge coolant air from the duct exterior to and through said first coolant channel means, and means for discharging the coolant from said first coolant channel means to the duct internal gas flow from at least one location longitudinally spaced from said entry means.

The invention will become readily apparent from the following description of exemplary embodiments thereof when taken in conjunction with the accompanying drawings, in which: Figures 1 shows a longitudinal section of a combustion turbine in which a transition duct is arranged to be cooled in accordance with a preferred embodiment of the invention; Figure 2 shows an elevational view of the transition duct along a longitudinal section thereof; Figure 3 shows an upstream end view of an entry portion of the duct; Figure 4 shows a downstream end view of the discharge or mouth portion of the duct; Figure 5 shows an enlarged view of area C of the downstream duct mouth portion shown in Figure 4; Figures 6 and 7 show enlarged views of respective areas A and B of the longitudinal duct section of Figure 2;; Figure 8 shows a portion of the duct crosssection along section line B-B of Figure 6; Figure 9 graphically illustrates the metal temperature performance of a duct constructed in accordance with the invention; Figure 10 shows an enlarged cross-section through a downstream portion of another duct embodiment shown in Figure 12; Figure 11 shows section Xl-Xl of Figure 10; Figure 12 shows another embodiment comprising a duct employing transpirating skin structure to provide enhanced cooling; Figure 13 shows an enlarged view of area D in Figure 12; Figure 14 shows an enlarged view of area Erin Figure 12; and Figure 15 shows a top view of the enlarged duct area of Figure 14.

More particularly, there is shown in Figure 1 a large combustion turbine 10 having a plurablity of generally cylindrical combustors 12. Fuel is supplied to the combustors through a nozzle structure 14 and air is supplied to the combustors 12 by a compressor 1 6 through air flow space 18 within a combustion casing 20.

Hot gaseous products of combustion are directed from each combustor 12 through an upstream end of a transition duct 22.to a downstream duct end where it is discharged into the annular space through which turbine blades rotate under the driving force of the expanding gases. In this case, three stages 24, 26, and 28 of turbine blades are provided along with corresponding stationary vanes 30,32 and 34.

Compressor discharge air provides external cooling for the tublar transition duct wall as it flows therearound as indicated by the illustrated flow paths in Figure 1.

To provide high turbine operating efficiency and extend transition duct and blade and vane operating life, a transition duct is structured with enhanced wall cooling in accordance with the principles of the invention. The preferred structure for the transition duct 22 is shown in Figures 2-8.

The duct 22 (Figure 2) is provided with an entry end portion 36 having a thin wall tubular structure of circular cross-section (Figure 3). A second portion 38 of the duct 22 is welded to the exit portion 36 and is provided with a thin tubular wall which is formed with a circular cross-section at its upper end. The duct portions 36 and 38 are generally inclined and shaped so that the duct channel extends somewhat radially inwardly along the turbine length.

A third or mouth duct portion 42 is provided with an outer tubular wall which is welded at its upstream end 43 to the exit end on the second duct portion 38. The tubular wall of the second and third duct portions 38 and 42 is graduated in shape along its length from its upstream circular cross-section to a cross-section at its downstream end 45 corresponding to a section of an annulus.

The mouth portion 42 is curved as indicated by the reference character 48 to direct the hot gases generally in the longitudinal direction across the annular space through which the turbine blades rotate.

Heat from the hot transition duct wall metal is transferred to circulating coolant compressor air from the outer duct wall surface. In addition, compressor collant air is directed into a coolant channel network within the duct wall structure to provide efficient and enhanced wall metal cooling which leads to extended duct, blade and vane life.

It is preferred that at least the duct mouth portion 42 be provided with structure which provides internal wall cooling. If desired, other duct portions can be provided with similar coolant provisions as warranted by turbine operating conditions.

Further, ducts provided with cooling features herein described can be provided in newly manufactured combustion turbines as well as retrofitted to turbines now operating in the field.

As shown in Figures 7 and 8, the mouth duct portion 42 is provided with an outer shell 50 having a support bracket 59 for attachment to the casing structure (Figure 1). An inwardly facing cooling cross-channel 52 is provided about the duct mouth inner periphery at a point approximately midway along the length of the duct mouth portion 42. A plurality of coollant entry holes 57 are provided through the shell 50, to pass coolant air from the combustion casing inner space to the cross-channel 52. The metal used for the duct shell 50, skin 54 and other duct wall structure can for example be either of the alloys previously noted herein.

An inner grooved metallic skin 54 is attached, as by welding, to the inner surface of the shell 50.

The outer surface of the skin 54 is provided with a plurality of outwardly facing collant channels 56 which preferably extend parallel to each other in the longitudinal direction to the upstream and downstream ends of the mouth direct portion. The channels 56 can for example be 03tut wide by .03" deep and spaced from each other by .03 inches.

All of the skin channels 56 are in communication with the transverse shell channel 54 to distribute coolant air, coming through the shell holes 57 from outside the duct, both upstream and downstream to the entry and discharge ends of the mouth portion 42.

Accordingly, coolant air is substantially distributed about the entire direct wall periphery as it flows within the duct wall in the upstream and downstream directions.

As shown in Figure 4, coolant air is directiy discharged from open downstream ends of the longitudinally duct skin coolant channels 65 inter the hot gas flow. At the upstream end of the duct channels, an annular channel space 59 is provided between the end of the wall of the duct portion 38 and the upstream ends of the longitudinal duct skin coolant channels 56 to provide for entry of the coolant into the downstream duct flow.

Calculated results achieved by use of the invention in cooling a typical ten inch length of duct mouth are shown in Figure 9.

Since the inner tubular skin surface is continuous and free of coolant outlet to the duct interior, it is readily adapted to accept a ceramic or other thermal barrier coating. Further, undesirable temperature profiles across the duct flow are avoided to avoid life shortening effects in the turbine blades and vanes. Further, needed cooling action is achieved with comparatively less coolant flow thereby minimizing effects of turbine efficiency.

An alternate system for cooling the duct wall is to employ a transpirating material rather than the solid homogenous skin 54 shown in Figures 6 and 8 with transpirating material 60 shown in Figures 10-1 5. Transpirating material such as "Lamilloy" manufactured by the DDA Division of General Motors Corporation or similar laminates like Rolls Royce's "Transply" may be used.

In this cooling arrangement, downward facing grooves 61 are machined into the inner surface of shell 62. The sides 64 of the groove are canted to minimize the contact surface 65 with the laminated skin 60.

Air management is similar to that previously described except the channels serve as a distribution system delivering coolant air to the porous top surface of the laminate. The cooling air does no exit at the ends of coolant channels 63, but flows instead through the porous skin 60. The longitudinal grooves 61 terminate at the edge of the skin 60, see Figures 13 and 14.

Claims (6)

1. A transition duct for a combustion turbine comprising an elongated outer tubular wall member arranged to be coupled with a combustion chamber at its upstream end and to be supported at its downstream end to direct the hot driving gases through the annular turbine blade space, characterized in that said tubular wall has at least a portion thereof structured for internal wall cooling including an outer tubular shell member, a skin member having a continuous inner tubular surface and an outer surface secured peripherally about and against an inward facing tubular surface of said shell member, first coolant channel means arranged peripherally about and in one of said shell member and skin member to face the other member and to direct coolant generally longitudinally of said duct portion between said shell and skin members, said shell member having entry means for directing compressor discharge coolant air from the duct exterior to and through said first coolant channel means, and means for discharging the coolant from said first coolant channel means to the duct internal gas flow from at least one location longitudinally spaced from said entry means.

2. A transition duct as set forth in claim 1 characterized in that said entry means comprises at least one second coolant channel means disposed peripherally about said tubular shell member and substantially traversely to the duct portion to face inwardly toward said tubular skin member, and a plurality of openings extending from said second collant channel means through said shell wall to the duct exterior and disposed peripherally about said shell member.

3. A transition duct as set forth in claim 1 characterized in that said first coolant channel means comprises a plurality of substantially parallel channels extending longitudinally between the upstream and downstream ends of said internally cooled turbine wall portion, said discharge means comprising openings from said channels through the downstream end of said skin member.

4. A transition duct as set forth in claim 3 characterized in that said entry means comprises at least one second coolant channel means disposed peripherally about said tubular shell member and substantially transversely to the duct portion to face inwardly toward said tubular skin member, and a plurality of openings extending form said second coolant channel means through said shell wall to the duct exterior and disposed peripherally about said shell member; said second coolant channel means is disposed substantially midway between the upstream and downstream ends of the substantially parallel channels of said first coolant channel means, said discharge means further including openings from said substantially- .

parallel channels through the upstream end of said skin member; and third channel means facing the duct interior and extending peripherally about said shell member at its upstream end to direct coolant air from the upstream ends of said skin channels into the duct gas flow.

5. A transition duct as set forth in claim 1 characterized in that said internally cooled tubular wall portion is a downstream end portion of said duct.

6. A transition duct as set forth in claim 1 characterized in that said first coolant channel means is formed in said shell member and said skin member is made of transpirating material.