Abstract:
A turbine engine exhaust diffuser assembly having a tuneable exhaust velocity profile is disclosed. The assembly includes structure that generates a desired, tunable velocity profile within working fluid exiting the exhaust diffuser assembly. The exhaust assembly includes an concentric boundary sleeves that extend along a central axis and form an exhaust flowpath between them. A flow deflecting member disposed within the flowpath longitudinally divides the flowpath into a first region and a second region. The first region is characterized by a first predetermined velocity profile, and the second region is characterized by a second predetermined velocity profile. The second velocity profile is determined by at least one pre-selected dimension of the flow deflecting member. Methods of maximizing HRSG energy extraction capacity and extending HRSG lifespan are also disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of internal to combustion engines and, more particularly, to an exhaust diffuser having tunable velocity profile properties. 
     BACKGROUND OF THE INVENTION 
     Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to the energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor shaft. The rotor shaft may, in turn, be linked to devices such as an electric generator to produce electricity. 
     To increase efficiency, engines are typically operated near the operational limits of the engine components. For example, to maximize the amount of energy available for conversion into electricity, the products of combustion (also referred to as the working gas or working fluid) often exit the combustion section at high temperature. This elevated temperature generates a large amount of potential energy, but it also places a great deal of stress on the downstream fluid guide components, such as the blades and vanes of the turbine section. 
     In many gas turbine applications, working fluid remains at high temperature even after passing through the turbine section of the engine. In these cases, additional energy may be extracted from the working fluid even after it exits the turbine section. Heat recovery steam generators (HRSGs), for example, may be used to harness energy remaining in the turbine section exhaust. Such HRSGs are typically located downstream of the engine combustion and turbine sections and use heat from the working fluid leaving the turbine section to boil water, or other suitable liquid, to produce steam. The steam is then directed to an associated steam turbine, where additional energy is recovered. In this way, return on fuel consumption increases and operational efficiency is desirably raised. 
     While HRSGs may be used in some situations to recover extra energy from turbine section exhaust, there are difficulties associated with this arrangement. For example, energy within the working fluid may not be removed efficiently in some engine arrangements. In other settings, HRSG components fail prematurely due to exposure to working fluid having localized unacceptably-high temperatures. Accordingly, there remains a need in this field to improve HRSG performance. 
     SUMMARY OF THE INVENTION 
     The present invention is an exhaust diffuser assembly that improves HRSG performance. The assembly includes structure that generates a desired, tunable velocity profile within working fluid exiting the exhaust diffuser assembly. The exhaust assembly includes an outer boundary member that extends along a central axis and an inner boundary member located radially-inward of the outer boundary; the inner boundary member also extends along the central axis. The boundary members, or sleeves, form an exhaust flowpath between them, and a flow deflecting member disposed within the flowpath longitudinally divides the flowpath into a first region and a second region. The first region is characterized by a first predetermined velocity profile, and the second region is characterized by a second predetermined velocity profile. The second velocity profile is determined by at least one pre-selected dimension of the flow deflecting member. Advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a side view of an combined cycle engine using the exhaust diffuser assembly of the present invention; 
         FIG. 2  is a close-up view of the exhaust diffuser assembly shown in  FIG. 1 ; 
         FIG. 3  is cross-section view of the exhaust diffuser assembly shown in  FIG. 2 , taken along cutting plane  3 - 3 ′ therein 
         FIG. 4  is a close-up, partial view of the exhaust diffuser assembly shown in  FIG. 2 , showing particular detail of the flow deflecting member; 
         FIG. 5  is a close-up view of the exhaust diffuser assembly shown in  FIG. 1 , showing a first velocity flow profile; and 
         FIG. 6  is a close-up view of the exhaust diffuser assembly shown in  FIG. 1 , showing an additional velocity flow profile. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made to the Figures generally, in which an exhaust diffuser assembly  10  according to the present invention is shown. The assembly  10  includes elements that allow the radial distribution or velocity profile of fluid  18  leaving the assembly to be strategically tuned without negatively impacting the operational performance of the engine turbine section  14 . According to one aspect of the invention, the exhaust diffuser assembly  10  increases combined cycle HRSG  12  effectiveness. According to another aspect of the invention, the diffuser assembly  10  maximizes combined cycle engine HRSG  12  life. 
     By way of overview, and with particular reference to  FIGS. 1 and 2 , the exhaust diffuser assembly  10  of the present invention will be described in the context of a combined cycle application, in which high-temperature working fluid  18  exiting the turbine section  14  of an industrial gas turbine engine  16  is directed through the present exhaust diffuser assembly  10  to a downstream heat recovery steam generator  12 , where additional heat remaining in the working fluid  18  is extracted and converted into steam. The exhaust diffuser assembly  10  includes a longitudinally-extending outer boundary sleeve  20  radially spaced apart from a longitudinally-extending inner boundary sleeve  22 . The sleeves  20 , 22  share a common axis  24  and cooperatively define an exhaust flowpath  26  that fluidly links the turbine section  14  with the HRSG  12 . An angled flow deflecting member  36  divides the flowpath  26  longitudinally into a first region  32  and a second region  34 , with the deflector member producing a strategically selected velocity profile  40  within the second region. As will be described more fully below, the nature of the second velocity profile  40  is determined by properties of the flow deflecting member  36  strategically selected to establish a desired radial distribution of heat, thereby providing customized interaction between the working fluid and the HRSG  12 . The exhaust diffuser assembly  10  of the present invention will now be described in detail. 
     As noted above, and with continued reference to  FIGS. 1 and 2 , a first embodiment of the exhaust diffuser assembly  10  of the present invention is especially suited for use in a combined cycle industrial gas turbine engine  16 . It is noted, however, that the exhaust diffuser assembly  10  may be advantageously employed in other applications, such as a simple cycle gas turbine with a need for a tuned exhaust profile and need not be used in combination with a HRSG  12 . For purposes of clarity, however, the present discussion will be directed to the combined cycle setting described above. 
     As noted above, the exhaust diffuser assembly  10  includes two boundary sleeves  20 , 22  that form a longitudinally-extending flowpath  26  between the turbine  14  and HRSG  12 . The boundary sleeves  20 , 22  are elongated and extend from a first end  28  of the assembly  10  to a downstream second end  30 . Radial spacing between the outer and inner sleeves  20 , 22  varies with longitudinal position along the flowpath  26 , and the spacing establishes at least two flow conditioning zones  42 ,  44  within the flowpath  26 . 
     As illustrated in  FIGS. 5 and 6 , the first conditioning zone  42  allows working fluid  18  to diverge after exiting the turbine  14  and introduces a gradual reduction in dynamic pressure. The second conditioning zone  44  promotes additional, more-rapid divergence, further lowering the overall velocity of the working fluid  18  and lengthening the residence time of the fluid within the HRSG  12 . With this arrangement, the conditioning zones  42 , 44  cooperatively ensure that working fluid  18  travels through the turbine  14  and into the exhaust diffuser assembly  10  in a free-flowing manner, at a velocity that promotes energy-transferring interaction between the working fluid and the HRSG  12 . 
     With respect to  FIG. 2 , the first conditioning zone  42  is bounded by a first portion  48  of the outer boundary sleeve  20  characterized by an increasing radius and an inner boundary sleeve first portion  50  characterized by a substantially-constant radius. It is noted, however, that the outer boundary sleeve  20  need not be characterized by a diverging section followed by non-diverging and diverging sections, as shown in  FIG. 2 ; the sleeve could also be characterized simply by an extend section of diverging radius (not shown), or a relatively short diverging section (not shown) and then a flow-wise-downstream continuous region of non-divergence. The contours of the outer boundary sleeve  20  are chosen in accordance with desired flow properties requirements. The second conditioning zone  44 , located immediately downstream of the first conditioning zone  42 , is bounded by an outer boundary sleeve second portion  52  characterized by a substantially-constant radius and an inner boundary sleeve second portion  54  characterized by a decreasing radius. The relative boundary sleeve spacing need not be produced in the manner described above; other arrangements that produce the divergence described in each conditioning zone  42 , 44  above would also suffice and are contemplated by this invention. 
     As seen with reference to  FIG. 4 , working fluid  18  leaving the engine turbine section  14  is characterized by a first velocity profile  38  upon entering the flowpath first region  32  and a second velocity profile  40  upon leaving the second region  34 . The second velocity profile  40  is generated by the flow deflecting member  36  as the working fluid  18  flows through the flowpath first region  32  and into the flowpath second region  34 . With continued reference to  FIG. 2 , and with additional reference to  FIGS. 3 and 4 , the flow deflecting member  36  encircles the inner boundary sleeve  22  and resembles a tapered ring that includes an upstream-facing guide surface  56 . The guide surface  56  is oriented to direct working fluid  18  into a desired course within the flowpath second region  34 . As seen with particular reference to  FIG. 4 , the guide surface  56  is substantially linear and is characterized by three defining dimensions: a surface length L gs , a deflection angle α, and an outer radius r ogs . The surface length L gs  is the distance from the distal end of the guide surface  56  to the inner boundary sleeve  22 ; the deflection angle α is measured between the central axis  24  and the guide surface  56 ; and the outer radius r ogs  is the distance between the central axis  24  and the radially-outward end of the guide surface. It has been determined that the flow deflecting member  36  functions best when the guide surface  56  deflection angle α lies below about forty-five degrees. Additionally, to allow appropriate working fluid  18  divergence within the flowpath first conditioning zone  42 , the flow deflecting member  36  is axially positioned within the second conditioning zone  44 , and more particularly, along the inner boundary sleeve second portion  54 . Other axial locations may be used if fluid divergence is not critical. 
     With a linear guide surface embodiment, as shown in  FIGS. 4 and 5 , the guide surface  56  defining dimensions L gs , α, r ogs  are interrelated and cooperatively determine the flowpath second profile  40  produced during operation. The flow deflecting member  36  may, as shown in the embodiment of  FIG. 5 , be configured to deliver heat to the HRSG  12  in a radially-uniform manner, ensuring maximum contact between the working fluid  18  and the heat extracting surfaces (not shown) of the HRSG, while reducing concentrations of unacceptably-high temperatures within the working fluid. This uniform heat distribution not only increases HRSG effectiveness but also improves HRSG life. 
     With continued reference to  FIG. 4 , this radial distribution is generated when the guide surface outer radius r ogs  is no larger than the outer radius r oib  of the inner boundary sleeve  22  in the inner boundary first portion  50 . As noted above, the deflection angle α provides optimal performance when below about forty-five degrees, and the defining dimensions are interrelated; therefore, once outer radius r ogs  is selected, the appropriate surface length L gs  is a matter of geometry, with the appropriate length generating a guide surface  56  having a desired outer radius r iob  and a deflection angle α of about forty-five degrees. 
     Other flow profiles may be produced. For example, it may be desired in some cases to direct a higher concentration of heat toward the outer boundary sleeve. In instances where an associated HRSG  12 ′ has been exposed to center-peaked, or “hub-strong” concentrations of heat for extended periods of time, it may be desirable to direct heat away from previously-damaged, radially-inward HRSG regions. As will be discussed further below, it is also possible to ensure a uniformly-distributed velocity profiles and outwardly-biased velocity profiles, as well. In engines  16  that do not have HRSGs  12 , it would still be advantageous to not have hot spots in the center portion of the exhaust ducting down stream of the diffuser section of the engine, for example in settings where plant ducting (not shown) makes a turn and centrally-located hot spots might damage the associated turning elbow (not shown). In such instances, the outer radius r ogs  is, as shown in  FIG. 6 , selected to be larger than the outer radius r iob  of the inner boundary sleeve  22 . It is noted that the outer radius r ogs  need not be larger than the radius r iob  of the inner boundary sleeve  22  in order to deflect the flow outward. The appropriate surface length L gs  is again a matter of geometry, with the appropriate length generating a guide surface  56 ′ having a desired outer radius r iob  and a deflection angle α of about forty-five degrees. This profile ′, while not necessarily maximizing energy extraction, will extend the useable life of a heat-stressed HRSG  12 ′ by diverting heat away from damaged regions. 
     It is noted that while the guide surface  56  has been shown and described as being substantially linear, other orientations, such as concave and/or convex contours or a combination of straight and convex/concave contours would also suffice. The guide surface could also, as shown in the Figures have a ramped section, followed by another section(s). It is noted, for example, that the guide surface could angle away from the centerline (as shown) and then have a straight section (not shown), and then angle back (not shown) toward the centerline. It is also noted that the guide surface need not intersect the inner boundary sleeve. 
     During operation, fuel (not shown) and air (not shown) combust in an engine  16  combustor section  15  to produce hot working fluid  18  that travels into a turbine section  14 , where energy is extracted. As shown in  FIG. 5 , the working fluid  18  then exits the turbine section  14  and travels through the exhaust diffuser assembly  10  to a downstream HRSG  12 . The flow deflecting member  36  in the exhaust diffuser assembly  10  produces a profile  40  within the working fluid  18  that provides strategically-selected/tuned interaction between the HRSG  12  and the working fluid. For example, if r ogs  is &lt;=r oib  then heat is distributed to the HRSG in an evenly-distributed profile, as shown in  FIG. 5 . If, however, r ogs  is &gt;r oib  then resulting profile would have a so-called “rim-strong or rim-centered” distribution, as shown in  FIG. 6 , tending to deliver more heat toward the outer boundary sleeve  20  and radially-outward regions of the flowpath  26 . 
     With the arrangement described above, the exhaust diffuser assembly  10  of the present invention advantageously allows the second flowpath region  34  velocity profile distribution to be tuned. Accordingly, with the exhaust diffuser assembly  10  of the present invention, heat may be delivered to the HRSG  12  in a variety of configurations, each of which help maximize HRSG life and effectiveness under diverse operational conditions. 
     According to one aspect of the invention, heat may be delivered in an evenly-distributed manner thereby enhancing post-turbine-section energy extraction. This approach helps recover working fluid  18  energy that would otherwise be lost in settings, for example, with turbine final stage components that generate center-peaked/hub-strong velocity profiles. Hub-strong velocity profiles often concentrate hot working fluid  18  in one radial area locally overloading the heat exchanger and decreasing the overall efficiency of the HRSG, and the present invention  10  can redistribute the working fluid to ensure maximum heat-extracting interaction. 
     According to another aspect of the invention, the exhaust diffuser assembly  10  allows turbine sections  14  designs that are largely HRSG-independent. That is, the present invention  10  compensates for turbine sections  14  that maximize energy extraction while producing HRSG-damaging velocity profiles. 
     According to another aspect of the invention, the exhaust diffuser assembly  10  also helps maximize HRSG  12  operational life by allowing concentrations of unacceptably-high temperatures within exhaust streams  26  to be reduced or eliminated before reaching the HRSG  12 , if needed. This beneficially allows operation at conditions, such as many low-power running conditions, which may produce center-peaked exhaust velocity profiles, allowing operation at these conditions for extended periods of time without sacrificing combined cycle efficiency or HRSG performance. 
     It is to be understood that while certain forms of the invention have been illustrated and described, it is not to be limited to the specific forms or arrangement of parts herein described and shown. It will be apparent to those skilled in the art that various, including modifications, rearrangements and substitutions, may be made without departing from the scope of this invention and the invention is not to be considered limited to what is shown in the drawings and described in the specification. The scope if the invention is defined by the claims appended hereto.