Patent Publication Number: US-2019170010-A1

Title: Methods, systems and apparatus relating to turbine engine exhaust diffusers

Description:
BACKGROUND OF THE INVENTION 
     This present application relates generally to turbine engines. More specifically, but not by way of limitation, the present application relates to exhaust diffusers for such turbine engines. 
     In turbine engines, an exhaust flow exiting the turbine or turbine section via the last row of turbine blades is typically directed through a diffuser, which, in general, is an outwardly flared flow passage of increasing cross-sectional area. Within a steam turbine engine, for example, the purpose of a diffuser is to lower the pressure of the steam exhaust at the exit of the turbine section so to, thereby, increase the amount of energy available to the last stage of rotor blades. Specifically, as a result of the increasing cross-sectional area through the diffuser, diffusion or deceleration occurs as the exhaust steam passes therethrough. This deceleration causes a decrease in the kinetic energy of the steam and an increase in pressure, with the desired net effect being that, at the inlet or upstream end of the diffuser, the exhaust steam has the lowest pressure within the diffuser flowpath, for example, between the turbine section and a condenser. With this achieved, the steam exhaust enters the diffuser a minimum pressure occurring just downstream of the last row of rotor blades and, thus, maximizes the velocity of steam flowing through those rotor blades and maximized the energy available to the turbine engine to do work. 
     From this minimum pressure, it is desirable for the diffuser to produce a steep rise in pressure or pressure recovery. However, the amount of diffusion a diffuser can produce is limited by the pressure gradient along the length of the diffuser, which is generally defined as the ratio of the pressure rise to diffuser length. Such pressure rise thus depends on the exit-to-inlet flow area ratio of the diffuser. As will be appreciated, if the pressure gradient becomes too large—for example, the walls of the diffuser diverge too steeply—the steam flow separates from the walls of the diffuser and the rate of diffusion can be seriously reduced, which negatively impacts performance. There is, therefore, a continuing need for improved methods and configuration related to diffusers and the operation thereof. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present application thus describes a turbine engine having a turbine section operably connected to an exhaust section, through which an exhaust flowpath is defined. The exhaust section includes a diffuser having a diffuser flowpath. The turbine engine further includes: diffuser walls that define and enclose the diffuser flowpath between an inlet and outlet of the diffuser, the diffuser walls comprising at least one stationary diffuser wall and at least one adjustable diffuser wall; and an actuator connected to the at least one adjustable diffuser wall. The actuator is configured to move the at least one adjustable diffuser wall in relation to the at least one stationary diffuser wall along an axis of movement that causes a modification to a cross-sectional flow area through the diffuser flowpath. 
     These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of this invention will be more completely understood and appreciated by careful study of the following more detailed description of exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional side view of the downstream or aft end of a turbine engine and an exemplary conventional diffuser; 
         FIG. 2  is a simplified cross-sectional view of a diffuser within which aspects of the present application may be practiced; 
         FIG. 3  is a simplified cross-sectional view of an exemplary diffuser in accordance with aspects of the present invention; 
         FIG. 4  is a simplified cross-sectional view of an exemplary diffuser in accordance with aspects of the present invention; 
         FIG. 5  is a simplified cross-sectional view of an exemplary diffuser in accordance with aspects of the present invention; and 
         FIG. 6  illustrates a side-by-side cross-sectional comparison of calculated flow data for low exit velocity conditions between a conventionally operating diffuser and a diffuser of the present invention operating with the downstream axial section of the inboard wall adjusted in the upstream direction, along with a plot of calculated data taken across a range of exit velocities. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the present application are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention. Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical designations to refer to features in the drawings. Like or similar designations in the drawings and description may be used to refer to like or similar parts of embodiments of the invention. As will be appreciated, each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. It is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. It is to be understood that the ranges and limits mentioned herein include all sub-ranges located within the prescribed limits, inclusive of the limits themselves unless otherwise stated. 
     Additionally, certain terms have been selected to describe the present invention and its component subsystems and parts. To the extent possible, these terms have been chosen based on terminology common to the technology field. Still, it will be appreciated that such terms often are subject to differing interpretations. For example, what may be referred to herein as a single component, may be referenced elsewhere as consisting of multiple components, or, what may be referenced herein as including multiple components, may be referred to elsewhere as being a single component. Thus, in understanding the scope of the present invention, attention should not only be paid to the particular terminology used, but also to the accompanying description and context, as well as the structure, configuration, function, and/or usage of the component being referenced and described, including the manner in which the term relates to the several figures, as well as, of course, the usage of the terminology in the appended claims. 
     The following examples are presented in relation to particular types of turbine engines. However, it should be understood that the technology of the present application may be applicable to other categories of turbine engines, without limitation, as would be appreciated by a person of ordinary skill in the relevant technological arts. Accordingly, unless otherwise stated, the usage herein of the term “turbine engine” is intended broadly and without limiting the usage of the claimed invention with different types of turbine engines, including various types of combustion or gas turbine engines as well as steam turbine engines. 
     Given the nature of how turbine engines operate, several terms may prove particularly useful in describing certain aspects of their function. For example, the terms “downstream” and “upstream” are used herein to indicate position within a specified conduit or flowpath relative to the direction of flow or “flow direction” of a fluid moving through it. Thus, the term “downstream” refers to the direction in which a fluid is flowing through the specified conduit, while “upstream” refers to the direction opposite that. These terms should be construed as referring to the flow direction through the conduit given normal or anticipated operation. Given the configuration of turbine engines, particularly the arrangement of the components about a common or central shaft or axis, terms describing position relative to an axis may be used regularly. In this regard, it will be appreciated that the term “radial” refers to movement or position perpendicular to an axis, for example, the center or central axis of the turbine engine. Related to this, it may be required to describe relative distance from the central axis. In such cases, for example, if a first component resides closer to the central axis than a second component, the first component will be described as being either “radially inward” or “inboard” of the second component. If, on the other hand, the first component resides further from the central axis than the second, the first component will be described as being either “radially outward” or “outboard” of the second component. As used herein, the term “axial” refers to movement or position parallel to the central axis, while the term “circumferential” refers to movement or position around the central axis. Unless otherwise stated or made apparent by context, these terms should be construed as relating to the center or central axis of the turbine (also “turbine central axis”) as defined by the central shaft extending therethrough, even when these terms are describing or claiming attributes of non-integral components—such as rotor or stator blades—that function therein. Finally, the term “rotor blade” is a reference to the blades that rotate about the turbine central axis during operation, while the term “stator blade” is a reference to the blades that remain stationary. 
     Referring now to the drawings, for background purposes,  FIG. 1  shows a portion of a turbine section  9  of a turbine engine that is operably connected to an exhaust section  10 . The exhaust section  10  may include a diffuser  11  having a conventional design. The turbine section  9  and exhaust section  10 , for example, may be part of a steam turbine engine. As illustrated, the turbine section  9  may include a shaft  12  that includes rotor wheels on which is mounted a plurality of circumferentially arrayed rotating buckets or rotor blades  16 . The shaft  12  defines what will be referred to as the central axis  14  of the turbine, which is the axis around which the turbine section  9  and diffuser  11  are formed. As shown, multiple axially stacked rows of the rotor blades  16  may be provided within the turbine section  9 , with intervening rows of stationary nozzles or stator blades  18  being placed between them. Thus, the turbine section  9  may include axially stacked rows of blades  16 ,  18 , which are alternated such that, within each stage, a row of stator blades  18  leads a row of rotor blades  16 . 
     The rotor and stator blades  16 ,  18  generally reside within an annular shaped working fluid flowpath  24  that is defined through the turbine section  9 . As will be appreciated, the working fluid flowpath  24  guides a working fluid—such as steam or, in the case of a combustion turbine engine, combustion gases—onto the blades  16 ,  18 . As indicated, the boundaries of the annular working fluid flowpath  24  are defined by concentrically formed inboard and outboard walls  25 ,  26 . In operation, a pressurized working fluid is expanded through the working fluid flowpath  24  of the turbine section  9 . The stator blades  18  direct this flow of working fluid onto the rotor blades  16  so to induce the rotor blades  16  to rotate. This rotation is translated to the shaft  12 , and, in this way, the energy of the flow of working fluid is transformed into the mechanical energy the rotating shaft  12 , which then may be used in a variety of applications, for example, to rotate the coils of a generator to produce electricity. 
     It will be appreciated that, once it is expelled from the working fluid flowpath  24  of the turbine section  9 , the working fluid is directed into the exhaust section  10 . As used herein, once the working fluid exits the turbine section  9  and enters the exhaust section  10 , it may be referred to as exhaust. Steam turbine engines—as well as combustion turbine engines—typically include diffusers  11  within the exhaust section  10 . In general, diffusers  11  are outwardly flared annular passages of increasing cross-sectional flow area. Within a steam turbine engine, for example, the purpose of a diffuser  11  is to lower the pressure of the exhaust at the exit of the turbine section  9  so to increases the amount of energy available to the last stage of rotor blades. More specifically, as a result of the increasing cross-sectional flow area, diffusion or deceleration occurs as the exhaust passes through the diffuser  11 . This deceleration of the exhaust causes a decrease in the kinetic energy and an increase in pressure, with the desired net effect being that, at the inlet or upstream end of the diffuser  11 , the exhaust has the lowest pressure level along the path through the diffuser  11 . This allows the working fluid exiting the last stage of rotor blades  16  to enter the diffuser  11  at a minimum pressure level, thus maximizing the velocity of working fluid flowing through the last stage of rotor blades  16  and increasing the output of the engine. 
     From this minimum pressure level at the inlet of the diffuser  11 , it is desirable for the diffuser  11  to induce a rapid recovery in pressure. However, the amount of diffusion possible within a diffuser  11  is limited by the longitudinal pressure gradient therewithin, which is generally defined as the ratio of the pressure rise to the length of the diffuser  11 , and such pressure rise typically depends on the exit-to-inlet area ratio of the diffuser  11 . As will be seen, if the pressure gradient becomes too large—for example, when the walls of the diffuser diverge too steeply from each other—the exhaust flow will separate from the walls of the diffuser  11 , and this will negatively impact diffuser performance. More specifically, adverse pressure gradients in the diffusing passage can cause boundary layer growth and stall, which generally leads to the exhaust flow separating from the diffuser walls and causing pressure losses that degrade performance. 
     As shown in  FIG. 1 , a diffuser  11  may generally form a diverging annular shaped diffuser flowpath  13  that enlarges in cross-sectional flow area as it extends axially in the downstream direction. The diffuser flowpath  13  of the diffuser  11  generally receives exhaust—which, as will be appreciated, constitutes the working fluid that is exiting the turbine section  9 —via an upstream end or inlet and then delivers that exhaust via a downstream end or outlet to another exhaust path component, which will be referred to herein as a collector  30 . The collector  30  may define a collector flowpath  31 . Thus, the exhaust section  10  may have an exhaust flowpath that includes an upstream section, which is defined by the diffuser flowpath  13  of the diffuser  11 , and a downstream section, which is defined by the collector flowpath  31  of the collector  30 . 
     As shown, the collector flowpath  31  may be configured so to deflect or turn the exhaust approximately 90 degrees. The collector flowpath  31  then directs the exhaust toward a desired downstream outlet  33  where it may be directed to another component. The collector  30  may include a backwall  32  that approximately opposes the outlet of the diffuser  11 . The backwall  32  may be oriented approximately perpendicular to the central axis  14  and, thus, deflect the exhaust exiting the diffuser  11  in a direction that is approximately perpendicular to the central axis  14 . As indicated by the flow arrows of  FIG. 1 , the turning of the exhaust may be initiated by a conical section within the diffuser  11  and then completed within the collector  30 . Thusly, the exhaust is turned from a flow that is generally parallel with the central axis  14  (as is the case as the exhaust enters the diffuser  11 ) to one that is generally perpendicular to the central axis  14  (as is the case once that exhaust is turned within the collector  30 ). The backwall  32 , as shown, of the collector  30  may include a surface that is oriented approximately perpendicular to the central axis  14 . The backwall  32  generally turns the flow of exhaust, as described, while the other boundary walls of the collector  30  direct the exhaust toward the outlet  33 . From the outlet  33  of the collector, the exhaust may be directed to a downstream component, such as, for example, a condenser. 
     The diffuser  11  is composed of several walls that define and enclose the annular shaped flowpath  13  defined through it, which will now be described. As will be seen, these walls generally extend: axially between upstream and downstream ends; and circumferentially 360 degrees about the central axis  14 . One way in which these walls may be differentiated is according to their concentric arrangement, in which an inboard wall  35  is surrounded by an outboard wall  36 . More specifically, as indicated, the diffuser  11  includes an inboard wall  35 , which defines an inboard boundary of the diffuser flowpath  13 , and an outboard wall  36 , which defines an outboard boundary of the diffuser flowpath  13 . 
     In regard to the inboard wall  35  of the diffuser  11 , for purposes herein, it may be described as having two axially stacked and adjacent sections, which will be referred to as: an upstream axial section  41 , which may be oriented approximately parallel to the central axis  14 ; and a conical or downstream axial section  42 , which is angled or canted in relation to the central axis  14 . The upstream axial section  41  resides upstream of the downstream axial section  42  in relation to the flow direction of exhaust through the diffuser  11 . The upstream axial section  41  is generally formed having the shape of a cylinder that wraps about the shaft  12  or central axis  14  of the turbine engine. As shown below in relation to  FIGS. 2 through 5 , the upstream axial section  41  also may have a conical or truncated cone shape. Axially, as shown, the upstream axial section  41  extends between an upstream end, which may be adjacent or connected to the inboard wall  25  of the working fluid flowpath  24 , and a downstream end, which may be adjacent to the downstream axial section  42 . 
     As shown, the conical or downstream axial section  42  of the diffuser  11  may take the shape of a truncated cone. The downstream axial section  42  may be oriented such that the smaller diameter end of the truncated cone shape is positioned upstream relative to the larger diameter end. The downstream axial section  42  may be radially symmetrical about a longitudinal axis defined through the central axis  14 . Axially, the downstream axial section  42  extends between an upstream end, which is adjacent or connected to the upstream axial section  41 , and a downstream end, which may be adjacent or connected to the backwall  32  of the collector  30 . The truncated cone shape may initiate the turning of the exhaust in anticipation of the backwall  32  of the collector  30 . In this way, the turning of the exhaust within the exhaust section  10  is more gradual, which reduces turbulent flow that may otherwise negatively impact performance. Thus, the downstream axial section  42  generally extends 360 degrees circumferentially about the central axis  14 , and enlarges in diameter as it extends axially in the downstream direction, thereby forming its truncated cone shape, which may terminate at or near the backwall  32  of the collector  30 . 
     In the case of the outboard wall  36  of the diffuser  11 , it also may have a conical shape, which extends in the downstream direction between an upstream end  43 , which may be smaller in diameter, and a downstream end  44 , which may be larger in diameter. More specifically, the outboard wall  36  may be shaped as a truncated cone, with the smaller diameter end of the truncated cone shape being positioned upstream relative to the larger diameter end. The truncated cone shape of the outboard wall  36  may be radially symmetrical about a longitudinal axis defined approximately by the central axis  14 . Additionally, the upstream end  43  of the outboard wall  36  may be adjacent or connected to the outboard wall  26  of the working fluid flowpath  24  of turbine section  9 , while the downstream end  44  extends into the collector  30 . It should be understood that the outboard wall  36  extends circumferentially 360 degrees about the central axis  14  and, thereby, forms the truncated cone shape that is oriented so that it enlarges axially between its upstream and downstream ends  43 ,  44 . As indicated, the downstream end  44  may include a curved or flared lip. This flared lip may include an outwardly curving piece, which is configured in this manner for guiding or turning the exhaust into the collector  30 . 
     Turning to  FIG. 2 , an enhanced cross-sectional view of an alternative diffuser  11  is provided, which has been labeled using the same numeral identifiers introduced in  FIG. 1 . The exemplary diffuser  11  of  FIG. 2  will be used as a template in several of the figures that follow to describe certain aspects the present invention. As designated in relation to the expected flow direction of exhaust through the diffuser  11 , dashed lines have been added to illustrate an upstream end of the diffuser  11 , which will be referred to herein as an inlet  53 , and a downstream end of the diffuser  11 , which will be referred to herein as an outlet  55 . As will be appreciated, the inlet  53  of the diffuser  11  is configured to accept the working fluid exiting the turbine section  9 . The turbine section  9  includes a last row of rotor blades  16  positioned just upstream of the inlet  53  of the diffuser  11 . In relation to this last row of rotor blades  16 , the inlet  53  and the upstream portion of the diffuser flowpath  13  contiguous to the inlet  53  form what will be referred to herein as an “exit area”, in that this is the area first encountered by the working fluid exiting the last row of the rotor blades  16 . In regard to the downstream end of the diffuser flowpath  13 , the outlet  55  discharges the exhaust moving through diffuser  11  into the collector  30 . As shown, between the inlet  53  and outlet  55 , the cross-sectional flow area through the diffuser flowpath  13  gradually enlarges. 
     As also shown in  FIGS. 2 through 5 , instead of the cylindrical shape described above, the upstream axial section  41  of the inboard wall  35  may be configured with a conical shape or truncated cone shape, which is oriented to such that the diameter of the truncated cone shape increases in the downstream direction. As shown, the conical shape of the upstream axial section  41  may have a shallower angle relative to the central axis  14  than that of the conically shape of the downstream axial section  42 . It should be appreciated that the present invention may be practiced with the upstream axial section  41  having either a cylindrical or conical shape. 
     With specific reference now to  FIGS. 3 through 5 , in accordance with embodiments of the present invention, a diffuser is described that has an adaptive or adjustable wall, the movement of which is controlled to modify the cross-sectional flow area through the diffuser flowpath. As will be seen, such modification to the diffuser flowpath may include narrowing and/or widening the diffuser outlet. Additionally, the modification of the diffuser flowpath may include narrowing and/or widening the diffuser inlet and the upstream portion of the diffuser, which, as defined above, represents an “exit area” in relation to the last stage of rotor blades. Further, according to preferred embodiments, the narrowing and/or widening may be done in response to the detection of a predefined operating condition, such as a low flow volume conditions or low exit velocity from the last stage of the rotor blades. As will be appreciated, when these conditions occur, the exhaust within the diffuser tends to separate from the diffuser walls, and this can cause a strong drop in pressure recovery and negatively affect power output from the last stage of rotor blades. Further, such conditions can make the exhaust begin to recirculate in the area just downstream of the last row of rotor blades—the above-described “exit area”—which is a result that can massively cut power output from the last row of rotor blades. As will be seen, in accordance with the present invention, if the conical section of the diffuser inboard wall is adjusted or repositioned axially when these conditions are satisfied, such flow separation and recirculation can be avoided and, hence, pressure recovery and power output significantly improved. As discussed below in relation to  FIG. 6 , the enthalpy recovery curve of the last stage of rotor blades generally includes a strong gradient at low volume flow conditions, which means the beneficial effect that the present invention has on power output is potentially sizeable. 
     In describing the present invention, several of the terms and concepts will be used in the same manner as they were in introducing the conventional systems of  FIGS. 1 and 2 . For example, it should be understood that the working fluid flowpath  24  is annularly shaped and defined between inboard and outboard walls  25 ,  26 . Similarly, the diffuser flowpath  13  also is annularly shaped and defined between inboard and outboard walls  35 ,  36 . Such inboard walls  25 ,  35  and outboard walls  26 ,  36  of the working fluid and diffuser flowpaths, respectively, are arranged concentrically about a central axis  14  that is defined through the turbine shaft  12 . Additionally, upstream and downstream directions through the working fluid flowpath  24  and the diffuser flowpath  13  are defined in relation to flow directions therethrough of the working fluid and the exhaust, respectively. 
     As shown in  FIGS. 3 and 4 , in accordance with the present invention, the diffuser  11  includes several walls (also “diffuser walls”) that define and enclose the diffuser flowpath  13  between an inlet  53  and outlet  55 . As will be seen, the present invention includes configuring at least one of these diffuser walls as an adaptive or adjustable diffuser wall and at least one of the diffuser walls as a stationary diffuser wall. As also shown, an actuator  61  may be provided for adjusting or moving the adjustable diffuser wall. The actuator  61  may include any conventional apparatus or structure for achieving the controlled movement described herein. The adjustable diffuser wall—which, as shown, may be the downstream axial section  42  of the inboard wall  35 —may be connected to the actuator  61  via any conventional structural arrangement—such as by an arm  65 . The actuator  61  and arm  65  may function to move the adjustable diffuser wall in relation to the stationary diffuser wall between two or more positions. Such movement may be effected along a particular axis of movement, which is chosen so that the flow area through the diffuser flowpath  13  is modified in a desirable way. As will be seen, such modification to the flow area through the diffuser flowpath  13  may be one that alternatively narrows and widens the outlet  55  of the diffuser  11 . 
     In addition, given the configuration of the adjustable diffuser wall, the modification to the flow area through the diffuser flowpath  13  may be one that also narrows and widens the inlet  53  of the diffuser  11  and/or upstream areas within the diffuser flowpath  13 . As will be appreciated, these upstream areas within the diffuser flowpath  13  represent an immediate exit area for the working fluid moving through and exiting the last stage of rotor blades. At very low volume flow conditions, flow separation within this exit area can cause a recirculation that negatively reacts on the airfoils of the rotor blades to cause a massive loss in power. As will be seen, the axial readjusting of the conical section of the inboard wall may narrow this “exit area” of the last stage of rotor blades and prevent this recirculation. The present invention also may be used to avoid low volume flow excitations of the last stage of rotor blades. Hence, it should be understood that the present invention may be used to both: 1) modify (reduce/enlarge) the diffuser area ratio by narrowing the outlet of the diffuser; and 2) modify (reduce/enlarge) the flow area through the inlet  53  and upstream areas of the diffuser  11  so to modify the exit area of the working fluid moving through the last stage of rotor blades. 
     According to exemplary embodiments, generally, the stationary diffuser wall of the diffuser  11  includes the outboard wall  36  of the diffuser  11 , while the adjustable diffuser wall includes the inboard wall  35  of the diffuser  11 . For purposes herein, one way in which the outlet  55  of the diffuser  11  is dimensionally defined is as the distance occurring between: a downstream most termination point or end of the outboard wall  36  of the diffuser  11  and a downstream most termination point or end of the inboard wall  35  of the diffuser. In the  FIGS. 3 through 6 , it will be appreciated that the downstream most end of the outboard wall  36  is designated by the numeral identifier  44  and the downstream most end of the inboard wall  35  is designated by the numeral identifier  72 . 
     According to an exemplary embodiment, the actuator  61  is configured to move the adjustable diffuser wall so to effectuate a narrowing of the diffuser flowpath  13 , generally, and, more specifically, a narrowing of the distance between the downstream most end of the outboard wall  36  and the downstream most end of the inboard wall  35 . The actuator  61  may be configured to move the adjustable diffuser wall through a range of positions occurring between a fully open condition and a fully narrowed condition. The movement over that range may be between preset intervals within that range or, alternatively, smoothly so that virtually any position within that range is attainable. At minimum, the present invention includes the actuator  61  being configured to move the adjustable diffuser between at least a first position (as depicted in  FIG. 3 ), in which the outlet  55  of the diffuser  11  is wide or in a fully open position, and a second position (as depicted in  FIG. 4 ), in which the outlet  55  has been narrowed. More specifically, in the first position of  FIG. 3 , the downstream most end of the inboard wall  35  of the diffuser  11  resides a first distance (D 1 ) from the downstream most end of the outboard wall  36  of the diffuser  11 . Whereas, in the second position of  FIG. 4 , the section of the inboard wall  35  connected to the actuator  61  has been axially relocated so that the downstream most end of the inboard wall  35  of the diffuser  11  resides a second (and comparatively shorter) distance (D 2 ) from the downstream most end of the outboard wall  36  of the diffuser  11 . According to exemplary embodiments, the second distance represents a significant reduction compared to the first distance. This, as illustrated, results in the outlet  55  of the diffuser  11 —as well as other portions of the diffuser  11 —being appreciably narrowed so to materially affect diffusion performance therewithin. The extent of this narrowing may be variable depending on many factors, such as desired performance as well as the overall configuration of the diffuser. According to certain preferred embodiments, the first distance is at least 1.25 times the second distance. 
     As discussed above, the inboard wall  35  of the diffuser  11  may be more specifically described in relation to certain axially defined sections, which include an upstream axial section  41  and a downstream axial section  42 . As shown in  FIG. 1 , the upstream axial section  41  of the inboard wall  35  may be in the shape of a cylinder or may have a conical or truncated cone shape (as shown in  FIGS. 2 through 5 . The downstream axial section  42  of the inboard wall may have a conical or truncated cone shape. The upstream axial section  41  of the inboard wall  35  extends between an upstream end, which is disposed adjacent to the inboard wall  35  of the working fluid flowpath, and a downstream end, which is disposed adjacent to the downstream axial section  42 . The downstream axial section  42  of the inboard wall  35  extends generally between an upstream end  71 , which is disposed adjacent to the upstream axial section  41  of the inboard wall  35 , and a downstream end  72 , which, as shown in  FIGS. 3 and 4 , may be disposed adjacent to the backwall  32  of the collector  30  or offset therefrom depending on the current position of the adjustable diffuser wall. 
     As will be appreciated, the truncated cone shape of the downstream axial section  42  of the inboard wall  35  is radially symmetrical about a longitudinal axis defined approximately at the central axis, with the truncated cone shape enlarging in diameter between a smaller diameter end and a larger diameter end. Thus, the upstream end  71  of the downstream axial section  42  of the inboard wall  35  includes the smaller diameter end of the truncated cone shape, while the downstream end  72  of the downstream axial section  42  of the inboard wall  35  includes the larger diameter end of the truncated cone shape. 
     The outboard wall  36  of the diffuser walls also may have a truncated cone shape that is radially symmetrical about a longitudinal axis defined approximately at the central axis  14 . The truncated cone shape of the outboard wall  36 , as illustrated, enlarges in diameter between a smaller diameter end and a larger diameter end, where an upstream end  43  of the outboard wall  36  includes the smaller diameter end of the truncated cone shape while the downstream end  44  of the outboard wall  36  includes the larger diameter end of the truncated cone shape. The outboard wall  36  may be configured to extend between an upstream end  43 , which is adjacent to the outboard wall  26  of the working fluid flowpath  24 , and a downstream end  44 . 
     Given this more detailed description of the diffuser flowpath  13 , in accordance with certain embodiments of the present invention, the stationary diffuser wall of the diffuser  11  may be more particularly identified as being both the outboard wall  36  and the upstream axial section  41  of the inboard wall  35 . And, the adjustable diffuser wall of the diffuser  11  of the present invention may be more particularly identified as being the downstream axial section  42  of the inboard wall  35 . Pursuant to this more detailed description, an outlet width dimension may be introduced that describes the cross-sectional flow area through the outlet  55  of the diffuser  11 . As indicated in  FIGS. 3 and 4 , the outlet width is the distance occurring between the downstream end  44  of the outboard wall  36  and the downstream end  72  of the downstream axial section  42  of the inboard wall  35 , which is depicted in  FIGS. 3 and 4  as D 1  and D 2 , which were previously referenced as first and second distances, respectively. In moving between the two different positions portrayed in  FIGS. 3 and 4 , the outlet width of the diffuser outlet  55  is varied between wider and narrower instances. According to preferred embodiments, as indicated above, the outlet width (D 2 ) of  FIG. 4  is one that is significantly reduced compared to the outlet width (D 1 ) of  FIG. 3 . For example, according to preferred embodiments, the outlet width (D 1 ) of  FIG. 3  may be described as being at least 1.25 times the outlet width (D 2 ) of  FIG. 4 . 
       FIG. 5  simultaneously depicts both of the axial positions between which the adjustable wall may be moved via to modify the flow area through the diffuser. The stationary diffuser wall of the diffuser  11  may be defined as the axially defined section of the outboard wall  36  that forms a conical or truncated cone shape. The adjustable diffuser wall of the diffuser  11  may be defined as the axially defined section of the inboard wall  35  that forms a conical or truncated cone shape. Given these generally definitions, the outlet width dimension introduced above may be described as being defined between: the downstream end of the axially defined section of the outboard wall  36  that forms the conical or truncated cone shape; and the downstream end of the axially defined section of the inboard wall  35  that forms the conical or truncated cone shape. In moving between the two exemplary axial positions depicted, it is demonstrated how the present invention narrows the outlet width of the diffuser outlet  55  and, thereby, appreciably reduces the cross-sectional flow area through the diffuser flowpath  13 . As further indicated in  FIG. 5 , movement between the two axial positions also may be marked relative to an axial offset that is created between the downstream end  72  of the downstream axial section  42  and the backwall  32  of the collector  30 . Specifically, in the first position, it will be appreciated that the downstream end  72  of the downstream axial section  42  resides very near or against the backwall  32  of the collector  30 . Whereas, in the second position, the downstream end  72  of the downstream axial section  42  is offset by a predetermined distance (D 3 ) from the backwall  32  of the collector  30 . 
     As should also be appreciated in  FIGS. 3 through 5 , the axial repositioning of the adjustable wall brings the upstream end  71  of the downstream axial section  42  of the inboard wall  35  much closer to the inlet  53  of the diffuser  11 , thereby narrowing the cross-sectional flow area within what is known as the exit area of the last row of rotor blades. This exit area, as previously explained, is the upstream portion of the diffuser flowpath  13 , and represents the area within the diffuser  11  first encountered by the working fluid as it exits the last row of the rotor blades  16 . Low volume flow conditions can cause the occurrence of recirculation in this area, which is particularly harmful to power output. The ability of the present invention to also narrow this portion of the diffuser flowpath  13  and, thereby, avoid such recirculation can boost power output considerably. 
       FIG. 6  illustrates calculated data at a range of exit velocities comparing performance characteristics during low exit velocity conditions between: 1) a conventional non-adjustable diffuser or a diffuser of the present invention operating in a conventional mode, i.e., with the downstream axial section of the inboard wall positioned fully in the downstream direction (which, in either case, will be referred to as “conventional diffuser  90 ”); and 2) a diffuser of the present invention with the downstream axial section of the inboard wall advantageously repositioned axially in the upstream direction (which will be referred to as “present diffuser  95 ”).  FIG. 6  also provides a side-by-side cross-sectional comparison showing flow separation differences at low exit velocity conditions. As shown in relation to the conventional diffuser  90 , at a low flow volume conditions—where the last stage of rotor blades has a low exit velocity—a high level of flow separation (see area indicated by arrow  91 ) takes place within the downstream axial section of the conventional diffuser  90 . This leads to boundary layer growth and stall, which generally causes the exhaust flow to separate from the diffuser walls and causes pressure losses that degrade performance. However, in the case of the present diffuser  95 , such flow separation is largely prevented due to the axial repositioning of the downstream axial section of the inboard wall, the upstream movement of which is indicated by arrows  96 . 
     As shown in the plotted data of  FIG. 6 —in which a curve  92  depicts results for the conventional diffuser  90  and a curve  97  depicts the results achievable by the present diffuser  95 —a significant performance benefit of about 4-6% additional power in the last stage rotor blade is possible. More specifically, included in the plotted data is an enthalpy recovery (“χ”) for the last stage of rotor blades over a range of exit velocities. As will be appreciated, the steep gradients occurring at low volume flow conditions indicate potentially sizeable gains are possible, and these are confirmed in the calculated data. Specifically, the performance benefit evidenced by the area between the curves  92 ,  97  at exit velocities below about 140 m/s is significant. As further indicated, as exit velocities increase above that 140 m/s threshold, the downstream axial section of the present invention can be relocated to its original position—thereby making the flow area through the flowpaths of the two diffusers  90 ,  95  equal—and, as expected, the data converges at these higher flow velocities. In this way, the present invention may be used to control the position of the adjustable wall in response to changing flow conditions so that high performance is maintained across a wider range of conditions. Specifically, at low volume flow and low exit velocity conditions, the flow area through the diffuser flowpath may be narrowed or reduced as described above, while at high volume and high exit velocity conditions, the flow area through the diffuser flowpath may be widened or increased to its original dimensions. In this way, the ability to manipulate the flow area through the diffuser flowpath may be used to reduce or eliminate flow separation and/or recirculation that would otherwise degrade performance. 
     For example, the present invention may include a method of operating a turbine engine. As before, the turbine engine may include a working fluid flowpath defined through a turbine section that operably connects to a diffuser flowpath defined through a diffuser, and the diffuser may include diffuser walls that define and enclose the diffuser flowpath between an inlet and outlet. The method may include the initial step of providing at least one stationary diffuser wall and at least one adjustable diffuser wall within the walls of the diffuser. As already described, the at least one adjustable diffuser wall may be one that is controllably adjustable or movable between at least a first position and a second position relative to the at least one stationary diffuser wall. (As will be appreciated, other potential embodiments of the present invention include the adjustable diffuser wall being adjustable between many other positions.) The second position may be one that reduces a cross-sectional flow area through a section of the diffuser flowpath—such as through the outlet and/or upstream portion of the diffuser flowpath—in comparison to the first position. The method may further include the step of sensing, via one or more sensors, a current value for an operating condition of the turbine engine and then comparing the current value to a threshold value. In response to the comparison, the method may include moving the at least one adjustable diffuser wall from the first position to the second position. As will be appreciated, the operating condition of the method may be one relating to flow volume or exit velocity through the turbine section, last row of rotor blades, or diffuser. Further, the step of comparing the current value to the threshold value may include making a determination as to whether the current value is less than the threshold value, with the threshold value serving as an indication that turbine engine is operating at a low flow volume or low exit velocity condition. 
     From the above description of preferred embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.