Patent Publication Number: US-10309409-B2

Title: Impeller shroud with pneumatic piston for clearance control in a centrifugal compressor

Description:
FIELD OF THE DISCLOSURE 
     The present invention relates generally to turbine engines having centrifugal compressors and, more specifically, to control of clearances between an impeller and a shroud of a centrifugal compressor. 
     BACKGROUND 
     Centrifugal compressors are used in turbine machines such as gas turbine engines to provide high pressure working fluid to a combustor. In some turbine machines, centrifugal compressors are used as the final stage in a multi-stage high-pressure gas generator. 
       FIG. 1  is a schematic and sectional view of a centrifugal compressor system  100  in a gas turbine engine. One of a plurality of centrifugal compressor blades  112  is illustrated. As blade  112  rotates, it receives working fluid at a first pressure and ejects working fluid at a second pressure which is higher than first pressure. The radially-outward surface of each of the plurality of compressor blades  112  comprises a compressor blade tip  113 . 
     An annular shroud  120  encases the plurality of blades  112  of the impeller. The gap between a radially inner surface  122  of shroud  120  and the impeller blade tips  113  is the blade tip clearance  140  or clearance gap. Shroud  120  may be coupled to a portion of the engine casing  131  directly or via a first mounting flange  133  and second mounting flange  135 . 
     Gas turbine engines having centrifugal compressor systems  100  such as that illustrated in  FIG. 1  typically have a blade tip clearance  140  between the blade tips  113  and the shroud  120  set such that a rub between the blade tips  113  and the shroud  120  will not occur at the operating conditions that cause the highest clearance closure. A rub is any impingement of the blade tips  113  on the shroud  120 . However, setting the blade tip clearance  140  to avoid blade  112  impingement on the shroud  120  during the highest clearance closure transient may result in a less efficient centrifugal compressor because working fluid is able to flow between the blades  112  and shroud  120  thus bypassing the blades  112  by flowing through gap  140 . This working fluid constitutes leakage. In the centrifugal compressor system  100  of  FIG. 1 , blade tip clearances  140  cannot be adjusted because shroud  120  is rigidly mounted to the engine casing  131 . 
     It is known in the art to dynamically change blade tip clearance  140  to reduce leakage of a working fluid around the blade tips  113 . Several actuation systems for adjusting blade tip clearance  140  during engine operation have been developed. These systems often include complicated linkages, contribute significant weight, and/or require a significant amount of power to operate. Thus, there continues to be a demand for advancements in blade clearance technology to minimize blade tip clearance  140  while avoiding rubs. 
     The present application discloses one or more of the features recited in the appended claims and/or the following features which, alone or in any combination, may comprise patentable subject matter. 
     SUMMARY 
     According to an aspect of the present disclosure, a compressor shroud assembly in a turbine engine having a dynamically moveable impeller shroud for encasing a rotatable centrifugal compressor and maintaining a clearance gap between the shroud and the rotatable centrifugal compressor, said assembly comprises: a static compressor casing; an air piston mounted to said casing, said piston comprising a chamber adapted to receive actuating air and an aft extending mounting arm which moves axially substantially maintaining a radial alignment when said piston is actuated; and an impeller shroud slidably coupled at a forward end to said casing and mounted proximate an aft end to said piston mounting arm, said impeller shroud moving relative to the rotatable centrifugal compressor in an axial direction while substantially maintaining a radial alignment when said piston is actuated. 
     In some embodiments the air piston chamber is adapted to receive air from the discharge of the rotatable centrifugal compressor. In some embodiments the air piston comprises a forward rigid member mounted at a forward end to said casing, an aft rigid member coupled at an aft end to said mounting arm, and a flexible member coupling said forward and aft rigid members to thereby form said piston chamber. In some embodiments the flexible member comprises a hoop having a U-shaped cross section. In some embodiments the flexible member comprises a bellows forming a hoop. In some embodiments the slidable coupling between said shroud and said casing is dimensioned to maintain an air boundary during the full range of axial movement of said shroud. In some embodiments the compressor shroud assembly further comprises a chamber bounded in part by said casing and at least a portion of the impeller shroud proximate the aft end thereof, said chamber being pressurized by exducer air. In some embodiments the compressor shroud assembly further comprises a chamber bounded in part by said casing and at least a portion of said impeller shroud proximate the forward end thereof, said chamber being pressurized by inducer air. In some embodiments the compressor shroud assembly further comprises one or more sensors for measuring the air pressure in said piston chamber, said piston being actuated or vented in response to the measured pressure in said piston chamber. In some embodiments the compressor shroud assembly further comprises one or more sensors for measuring the clearance gap between said shroud and the rotatable centrifugal compressor, said piston being actuated or vented in response to the clearance gap measure by the one or more sensors. 
     According to another aspect of the present disclosure, a compressor shroud assembly in a turbine engine having a dynamically moveable impeller shroud for encasing a rotatable centrifugal compressor and maintaining a clearance gap between the shroud and the rotatable centrifugal compressor, said assembly comprises: a static compressor casing; an air piston mounted to said casing, said piston comprising a chamber adapted to receive actuating air and an aft extending mounting arm which moves axially while substantially maintaining a radial alignment when said piston is actuated; and an impeller shroud mounted at a forward end to said casing and mounted proximate an aft end to said piston mounting arm, said impeller shroud moving relative to the rotatable centrifugal compressor in a cantilevered manner from said forward end thereof when said piston is actuated. 
     In some embodiments the air piston chamber is adapted to receive air from the discharge of the rotatable centrifugal compressor. In some embodiments the air piston comprises a forward rigid member mounted at a forward end to said casing, an aft rigid member coupled at an aft end to said mounting arm, and a flexible member coupling said forward and aft rigid members to thereby form said piston chamber. In some embodiments the flexible member comprises a hoop having a U-shaped cross section. In some embodiments the flexible member comprises a bellows forming a hoop. 
     According to an aspect of the present disclosure, a method of dynamically changing a clearance gap between a rotatable centrifugal compressor and a shroud encasing the rotatable centrifugal compressor, said method comprises mounting a pressure-actuated piston to a static casing; mounting a shroud to the piston; and actuating the piston to thereby move the shroud relative to a rotatable centrifugal compressor. 
     In some embodiments the method further comprises providing air from the discharge of the rotatable centrifugal compressor to actuate the piston. In some embodiments the method further comprises slidably coupling the forward end of the shroud to the casing, wherein the shroud moves relative to the rotatable centrifugal compressor in an axial direction while substantially maintaining a radial alignment when the piston is actuated. In some embodiments the method further comprises sensing the fluid pressure in an actuating chamber of the piston and actuating the piston in response to the sensed fluid pressure. In some embodiments the method further comprises sensing the clearance gap between the rotatable centrifugal compressor and the shroud and actuating the piston in response to the sensed clearance gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following will be apparent from elements of the figures, which are provided for illustrative purposes and are not necessarily to scale. 
         FIG. 1  is a schematic and sectional view of a centrifugal compressor system in a gas turbine engine. 
         FIG. 2A  is a schematic and sectional view of a centrifugal compressor system having a clearance control system in accordance with some embodiments of the present disclosure. 
         FIG. 2B  is an enlarged schematic and sectional view of the clearance control system illustrated in  FIG. 2A , in accordance with some embodiments of the present disclosure. 
         FIG. 3  is a schematic and sectional view of another embodiment of a clearance control system with a bellows-type air piston in accordance with the present disclosure. 
         FIG. 4  is a schematic and sectional view of another embodiment of a clearance control system in accordance with the present disclosure. 
         FIG. 5  is a schematic and sectional view of another embodiment of a clearance control system in accordance with the present disclosure. 
         FIG. 6  is a schematic and sectional view of another embodiment of a clearance control system in accordance with the present disclosure. 
         FIG. 7  is a schematic and sectional view of the pressure regions of a clearance control system in accordance with some embodiments of the present disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims. 
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to a number of illustrative embodiments illustrated in the drawings and specific language will be used to describe the same. 
     This disclosure presents embodiments to overcome the aforementioned deficiencies in clearance control systems and methods. More specifically, the present disclosure is directed to a system for clearance control of blade tip clearance which avoids the complicated linkages, significant weight penalties, and/or significant power requirements of prior art systems. The present disclosure is directed to a system which supplies high pressure actuating air to an air piston to cause axial deflection of an impeller shroud. 
       FIG. 2A  is a schematic and sectional view of a centrifugal compressor system  200  having a clearance control system  260  in accordance with some embodiments of the present disclosure. Centrifugal compressor system  200  comprises centrifugal compressor  210  and clearance control system  260 . 
     The centrifugal compressor  210  comprises an annular impeller  211  having a plurality of centrifugal compressor blades  212  extending radially from the impeller  211 . The impeller  211  is coupled to a disc rotor  214  which is in turn coupled to a shaft  216 . Shaft  216  is rotatably supported by at least forward and aft shaft bearings (not shown) and may rotate at high speeds. The radially-outward surface of each of the compressor blades  212  constitutes a compressor blade tip  213 . 
     As blade  212  rotates, it receives working fluid at an inlet pressure and ejects working fluid at a discharge pressure which is higher than the inlet pressure. Working fluid (e.g. air in a gas turbine engine) is typically discharged from a multi-stage axial compressor (not shown) prior to entering the centrifugal compressor  210 . Arrows A illustrate the flow of working fluid through the centrifugal compressor  210 . Working fluid enters the centrifugal compressor  210  from an axially forward position  253  at an inlet pressure. Working fluid exits the centrifugal compressor  210  at an axially aft and radially outward position  255  at a discharge pressure which is higher than inlet pressure. 
     Working fluid exiting the centrifugal compressor  210  passes through a diffusing region  250  and then through a deswirl cascade  252  prior to entering a combustion chamber (not shown). In the combustion chamber, the high pressure working fluid is mixed with fuel and ignited, creating combustion gases that flow through a turbine (not shown) for work extraction. 
     In one embodiment, the clearance control system  260  comprises a high pressure air source  262 , an air piston  264 , an annular shroud  220 , and a slidable coupling  266 . Clearance control system  260  can also be referred to as a compressor shroud assembly. 
     High pressure air source  262  provides high pressure actuating air to air piston  264 . In some embodiments high pressure air source  262  is supplied from centrifugal compressor discharge air. 
     Air piston  264  is adapted to receive high pressure air from high pressure air source  262 . Air piston  264  comprises a forward rigid member  271 , aft rigid member  272 , and a central flex member  273  disposed between forward rigid member  271  and aft rigid member  272 . Together, forward rigid member  271 , aft rigid member  272 , and central flex member  273  define a piston chamber  274 . 
     In some embodiments, as illustrated in  FIGS. 2A and 2B , central flex member  273  comprises a ring  279  or hoop having a U-shaped cross section which extends radially outward from forward rigid member  271  and aft rigid member  272  and adapted to expand, contract, or flex primarily in an axial direction. In other words, expansion and contraction of air piston  264  results in axial movement while substantially maintaining a radial alignment. 
     In some embodiments high pressure air is received from high pressure air source  262  via a receiving chamber  275  which is in fluid communication with piston chamber  274 . In some embodiments receiving chamber  275  includes a regulating valve which regulates movement of high pressure air into and out of piston chamber  274 . In some embodiments receiving chamber  275  further includes a member for venting piston chamber  274  to atmospheric pressure or to a pressure which is lower than that of piston chamber  274 . 
     Air piston  264  is axially disposed between a portion of engine casing  231  and shroud  220 . A forward-extending arm  276  extends axially forward from forward rigid portion  271  and is coupled to engine casing  231  at first mounting flange  233 , thus mounting air piston  264  to the casing  231 . An aft-extending arm  277  extends axially aft from aft rigid portion  272  and is coupled to a mounting arm  278  extending axially forward from shroud  220 . Aft-extending arm  277  and mounting arm  278  are coupled at mounting flange  237 . 
     In some embodiments air piston  264  is an annular piston. In other embodiments, a plurality of discrete air pistons  264  are circumferentially disposed about shroud  220  and each act independently upon the shroud  220 . 
     Shroud  220  is a dynamically moveable impeller shroud. Shroud  220  encases the plurality of blades  212  of the centrifugal compressor  210 . Shroud  220  comprises a forward end portion  223  terminating at slidable coupling  266 , a central portion  224 , and a aft end portion  225 . In some embodiments, surface  222  of shroud  220  comprises an abradable surface. In some embodiments, a replaceable cover is provided which covers the surface  222  and is replaced during engine maintenance due to impingement of blade tips  213  against surface  222 . 
     In some embodiments aft end portion  225  is defined as the radially outward most third of shroud  220 . In other embodiments aft end portion  225  is defined as the radially outward most quarter of shroud  220 . In still further embodiments aft end portion  225  is defined as the radially outward most tenth of shroud  220 . In embodiments wherein mounting arm  278  extends axially forward from aft end portion  225 , these various definitions of aft end portion  225  as either the final third, quarter, or tenth of shroud  220  provide for the various radial placements of mounting arm  278  relative to shroud  220 . 
     Slidable coupling  266  comprises an axial member  280  coupled to forward end portion  223  of shroud  220 . Slidable coupling  266  is adapted to allow sliding displacement between axial member  280  and forward end portion  223 . In some embodiments one or more surfaces of forward end portion  223  and/or axial member  280  comprise a lubricating surface to reduce friction and wear between these components. In some embodiments the lubricating surface is a coating. 
     Clearance control system  260  is coupled to the engine casing  231  via a first mounting flange  233  and second mounting flange  235 . In some embodiments engine casing  231  is at least a portion of a casing around the multi-stage axial compressor. 
     The gap between a surface  222  of shroud  220  which faces the impeller  211  and the impeller blade tips  213  is the blade tip clearance  240 . In operation, thermal, mechanical, and pressure forces act on the various components of the centrifugal compressor system  200  causing variation in the blade tip clearance  240 . For most operating conditions, the blade tip clearance  240  is larger than desirable for the most efficient operation of the centrifugal compressor  210 . These relatively large clearances  240  avoid rubbing between blade tip  213  and the surface  222  of shroud  220 , but also result in high leakage rates of working fluid past the impeller  211 . It is therefore desirable to control the blade tip clearance  240  over a wide range of steady state and transient operating conditions. The disclosed clearance control system  260  provides blade tip clearance  240  control by positioning shroud  220  relative to blade tips  213 . 
       FIG. 7  is a schematic and sectional view of the pressure regions P 1 , P 2 , and P 3  of a clearance control system  260  in accordance with some embodiments of the present disclosure. A first pressure region P 1  is defined as piston chamber  274  and receiving member  275 . A second pressure region P 2  is disposed radially inward from air piston  264  and radially outward from shroud  220  and axial member  280 . A third pressure region P 3  is disposed radially outward from air piston  264  and radially inward from a casing arm  702 . 
     In some embodiments, second pressure region P 2  and third pressure region P 3  are maintained at or near atmospheric pressure, meaning that regions P 2  and P 3  are neither sealed nor pressurized. First pressure region P 1  receives high pressure air from high pressure air source  262 , which in some embodiments is compressor discharge air. However, in such an embodiment, a relatively large piston chamber  274  is required to overcome the large differential pressure across the shroud  220  (i.e. differential pressure between the pressure of regions P 2  and P 3  and the pressure of the centrifugal compressor  210 . In other words, the large differential pressure makes it more difficult to deflect or cause axial movement in shroud  220 , thus requiring a larger air piston  264  to perform the work. 
     Thus in other embodiments second pressure region P 2  and third pressure region P 3  are sealed and pressurized to reduce the differential pressure across the shroud  220 . For example, in some embodiments second pressure region P 2  and third pressure region P 3  are pressurized using one of inducer air, exducer air, or intermediate stage compressor air. Supplying compressor discharge air to piston chamber  274  still creates a differential pressure across the air piston  264  that causes axial deflection, but the force required to move shroud  220  is greatly reduced due to the lower differential pressure across the shroud  220 . 
     In embodiments with second pressure region P 2  sealed and pressurized using inducer air and third pressure region P 3  sealed and pressurized using exducer air, the selection of the location of mounting arm  278  between forward end  223  and aft end  225  is significant because a greater exposure of shroud  220  to exducer pressure results in less work required by the the air piston  264  to move shroud  220 . In addition, it can be undesirable to locate mounting arm  278  adjacent to aft end  225  due to the risk that the air piston  264  will overly bend the upper tip of shroud  220 . 
     In some embodiments second pressure region P 2  and third pressure region P 3  are merged as a single, sealed pressure region and are thus pressurized at equal pressures. 
       FIG. 2B  is an enlarged schematic and sectional view of the clearance control system  260  illustrated in  FIG. 2A , in accordance with some embodiments of the present disclosure. The operation of clearance control system  260  will be discussed with reference to  FIG. 2B . 
     In some embodiments during operation of centrifugal compressor  210  blade tip clearance  240  is monitored by periodic or continuous measurement of the distance between surface  222  and blade tips  213  using a sensor or sensors positioned at selected points along the length of surface  222 . When clearance  240  is larger than a predetermined threshold, it may be desirable to reduce the clearance  240  to prevent leakage and thus improve centrifugal compressor efficiency. Pressure inside the piston chamber  274  may be adjusted based on measured blade tip clearance  240  to move shroud  220  and thus adjust the blade tip clearance  240  as desired. 
     In other embodiments, engine testing may be performed to determine blade tip clearance  240  for various operating parameters and a piston chamber  274  pressure schedule is developed for different modes of operation. For example, based on clearance  240  testing, piston chamber  274  pressures may be predetermined for cold engine start-up, warm engine start-up, steady state operation, and max power operation conditions. As another example, a table may be created based on blade tip clearance  240  testing, and piston chamber  274  pressure is adjusted according to operating temperatures and pressures of the centrifugal compressor  210 . A sensor may be used to monitor pressure in piston chamber  274 . Thus, based on monitoring the operating conditions of the centrifugal compressor  210  such as inlet pressure, discharge pressure, and/or working fluid temperature, a desired blade tip clearance  240  is achieved according to a predetermined schedule of pressures for piston chamber  274 . 
     Regardless of whether clearance  240  is actively monitored or controlled via a schedule, in some operating conditions it may be desirable to reduce the clearance in order to reduce leakage past the centrifugal compressor  210 . In order to reduce the clearance  240 , high pressure gas is supplied by high pressure gas source  262  to piston chamber  274 . Piston chamber  274  expands between forward rigid member  271  and aft rigid member  272  due to the admission of high pressure gas. Central flex member  273  enables this expansion in an axial direction. With air piston  264  rigidly coupled, or “grounded”, to casing  231  via forward-extending arm  276 , expansion of the air piston  264  is enabled in the axially aft direction as indicated by arrow  291  in  FIG. 2B . 
     The axially aft expansion of air piston  264  displaces aft-extending arm  277  and mounting arm  278 . Mounting arm  278  is coupled to and imparts a force on the aft end portion  225  of shroud  220 , thus moving the aft end portion  225  in an axially aft direction as indicated by arrow  292 . This movement of aft end portion  225  is translated to a similar axially aft movement at the slidable coupling  266 , where forward end portion  223  is displaced in an axially aft direction relative to axial member  280  as indicated by arrow  293 . Additionally, as discussed with reference to  FIG. 7 , the application of air pressure at third pressure region P 3  imparts a force on aft end portion  225 . Shroud  220  thus moves relative to the centrifugal compressor  210  in an axial direction while substantially maintaining the radial alignment of shroud  220 . 
     The axially aft movement of shroud  220  caused by air piston  264  expansion results in shroud  220  moving closer to blade tips  213 , thus reducing the clearance  240  and leakage. During many operating conditions this deflection of shroud  220  in the direction of blade tips  213  is desirable to reduce leakage and increase compressor efficiency. 
     Where monitoring of blade tip clearance  240  indicates the need for an increase in the clearance  240 , high pressure air is bled from piston chamber  274 . As piston chamber  274  contracts, central flex member  273  enables the contraction to be primarily in the axial direction, resulting in axially forward movement of aft-extending arm  277 , mounting arm  278 , and aft end portion  225 . The axially forward movement of aft end portion  225  results in similar movement of shroud  220 , including the sliding displacement in an axially forward direction of forward end portion  223  against axial member  280 . Thus, by bleeding air from piston chamber  274  shroud  220  is moved axially forward, away from blade tips  213  and increasing blade tip clearance  240 . Slidable coupling  266  is dimensioned such that an air boundary is maintained through the full range of axial movement of shroud  220 . 
       FIG. 3  is a schematic and sectional view of another embodiment of a clearance control system  360  with a bellows-type air piston  364  in accordance with the present disclosure. The clearance control system  360  illustrated in  FIG. 3  is substantially similar to the clearance control system  260  illustrated in  FIG. 2 . Air piston  364  comprises a bellows  379  as central flex member  273  forming a hoop disposed between forward rigid member  271  and aft rigid member  272 . Like flexible protrusion  279 , bellows  379  is adapted to expand, contract, or flex primarily in an axial direction. The operation of clearance control system  360  is substantially the same as the operation of clearance control system  260  as described above. Bellows  379  is interchangeable with flexible protrusion  279 , and central flex member  273  can take many forms. 
       FIG. 5  is a schematic and sectional view of another embodiment of a clearance control system  560  in accordance with the present disclosure. Clearance control system  560  includes shroud  220  which comprises an extended forward end portion  503 , central portion  224 , and aft end portion  225 . Extended forward end portion  503  is coupled to casing  231  at mounting flange  235 . Supplying high pressure air to piston chamber  274  results axial expansion of air piston  264 , which in turn causes an axially aft movement of mounting arm  278 . Shroud  220  flexes in an axially aft and radially inward direction as indicated with arrow  501 , toward the blade  212 . Thus the embodiment of  FIG. 5  illustrates a shroud  220  which is more rigidly coupled to casing  231  and which deflects in a radially inward and axially aft direction as indicated by arrow  501 . Evacuation of air from piston chamber  274  results in contraction of the air piston  264 , axially forward movement of mounting arm  278 , and a radially outward and axially forward deflection of shroud  220 . 
       FIG. 4  is a schematic and sectional view of another embodiment of a clearance control system  460  with a modified mounting arm  278  placement in accordance with the present disclosure. In the embodiment of  FIG. 4 , mounting arm  278  is coupled to shroud  220  at central portion  224 . As in the embodiment of  FIG. 5 , shroud  220  which comprises an extended forward end portion  503 , central portion  224 , and aft end portion  225 . Extended forward end portion  503  is coupled to casing  231  at mounting flange  235 . 
     Axial expansion of air piston  264  caused by supplying high pressure air to piston chamber  274  results in axially aft movement of mounting arm  278 . In the embodiment of  FIG. 4  the central placement of mounting arm  278  results in different response and deflection characteristics along the shroud  220  and a different force required in order to effect axial movement of the shroud  220 . With the shroud  220  anchored by extended forward portion  503 , the axially aft motion of mounting flange  278  results in shroud  220  moving in an axially aft and radially inward direction as indicated by arrow  401 . 
     In some embodiments central portion  224  is defined as the centermost third of shroud  220  along its axial length. In other embodiments central portion  224  is defined as the centermost quarter of shroud  220  along its axial length. In still further embodiments central portion  224  is defined as the centermost tenth of shroud  220  along its axial length. In embodiments wherein mounting arm  278  extends axially forward from central portion  224 , these various definitions of central portion  224  as either the centermost third, quarter, or tenth of shroud  220  provide for the various radial placements of mounting arm  278  relative to shroud  220 . 
       FIG. 6  is a schematic and sectional view of another embodiment of a clearance control system  660  in accordance with the present disclosure. Clearance control system  660  has a hinged joint  601  comprising an annular pin  603  received by a forward portion  605  of shroud  220  and a receiving portion  606  of axial member  280 . 
     As with the embodiment of  FIG. 5 , axial deflection of air piston  264  causes shroud  220  to deflect in a radially inward and axially aft direction as indicated by arrow  607 . Axial deflection of air piston  264  caused by supplying high pressure air to piston chamber  274  results in axially aft movement of mounting arm  278 . With a hinged joint  601 , shroud  220  pivots about the annular pin  603  causing motion in a radially inward and axially aft direction as indicated by arrow  607 . 
     The present disclosure provides many advantages over previous systems and methods of controlling blade tip clearances. The disclosed clearance control systems allow for tightly controlling blade tip clearances, which are a key driver of overall compressor efficiency. Improved compressor efficiency results in lower fuel consumption of the engine. Further, utilizing compressor discharge as the high pressure gas source obviates the need to attach an actuator external to the compressor or engine. The use of an air piston provides for manufacturing the shroud from a rigid or primarily rigid material, with the piston chamber supplying axial deflection of the shroud. Additionally, the present disclosure eliminates the use of complicated linkages, significant weight penalties, and/or significant power requirements of prior art systems. 
     Although examples are illustrated and described herein, embodiments are nevertheless not limited to the details shown, since various modifications and structural changes may be made therein by those of ordinary skill within the scope and range of equivalents of the claims.