Patent Publication Number: US-10309410-B2

Title: Impeller shroud with deflecting outer member 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 . 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 actuator mounted to said casing; and a double-walled impeller shroud comprising a pair of nested arcuate shroud members shaped to follow the contour of the rotatable centrifugal compressor, a forward member mounted to said casing and an aft member deflectively coupled to said forward member and operatively coupled to said actuator to effect deflection of said aft member relative to said forward member and to the rotatable centrifugal compressor when said actuator is activated. 
     In some embodiments said nested members form a chamber adapted to receive actuating air, said aft member being deflected responsive to the provision of actuating air to said chamber. In some embodiments the actuation air is drawn from the discharge air of the rotatable centrifugal compressor. In some embodiments the deflective coupling between said shroud members comprises a bellows coupling at the aft end of said shroud members. In some embodiments said shroud members are statically coupled at a forward end thereof. In some embodiments said actuator comprises a mechanical driver coupled to the aft end of said aft member to effect deflection of said aft member responsive to the provision of a driving force to said member by said mechanical driver. In some embodiments the deflective coupling between said shroud members comprises a bellows coupling at the aft end of said shroud members. In some embodiments said actuator comprises a cylindrical member coupled to said casing wherein the axial motion of said cylindrical member effects axial translation of said aft shroud member. In some embodiments said actuator comprises a pneumatic piston coupled to said casing wherein the actuation of said piston effects axial translation of said aft shroud member. In some embodiments the assembly further comprises one or more sensors for measuring the fluid pressure in said chamber, said chamber being actuated or vented in response to the measured fluid pressure in said chamber. In some embodiments the assembly further comprises one or more sensors for measuring the clearance gap between said shroud and the rotating centrifugal compressor, said chamber being actuated or vented in response to the clearance gap measure by the one or more sensors. In some embodiments the 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 at a pressure between ambient pressure and 450 pounds per square inch. 
     According to another aspect of the present disclosure, a method of dynamically changing a clearance gap between a rotatable centrifugal compressor and a shroud encasing the rotating centrifugal compressor, said method comprises: mounting a forward shroud member to a static casing; nesting and deflectively coupling an aft shroud member to the forward shroud member forming a double-walled shroud following the contour of the rotatable centrifugal compressor; and deflecting the aft shroud member relative to the forward shroud member and the rotatable centrifugal compressor. 
     In some embodiments the method further comprises deflecting the aft shroud member by providing actuation air to a chamber formed between the shroud members. In some embodiments the method further comprises providing actuating air from the discharge of the rotatable centrifugal compressor. In some embodiments the method further comprises sensing the air pressure in the chamber and providing actuating air to the chamber in response to the sensed air pressure. In some embodiments the method further comprises sensing the clearance gap between the rotatable centrifugal compressor and the shroud and providing actuating air to the chamber in response to the sensed clearance gap. In some embodiments the method further comprises deflecting the aft shroud member by providing a mechanical force to the aft shroud member from a mechanical driver. In some embodiments the method further comprises sensing the clearance gap and deflecting the aft shroud member responsive to the sensed clearance gap. In some embodiments the clearance gap is sensed by more than one clearance gap sensor positioned along the length of the aft shroud member. 
    
    
     
       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 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 uses a two piece joined shroud construction to deflect an aft portion of the shroud toward or away from the blade tips. 
       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  and an annular shroud  220 . Clearance control system  260  may also be referred to as a compressor shroud assembly. 
     Annular shroud  220  comprises an aft member  221  and an forward member  224 . Aft member  221  has a surface  222  opposing impeller blade tips  213 . Aft member  221  and forward member  224  are statically coupled at a forward end  273 . Aft member  221  and forward member  224  are variably coupled at a aft end  271  by bellows  228 . Bellows  228  forms a deflective coupling between aft member  221  and forward member  224 . A chamber  225  is defined between aft member  221  and forward member  224 . Aft member  221 , forward member  224 , and chamber  225  are annular and follow the contour of the centrifugal compressor  210 . In some embodiments a sensor may be disposed in or in fluid communication with chamber  225  and adapted to measure a fluid pressure or fluid temperature of chamber  225 . 
     Shroud  220  is a dynamically moveable impeller shroud. Shroud  220  may be referred to as a double-wall shroud. Both aft member  221  and forward member  224  follow the contour of an impeller blade  212 . Shroud  220  is therefore a double-walled impeller shroud comprising a pair of nested arcuate shroud members  221 ,  224  which follow the contour of the centrifugal compressor  210 . Aft member  221  is deflectively coupled to forward member  224 . Aft member  221  deflects relative to forward member  224  and blade tips  213 . 
     Shroud  220  is coupled to at least a portion of the engine casing. In the illustrated embodiment, shroud  220  is coupled to a first casing portion  231  and second casing portion  232  at mount flange  233 . In some embodiments first casing portion  231  and second casing portion  232  are at least a portion of a casing around the multi-stage axial compressor. Shroud is coupled via radial arm  226  and axial arm  227 . In some embodiments, as illustrated in  FIG. 2A , radial arm  226 , axial arm  227 , and forward member  224  are formed as a unitary component. In other embodiments, radial arm  226 , axial arm  227 , and forward member  224  are formed separately and joined. 
     A receiving member  272  extends radially outward from forward member  224  and receives a feed tube  274 . A sealing member  229  provides a seal between receiving member  272  and feed tube  274 . An interior  275  of feed tube  274  is in fluid communication with chamber  225 . In some embodiments a plurality of feed tubes  274  are circumferentially disposed about shroud  220  and fluidly communicate with the annular chamber  225  in a plurality of locations. In some embodiments annular chamber  225  is segregated into a plurality of cavities and each of these cavities is supplied by a one of a plurality of feed tubes  274 . 
     In some embodiments feed tube  274  includes a regulating valve which regulates movement of high pressure air into and out of chamber  225 . In some embodiments feed tube  274  further includes a member for venting chamber  225  to atmospheric pressure or to a pressure which is lower than that of chamber  225 . 
     High pressure air source  262  provides high pressure air to chamber  225  via feed tube  274 . In some embodiments high pressure air source  262  is supplied from centrifugal compressor discharge air. 
     In some embodiments high pressure air source  262  and bellows  228  comprise an actuator for actuating the deflection of aft shroud  221  from forward shroud  224 . 
     Shroud  220  encases the plurality of blades  212  of the centrifugal compressor  210 . 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 . 
     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  212  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. 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. 
     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 . 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  240  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 chamber  225  via interior  275  of feeder tube  274  as indicated by arrow  291 . Chamber  225  expands due to the admission of high pressure gas. With forward member  224  rigidly coupled, or “grounded”, to the engine casing, the expansion of chamber  225  is directed toward aft member  221 . Bellows  228  expands in an axially aft direction, and aft member  221  deflects in a simultaneously radially inward and axially aft direction as indicated by arrows  292  and  293 . The deflective motion of aft member  221  is controlled by the rigid coupling to forward member  224  at the forward end  223  and the variable or flexible coupling to forward member  224  at aft end  271 . 
     The axially aft deflection of aft member  221  results in the surface  222  moving closer to blade tips  213 , thus reducing the clearance  240  and leakage. During many operating conditions this deflection of aft member  221  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 chamber  225 . As chamber  225  contracts, aft member  221  moves in a simultaneously radially outward and axially forward direction. Thus, by bleeding air from chamber  225 , surface  222  is moved axially forward, away from blade tips  213  and increasing blade tip clearance  240 . 
       FIG. 3  is a schematic and sectional view of another embodiment of a clearance control system  360  in accordance with the present disclosure. Clearance control system  360  comprises an annular shroud  320 , an mechanical driver  310 , and a driving arm  312  coupled between the annular shroud  320  and mechanical driver  310 . In some embodiments mechanical driver  310  may be an actuator. 
     Shroud  320  comprises an inner member  321  and outer member  324 . Inner member  321  has a surface  322  which faces the impeller blades  212  of centrifugal compressor  210 . Both inner member  321  and outer member  324  follow the contour of an impeller blade  212 . Inner member  321  and outer member  324  are coupled at a forward end  325  of shroud  320 . A gap  326  is defined between inner member  321  and outer member  324 . Inner member  321  is coupled to driving an  213  at an aft end  327 . Outer member  324  is shorter than inner member  321  and has a free end  328 . 
     Outer member  324  is coupled to casing arm  330 . In some embodiments, outer member  324  and casing arm  330  are formed as a unitary structure, while in other embodiments outer member  324  and casing arm  330  are formed separately and joined. Casing arm  330  is coupled to a first casing portion  341  and a second casing portion  342  at mounting flange  343 . Thus casing arm  330  anchors or “grounds” the shroud  320  to at least a portion of the engine casing. In some embodiments, one or both of first casing portion  341  and second casing portion  342  are portions of a compressor casing. 
     In some embodiments, mechanical driver  310  passes through or is coupled to mounting flange  343 . Mechanical driver  310  in the illustrated embodiment comprises a cylindrical member  350  adapted to be received by an orifice portion  351  of second casing portion  342 . In some embodiments seals (not shown) may be provided between cylindrical member  350  and orifice portion  351 . Cylindrical member  350  is adapted to be axially driven by a motive force provider (not shown). Cylindrical member  350  imparts axial movement on driving arm  312 , which in turn imparts axial movement on the aft end  327  of inner member  321  of shroud  320 . 
     In some embodiments cylindrical member  350  may be threadably disposed in orifice portion  351 , and motive force provider may be adapted to provide rotating motion to threaded cylindrical member  350 . The rotational movement of cylindrical member  350  may be translated into axial movement. Cylindrical member  350  imparts axial movement on driving arm  312 , which in turn imparts axial movement on the aft end  327  of inner member  321  of shroud  320 . 
     Movement in the axially aft direction of driving arm  312  causes deflection of inner member  321 . The deflective motion of aft member  221  is controlled by the rigid coupling to outer member  324  at the forward end  325  and the variable or flexible coupling to driving arm  312  at aft end  327 . The axially aft deflection of inner member  321  results in the surface  322  moving closer to blade tips  213 , thus reducing the clearance  240  and leakage. During many operating conditions this deflection of aft member  221  in the direction of blade tips  213  is desirable to reduce leakage and increase compressor efficiency. 
     Where movement away from blade tips  213  is desired in order to provide greater blade tip clearance  240 , cylindrical member  350  is rotated in the opposite direction. 
     In some embodiments, mechanical driver  310  is a pneumatic piston and actuation of the piston effects axial translation of aft member  221 . 
     In some embodiments a sealed, pressurized cavity is formed proximal the forward side of forward member  224 . The cavity may be bounded by forward member  224 , and portions of casing  231 ,  232 ,  226 . This cavity may be pressurized using an intermediate stage compressor air, inducer air, or discharge air from the centrifugal compressor  210 . By pressurizing the forward side of forward member  224 , the differential pressure across shroud  220  is reduced, thus reducing the amount of work required to translate aft member  221  axially forward and aft. 
     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. 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.