Patent Publication Number: US-8122724-B2

Title: Compressor including an aerodynamically variable diffuser

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTROSS-REFERENCES TO RELATED APPLICATIONS 
     This invention was made with Government support under Contract Number DAA-H10-02-0003 awarded by the U.S. Army. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to compressors and, more particularly, to a compressor that includes an aerodynamically variable diffuser. 
     BACKGROUND 
     Aircraft main engines not only provide propulsion for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main engines may not be capable of supplying power. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may additionally be used to start the main propulsion engines. 
     An APU is, in most instances, a gas turbine engine that includes a combustor, a power turbine, and a compressor. During operation of the APU, compressor draws in ambient air, compresses it, and supplies compressed air to the combustor. The combustor receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy compressed air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive the compressor. In some instances, an APU may additionally include a starter-generator, which may either drive the turbine or be driven by the turbine, via the turbine output shaft. Some APUs additionally include a bleed air port between the compressor section and the turbine section. The bleed air port allows some of the compressed air from the compressor section to be diverted away from the turbine section, and used for other functions such as, for example, main engine starting air, environmental control, and/or cabin pressure control. 
     Although most APUs, such as the one generally described above, are robust, safe, and generally reliable, some APUs do suffer certain drawbacks. For example, when some APUs are operated at part power, the surge margin of the APU compressor, or at least one or more stages of the compressor, can be reduced. Thus, in many instances APU compressors include a diffuser that is not optimally designed for operations over the entire operational envelope of the APU, which can result in reduced operational efficiency (e.g., increased specific fuel consumption (SFC)) of the APU. Alternatively, some APUs include a surge valve, or mechanically variable diffuser vanes, which allow the geometry of the vanes within the diffuser to be varied. However, these systems and methods can be inefficient, complex, and costly. 
     Hence, there is a need for a system and method of controlling the surge margin, and thus improving the overall operational efficiency, of a compressor, that is more efficient, less complex, and less costly than existing systems and methods. The present invention addresses one or more of these needs. 
     BRIEF SUMMARY 
     The present invention provides a system and method of controlling the surge margin, and thus improving the overall operational efficiency, of a compressor. 
     In one embodiment, and by way of example only, a compressor includes a housing, an impeller, a diffuser, a recirculation duct, and a valve. The impeller is rotationally mounted within the housing and has a leading edge and a trailing edge. The impeller is operable, upon rotation thereof, to discharge a flow of relatively low pressure, high velocity air from the trailing edge. The diffuser has at least an air inlet and an air outlet. The diffuser air inlet is in fluid communication with the impeller trailing edge to thereby receive the flow of air supplied therefrom. The diffuser is configured to reduce the velocity magnitude of the air, increase the pressure magnitude of the air, and discharge the air at the increased pressure magnitude from the diffuser air outlet. The recirculation duct has an air inlet and an air outlet. The recirculation duct air inlet is in fluid communication with the diffuser air outlet, and the recirculation duct air outlet is in fluid communication with the diffuser air inlet. The valve is disposed at least partially within the recirculation duct and is moveable between at least a closed position, in which the recirculation duct air inlet is fluidly isolated from the recirculation duct air outlet, and an open position, in which the recirculation duct air inlet is in fluid communication with the recirculation duct air outlet, to thereby inject at least a portion of the relatively high pressure air discharged from the diffuser air outlet into the diffuser air inlet. 
     In another exemplary embodiment, a gas turbine engine includes a housing, a compressor, a combustor, a turbine, a recirculation duct, and a valve. The compressor, combustor, and turbine are all mounted in flow series within the housing, and the compressor includes at least an impeller and a diffuser. The impeller has a leading edge and a trailing edge, and is configured to rotate and is operable, upon rotation, to supply a flow of air having a velocity magnitude and a pressure magnitude. The diffuser has at least an air inlet and an air outlet. The diffuser air inlet is in fluid communication with the impeller trailing edge to thereby receive the flow of air supplied therefrom. The diffuser is configured to reduce the velocity magnitude of the air, increase the pressure magnitude of the air, and discharge the air at the increased pressure magnitude from the diffuser air outlet. The recirculation duct has an air inlet and an air outlet. The recirculation duct air inlet is in fluid communication with the diffuser air outlet, and the recirculation duct air outlet is in fluid communication with the diffuser air inlet. The valve is disposed at least partially within the recirculation duct and is moveable between at least a closed position, in which the recirculation duct air inlet is fluidly isolated from the recirculation duct air outlet, and an open position, in which the recirculation duct air inlet is in fluid communication with the recirculation duct air outlet, to thereby inject at least a portion of the air discharged from the diffuser air outlet into the diffuser air inlet. 
     In still another exemplary embodiment, a turbocharger includes a housing, a turbine, and a compressor, a recirculation duct, and a valve. The turbine is rotationally mounted within the housing, is configured to receive a flow of gas, and is operable, upon receipt of the gas flow, to supply a drive force. The compressor is mounted in within the housing and is coupled to receive the drive force from the turbine, and includes at least an impeller and a diffuser. The impeller has a leading edge and a trailing edge, and is configured to rotate and is operable, upon rotation, to supply a flow of air having a velocity magnitude and a pressure magnitude. The diffuser has at least an air inlet and an air outlet. The diffuser air inlet is in fluid communication with the impeller trailing edge to thereby receive the flow of air supplied therefrom. The diffuser is configured to reduce the velocity magnitude of the air, increase the pressure magnitude of the air, and discharge the air at the increased pressure magnitude from the diffuser air outlet. The recirculation duct has an air inlet and an air outlet. The recirculation duct air inlet is in fluid communication with the diffuser air outlet, and the recirculation duct air outlet is in fluid communication with the diffuser air inlet. The valve is disposed at least partially within the recirculation duct and is moveable between at least a closed position, in which the recirculation duct air inlet is fluidly isolated from the recirculation duct air outlet, and an open position, in which the recirculation duct air inlet is in fluid communication with the recirculation duct air outlet, to thereby inject at least a portion of the air discharged from the diffuser air outlet into the diffuser air inlet. 
     In yet a further exemplary embodiment, in a gas turbine engine including at least compressor that has an impeller, and a diffuser disposed downstream of the impeller, and in which the diffuser includes at least an air inlet and an air outlet, a method of varying the surge margin of the compressor includes determining an operational state of the engine, and selectively supplying air discharged from the diffuser air outlet to the diffuser air inlet based at least in part on the determined operational state. 
     Other independent features and advantages of the preferred gas turbine engine and compressor systems and methods will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an auxiliary power unit (APU) according to an exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  are cross section view of an exemplary and alternative compressor, respectively, that may be used in the APU of  FIG. 1 ; 
         FIGS. 3 and 4  are three dimensional partial cross section views of an exemplary shroud that may be used in the compressor of  FIG. 2A  or  2 B; and 
         FIG. 5  is a graph depicting the operational characteristics of an exemplary diffuser in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Before proceeding with a detailed description, it is to be appreciated that the described embodiment is not limited to use in conjunction with a particular type of turbine engine or particular type of compressor. Thus, although the present embodiment is, for convenience of explanation, depicted and described as being implemented in a single-stage centrifugal compressor, and in an auxiliary power unit, it will be appreciated that it can be implemented as various other types of compressors, engines, turbochargers, and various other fluid devices, and in various other systems and environments. 
     Turning now to the description, and with reference first to  FIG. 1 , an embodiment of an exemplary auxiliary power unit (APU)  100  is shown in simplified schematic form. The APU  100  includes a compressor  102 , a combustor  104 , a turbine  106 , and a starter-generator unit  108 , all preferably housed within a single containment housing  110 . During operation of the APU  100 , the compressor  102  draws ambient air into the containment housing  110 . The compressor  102  compresses the ambient air, and supplies a portion of the compressed air to the combustor  104 , and may also supply compressed air to a bleed air port  105 . The bleed air port  105 , if included, is used to supply compressed air to a non-illustrated environmental control system. It will be appreciated that the compressor  102  may be any one of numerous types of compressors now known or developed in the future. In a particular preferred embodiment, however, the compressor is a two-stage centrifugal compressor, an embodiment of which is described in more detail further below. 
     The combustor  104  receives the compressed air from the compressor  102 , and also receives a flow of fuel from a non-illustrated fuel source. The fuel and compressed air are mixed within the combustor  104 , and are ignited to produce relatively high-energy combustion gas. The combustor  104  may be implemented as any one of numerous types of combustors now known or developed in the future. Non-limiting examples of presently known combustors include various can-type combustors, various reverse-flow combustors, various through-flow combustors, and various slinger combustors. 
     No matter the particular combustor configuration  104  used, the relatively high-energy combustion gas that is generated in the combustor  104  is supplied to the turbine  106 . As the high-energy combustion gas expands through the turbine  106 , it impinges on the turbine blades (not shown in  FIG. 1 ), which causes the turbine  106  to rotate. It will be appreciated that the turbine  106  may be implemented using any one of numerous types of turbines now known or developed in the future including, for example, a vaned radial turbine, a vaneless radial turbine, and a vaned axial turbine. In a particular preferred configuration, several embodiments of which are described further below, the turbine  106  is implemented as a vaneless radial turbine. No matter the particular type of turbine that is used, the turbine  106  includes an output shaft  114  that drives the compressor  102 . Moreover, depending on the mode in which the APU  100  is operating, the turbine  106 , via the output shaft  114 , may also drive the starter-generator unit  108 , or alternatively the turbine  106  may be driven by the starter-generator unit  108 . 
     Turning now to  FIG. 2A , a more detailed description of the compressor  102  will be provided. In the depicted embodiment, the compressor  102  is a single-stage centrifugal compressor and includes an impeller,  206 , a shroud  208 , and a diffuser  210 . The impeller  206  is mounted on the output shaft  114 , via a hub  212 , and is thus rotationally driven by either the turbine  106  or the starter-generator  108 , as described above. A plurality of spaced-apart blades  214  extend generally radially from the hub  212  and together therewith define a leading edge  201  and a trailing edge  203 . As is generally known, when the impeller  206  is rotated, the blades  214  draw air into the impeller  206 , via the leading edge  201 , and increase the velocity of the air to a relatively high velocity. The relatively high velocity air is then discharged from the impeller  206 , via the trailing edge  203 . 
     The shroud  208  is disposed adjacent to, and partially surrounds, the impeller blades  214 . The shroud  208 , among other things, cooperates with an annular inlet duct  218  to direct the air drawn into the APU  100  by the compressor  102  into the impeller  206 . As will be described in more detail further below, the shroud  208 , at least in the depicted embodiment, additionally includes a plurality of injection passages  220  (only one shown in  FIG. 2 ), which are used to inject air from downstream of the diffuser  210  back into the region upstream of the diffuser  210 . 
     The diffuser  210  is disposed adjacent to, and surrounds a portion of, the impeller  206 , and includes an air inlet  222  and an air outlet  224 . In the depicted embodiment, the diffuser  210  is a radial vaned diffuser and, thus, further includes a plurality of diffuser vanes  226 . However, it will be appreciated that the diffuser  210  could be implemented as any one of numerous other diffusers, including a vaneless radial diffuser. The diffuser vanes  226  are arranged substantially tangential to the impeller trailing edge  203  and, similar to the impeller blades  214 , define a leading edge  209  and a trailing edge  211 . As shown in  FIG. 2A , the diffuser air inlet  222  is in fluid communication with the impeller trailing edge  203 . Thus, the relatively high velocity air discharged from the impeller  206  flows into and through the diffuser air inlet  222 . As the air flows through the diffuser  210 , the diffuser  210  reduces the velocity of the air and increases the pressure of the air to a higher magnitude. 
     During some operating conditions, such as during part speed operations, the inlet flow of the compressor  102  can be reduced and, in some instances, can result in a surge condition. To increase the surge margin of the compressor  102  for a given airflow into the compressor  102 , the compressor  102  further includes a recirculation system  250 . The recirculation system  250  includes a recirculation duct  252  and one or more flow control valves  254 . In the depicted embodiment, the recirculation duct  252  is coupled between a discharge duct  228  downstream of the diffuser outlet  224  and the shroud  208 , and includes an air inlet  256  and an air outlet  258 . Thus, the recirculation duct air inlet  256  is in fluid communication with the diffuser air outlet  224 , via the flow duct  228 , and the recirculation duct air outlet  258  is in fluid communication with the diffuser air inlet  222 , via the shroud  208 . 
     In the depicted embodiment, the flow control valve  254  is mounted on, and extends into, the recirculation duct  252 . The flow control valve  254 , which may be any one of numerous types of valves, is moveable between a closed position and an open position. In the closed position, the recirculation duct air inlet  256  is fluidly isolated from the recirculation duct air outlet  258 , and air flow through the recirculation duct  252  is prevented. Conversely, when the flow control valve  254  is in the open position, which may be the full-open position or any one of numerous throttled positions, the recirculation duct air inlet  256  is in fluid communication with the recirculation duct air outlet  258 , and a portion of the air discharged from the diffuser air outlet  224  flows through the recirculation duct  252  and is injected into the diffuser air inlet  222 . It will be appreciated that although a single flow control valve  254  is shown, a plurality of flow control valves  254  could be used, and the positions of each scheduled to provide a desired flow through the recirculation duct  252 . 
     Before proceeding further, it should be appreciated that although the recirculation duct  252  is depicted as being implemented as a separate duct or conduit, this is merely exemplary of any one of numerous implementations. For example, as shown in  FIG. 2B , an open space or cavity within the containment housing  110  could be used to define the recirculation duct  252 . Moreover, although the recirculation duct air inlet  256  is shown coupled immediately downstream of the diffuser air outlet  225 , it will be appreciated that it could be coupled at any one of numerous locations downstream of the diffuser air outlet  224 . It will additionally be appreciated that mounting the flow control valve  254  on the recirculation duct  252  is merely exemplary, and that it could be mounted elsewhere. For example, the flow control valve  254  could be mounted on the compressor discharge duct  228 , as is shown in  FIG. 2B , or on the shroud  208 . 
     Returning once again to the description, the position of the flow control valve  254  is preferably controlled via valve position commands supplied by a controller  262 . As such, the flow control valve  254  includes a valve actuator  264  that is configured to receive the valve position commands and, in response to the commands, move the flow control valve  254  to the commanded position. It will be appreciated that the valve actuator  264  may be any one, or combination, or numerous types of actuators including, for example, pnuematic, hydraulic, or electric. It will additionally be appreciated that the controller  262  may be a dedicated valve controller, or may be implemented as part of another system controller. For example, the controller  262  may be an engine controller, such as a FADEC, that includes the additional functionality of positioning the flow control valve  254 . No matter the specific manner in which the controller  262  is implemented, the controller  262  preferably receives one or more sensor signals from one or more sensors  266 . These sensor signals are one or more signals representative of engine operating conditions such as, for example, impeller rotational speed, shaft speed, and bleed air flow just to name a few. In response to the sensor signals, the controller  262  determines whether the flow control valve  254  should be commanded to the open or closed position. 
     As was noted above, when the flow control valve  254  is in the open position, a portion of the air discharged from the diffuser air outlet  224  flows through the recirculation duct  252  and is injected into the diffuser air inlet  222 . Preferably, when the air from the diffuser air outlet  224  is injected into the diffuser air inlet  222 , the flow is injected such that the effect on diffuser performance is minimal. To implement this desired functionality, the air in the recirculation duct  252  is injected into the diffuser air inlet  222  via a plurality of injection passages  220 . As was previously noted, these injection passages  220  are preferably formed through the shroud  208 . In the embodiment depicted in  FIG. 2A , the injection passages  220  are implemented as a plurality of flow passages. However, as depicted in the alternative embodiment depicted in  FIG. 2B , instead of a plurality of injection passages  220 , one (or more) injection passage  220 , implemented as an annular slot  223 , could be used. It will additionally be appreciated that in other alternative embodiments the injection passage(s)  220  could be formed as part of the recirculation duct air outlet  258 , the diffuser  210 , or various other portions of the compressor  102 , and not just as part of the shroud  208 . 
     Turning now to  FIGS. 3 and 4 , an exemplary physical implementation of a portion of the shroud  208  is shown, and depicts a particular preferred configuration of the injection passages. As was just noted, the injection passages  220  are preferably configured such that when air from the recirculation duct  252  is injected into the diffuser inlet  222 , its effect on performance is minimal. Thus, in a particular preferred embodiment, the injection passages  220  are configured to inject a predetermined amount of flow into the diffuser air inlet  222 . In addition, as shown most clearly in  FIG. 3 , the injection passages  220  are preferably configured to inject the flow at a predetermined side (or meridional) injection angle (α M ), and at predetermined swirl (or tangential) injection angle (α T ) that yield the minimal impact on diffuser  210  performance. The amount of flow that is injected is based, at least in part, on the increase in surge margin desired. The injection passages  220  are preferably configured to control the amount of flow, and to provide enough guidance to direct the flow at the appropriate meridional angle (α M ) and tangential angle (α T ). 
     It will be appreciated that each of the above-described parameters may vary from application to application. However, in a particular preferred exemplary implementation, the meridional angle (α M ) is preferably less than or equal to about 20-degrees, and the tangential angle (α T ) is preferably less than or equal to about 65-degrees and most preferably close to the diffuser leading edge angle. Moreover, the injection passages  220  are preferably configured to inject 10% flow into the diffuser air inlet  222 . In one particular physical implementation, it has been found that including 67 injection passages  220  in the shroud  208  that are formed at the predetermined meridional angle (α M ) and tangential angle (α T ), and having an effective length-to-diameter ratio of about 4, provide the desired characteristics. 
     Having described the configurations of the engine  100 , the compressor  102 , and the recirculation system  250 , and the general functionality of each, a more detailed description of the overall operation and interaction of the engine  100  and recirculation system  150  will now be provided. As noted above, the engine  100  includes various sensors  266  that sense one or more engine operating conditions, and supply sensor signals representative of the sensed parameters to the controller  262 . The controller  262 , based at least in part on the sensor signals, determines the operational state of the engine  100  and supplies, among other things, appropriate command signals to the valve actuator  264 , to thereby move the valve  254  to the appropriate position. 
     For example, if the controller  262  determines that the operating conditions of engine  100  are such that a surge condition is not likely, such as at full power on a turbine engine or bleed mode in an APU, the controller  262  will issue appropriate command signals that ensure the flow control valve  254  is in, or is moved to, the closed position. As a result, no air discharged from the compressor  202  is circulated back to the diffuser air inlet  222 . Conversely, if the controller  262  determines that the operating conditions of the engine  100  are such that the surge margin is sufficiently reduced, or a surge condition is potentially likely, such as operation at part speed on a turbine engine or shaft output only on an APU, the controller  262  will issue appropriate valve position command signals to cause the flow control valve  154  to move to the open position. As a result, a portion of the air discharged from the compressor  202  will be bled away from the compressor second stage  204 , and into the recirculation duct  252 . The air in the recirculation duct is then directed into the diffuser air inlet  222 . 
     The system and method described herein aerodynamically varies the effective area at the diffuser leading edge  209 . Specifically, when air discharged from the compressor  202  is circulated back, and injected into, the diffuser air inlet  222 , the injected flow reduces the effective flow area at the diffuser air inlet  222 . As shown in  FIG. 5 , this reduced effective flow area shifts the diffuser loss bucket, thereby providing a more optimal impeller/diffuser match at lower power or non-bleed-air conditions and moves the diffuser away from surge. 
     Although the compressor  102  was depicted and described herein as being implemented as a single-stage centrifugal compressor, and in an auxiliary power unit, it will be appreciated that it can also be implemented as various other types of compressors, and in various types of engines, turbochargers, and various other fluid devices, and in various other systems and environments. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.