Abstract:
A system and a method for adjusting an angle of at least one stator vane of a low pressure compressor (LPC) of a two spool gas turbine engine is disclosed. Variable stator vanes are rotatably coupled to a stationary case in one of the stages of the LPC. An actuator is coupled to at least one of the variable stator vanes for imparting rotation of the stator vane about a radius of the case. Various coupling or linkage arrangements may be made so that rotation of one vane results in rotation of the other vanes disposed along the case. The controller includes the stored constraints, the ability to estimate operating condition, and the ability to estimate optimum targets.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to gas turbine engines, and more particularly to dual-spool turbofan or turboprop engines. Still more specifically, this disclosure relates to variable stator vanes of the low pressure compressor (LPC) of such engines and control schemes for such variable stator vanes. 
       BACKGROUND 
       [0002]    Two spool turbofan and turboprop jet engines typically include three sections in the core. The first section is the compressor section, which includes a low pressure compressor (LPC) followed by a high pressure compressor (HPC). A combustor section is disposed between the compressor section and a turbine section, which includes a high pressure turbine (HPT) followed by a low pressure turbine (LPT). The LPT is connected to and drives the LPC via one shaft and the HPT is connected to the HPC via a second shaft. 
         [0003]    Turbofan and turboprop engines operate by combusting fuel in air that has been compressed by the LPC and HPC of the compressor section. The combustion takes place in the combustor section to create heated gases with increased pressure and density. The heated gases are used to rotate the HPT and LPT in the turbine section that are used to produce thrust or power. For example, in a turbofan, the heated gases are ultimately forced through an exhaust nozzle at a velocity higher than which inlet air is received into the engine to produce thrust for driving an aircraft. The heated gases also rotate the HPT and LPT, which are used to drive the HPC and LPC respectively, which generate the compressed air necessary to sustain the combustion process. 
         [0004]    The compressor (LPC and HPC) and turbine (HPT and LPT) sections of a turbofan or turboprop engine typically comprise a series of stator vane and rotor blade stages. The stator vanes of each stage are positioned in front of a rotor to efficiently direct air flow to the blades of the rotor. In general, the stator vanes redirect the trajectory of the air coming off the rotors of the preceding stage for flow into the next stage. 
         [0005]    In the compressors, the stator vanes convert kinetic energy of the moving air into pressure, while, in the turbines, the stator vanes accelerate pressurized air to extract kinetic energy. Turbofan and turboprop efficiencies are, therefore, closely linked to the ability of the engine to efficiently direct air flow within the compressor and turbine sections of the engine. Air flows through the compressor and turbine sections differ at various operating conditions of the engine, with more air flow being required at higher output levels and vice versa. 
         [0006]    Variable stator vane assemblies are utilized to improve the performance and operability of the engine. Variable stator vane assemblies typically include variable stator vanes which extend forward of rotor blades. The variable stator vanes are rotatable about substantially radial axes. The orientation of the variable stator vanes varies the attack angle of the vanes in a controlled fashion. This allows the vanes to be realigned to change the impingement angle of compressed air onto the following rotor blades as the operating condition of the engine changes. The position of the vanes may be changed by many different means, including, but not limited to a lever arm attached to an actuator ring on the outside of the compressor case or a gear driven arrangement. Thus, air flow through the engine can be controlled, in part, by using variable stator vanes and variable stator vanes have been used to advantageously control the incidence of air flow onto rotor blades of subsequent compressor under different operating conditions. 
         [0007]    However, schemes for controlling variable stator vanes are generally lacking. Engines without LPC variable stator vanes modify the stability of the LPC using bleed air, which detracts from the performance and efficiency of the engine. 
         [0008]    Therefore, there is a need for a control scheme for altering the positions of the LPC stator vanes that is more flexible than the currently available control schemes and which can be used to advantageously control various operating parameters including overall compressor pressure ratio (i.e., the ratio of the pressure at the aft end of the HPC to the pressure at the forward end of the LPC), compressor corrected air flow, bypass ratio (i.e., the ratio of the air entering the core shroud to the air entering the inlet shroud), engine temperatures, spool speed (rpm), and compressor operating line, while reducing fuel consumption. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    In an aspect, the system for adjusting an angle of at least one stator vane of a low pressure compressor (LPC) of a two spool gas turbine engine is disclosed. The system includes at least one stator vane rotatably coupled to a stationary case and directed inwardly along a radius of the case. The system also includes an actuator coupled to at least one stator vane for imparting rotation to the stator vane about said radius. The actuator is linked to a controller. The controller includes a memory stored with constraints defined by corresponding LPC compressor pressure ratios and LPC compressor corrected air flows. The controller sends a signal to rotate at least one stator vane to alter at least one of the compressor pressure ratio or compressor corrected air flow values to change the current operating condition of the LPC to a target operating condition such that the engine remains within its stability constraint. 
         [0010]    In another aspect, a system is disclosed for adjusting an angle of least one stator vane of a low pressure compressor of a two spool gas turbine engine. The disclosed system includes at least one stator vane being rotatably coupled to a circular stationary case and directed inwardly along a radius of the case. The system also includes an actuator coupled to at least one stator vane for imparting rotation to the stator vane about said radius. The actuator is linked to a controller. The controller includes a memory with stability constraints for the LPC stored therein for a plurality of operating conditions. The map includes a stability constraint defined by LPC compressor pressure ratios along a y-axis and LPC compressor corrected air flows along an x-axis. The constraint is disposed below the stall line on the stall map. The controller is programmed to send a signal to rotate at least one stator vane to alter at least one of the compressor pressure ratio or compressor corrected air flow to change a current operating condition of the LPC to a target operating condition that delivers more higher performance but within its stability constraint. 
         [0011]    A method is also disclosed for optimizing performance of a two spool gas turbine engine. The engine includes a low pressure compressor (LPC) having at least one stage that includes a ring of stator vanes that are rotatably coupled to and directed radially inward from the case. The ring of stator vanes provides a design space within the low pressure compressor whereby rotating the stator vanes enables the operating conditions of the engine to be optimized as the vanes can be rotated and the operating conditions can be evaluated to ensure that the engine is operating within established operating and temperature constraints. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine in which a disclosed variable vane control system may be used. 
           [0013]      FIG. 2  is a flow diagram of one disclosed control scheme. 
           [0014]      FIG. 3  is a compressor map that graphically illustrates the relationship between compressor pressure ratio and compressor corrected air flow and the changing of the operating conditions to move from a current operating point to a target operating point. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a schematic cross section of a gas turbine engine  10  which includes a variable vane actuation mechanism  11 . In the embodiment shown, the gas turbine engine  10  includes a dual-spool high bypass ratio turbofan engine having a variable vane turbine section that incorporates the actuation mechanism  11 . In other embodiments, the gas turbine engine  10  may be another type of gas turbine engine used for aircraft propulsion or power generation, or other systems incorporating variable stator vanes. Although the actuation mechanism  11  is well suited for the low pressure compressor (LPC)  12 , the disclosed system is readily applicable to the high pressure compressor (HPC)  13 , the high pressure turbine (HPT)  14  as well as the low pressure turbine (LPT)  15 . 
         [0016]    The operating principals of the gas turbine engine  10  are well known in the art. Briefly, the gas turbine engine  10  includes a fan  16 , followed by the LPC  12 , HPC  13 , combustor section  17 , HPT  14 , and LPT  15 , all of which are disposed about an axis  18  of the engine  10 . The fan  16 , LPC  12 , HPC  13 , HPT  14 , LPT  15  and other engine components are enclosed at their outer diameters within various engine casings that are disposed within a nacelle or core nacelle (not shown). The fan  16  is disposed within the fan case  18 . The LPC  12  is disposed within a LPC case  19 ; the HPC  13  is disposed within a HPC case  21 ; the HPT  14  is disposed within a HPT case  22  and the LPT  15  is disposed within an LPT case  23 . The fan  16  and LPC  14  are connected to the LPT  22  through the shaft  24 , which is supported by the bearings  25 ,  26  at its forward end and the bearing  27  at its aft end. Together, the fan  16 , LPC  12 , LPT  15  and shaft  24  collectively form a low pressure spool. HPC  13 , is connected to HPT  14  through the shaft  28 , which is supported within the engine by the bearings  31 ,  32 . Together, the HPC  13 , HPT  14  and shaft  28  form the high pressure spool. 
         [0017]    Inlet air A enters the engine  10  whereby it is divided into streams of primary air A p  and secondary air A s  after passing through the fan  16 . The bypass ratio is the ratio of the primary A p  over the secondary air A s . The fan  16  is rotated by the low pressure turbine  15  through the shaft  24  to accelerate the secondary air A s  (also known as bypass air) through the exit guide vanes  33 , thereby producing a significant portion of thrust output of the engine  10 . Primary air A p  (also known as gas path air) is directed first to the LPC  12  and then to the HPC  13 . The LPC  12  and HPC  13  work together to incrementally increase the pressure and temperature of the primary air A p . The HPC  13  is rotated by the HPT  14  through the shaft  28  to provide compressed air to the combustor  17 . The compressed air is delivered to the combustor  17 , along with fuel from the injectors  35 ,  36 , such that a combustion process can be carried out to produce high energy gases necessary to rotate the HPT  14  and LPT  15 . Primary air A p  continues through the engine  10  where it is typically passed through an exhaust nozzle to produce additional thrust. 
         [0018]    Flow of primary air A p  through the engine  10  is enhanced by the use of variable stator vanes. In particular, LPC  12  includes variable stator vanes  38 , which may be disposed between rotor blades  39  or, the rotary vanes  38  may be disposed at the forward stage of the LPC  12  as shown in  FIG. 1 , thereby placing the rotary stator vanes  38  in front of the rotor blades  39 . One or more sets of variable stator vanes  38  may be employed in the LPC  12 , HPC  13 , HPT  14  or LPT  15  in accordance with this disclosure. 
         [0019]    The pitch of the variable vanes  38  may be adjusted by the actuator  41 . A variety of means of adjusting the attack angle of the vanes  38  are available and will be apparent to those skilled in the art. Gear mechanisms, lever mechanisms and combinations of the two are available. The variable stator vanes  38  are accommodated within a circular case indicated schematically at  42 . The vanes  38  may be rotatably coupled to the case  42  and directed radially inwardly towards the axis  18  or along a radius of the annular case  42 . The vanes  38  rotate about their respective radial axes, which extend at least substantially perpendicular to the engine axis  18 . When actuated, if multiple variable vanes  38  are involved, the vanes  38  are rotated to adjust the flow of the primary air A p  through the engine  10  for different operating conditions. For example, when the engine  10  undergoes a transient loading such as during a take-off operation, the mass flow of the primary air A p  increases as the engine  10  goes from an idle to a high-throttle operation. As such, the pitch of the variable vanes  38  may be continually altered to among other things, improve air flow and prevent stalling. The actuator  41  for the variable stator vanes  38  is linked to a controller  42  as shown in  FIG. 1 . 
         [0020]    Turning to  FIGS. 2-3 , the controller  42  includes the ability to know its operating point, limits, and target on the compressor map  44  as shown in  FIG. 3 . The y-axis  45  of the compressor map  44  represents the compressor pressure ratio, or the ratio of the pressure of the primary air A p  exiting the LPC  12  divided by the pressure of the primary air A p  entering the LPC  12 . The x-axis  46  represents the compressor corrected air flow. 
         [0021]    The compressor map  44  of  FIG. 3  includes two lines or plots  47 ,  48 . The line  47  represents a stall line. That is, compressor pressure ratio/compressor corrected air flow coordinates falling on line  47  or above line  47  may result in the LPC  12  stalling. Below the line  47  is the line  48 , which can be considered to be a stability limit line or a “not-to-exceed” operating line. Thus, operating conditions may approach the line  48  without fear of stalling the LPC  12  because of the protective operating margin between the lines  48  and  47 . To adjust the operation of the engine  10  from the operating point  51  to the target operating point  52 , the method of  FIG. 2  is carried out. 
         [0022]    Referring back to  FIG. 2 , various sensors and controllers associated with the engine  10  detect or calculate operating parameters at step  54 . The controller  42  may calculate an optimum performance target at step  55  which may or may not be independent from the measurements and calculations carried out in step  54 . At step  56 , the controller  42  is in communication with the actuator  41  but the actuator  41  has yet to move the adjustable vanes  38 . At step  57 , the controller  42  determines whether the current operating point  51  ( FIG. 3 ) is sufficiently different or not within tolerance of the target operating point  52 . If the operating point  51  is not within the desired tolerance, the controller  42  sends a signal to the actuator  41  and the vanes  38  are adjusted at step  56  before the comparison of step  57  is repeated. Once the current operating point  51  is sufficiently close to the target operative point  52  so as to be within the desired tolerance, the controller  42  determines whether the engine  10  is operating within desired limits at step  58 . In other words, at step  58 , the controller  42  determines whether the new operating point  51  is above or within the limit line  48  of  FIG. 3 . Assuming the new operating point  51  is disposed within the limit line  48 , the variable vane  38  setting is accepted at step  59  and the process is repeated as a continuous loop. 
       INDUSTRIAL APPLICABILITY 
       [0023]    Thus, a system and a method for adjusting the angle of variable stator vanes of a LPC of a two spool gas turbine engine are disclosed. An actuator is coupled to at least one of the variable stator vanes for imparting rotation to the stator vane to adjust the angle. The stator vane coupled to the actuator may be linked or coupled to the other stator vane in the stage using known linkage, lever or gear mechanisms. The system for adjusting the stator vanes may utilize two or more parameters, including the compressor pressure ratio and the compressor corrected air flow. Sensors and calculations are used to measure the compressor pressure ratio and the compressor corrected air flow to determine the current operating point. If the current operating point is sufficiently different from the closest stall margin value, the controller will command the actuator to rotate the variable vanes to increase the compressor pressure ratio and/or the compressor corrected air flow but not to a point where the desired target operating point is at or beyond limits. 
         [0024]    The disclosed control system may be used as original equipment on new engines or may be added as a retrofit to existing turbofan, turboprop or other dual-spool gas turbine engines.