Abstract:
Cooling air to the blades and disks of a gas turbine may be modulated to provide a variable turbine cooling flow. A bellows may be extended by providing a high pressure compressor discharge flow to an interior of the bellows. The bellows may be compressed when the interior of the bellows communicates with ambient pressure air. The extension/compression of the bellows moves an arm over orifices in a cooling air flow path. The pressure inside of the bellows is metered to move the arm over at least one orifice, thereby restricting cooling air flow when the engine is running at low power. The pressure inside of the bellows is metered to move the arm to uncover all of the slots to provide maximum cooling flow when the engine is running at high power. The resulting variable cooling flow system results in less need for cooling air at low powers, thus reducing engine fuel consumption.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to a system for providing cooling flow to the turbine components of a gas turbine and, more particularly, to a system for providing a variable turbine cooling flow to the turbine at various power and altitude conditions.  
         [0002]     A conventional turbine rotor cooling system may be designed to satisfy cooling needs of the high pressure turbine (HPT) blade and of the turbine disk at maximum turbine inlet temperature and compressor discharge conditions. This condition usually occurs at maximum engine power. At lower power settings, the turbine coolant air and gaspath temperatures are much lower. Coolant air temperature is lower at altitude than at sea level, even when gaspath temperature is high. In these cases, since the cooling circuit geometry is fixed the coolant flow rate requirements are less than the actual flow rate. The source of the cooling air is compressor discharge air. Use of an excess of this air, because it has had compression work done on it by the engine, adversely impacts the engine performance. For many applications, the desire to minimize fuel burn is most critical at lower power levels, because more operating time is spent at lower power levels as compared to time spent at high power.  
         [0003]     Referring to  FIG. 1 , there is shown a conventional HPT cooling system. Turbine coolant air,  100 , is delivered from the compressor section of the engine. Compressor discharge air flows as indicated by arrow  102  passes through passages  104 . After passing through passages  104 , the cooling air is divided into three separate cooling flows, as indicated by arrows  105 , 106 ,  108 . Cooling flow  105  provides cooling air for a downstream turbine disk (not shown). Cooling flow  106  provides cooling air for the high pressure turbine disk  110  and turbine blade,  112 . Cooling flow  108  provides cooling air for the sealplate forward of the turbine disk.  
         [0004]     U.S. Pat. No. 5,996,331 discloses a passive system in which a modulating device is arranged in the engine such that pressures generated within the engine may naturally activate a bellows which axially positions a sleeve over a flow control restriction. This passive system relies on pressures generated in the engine to naturally activate the bellows. There is no active control relationship between the temperature and/or power level angle and the amount of cooling flow through the turbine.  
         [0005]     U.S. Pat. No. 4,217,755 discloses a control valve for effecting flow modulation using plenum discharge/ambient pressures for flow control. Axial annular member  140  acts as an axial spring to push leaf spring  144  axially closed or pull leaf spring axially open ( FIG. 1 ). Annular member  140  and leaf spring  144  are assembled in a configuration that is prone to vibration oscillation and potential “Hemholtz” cavity resonation which may cause vibrational fatigue in gas turbine engines. The radial cooling over the leaf spring end  152  may also result in “reeding” and increased noise in the engine. In addition, this system has very little vibrational damping. This could result in component fatigue and cooling flow oscillation as the leaf spring system resonates. This patent requires that flow migrate past the flow control leaf spring outer edge  152  to provide full turbine cooling flow. Additionally, this patent uses a large radial leaf spring  144  to effect flow modulation. Many applications do not have the radial space to incorporate this size of an assembly. Further, this patent requires evacuation of the undercut cavity  130  to produce the required pressure differential across the leaf spring to open the flow circuit and adequately cool the turbine at high temperature conditions. Thus, if the exhaust circuit  164  gets plugged with dirt, compressor abraidable, etc. then the system may fail to deliver acceptable turbine cooling.  
         [0006]     As can be seen, there is a need for an improved turbine cooling flow system that varies the amount of coolant flow being delivered to the turbine blades and disk at low power conditions.  
       SUMMARY OF THE INVENTION  
       [0007]     In one aspect of the present invention, a variable flow cooling system comprises a source of cooling fluid providing a cooling fluid flow along a flow path; a flow restriction element in the flow path having at least two sets of orifices therein; a source of pressurized fluid; a bellows consisting of two parallel wave seals or springs having a flow line connecting an interior of the bellows with either the source of pressurized fluid or with ambient air; an arm moveably attached to the bellows wherein movement of the bellows moves the arm to either an open position, covering none of the orifices, or a closed position, covering at least one set of the orifices, thereby restricting flow of the cooling fluid through the flow path.  
         [0008]     In another aspect of the present invention, a variable flow cooling system for providing a cooling air flow to components of a gas turbine engine comprises a source of cooling air providing the cooling air flow along a flow path; a flow restriction element in the flow path having at least two sets of orifices therein; a source of pressurized air; a bellows having a flow line connecting an interior of the bellows with either the source of pressurized air or with ambient air; an arm moveably attached to the bellows; a bellows housing; the interior of the bellows is defined by the two springs of the bellows, the housing, and the arm; the arm connected to the bellows free end, thereby moving the arm with the bellows free end, wherein movement of the bellows free end moves the arm to either an open position, covering none of the orifices, or a closed position, covering at least one of the two sets of orifices thereby restricting flow of the cooling fluid through the flow path.  
         [0009]     In yet another aspect of the present invention, a variable flow cooling system for providing a cooling air to components of a gas turbine engine comprises a source of cooling air providing the cooling air along a flow path; a flow restriction element in the flow path having two sets of orifices therein; a source of pressurized air provided by a turbine engine compressor discharge; a bellows having a flow line connecting an interior of the bellows with either the source of pressurized air or with ambient air; an arm moveably attached to the bellows; a bellows housing, a bellows free end and the arm; the arm connected to the bellows free end, thereby moving the arm with the bellows free end, wherein movement of the bellows free end moves the arm to either an open position, covering none of the orifices, or a closed position, covering one of the two sets of orifices, thereby restricting flow of the cooling fluid through the flow path; a stop, the arm contacting the stop when the arm is in a closed position, covering one of the sets of orifices, the stop preventing the arm from covering both sets of orifices; and a low friction coating on either the arm or that portion of the bellows housing on which the arm moves when the bellows is extended or compressed wherein the pressure difference on the bellows and arm provides a force to pull the bellows free end over the slots when the interior is at ambient pressure; and wherein the bellows is resiliently restored to its unloaded state when the flow line communicates with the pressurized fluid, thereby moving the arm to uncover the slots.  
         [0010]     In a further aspect of the present invention, a gas turbine engine having variable flow cooling system comprises a source of cooling fluid providing a cooling fluid flow along a flow path; a flow restriction element in the flow path having at least two sets of orifices therein; a source of pressurized fluid; a bellows having a flow line connecting an interior of the bellows with either the source of pressurized fluid or with ambient air; and an arm moveably attached to the bellows wherein movement of the bellows moves the arm to either an open position, covering none of the orifices, or a closed position, covering at least one of the two sets of orifices, thereby restricting flow of the cooling fluid through the flow path.  
         [0011]     In still a further aspect of the present invention, a method for modulating cooling flow to a turbine, comprises providing the cooling flow along a flow path; disposing a flow restriction element in the flow path having at least two sets of orifices therein; connecting an interior of a bellows with either a source of pressurized air or with ambient air; defining the interior of the bellows by a bellows housing, a bellows free end attached to the arm; movably connecting the arm to the bellows free end, thereby allowing movement of the arm with movement of the bellows free end, wherein movement of the bellows free end moves the arm to either an open position, covering none of the orifices, or a closed position, covering at least one of the two sets of orifices, thereby restricting flow of the cooling fluid through the flow path.  
         [0012]     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a cross-sectional view showing a conventional turbine cooling system;  
         [0014]      FIG. 2  is a cross-sectional view showing a variable flow cooling system in an open position, according to the present invention; and  
         [0015]      FIG. 3  is a cross-section view of the variable flow cooling system of  FIG. 2 , in a closed position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0017]     The present invention provides a turbine rotor (disk, blades and sealplate) cooling system that provides a variable flow of cooling air dependent upon engine conditions such as turbine gas temperature or power level angle. The cooling system of the present invention reduces the amount of coolant flow being delivered to the turbine blades and disk at low power conditions. As discussed in greater detail below, the present invention provides a cooling system that is simple to manufacture, overcomes the effects of varying altitudes, and gives a failure mode cooling sufficient to prevent engine damage. Moreover, another benefit of the cooling system of the present invention is that lower coolant flow at lower power results in a higher blade temperature. Due to thermal expansion of the blade, this higher blade temperature results in a reduced turbine blade tip clearance, yielding improved aerodynamic efficiency. This is unlike conventional coolant flow systems which, among other things, are designed to provide coolant flow sufficient for high power/high temperature operation.  
         [0018]     The variable flow turbine cooling system of the present invention may be useful in a variety of different turbine engines. In particular, the variable flow cooling system may be most beneficial in rotorcraft and tanks which operate much of their engine running times at a lower power point. In these types of turbine engine containing apparatus where much of the engine running time is spent at a lower power point, fuel consumption is significantly reduced with the reduction in the required coolant air flow.  
         [0019]     Referring to  FIGS. 2 and 3 , there are shown cross-sectional views of a variable flow cooling system  10 , according to the present invention, in an open position ( FIG. 2 ) and in a closed position ( FIG. 3 ). Under maximum power conditions, a bellows  28  may be fully extended as shown in  FIG. 2 .  
         [0020]     Referring to  FIG. 2 , air may enter air passages  14  via a path indicated by arrow  16 . Downstream of air passages  14 , the cooling air flow may be unrestricted and flow via cooling paths  80 ,  82  and  84  to cool a downstream turbine (not shown), a turbine blade (not shown), a turbine disk  88  and sealplate  92 .  
         [0021]     Just upstream of air passages  14 , a flow restricting element  18  may have sets of axially separated orifices  20  therethrough. Each set of orifices  20  may be located around the flow restricting element  18  at the same axial location. Orifices  20  may be of any shape i.e. slots, holes, or the like. The air pressure on the interior  12  of bellows  28  may be just below the compressor discharge pressure. A flow line  22  may connect the volume between the interior  12  of bellows  28  to a valve  90  just outboard the compressor. Valve  90  can vent the air within bellows  28  either overboard (as shown by arrow B) or to compressor discharge pressure (as shown by arrow A). At low power, that is, at or below a predetermined power level where the engine cooling requirements may be met by a reduced cooling flow, valve  90  may be actuated to vent overboard, which drops the pressure within bellows  28  to ambient pressure. The resulting pressure load on bellows  28  may axially displace a cylindrical arm  24  over some of the orifices  20  in flow restricting element  18 , thereby restricting part of the cooling flow. Flow line  22  may be sized for rapid response of bellows  28  in order to quickly close some of the orifices  20 . The low power level may be determined experimentally, by measuring engine component temperature to determine at which power level a reduced cooling flow may be effective. At or below this power level may be considered low power.  
         [0022]     Bellows  28  may be designed in any suitable manner so long as it has the ability to move cylindrical arm  24  in response to changes in the interior  12  air pressure. For example, bellows  28  may be designed as a piston with the piston&#39;s cylinder acting as bellows housing  30  and the arm of the piston acting as cylindrical arm  24 . A return spring would be used to react against the piston, ensuring that at low differential pressure across the piston, the arm is positioned to provide unrestricted cooling flow. Valve  90  may be tied to any parameter indicative of the temperature of the components designed to be cooled by the cooling flow system. For example, the temperature of the engine gas flow may be measured as being indicative of component temperature. The temperatures of the actual components themselves may also be measured. Further, the power level angle may also be measured so that once the operator puts the engine at a specific power level angle or higher, valve  90  will connect flow line  22  with compressor discharge pressure.  
         [0023]     Cylindrical arm  24  of bellows  28  may be attached to a bellows free end  26 . The other end of the bellows may be attached to bellows housing  30 . Bellows housing  30 , bellows springs, and bellows free end  26  define the interior  12  of bellows  28 , into which flow line  22  communicates. When valve  90  connects flow line  22  with compressor discharge air, bellows free end  26  is in its free state, i.e. extended to an open position as shown in  FIG. 2 . When valve  90  connects flow line  22  with ambient air, the pressure loading can pull bellows free end  26  toward bellows housing  30 , to place the variable flow system in a closed position, as shown in  FIG. 3 .  
         [0024]     In order to accommodate the expected tolerance range in the spring rate of bellows  28 , and to also accommodate the variation in compressor discharge pressure to overboard (ambient) pressure at various flight altitudes, cylindrical arm  24  may block off at least one set of orifices  20  in flow restricting element  18 . A stop  34  on flow restricting element  18  may keep cylindrical arm  24  from moving too far shut. By appropriately choosing the spring rate of bellows  28 , the line size  22 , and the axial position of the orifices  20 , tolerance and altitude pressure variation in displacement will have no bearing on flow through orifice  20 . A surface  36  over which cylindrical arm  24  passes may have a low friction coating applied to prevent binding and allow for rapid bellows response to switching of valve from compressor discharge air to ambient air.  
         [0025]     Bellows  28  as used in the example depicted in the drawing may be of any suitable configuration and may be made of any suitable material, such as a high temperature nickel alloy or Inconel  100 . Bellows  28  and surrounding components may be made of a material capable of withstanding the operating temperatures of a gas turbine engine. These temperatures may typically range from about 600-1100° F.  
         [0026]     The system may be designed so that failure modes are benign. More specifically, a failure of bellows  28  may drive cylindrical arm  24  to an open position by letting supply air pressure into the bellows cavity  12 . Valve  90  may be designed to fail so that flow line  22  communicates with compressor discharge air, thereby keeping the variable flow system open. A failure of line  22  will let compressor air into the bellows plenum  12 , thereby positioning the orifices  20  open.  
         [0027]     While the above description describes the variable flow system having two sets of orifices  20 , i.e. axial slots or holes, one of which may be closed by cylindrical arm  24 , the system may also be designed with more than two sets of orifices, or may even have one elongated orifice over which cylindrical arm  24  covers a portion thereof at lower power. Alternatively, cylindrical arm  24  may be used to turn a valve within the orifices in order to limit the amount of cooling air flowing therethrough. In the alternate embodiment having three sets of orifices in flow restricting element  18 , for example, valve  90  may communicate flow line  22  with not only ambient air and compressor discharge air, but also with an air source having a pressure somewhere between the air pressure of ambient air and compressor discharge air such as an intercompressor source. Flow restricting element  18  may then have three sets of orifices therethrough. When the valve connects flow line  22  with compressor air, the bellows is in an open position. When the valve moves to connect flow line  22  with the middle pressure air source (not shown), cylindrical arm  24  may partially close, closing off one of the three sets of orifices. Finally, when the valve moves to connect flow line  22  with ambient air, cylindrical arm  24  may close all the way to stop  34 , closing off two of the three sets of orifices in flow restricting element  18 , leaving at least one set of orifices always open.  
         [0028]     The variable flow cooling system of the present invention may result in about a 20 to about 80% reduction in cooling air requirements. Preferably, the variable flow cooling system of the present invention results in up to about a 40 to about a 70% reduction, and more preferably, up to about a 50% reduction or more in cooling air requirements. This reduction in the need for compressor discharge air increases the fuel efficiency of the gas turbine engine by not wasting energy in the form of air compression work on unnecessarily large amounts of compressor discharge cooling air.  
         [0029]     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.