Abstract:
A clearance control arrangement includes first and second components defining a clearance therebetween. The first component includes a surface portion having at least a layer of material that changes thickness when actuated. The clearance control arrangement also includes an actuator to actuate the layer of material.

Description:
BACKGROUND 
     The present invention relates to a clearance control arrangement, in particular, although not exclusively, a tip clearance control arrangement for controlling the clearance of rotor blades in casings in gas turbine engines. 
     A gas turbine engine  10  is shown in  FIG. 1  and comprises an air intake  12  and a propulsive fan  14  that generates two airflows A and B. The gas turbine engine  10  comprises, in axial flow A, an intermediate pressure compressor  16 , a high pressure compressor  18 , a combustor  20 , a high pressure turbine  22 , an intermediate pressure turbine  24 , a low pressure turbine  26  and an exhaust nozzle  28 . A nacelle  30  surrounds the gas turbine engine  10  and defines, in axial flow B, a bypass duct  32 . 
     Each of the compressors and turbines comprise alternating stages of rotating blades and stationary stators surrounded by a casing. Engine efficiency is improved as the clearance between the rotating blade tips and the casing is minimised so that the working fluid passes over the blade surfaces and does not leak over the tips. However, differential heating of the components occurs during operation of the gas turbine engine. For an aero gas turbine engine used to power an aircraft, the radial growth of the blades is quicker than the radial growth of the casing when the engine accelerates, for example during take off and climb manoeuvres. Thus the minimum clearance between the blade tips and the casing must be set for the worst case scenarios, e.g. take off and step climb. At other flight phases the blades radially shrink more than the casing so that the clearance is larger than optimal. 
     Selective case cooling has been used to decrease the clearance during aircraft cruise. However, the clearance cannot be wholly minimised because the casing must be able to radially grow quickly enough that the blade tips do not rub against the casing during step climb manoeuvres. 
     Another method of decreasing the clearance during cruise has been to provide radially moveable segments of casing. These are generally mechanically complex and heavy, requiring significant actuation components. Additionally a casing segment is a relatively large component to move which cannot respond rapidly enough to transient aircraft manoeuvres such as step climb. Thus the minimum clearance must be specified at all flight phases as that required for take off, climb and step climb meaning that the engine is more inefficient in cruise. This is particularly expensive as cruise generally comprises the greatest proportion of the flight, at least for passenger and freight aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention provides a clearance control arrangement that seeks to address the aforementioned problems. 
     Accordingly the present invention provides a clearance control arrangement comprising: first and second components defining a clearance therebetween; the first component comprising a surface portion having at least a layer of material that changes thickness when actuated; and an actuator to actuate the layer of material. 
     Advantageously this arrangement has a rapid response time which makes it ideal for fast transient control. 
     The clearance control arrangement may comprise a plurality of first components each defining a clearance relative to the second component and each having at least a layer of material that changes thickness when actuated. The actuator may actuate all the layers of material in synchronicity. Alternatively an actuator may be provided to actuate each layer of material individually. 
     The layer of material may comprise piezoelectric material and the actuator may comprise a voltage source. The surface portion may comprise the layer of piezoelectric material sandwiched between layers of metal. The voltage source may be connected to the layers of metal across the layer of piezoelectric material. 
     The layer of material may comprise a flexible membrane and the actuator may comprise an air supply. Alternatively the layer of material may comprise shape memory alloy and the actuator may comprise a heat source. 
     The clearance control arrangement may further comprise a controller to control actuation of the actuator. 
     The clearance control arrangement may further comprise a sensor to determine the clearance. The clearance control arrangement may further comprise a feedback loop between the sensor output and the controller. The sensor may be coupled to the surface portion or may be coupled to another part of the first component. 
     The present invention also provides a gas turbine engine comprising a clearance control arrangement as defined. The gas turbine engine may comprise a clearance control arrangement as defined for tip clearance control wherein the first component comprises a casing segment and the second component comprises a rotating blade. 
     The gas turbine engine may further comprise a cooling arrangement for controlling the temperature of the first component. 
     The present invention further provides a gas turbine engine comprising a clearance control arrangement for fan duct acoustic control. 
     The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side view of a gas turbine engine. 
         FIG. 2  is a schematic graph of clearance against time for an exemplary flight plan. 
         FIG. 3  is a schematic sectional view of a first embodiment of a clearance control arrangement according to the present invention. 
         FIG. 4  is a schematic sectional view of part of a clearance control arrangement according to the present invention. 
         FIG. 5  is a schematic diagram of a first voltage source arrangement. 
         FIG. 6  is a schematic diagram of a second voltage source arrangement. 
         FIG. 7  is a schematic sectional view of a second embodiment of a clearance control arrangement according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a blade tip clearance  34  for a typical flight plan for a civil aircraft comprising sequential phases of runway taxi, take off, climb to altitude, cruise, step climb to a higher altitude and further cruise. It will be apparent to the skilled reader that there are further phases in a typical flight plan, including descent, landing and taxi, and that the step climb and further cruise phases may not occur or may be repeated.  FIG. 2  also illustrates a target tip clearance  36  that is calculated as the sum of a clearance required to take account of measurement uncertainties or errors  38  and a clearance required to take account of asymmetries in the radial extent of the blade set  40 . The blade tip clearance  34  is coextensive with the target tip clearance  36  when the engine  10  runs hottest, during take off, climb and step climb. During taxi the engine  10  has not reached full operating temperature and therefore the blade tip clearance  34  is much larger than the target tip clearance  36 . During the cruise phases the blade tip clearance  34  increases beyond the target tip clearance  36 . The area  42  between the blade tip clearance  34  and the target tip clearance  36  represents ‘wasted’ efficiency, since this is excess clearance beyond the minimum requirement. 
     The clearance control arrangement of the present invention reduces the blade tip clearance  34  towards the target tip clearance  36  during the cruise phase thereby improving the efficiency of the engine  10 . This results in specific fuel consumption improvement, which equates to a significant fuel saving and thus cost saving over each flight. 
     An exemplary embodiment of the present invention is shown in  FIG. 3  and illustrates a blade  44  rotating within a casing  46 . The blade  44  is part of a rotor blade set, for example of the high pressure turbine  22 . Coupled to the casing  46  is a shroud segment  48  that has a surface portion  50  which will be described in greater detail with respect to  FIG. 4 . A clearance  52  is defined between the surface portion  50  of the segment  48  and the tip  54  of the blade  44 . It is this clearance  52  that is controlled by the clearance control arrangement of the present invention. 
     As shown in  FIG. 4 , the surface portion  50  of the segment  48  comprises a pair of metal layers  56  that sandwich a layer of material  58 . The metal layers  56  may comprise nickel-alloy which is typically used for shroud segments  48 . In one embodiment the layer of material  58  comprises a piezoelectric material. One terminal of a voltage source  60  is connected to each of the metal layers  56  so that a voltage is applied across the layer of piezoelectric material  58 . The amount of voltage supplied by the voltage source  60  is controlled by a controller  62 , for example in response to a feedback control loop. The controller  62  may be the engine electronic controller of the gas turbine engine  10  or a separate controller. The layer of piezoelectric material  58  increases in thickness with increasing applied voltage thereby causing the clearance  52  to reduce. In this way the blade tip clearance line  34  is brought closer to the target tip clearance line  36  on  FIG. 2  and the efficiency and fuel consumption of the engine  10  is improved, particularly during cruise. 
     Although only two wires are shown connecting the voltage source  60  to the metal layers  56  it will be apparent to the skilled reader that there may be multiple pairs of wires deployed in parallel so that the voltage is applied evenly across the whole of the layer of piezoelectric material  58 . 
     There is usually a plurality of shroud segments  48  in a circumferential array around a set of rotor blades  44 . Thus in accordance with the clearance control arrangement of the present invention, each segment  48  comprises a surface portion  50  having a layer of material  58 .  FIG. 5  and  FIG. 6  show two ways of actuating the layers of piezoelectric material  58 . In  FIG. 5 , the controller  62  sends a control signal to the voltage source  60  which actuates all of the layers of material  58  simultaneously. Thus this represents ganged control which is light as only one voltage source  60  is required. In  FIG. 6 , there is a voltage source  60  provided for each layer of material  58  so that the controller  62  sends control signals to each voltage source  60  separately. This enables individual control of the clearances  52  between the surface portion  50  of each segment  48  and the passing rotor blades  44 . This is advantageous where there is asymmetry between the various segments  48  but is heavier than the  FIG. 5  arrangement because there are multiple voltage sources  60 . The  FIG. 6  arrangement may be beneficial as it offers the potential for redundancy as, in the event that one or more of the voltage sources  60  fails, voltage may be supplied to more than one layer of material  58  by a single voltage source  60 . 
     The clearance control arrangement may also comprise a tip clearance sensor  64 , as shown in  FIG. 3 , that forms part of a feedback control loop with the controller  62 . The sensor  64  may be coupled to the outer metal layer  56  so that it measures the true clearance  52 . Alternatively, it may be coupled to the segment  48  and arranged to measure displacement of the layer of material  58  in order to derive the remaining clearance  52 . 
       FIG. 7  shows a second embodiment of a clearance control arrangement according to the present invention and shares many features with  FIG. 3 . Thus the casing  46  comprises a shroud segment  48  that has a surface portion  50 . A blade  44 , which is part of a rotor blade set, has a tip  54  spaced from the surface portion  50  to define a clearance  52 . A sensor  64  is arranged to measure the clearance  52 , either directly or indirectly by measuring the displacement of the surface portion  50 , and to feed back a signal to a controller  62 . The controller  62  may be unique to the clearance control arrangement or may be integrated with the engine electronic controller or another extant controller in the engine  10 . 
     In the second embodiment the surface portion  50  comprises a layer of material  58  that is a flexible membrane. An arrangement of three air pipes  66  meet at a three-way valve  68 . One of the air pipes  66  couples a source of air, for example compressor exit air or bypass duct air, to the three-way valve  68 . A second of the air pipes  66  couples the three-way valve  68  to a discharge aperture, such as downstream of the blades  44  or into the bypass duct  32 . A third of the air pipes  66  couples the three-way valve  68  to a cavity  70 , one side of which is formed by the flexible membrane  58 . The three-way valve  68  acts in a first orientation to divert air from the source through the first of the air pipes  66  to bypass the clearance control arrangement and discharge the air through the second of the air pipes  66 . In a second orientation the three-way valve  68  diverts the air from the source through the first of the air pipes  66  and into the third of the air pipes  66  to fill the cavity  70  and thereby distend the flexible membrane to reduce the clearance  52 . In a third orientation of the three-way valve  68 , air is diverted from the source through both the second and third air pipes  66  so that the flexible membrane  58  is distended to a lesser extent and the additional air is expelled through the second of the air pipes  66 . There may be continuous movement of the three-way valve  68  between the three orientations, rather than three discrete orientations. 
     A controller  62  is coupled to the three-way valve  68  to control its actuation between the three orientations. By using the feedback signal from the sensor  64  and modulating the orientation of the three-way valve  68  accordingly, the flexible membrane  58  can be distended towards the tip  54  of the rotor blade  44  by a sufficient distance to minimise the clearance  52 . As in the first embodiment, the sensor  64  may be mounted to the flexible membrane to measure the actual clearance  52  or may be mounted to the segment  48  or elsewhere to measure the displacement of the flexible membrane  58  from the segment  48  from which the clearance  52  may be derived. 
     Advantageously, the clearance control arrangement of the present invention has a rapid response time which makes it ideal for fast transient control, as is required to compensate for the differential component growth caused by step climb and similar manoeuvres. This means that the clearance  52  is minimised throughout the flight cycle and therefore results in reduced fuel consumption and increased engine efficiency. All the described embodiments can be arranged to cause a range of movement of the surface portion  50  to control the clearance  52 . By providing individual control of the actuators, voltage source  60  or three-way valve  68 , the clearance control arrangement of the present invention is able to compensate for asymmetries in the system, such as irregular length blades  44 , segment imperfections and irregularities, transient thermal asymmetry of the casing, vibrations, and other combinations of thermomechanical effects. 
     The clearance control arrangement of the present invention could be used in conjunction with a cooling arrangement for controlling the temperature of the casing. In this way, the cooling arrangement provides relatively large movements of the casing over longer periods, to accommodate blade growth due to creep of the respective blades for example. In such an arrangement, the clearance control arrangement of the present invention provides “fine tuning” of the blade clearance, i.e. relatively small movements of the casing over much shorter periods to accommodate blade growth or shrinkage due to transient temperature changes of the respective blades. As the clearance control arrangement is not required to accommodate large changes in blade length, the control arrangement can be made to be lighter, and therefore respond more quickly to transient changes in blade length. The clearance control arrangement, when used in conjunction with a cooling arrangement, is also simpler and more robust in comparison to prior mechanical clearance control arrangements. 
     Although specific embodiments of the clearance control arrangement according to the present invention have been described with respect to tip clearance control in a gas turbine engine  10 , other embodiments and variations fall within the scope of the claimed invention. The layer of material  58  may comprise a shape memory alloy or other heat-affected material. In this case the actuator would be a heat source instead of the voltage source  60  of the first described embodiment. 
     For the second embodiment, the flexible membrane  58  may be formed as a plurality of pockets so that the deformation of the flexible membrane  58  is more even across the surface portion  50 . The third of the air pipes  66  may split into multiple end portions to feed such pockets. Although a single layer of material  58  has been shown for the surface portion  50  of each segment  48 , a plurality of layers of material  58  may be provided instead. 
     The controller  62  may, control one actuator, the voltage source  60  or the three-way valve  68  or a heat source, which actuates a plurality of layers of material  58 . Alternatively the controller  62  may actuate multiple actuators, each of which actuates one or a plurality of layers of material  58  forming part of one or several surface portions  50 . Some redundancy may be designed into the system whereby multiple actuators are cross-coupled to the same layers of material  58  so that a failed actuator can be bypassed. 
     Although a three-way valve  68  has been described, a different arrangement of valves having the same effect may be used instead. For example, three separate valves in combination. 
     Although the clearance control arrangement of the present invention has been described with respect to application of tip clearance control in an aero gas turbine engine  10 , it finds equal utility for tip clearance control in marine or industrial gas turbine engines, and in propeller gas turbine engines. The present invention can also be used with equal felicity for fan duct acoustic control of a gas turbine engine  10  by changing the shape of the fan inlet duct to change the acoustic properties thereof. 
     The present invention may find utility in other industries, for example in controlling the clearance of contra-rotating shafts and co-rotating shafts e.g. in the automobile and manufacturing industries.