Abstract:
A gas turbine engine comprises a bleed duct for receiving bleed flow and a plurality of valve members pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct. The gas turbine engine also comprises an actuation mechanism for pivoting the valve members relative to the bleed duct, wherein the actuation mechanism is configured such that pivoting of the valve members is staggered.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure concerns a gas turbine engine, and/or a method of operating a gas turbine engine. 
       BACKGROUND 
       [0002]    Gas turbine engines are typically employed to power aircraft. Typically a gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven off an additional lower pressure turbine in the engine core. 
         [0003]    The fan comprises an array of radially extending fan blades mounted on a rotor and air travelling through the fan will provide a large percentage of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core. 
         [0004]    It is known to extract air from the working gas path of a gas turbine engine, either to manage the engine airflow and operating conditions or to provide an air supply for the passenger cabin or for other purposes. Generally, this air is extracted through one or more bleed valves in an engine casing, which are actuated (for example, by a solenoid or hydraulic actuator) to either an open position in which air can flow through it or a closed position in which no air can flow through it. 
         [0005]    Air may be bled from the compressor to manage the operating conditions of the engine and/or for auxiliary functions such as providing air to a cabin of an aircraft. A valve is usually provided to control the bleed from the compressor. Conventionally the valve is a binary valve, that is the valve is of the type where it is either open or closed. 
         [0006]    However, it has been suggested that a butterfly valve could be used to control the bleed from the compressor. A butterfly valve has a valve member that is provided in a valve housing and is pivotable thereto so as to vary the flow of fluid through the butterfly valve between an open position and a closed position. 
       SUMMARY OF DISCLOSURE 
       [0007]    According to a first aspect there is provided a gas turbine engine comprising a bleed duct for receiving bleed flow (e.g. bleed flow from a compressor of the gas turbine engine). A plurality of valve members are pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct. An actuation mechanism is provided for pivoting the valve members relative to the bleed duct. The actuation mechanism is configured such that pivoting of the valve members is staggered. 
         [0008]    It may be considered that the valve members are pivoted non-simultaneously. When the actuation mechanism moves the valve members a first valve member may be pivoted before a second valve member is pivoted. For example, when the actuation mechanism moves the valve members from defining a position where the bleed duct is closed to a position where the bleed duct is open, a first valve member of the plurality of valve members starts to open before a second valve member of the plurality of valve members. When the actuation mechanism moves the valve members from defining a position where the bleed duct is open to a position where the bleed duct is closed, the first (or second) valve member of the plurality of valve members starts to close before the second (or first) valve member of the plurality of valve members. 
         [0009]    The valve members may be sequentially operated. For example, pivoting of one of the plurality of valve members commences before pivoting of another valve member commences. 
         [0010]    The actuation mechanism may include one or more link members that link the plurality of valve members. 
         [0011]    The link members may be arranged such that actuation of one valve member initiates actuation of other valve members, e.g. valve members connected directly or indirectly to said one valve member via the link members. 
         [0012]    The link members may comprise pins and/or slots arranged such that a pin of one link member is received in a slot of an adjacent link member. The slots may be curved. 
         [0013]    The slots of the link members may be defined such that that a pin received in the slot starts to move in the slot before rotation of a respective valve member. 
         [0014]    Each valve member may have a link member connected to it, e.g. at a position substantially aligned with an axis of rotation of the valve member. 
         [0015]    The valve member may be a disc, e.g. a circular disc. 
         [0016]    The bleed duct may comprise a plurality of housings at an entrance thereof. Each housing may house one valve member of the plurality of valve members. 
         [0017]    The housings may define a cylindrical fluid flow path. The housings may be positioned axially adjacent to each other. 
         [0018]    The actuation mechanism may comprise a ram, e.g. a hydraulic, pneumatic, or electrical ram for moving the link members, e.g. the ram may be connected to one of the link members and actuate said one of the link members. Actuation of said one of the link members may actuate directly or indirectly the remaining link members. 
         [0019]    In alternative examples, the actuation mechanism may comprise rack and pinion gears, or a geared arrangement. 
         [0020]    The gas turbine engine may comprise a bypass duct and a compressor. The bleed duct may extend to bleed air from the compressor to the bypass duct. 
         [0021]    A brush seal may define a seal between the valve member and a housing that defines a portion of the bleed duct. 
         [0022]    A portion of the bleed duct may be defined by a plurality of valve housings. One valve member may be positioned in each housing and be pivotable relative thereto. 
         [0023]    The brush seal may be provided on the edge of the valve member. Alternatively, the brush seal may be provided on the housing. 
         [0024]    The brush seal may comprise a plurality of bristles extending radially outwardly relative to the valve member. The brush seal may comprise a plurality of bristles arranged helically around the valve member. The bristles may be arranged helically in a clockwise and/or an anticlockwise direction. 
         [0025]    The surface of the valve member may be shaped (e.g. contoured or comprises profiling elements) to optimise the position of the centre of pressure acting on the butterfly valve member. 
         [0026]    According to a second aspect there is provided a valve arrangement comprising a plurality of valve members pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct; and an actuation mechanism for pivoting the valve members relative to the bleed duct. The actuation mechanism is configured such that pivoting of the valve members is staggered. 
         [0027]    The valve arrangement of the second aspect may have one or more of the optional features of the first aspect, where the optional features relate to features of a valve arrangement. 
         [0028]    According to a third aspect there is provided a method of operating a gas turbine engine, the gas turbine engine comprising a bleed duct for receiving bleed flow (e.g. bleed flow from a compressor of the gas turbine engine), and a plurality of valve members pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct, the method comprising pivoting each valve member of the plurality of valve members in a staggered relationship to each other. 
         [0029]    The gas turbine engine may be the gas turbine engine of the first aspect. 
         [0030]    According to a fourth aspect there is provided a gas turbine engine comprising a bleed duct for receiving bleed flow (e.g. bleed flow from a compressor of the gas turbine engine); a valve member pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct; and a brush seal provided between the bleed duct and the valve member. 
         [0031]    According to a fifth aspect there is provided a butterfly valve comprising a housing, a valve member pivotable relative to the housing, and a brush seal provided between the housing and the valve member. 
         [0032]    According to a sixth aspect there is provided a gas turbine engine comprising a bleed duct for receiving bleed flow (e.g. bleed flow from a compressor of the gas turbine engine); a valve member pivotable relative to the bleed duct for variably controlling fluid flow along the bleed duct, wherein the valve member (e.g. one or more faces of the valve member) is profiled such that a centre of pressure on the valve member during operation of the valve is close to or coincident with an axis of rotation of the valve member. 
         [0033]    The valve member may be contoured and/or elements may be attached to the valve member. 
         [0034]    According to a seventh aspect there is provided a butterfly valve comprising a housing, a valve member pivotable relative to the housing, and a brush seal provided between the housing and the valve member. 
         [0035]    The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0036]    Embodiments will now be described by way of example only, with reference to the Figures, in which: 
           [0037]      FIG. 1  is a sectional side view of a gas turbine engine; 
           [0038]      FIG. 2  is a sectional schematic of a bleed arrangement between a bypass duct and compressor of the engine of  FIG. 1 ; 
           [0039]      FIG. 3A  is a schematic of a valve member of the bleed arrangement of  FIG. 2  in a closed position; 
           [0040]      FIG. 3B  is a schematic of a valve member of the bleed arrangement of  FIG. 2  in an open position; 
           [0041]      FIG. 4  is a schematic sectional view of the bleed arrangement of  FIG. 2  in a closed position and illustrating link members that connect the valve members of the bleed arrangement; 
           [0042]      FIG. 5  is a schematic sectional view of the bleed arrangement of  FIG. 2  in an open position and illustrating link members that connect the valve members of the bleed arrangement; 
           [0043]      FIG. 6  shows a graph illustrating valve control accuracy A against percentage of valve opening (i.e. the valve in a closed position is indicated by 0 and the valve in the open position is indicated by 100) for a bleed arrangement having a single butterfly valve and for the bleed valve arrangement having multiple valve members (e.g. five valve members); 
           [0044]      FIGS. 7A to 7D  schematically illustrate alternative arrangements of valve and link members; 
           [0045]      FIG. 8  illustrates a valve member; 
           [0046]      FIG. 9  illustrates an alternative valve member; and 
           [0047]      FIG. 10  illustrates bristles that may be provided around the edge of the valve member of  FIG. 8  or  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , and intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
         [0049]    The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
         [0050]    The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high  17 , intermediate  18  and low  19  pressure turbines drive respectively the high pressure compressor  15 , intermediate pressure compressor  14  and fan  13 , each by suitable interconnecting shaft. 
         [0051]    Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
         [0052]    Referring now to  FIG. 2 , a bleed arrangement from the compressor, in this case the intermediate pressure compressor  14  is indicated generally at  24 . The bleed arrangement includes a bleed duct  26  for receiving a bleed flow from the compressor. A flexible seal  28  is provided and circumferentially surrounds a portion of the bleed duct. The flexible seal is provided to prevent leakage of bypass air into a fire zone that is located around the bleed duct, and to prevent leakage of fire zone ventilation air into the bypass duct  22 . A valve arrangement  30  is provided between an engine casing  29  proximal to the compressor and the bypass duct  22 . 
         [0053]    The cross sectional area of the bleed duct  26  is greater downstream of the valve arrangement  30  than upstream of the valve arrangement (upstream and downstream referring to the general direction of air flow through the bleed duct). The walls of the bleed duct are substantially parallel, but in alternative embodiments the walls may converge towards the bypass duct  22 . In the present embodiment the bleed duct is open directly to the bypass duct  22 ; however in alternative embodiments a barrier may be provided between the bypass duct  22  and the bleed duct  26 . The barrier may be a perforated sheet or a wire mesh. The perforations in the sheet or the holes formed in the mesh may be small enough to prevent debris such as bolts entering the bleed duct. In some embodiments the barrier may be a “pepper pot”. In such embodiments, the perforations or holes may be small enough to reduce noise (for example they may have a diameter of approximately 3 mm). In further alternative embodiments, the barrier may be a slotted barrier, for example a louvered barrier. 
         [0054]    Referring to  FIGS. 2, 3A and 3B , the valve arrangement  30  includes a plurality of butterfly valves (in this example three butterfly valves). Each valve includes a valve member  32  and a housing  34 . The housing  34  defines a fluid flow path for fluid flow through the butterfly valves, and defines a portion of the bleed duct  26 . In the present example each housing is cylindrical. 
         [0055]    Each valve member  32  is disc shaped. In the present example the faces of each valve member are planar, but in alternative embodiments the faces of the valve member may have any suitable profile. For example, the face of the valve member may be profiled, e.g. be contoured or have elements attached to the face. The profiling of the valve member may be such that the centre of pressure of the butterfly valve is positioned closer to a physical axis of the valve (i.e. a rotational axis (discussed below) about which the valve member pivots) during the highest aerodynamically loaded conditions during operation of the gas turbine engine. 
         [0056]    Each valve member  32  is pivotally connected to the respective housing  34 . Each valve member is connected to the respective housing at two diametrically opposed positions. The valve member  32  is connected to the housing  34  such that the valve member is pivotable about a rotational axis  36 . The rotational axis  36  extends between the two positions of connection with the housing. 
         [0057]    An actuator  38  is provided. The actuator  38  is configured to move the valve members  32  directly or indirectly between an open and a closed position and variable positions there between. In the present example the actuator is a ram, e.g. a hydraulic or pneumatic ram 
         [0058]    The closed position of the valve member  32  is illustrated in  FIG. 3A . In the closed position the valve member substantially blocks the fluid flow path defined by the housing  34 . It could be considered that in the closed position, when the butterfly valve is viewed from an axial end, the valve member is concentric the fluid flow path (or bore) of the housing. The open position of the valve member  32  is illustrated in  FIG. 3B . In the open position the valve member is positioned so as to block the fluid flow path by a minimal amount. In the present example, the valve member  32  is positioned orthogonal to the closed position. 
         [0059]    When the butterfly valve is viewed from an axial end, in the open position only a portion of the circumferential face of the valve member is visible. 
         [0060]    Referring now to  FIGS. 4 and 5 , the valve members of the valve arrangement  30  are connected by link members  40   a,    40   b.  In the present example two types of link members are provided. The first type of link member  40   a  is a straight member having a pin  44  positioned at one end. An opposite end of the link member  40   a  is connected to the valve member  32  at a position  46   a  substantially aligned with the axis of rotation of the valve member. In the valve arrangement  30  shown in  FIGS. 4 and 5 , an array of three valves (i.e. three valve members and respective housings) are arranged axially adjacent each other. One of the first type of link members is connected to the end valve members of the array of three (i.e. to each valve member adjacent the central valve member of the array). 
         [0061]    The second type of link member  40   b  is a curved link member. The curved link member includes a central portion that is straight and two end portions that are curved. The two end portions are curved in the same direction. A slot  42  is provided in each end portion. The slot  42  is curved. In the present example the slot  42  follows the curved profile of the end portions. The pins  44  of the first type of link member  40   b  are received in the slot  42  at the respective end of the second type of link member. The second type of link member is connected to the valve member at a position  46   b  substantially aligned with the axis of rotation of the valve member. In the present example, the second type of link member is connected to the central valve member of the array of three. 
         [0062]    An axle or similar may be provided through the valve member and the link member may be connected to one end of said axle. The link member may be connected (e.g. rigidly connected to the axle or similar) to the valve member such that the link member rotates with the valve member or the valve member rotates with the link member. That is, relative movement between a link member and the respective connected valve member is limited. 
         [0063]    During operation of the gas turbine engine, to open the valves fully or by a certain degree, the actuator rotates one of the valve members, for example a valve member at one end of the array, about the rotational axis of the valve member. As the valve member rotates, the respective link member rotates. Rotation of the link member causes the respective pin to move in the respective slot of the or an adjacent link member. Movement of the pin in the slot causes rotation of the adjacent link member, which actuates opening of the respective valve member by rotating the valve member about its rotational axis. In this way all linked link members actuate rotation of the valve members when one valve member is actuated. In the arrangements shown, the valve members are actuated sequentially, i.e. directly linked valve members commence opening one after another. Indeed, in the present example, a first valve is opened, then a second valve is opened and then a third valve is opened. That is the pin of the respective link member slides in the slot and when it comes to a stop actuation of the respective linked valve member is initiated. 
         [0064]    To close the butterfly valves a similar process as to open the butterfly valves is followed, but the actuator rotates the butterfly valve in the reverse direction. 
         [0065]    The described arrangement provides increased accuracy of control of the bleed flow through the bleed duct. Referring to  FIG. 6 , the accuracy of control (A) is indicated on the y-axis and the amount the duct is open is indicated on the X-axis. As can be seen from this graph, when a single butterfly valve is used (indicated by line  48 ) the accuracy of control when the valve is close to being fully open or fully closed is reduced compared to when the valve is fully open. 
         [0066]    The mean accuracy of control is indicated by dotted line  50 . 
         [0067]    In the described example where multiple valves are used having staggered opening permits a wider range of accuracy of opening of the valves, as indicated by the accuracy of operation lines  52 , and the mean accuracy of control indicated by dotted line  54 . It can be seen from a comparison of the mean lines that the described example a wider range of control with increased accuracy can be achieved. 
         [0068]    As will be appreciated by the person skilled in the art, the valve members and link members can be arranged in a variety of ways, for example depending on the space requirements or needs of the local environment. Various alternative example valve arrangements are illustrated in  FIGS. 7A to 7D . 
         [0069]    Referring now to  FIGS. 8 to 10 , the housing of the butterfly valve or the valve member  32  may include a brush seal. In the examples shown in  FIGS. 8 and 9 , the bush seal is connected to the valve member. Bristles  54  extend radially outwardly from the valve member  32 . The bristles may extend directly radially outwardly, or they may extend radially outwardly and be angled towards one side of the valve member. In some examples, the bristles may be spirally arranged in either a clockwise or a counter clockwise direction. The bristles in the present example are overlapping. The bristles may extend radially from the valve member (or the housing) by a distance of approximately 0.5 mm. 
         [0070]    The use of a brush seal reduces leakage between the valve member and the valve housing. 
         [0071]    In the present example a single actuator is provided, but in alternative embodiments multiple actuators may be provided. In such examples, each actuator may be arranged to commence movement of the valve members at a different time to when movement of another valve member is commenced. 
         [0072]    In the present example the valve members are connected by link members, but in alternative embodiments the valve members may be connected by an alternative arrangement, for example by rack and pinion gears, quarter gears intermeshing only at a time when staggered opening is required, or a worm gear. When gears are used, gear ratios can be selected to provide the desired control of rate of opening and closing of the butterfly valves. The gears may be provided with regions free of teeth. In this way, the gears can be arranged so that there is a delayed opening on one valve which when engaged has an accelerated opening via its gear ratio. The gears may by non-circular gears. For example, a smaller diameter gear and a larger diameter gear may be provided. The gears may have a region that is substantially defined by a circle. A portion of the gears adjacent said region may on the smaller gear include a concave surface and on the larger gear include a convex surface. The convex surface is configured to engage the concave surface. 
         [0073]    The present example has been described in relation to valves arranged within the vicinity of the same bleed duct, but in alternative embodiments the valves may be mounted on different sides of the engine, for example a port side and a starboard side of the engine. 
         [0074]    It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.