Abstract:
An example bypass valve includes a sleeve providing a bore that extends along an axis. The sleeve has at least one window. The bypass valve also includes a spool received within the bore. The spool is configured to move within the bore between a first position that restricts flow through at least one window and a second position that permits flow through the at least one window of the sleeve. An outer diameter of the spool is from 99.90 to 99.95 percent of a diameter of the bore.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to a valve and, more particularly, to a bypass valve for controlling a turbomachine fuel flow. 
         [0002]    Turbomachines, such as gas turbine engines, typically include a fan section, a compression section, a combustion section, and a turbine section. Air flow enters the turbomachine through the fan section. The air is compressed in the compression section. The compressed flow is mixed with fuel and combusted in the combustion section. The products of combustion are expanded through the turbine section to drive rotors of the turbomachine. 
         [0003]    Many turbomachines include a fuel control assembly. Fuel delivery valves of the fuel control assembly deliver fuel to the combustion section. Fuel windmill bypass valves of the fuel control assembly communicate flow away from the fuel delivery valves during engine shutdown. The windmill bypass valve maintains sufficient muscle pressure to position the actuators during windmilling and start. Shutdown of the engine may occur on the ground, or in flight during, for example, an over-speed condition. 
       SUMMARY 
       [0004]    An example windmill bypass valve includes a sleeve providing a bore that extends along an axis. The sleeve has at least one window. The windmill bypass valve also includes a spool received within the bore. The spool is configured to move within the bore between a first position that restricts flow through at least one window and a second position that permits flow through the at least one window of the sleeve. An outer diameter of the spool is from 99.90 to 99.95 percent of a diameter of the bore. 
         [0005]    Another example windmill bypass valve includes a sleeve providing a bore that extends along an axis. The sleeve has at least one window. The valve also includes a spool translates axially within the bore. The spool is configured to move within the bore between a first position that restricts flow through at least one window and a second position that permits flow through the at least one window of the sleeve. The Flow Gain through the windows is from 0.00 square inches/0.001 inches to 0.001185 square inches/0.001 inches. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0006]    The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows: 
           [0007]      FIG. 1  is a cross-section view of an example turbomachine having a fuel delivery arrangement. 
           [0008]      FIG. 2A  is a highly schematic view of the fuel delivery arrangement in a fuel delivery position. 
           [0009]      FIG. 2B  is a highly schematic view of the fuel delivery arrangement in a fuel bypass position. 
           [0010]      FIG. 3  is an exploded view of a fuel windmill bypass valve of the  FIG. 2A  arrangement. 
           [0011]      FIG. 4  is a section view of the fuel windmill bypass valve of  FIG. 3 . 
           [0012]      FIG. 5  is a section view of a valve sleeve of the fuel windmill bypass valve of  FIG. 3 . 
           [0013]      FIG. 6  is a side view of the valve sleeve of  FIG. 5  with a spool. 
           [0014]      FIG. 7  is a section view at line  7 - 7  in  FIG. 6 . 
           [0015]      FIG. 8  shows a close-up view of a window of the fuel windmill bypass valve of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a gas turbine engine  10  is used to propel an aircraft. The gas turbine engine  10  is a type of turbomachine. 
         [0017]    The example gas turbine engine  10  includes a fan section  12 , a compressor section  14 , a combustion section  16  and a turbine section  18 . Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section  12  drives air along a bypass flow path B while the compressor section  14  draws air in along a core flow path C where air is compressed and communicated to a combustion section  16 . In the combustion section  16 , air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through the turbine section  18  where energy is extracted and utilized to drive the fan section  12  and the compressor section  14 . In this example, the turbine section  18  drives the fan section  12  through a geared architecture  15  such that the fan section  12  may rotate at a speed different than the turbine section  18 . 
         [0018]    Although the disclosed non-limiting embodiment depicts a turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section  14 . 
         [0019]    The example gas turbine engine includes a fuel system  16  that supplies fuel from a fuel supply to the combustion section  16  and also to other devices within the gas turbine engine that may utilize fuel for heat exchanging or for powering actuators. 
         [0020]    Referring to  FIGS. 2A and 2B  with continuing reference to  FIG. 1 , the fuel system or (fuel control assembly  30 ) forms a portion of a fuel delivery arrangement  34 . During normal operation, components of the fuel delivery arrangement  34  are in a fuel delivery position. Under some conditions, such as a shutdown of the gas turbine engine  10 , components of the fuel delivery arrangement  34  are moved to a fuel bypass position. The gas turbine engine  10  may be windmilling when the gas turbine engine  10  is shutdown. 
         [0021]    The fuel control assembly  30  includes at least one fuel delivery valve  38 . In some examples, eight fuel delivery valves are used. When the fuel delivery arrangement  34  is in the fuel delivery position, fuel from a fuel supply  42  moves through the fuel delivery valve  38  to the combustion section  16 . 
         [0022]    The fuel control assembly  30  also includes at least one fuel bypass valve  50 . In some examples, two fuel bypass valves are used. When the fuel delivery arrangement  34  is in the fuel bypass position, fuel from the fuel supply  42  moves through the fuel bypass valve  50  to an overflow area  54 . Depending upon the architecture of the engine  10 , the bypass flow will either return to the fuel supply  42  or recirculate within fuel lines until the engine  10  spools down and there is no more demand for fuel flow thru the lines. In some examples, after shutdown, many engines drain manifold lines between the fuel control assembly  30  and the combustion section  16  by either a valve piston that, when translated, has enough volume capacity to handle the fuel lines or a separate small reservoir or bowl that will accept the manifold fuel. 
         [0023]    When in the fuel bypass position, the fuel delivery arrangement  34  is able to receive fuel from the fuel supply  42  without delivering fuel to the combustion section  16 . Delivering fuel to the combustion section  16  during an engine shutdown, for example, may be problematic as is known. 
         [0024]    In this example, a solenoid  56  is selectively activated to move the fuel bypass valve  50 . Moving the fuel delivery valve  38 , the fuel bypass valve  50 , or both moves the fuel delivery arrangement  34  between the fuel delivery position and the fuel bypass position. 
         [0025]    Referring now to  FIGS. 3 to 8  with continuing reference to  FIGS. 1 to 3 , the fuel bypass valve  50  includes a sleeve  58  that receives a spool  62 . The sleeve  58  and the spool  62  make up a valve set. In this example, the fuel bypass valve  50  also includes a cap  66 , a cap seal  68 , a spring  70 , a spool spacer  72 , a spool seal  74 , a face seal  76 , a sleeve seal  80 , and a housing  82 . The components of the fuel bypass valve are arranged about an axis A. 
         [0026]    To move the fuel delivery arrangement  34  between the fuel delivery position and the fuel bypass position, the solenoid  56  moves the spool  62  axially relative to the sleeve  58 . A person having skill in this art and the benefit of this disclosure would understand how to configure the solenoid  56  to move the spool  62  relative to the sleeve  58  in this way. 
         [0027]    The spool  62  is held within a bore  84  of the sleeve  58 . An outer diameter D 1  of the spool  62  is smaller than a diameter D 2  of the bore  84 . In some examples, the outer diameter D 1  is from 99.90 to 99.95 percent the diameter D 2 . Making the outer diameter D 1  smaller than the diameter D 2 , and within the above range, has been found to facilitate axial movement of the spool  62  within the bore  84 . 
         [0028]    The sleeve  58  includes, in this example, three windows  88  that are able to communicate fuel when not blocked by the spool  62 . In the fuel delivery position, the spool  62  blocks movement of fuel through the windows  88  so that the fuel moves to the combustion section  16 . Moving the spool  62  axially relative to the sleeve  58  allows fuel to communicate radially through the windows  88  to the overflow area  54 . 
         [0029]    The windows  88  include a low gain portion  90  and a high gain portion  92 . As the spool  62  is moved axially within the sleeve  58  in a direction Y, the low gain portion  90  of the windows  88  are initially opened for communicating fuel F from the fuel supply  42  to the overflow area  54 . 
         [0030]    In  FIG. 6 , the spool  62  has been moved by the solenoid  56  in the direction Y to a position having the low gain portion  90  fully opened and the high gain portion  92  partially opened. 
         [0031]    After more movement of the spool  62  relative to the sleeve  58  in the direction Y, the high gain portion  92  is also fully opened. When the windows  88  are fully opened, the area of the low gain portion  90  that is available for communicating fuel is lower than the area of the high gain portion  92  that is available for communicating fuel. 
         [0032]    Making the low gain portion  90  available for communicating fuel prior to the high gain portion  92  facilitates a gradual increase of fuel moving through the windows  88 . The gradual increase facilitates reducing resonance associated with fuel movement through the windows. 
         [0033]    The axial position of the spool  62  within the sleeve  58  determines the amount of low gain portion  90  and high gain portion  92  available for communicating flow. 
         [0034]    In this example, a ratio of the area of the windows  88  to the axial position of the spool  62  relative to the sleeve  58  is from 0.00 square inches/0.001 inches to 0.001185 square inches/0.001 inches. This ratio may be referred to as gain. Gain is the change in fuel flow going thru the window  88  as a function of the stroke of the fuel bypass valve  50 . How much the area of the window  88  changes as a function of the fuel bypass valve  50  stroke is a relationship that is adjusted to reduce undesired flow characteristics of fuel moving through the fuel bypass valve  50 , this could be stabilizing flow perturbations, etc. 
         [0035]    In some positions, an end  96  of the spool  62  contacts the face seal  76 . In this example, an outer perimeter of the end  96  has a chamfer  98  that is from 0.001 to 0.005 inches (0.0025 to 0.0127 centimeters). Maintaining the chamfer  98  within this range has been found to reduce damage to the face seal  76 . 
         [0036]    An edge  100  associated with the windows  88  has a maximum break of 0.001 inches (0.0025 cm). There is some gain with associate with the break at the edge  100 . Keeping the break at the edge at or below this size has been found to desirably minimize this gain such that the areas of the low gain portion  90  and the high gain portion  92  may be primarily used to control flow. 
         [0037]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.