Abstract:
A method and system for providing fuel to primary and secondary fuel nozzles in a gas turbine engine fuel system comprises generating a fuel flow and routing primary fuel from the fuel flow to a primary fuel nozzle. Backpressure on the fuel flow is maintained using a valve. The valve is opened at increased fuel flow to route secondary fuel from the fuel flow to a secondary fuel nozzle. The valve is progressively opened under increasing fuel flows to reduce a pressure drop across the valve produced by the secondary fuel.

Description:
BACKGROUND 
     The present invention relates generally to fuel systems for gas turbine engines. More particularly, the present invention relates to systems for delivering fuel to nozzles within combustors of the gas turbine engines. 
     Combustors within gas turbine engines are generally of the annular configuration wherein an inner diameter wall circumscribes the engine centerline and an outer diameter wall circumscribes the inner diameter wall to define a combustion chamber therebetween. A ring-like dome typically connects the walls at their upstream end. Fuel nozzles are provided in the dome to inject fuel into a flow of compressed air flowing through the dome. The fuel is injected through small orifices that atomize the fuel to increase combustion efficiency. The nozzles are distributed within the dome evenly around the circumference of the combustor. Recent advancements in combustor design have incorporated the use of primary and secondary fuel nozzles to better control fuel injection during low flow operating states, such as during ignition, ground idle and flight idle. A few primary nozzles that are used during the low-flow conditions are dispersed around the dome and have small injector orifices. A greater number of secondary nozzles having larger orifices are interspersed between the primary nozzles and are brought into use at higher flow conditions, such as during take-off and cruise. The pressure required to properly atomize the fuel in the primary and secondary nozzles can vary widely due to the difference in orifice size. 
     The primary nozzles open first when fuel flow is initiated, then the secondary fuel nozzles open as fuel flow increases. The valve maintains a minimum backpressure to the primary nozzles to ensure atomization at low flow conditions. The high atomization pressure required by the primary nozzles therefore requires the valve to have a high opening pressure, thereby introducing a point of high pressure drop at the valve during all operating conditions of the engine when the valve is open. It is, however, undesirable to have such a high pressure drop located within the fuel flow. For example, a high pressure drop within the system increases the working pressure and power of the fuel pump, which introduces heat into the fuel system. The heat is an indication of fuel flow inefficiency and, in any event, must be dealt with or dissipated by the engine fuel management system. There is, therefore, a need for controlling flow to primary and secondary nozzles within gas turbine engine combustors without introducing unnecessary high pressure drops within the system. 
     SUMMARY 
     The present invention is directed to a method for providing fuel to primary and secondary fuel nozzles in a gas turbine engine fuel system. The method comprises generating a fuel flow and routing primary fuel from the fuel flow to a primary fuel nozzle. Backpressure on the fuel flow is maintained using a valve. The valve is opened at increased fuel flow to route secondary fuel from the fuel flow to a secondary fuel nozzle. The valve is progressively opened under increasing fuel flows to reduce a pressure drop across the valve produced by the secondary fuel. 
     The present invention is directed to a fuel system for dividing fuel between primary and secondary nozzles in a gas turbine engine. The fuel system comprises primary fuel nozzles, secondary fuel nozzles, a fuel pump and a flow divider valve. The primary and secondary fuel nozzles are coupled to a combustor in the gas turbine engine. The fuel pump generates a fuel flow. The flow divider valve receives the fuel flow and divides fuel to the primary and secondary fuel nozzles. The flow divider valve comprises a valve housing having an inlet for receiving the fuel flow, a piston disposed within the housing to maintain a backpressure on the fuel flow, means for providing primary fuel to the primary fuel nozzles, and means for providing secondary fuel to the secondary fuel nozzles while progressively decreasing the pressure drop across the means for providing the secondary fuel to the secondary fuel nozzles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified schematic of a gas turbine engine fuel system that provides continuous flow to primary nozzles and equalizing flow to secondary nozzles using a flow divider valve. 
         FIG. 2  shows a graph depicting pressure drop in a flow divider valve versus fuel flow for conventional divider valves and passive equalization divider valves of the present invention. 
         FIG. 3  shows a schematic of a portion of the fuel system of  FIG. 1  in which the passive equalization divider valve of the present invention comprises a series-flow, dual-valve system. 
         FIG. 4  shows a schematic of a portion of the fuel system of  FIG. 1  in which the passive equalization divider valve of the present invention comprises a double-action, linear piston valve. 
         FIG. 5  shows a schematic of a portion of the fuel system of  FIG. 1  in which the passive equalization divider valve of the present invention comprises a double-action, parallel-flow piston valve. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a simplified schematic of gas turbine engine  10  having fuel system  12  in which a flow dividing valve system of the present invention is used. Gas turbine engine  10  includes compressor  14 , combustor  16 , turbine  18  and shaft  20 . Fuel system  12  includes fuel tank  22 , fuel pump  24 , Fuel Metering Unit (FMU)  26  and flow divider valve  28 . Combustor  16  includes primary fuel nozzles  30  and secondary fuel nozzles  32 . 
     FMU  26  comprises an electronic valve module that regulates fuel flow from pump  24  based on sensed needs of engine  10  from, for example, communications with a Full Authority Digital Engine Controller (FADEC) (not shown). Fuel system  12  dispenses fuel from fuel tank  22  to engine  10 . Fuel is drawn into pump  24  through fuel line  34  and provided to FMU  26  through fuel line  36 . Fuel not needed by FMU  26  is routed back to pump  24  through return line  38 . After operation of engine  10  ceases, any remaining fuel in fuel system  12  or combustor  16  is routed to a fuel ecology system  39  via fuel line  40 A. Ecology system  39  purges unused fuel from combustor  16  and divider valve  28  to reduce coking and leakage of fuel after shutdown, as is known in the art. FMU  26  is fluidly coupled to divider valve  28  via fuel line  42 . Primary nozzles  30  receive fuel directly from FMU  26  through fuel lines  42  and  44 . Secondary nozzles  32  receive fuel from divider valve  28  through fuel line  46 . Valve  28  can be provided with control pressure through fuel line  40 B that connects into an upstream portion of the fuel system, such as fuel line  34 . 
     Compressor  14  intakes and compresses a gas, such as atmospheric air, and forces the compressed gas into combustor  16 . Combustor  16  also receives fuel from fuel pump  24  at primary nozzles  30  and secondary nozzles  32 . Within combustor  16 , the compressed gas and fuel are mixed and ignited to force expanded gas into turbine  18 . Turbine  18  extracts energy from the expanded gas to cause rotation of shaft  20  before the gas is expelled from engine  10  as exhaust. Shaft  20 , in turn, powers compressor  14  and other subsidiary systems. For example, power from shaft  20  is typically used to turn a tower shaft and gear system for providing input to fuel pump  24  and other accessory systems, such as a generator (not shown). 
     Fuel pump  24  operates based on the speed of shaft  20  and thus provides an unregulated amount of fuel to FMU  26 . FMU  26  receives various engine control signals from various sensors, such as pressure and temperature sensors, within engine  10  to determine various engine needs based on performance demands being placed on engine  10 . For example, the amount of fuel needed by engine  10  depends on, among other things, a throttle position actuated by an operator. Engine  10  requires different amounts of fuel under different operating conditions. For example, under start-up conditions, only a low amount of fuel flow is needed by engine  10  as compared to take-off and cruise conditions where a higher amount of fuel flow is required. 
     FMU  26  meters flow to combustor  14 . Primary nozzles  16  are directly connected to the flow of metered fuel, as they are required to receive fuel under all operating conditions of the gas turbine engine. In particular, primary nozzles receive fuel under low-flow conditions, such as start-up. In the embodiment shown, fuel line  44  extends directly between fuel line  42  and nozzles  30 , as is discussed with reference to  FIG. 3 . Fuel line  44  may, however, extend from valve  28  to connect to nozzles  30 , as is discussed with reference to  FIGS. 4 and 5 . Primary nozzles  30  receive a relatively small fraction of the fuel flow from line  42 . The remainder of the flow passes thru flow divider valve  28  and to secondary nozzles  32 . Flow divider valve  28  is configured to open at a particular backpressure within line  42  to maintain pressurization for atomizing fuel at primary nozzles  30  at low-flow conditions. Flow divider valve  28  provides an equalizing flow of fuel to secondary nozzles  32 . Flow divider valve  28  of the present invention allows secondary nozzles  32  to be passively brought into flow communication with combustor  16  as gas turbine engine  10  transitions from low-flow operating conditions to high-flow operating conditions, such as during cruise or flight maneuvers. Valve  28  operates based on fuel pressure and flow rate and is not actively controlled. Furthermore, at high fuel flow rates, valve  28  of the present invention operates to reduce the high backpressure it produces at low fuel flow rates to improve efficiency of fuel system  12 . 
     Primary nozzles  30  are configured with small atomization orifices to provide optimal combustion conditions for start-up with a low amount of fuel. Primary nozzles  30  therefore require a large backpressure in fuel line  44  to properly operate. Secondary nozzles  32  are configured with larger atomization orifices to permit larger volumes of fuel flow such as at cruise conditions. As such, secondary nozzles do not require as large of backpressure within fuel line  46 . Flow divider valve  28  maintains backpressure within fuel line  44  at low-flow conditions, but opens under high-flow conditions to increase the size of its flow restriction and reduce the pressure drop it produces. Adequate backpressures are maintained in fuel lines  44  and  46  at high flow conditions due to the inherent static fuel pressures at such elevated flow rates. 
       FIG. 2  shows a graph depicting pressure drop in a flow divider valve versus fuel flow for prior art divider valves and passive equalization divider valves of the present invention. The x-axis indicates fuel flow in a gas turbine engine, indicating zero flow at the y-axis, start or ground/flight idle conditions within zone A, equalization in zone B, and takeoff and cruise within zone C. The y-axis indicates pressure drop, ΔP, across the divider valve, increasing from zero to pressures above what is required to operate a typical flow divider valve. Pressure drop for conventional, non-equalizing divider valves is shown by solid line P 0 . Pressure drop for the passive equalization divider valves of the present invention is shown by dashed line PE. 
     During low flow conditions, fuel flow increases until the back pressure in the fuel system (fuel line  44 ) reaches point D, beyond which point the pressure drop of a conventional divider valve would continue to increase along solid line P 0 . However, as mentioned, continuously having to overcome the pressure drop produced by the flow divider valve after the primary nozzles have been primed at point D is inefficient. During high flow conditions the static fuel pressure is sufficient such that the need for a restriction with a large pressure drop to maintain backpressure is not needed. For example, the restriction increases the operating burden of fuel pump  24  and the rest of the thermal management system of engine  10 . 
     Passive equalization divider valves  28  of the present invention operate in a two-stage manner to 1) provide adequate backpressure during low-flow conditions to prime primary nozzles  30 , and 2) to reduce the pressure drop signature produced by the divider valve during high-flow conditions when fuel pressure is adequate to maintain pressurization of primary nozzles  30 , while providing fuel to secondary nozzles  32 . As shown in  FIG. 2 , during start-up and ground/flight idle conditions in zone A, the passive equalization divider valves of the present invention permit backpressure in line  44  to build to a particular ΔP at point D, as do conventional divider valves. At point D, passive equalization divider valve  28  continue to open to permit increasing fuel flow into secondary nozzles  32 . However, rather than merely opening and then continuously being maintained open by fuel flow at the same ΔP, the passive equalization divider valves are configured to reduce the total pressure drop across the valve as the fuel flow rate increases. As shown in  FIG. 2 , ΔP drops linearly with respect to fuel flow in zone B. In other embodiments, ΔP may be configured to drop at faster or slower rates at different points of the fuel flow rate (i.e. the fuel flow rate can be non-linear in zone B). At point E, ΔP across passive equalization divider valve  28  levels off to reduce the workload of pump  24  under high-flow conditions. 
       FIG. 3  shows a schematic of a portion of fuel system  12  in which passive equalization divider valve  28  of the present invention comprises a series-flow, dual-valve system having primary pressurization valve  48  and flow divider valve  50 . Valves  48  and  50  are integrated into housing  52 , which includes inlet  54 , piston cylinder  56 , window  58 , feed  60  and outlets  62 . Primary pressurization valve  48  includes piston  64  and spring  66 . Flow divider valve  50  comprises a divider valve of conventional design. Primary valve  48  is provided with control pressure through line  40 B, which is connected to fuel line  34 . Flow divider valve  50  is provided with a drain outlet at line  40 A. 
     During operation of engine  10 , fuel flows into inlet  54  of housing  52  from fuel line  42  ( FIG. 1 ). During start-up conditions, the fuel pressure is low so that valve  48  does not open. Fuel pressure from line  40 B balances static fuel pressure from inlet  54 . In other embodiments, line  40 B may be connected to another low pressure point within fuel system  12 , such as fuel line  38 . Thus, the spring force of spring  66  biases piston  64  against inlet  62  thereby preventing fuel flow to window  58 . Fuel, however, continues to flow from line  42  into line  44  and on to primary nozzles  30 . As the start-up of engine  10  continues, backpressure builds within line  44  allowing fuel to be atomized at nozzles  30 . At a threshold pressure, the magnitude of which is sufficient to atomize the fuel at nozzles  30 , primary valve  48  begins to open due to increased fuel flow rates, which overcomes the force applied by spring  66 . Piston  64  retreats within piston cylinder  56  to uncover window  58 , permitting fuel to leak into feed  60 . This occurs within region A in  FIG. 2 . Divider valve  50  operates in a conventional manner to split the fuel flow into multiple paths at outlets  62  for feeding each secondary nozzle  32  individually or for feeding zones of injectors, such as with a manifold or manifolds. The pressure required to open valve  50  is much lower than the pressure required to open valve  48 , as provided by spring  66 . As fuel pressure continues to increase as engine  10  moves through equalization (zone B of  FIG. 2 ), piston  64  retreats to fully uncover window  58 . At such point the pressure drop produced by primary valve  48  becomes negligible, leaving only the pressure drop produced by equalizing valve  50 . This occurs at point E in  FIG. 2 . As such, overall pressure drop in valve  28  is lowered from that provided by valve  48  to that provided by valve  50 , thereby lowering the work needed to be done by pump  24  ( FIG. 1 ). Divider valve  50  is provided with a drain outlet at line  40 A, which is connected to ecology system  39  ( FIG. 1 ) to permit unused fuel remaining in valve  50  to be removed from the fuel system. 
       FIG. 4  shows a schematic of a portion of fuel system  12  in which passive equalization divider valve  28  of the present invention comprises a double-action, linear piston valve having piston  68  disposed within housing  70 . Housing  70  includes inlet  72 , piston cylinder  74 , window  76 , cross-port  78 , primary outlet  80 , secondary outlets  82 , drain line  84  and drain  86 . Piston  68  includes actuation face  88 , equalization port  90 , spring pocket  92 , drain port  94 , balance port  96 , spring  98  and orifice  99 . 
     During operation of engine  10 , fuel flows into inlet  72  of housing  70  from fuel line  42  ( FIG. 1 ). Fuel also flows into balance port  96  and into spring pocket  92  to allow static fuel pressure to maintain a force balance on piston  68 . The force of the motive flow of fuel, however, acts against piston face  88  to counteract spring force from spring  98  to open valve  28 . During start-up and ground/flight idle conditions, with low fuel flow, piston  68  moves to uncover window  76  and partially uncover window  90 . Window  76  is contoured to maintain a minimum pressure drop and to produce a linear valve position versus fuel flow relationship. Fuel flows through window  76 , into cross-port  78  and out to primary outlet  80 . Thus, fuel is provided to primary nozzles  30  ( FIG. 1 ). Flow above that going to primary nozzles  30  passes thru equalization port  90  to outlets  82  to secondary nozzles  32 . The pressure drop across window  90  maintains a backpressure within line  78  such that fuel provided to the primary nozzles through outlet  80  is sufficiently atomized. 
     Under mid to high flow conditions, piston  68  moves further to the right (with reference to  FIG. 4 ), continuing to open equalization port  90 . Equalization port  90  is shaped such that, as it continues to open, the pressure drop across it decreases to being negligible compared to the pressure drop produced by window  76 . This is low enough to not produce undue burden on pump  24  and the thermal management system of engine  10 . 
     Housing  70  is also connected to fuel line  40 A ( FIG. 1 ) to permit fuel to drain from valve  28  when engine  10  is shut down. Specifically, drain lines  84  are fluidly coupled with secondary outlets  82  to permit fuel to drain back to ecology system  39  through fuel line  40 A. Drain lines  84  are fluidly coupled to drain  86  via drain port  94  when piston  68  is fully closed, or all the way to the left with reference to  FIG. 4 . 
       FIG. 5  shows a schematic of a portion of fuel system  12  in which the passive equalization divider valve  28  of the present invention comprises a double-action, parallel-flow piston valve having piston  100  disposed within housing  102 . Housing  102  includes inlet  104 , barrier  106 , screen  108 , piston cylinder  110 , primary outlet  112 , secondary outlet  114 , control passage  116 , fixed orifice  117 , drain valve  118 , window  120 , control outlet  122  and drain passages  124 A,  124 B and  124 C. Piston  100  includes actuation face  126 , actuation flange  128 , orifice  130 , drain window  132 , drain window  134  and spring  136 . 
     During operation of engine  10 , fuel flows into inlet  72  of housing  70  from fuel line  42  ( FIG. 1 ). Inlet fuel F I  contacts barrier  106  and is pushed outward into screen  108  to remove particulates from the fuel flow. Inlet fuel F I  then engages actuation face  126  and pushes piston  100  to the right (with reference to  FIG. 5 ). Initially, during start-up conditions, the fuel flow is low so that piston  100  uncovers primary outlet  112 , so that most of the fuel can flow out to primary nozzles  30  ( FIG. 1 ). A small portion of the fuel flows into line  115  as actuation fuel F A . At fuel flows above start-up, window  137  opens to flow to secondary nozzles  32  via line  114 . 
     Actuation fuel F A  within passage  116  travels to the inside of piston cylinder  110  behind piston  100  and within actuation flange  128 . From piston cylinder  110 , actuation fuel F A  travels through orifice  130  and window  120  and into control outlet  122 . Window  120  is contoured to provide a smaller restriction with a larger pressure drop at low flow rates (when piston  100  is toward the left in  FIG. 5 ) and to provide a larger restriction with a smaller pressure drop at high flow rates (when piston  100  is toward the right in  FIG. 5 ). Window  137  realizes the total pressure drop in valve  28 , including that of orifice  117  plus window  120 . This same total pressure drop is realized by primary nozzles  30 . Thus, primary nozzles  30  see sufficient pressure at low flows for proper fuel atomization. 
     Movement of piston  100  is dictated by the pressure across orifice  117 . Initially, the summation of the pressure drops in orifice  117  and window  120  provides a backpressure so that low fuel flow will be forced into primary outlet  112 . At low fuel flows, window  120  provides a large pressure drop that limits flow into passage  116 . As increased fuel flow continues to stroke piston  100 , window  120  opens to increase its restriction size and to reduce backpressure downstream of orifice  117 . Thus, more actuation fuel F A  is permitted to flow into control passage  116 . As piston  100  continues to stroke open, secondary fuel F S  increases thru secondary outlet  114 , where it is joined by actuation fuel F A  for distribution to secondary nozzles  32  ( FIG. 1 ). Actuation fuel F A  ultimately joins with secondary outlet  114  for distribution to secondary nozzles  32 . Housing  102  can be provided with a plurality of secondary outlets  114  for distributing fuel to a plurality of secondary nozzles or secondary manifolds. However, only one of the secondary outlets  114  need be provided with control passage  116 , fixed orifice  117  and window  120  to control the position of piston  100 . 
     Housing  102  and piston  100  also include drain lines and windows to permit fuel to drain from valve  28  at shut-down of engine  10 . Specifically, drain fuel F D  is permitted into the interior of piston  100  through drain line  124 A, which engages drain window  132  across the entire stroke length of piston  100 . When piston  100  retreats under lack of fuel pressure drain window  132  engages drain line  124 C to let fuel drain out to primary nozzles  30 . Additionally, with piston  100  retracted, drain valve  118  can be configured to open to allow fuel to leave valve housing  102  at drain line  124 B, which connects to fuel line  39  through fuel line  40 A ( FIG. 1 ). 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.