Abstract:
A method and apparatus for dumping surge bleed air into a primary nozzle of a free gas turbine engine. The surge bleed air is introduced into gas turbine exhaust flow within the primary nozzle to create a mixed flow which may be used as a combined driver flow to compensate for reduced engine exhaust flow during periods when operation of the turbine engine may be exclusively dedicated to only electric load operation. The surge bleed air may not be the educted flow or the secondary driven flow, while cooling air passing through an oil cooler may be an educted flow. Surge bleed air may flow through, for example, mixer lobes, hollow struts, or the center body before mixing with the gas turbine exhaust flow.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to a method and apparatus for cooling the heat generated by a gas turbine engine mounted in a compartment and, in particular, to an eductor system that directs gas turbine exhaust gas and surge bleed air into a primary nozzle of the eductor to entrain sufficient cooling airflow to cool the compartment and to cool the gearbox and generator oil.  
         [0002]     In addition to their traditional propulsion functions, gas turbine engines are used as auxiliary power units (APUs) aboard many types of aircraft, ground vehicles, and stationary installations to provide continuous shaft and/or pneumatic power. The shaft power is used to drive electric generators, load compressors, hydraulic pumps, or other equipment. The pneumatic power is used by air turbine motors for main engine starting, cabin air-conditioning and pressurization, de-icing, or other components requiring compressed air. When used aboard an aircraft, for example, the APU is typically mounted in a compartment located within the tail cone of the aircraft.  
         [0003]     Historically, APUs have only been operated when the aircraft was on the ground. Currently, aircraft need an additional source of power while in flight. To meet this need an APU may be started and operated in flight at high altitudes. During the operation of the APU, heat is rejected into the compartment from numerous sources including the engine skin; exhaust gases, the tailpipe, as well as the engine oil cooler, generator, and other compartment accessories. To prevent the temperature in the compartment from reaching unacceptable levels, a ventilating or cooling airflow must be provided through the compartment.  
         [0004]     To remove this heat, an axial, vane type fan driven by the APU gearbox is usually provided to pump cooling air past the oil cooler as well as through the compartment. However, because of their multiplicity of high speed, rotating parts, these fans are susceptible to mechanical failures, which may require that the aircraft be removed from operation. These fans sometimes leak oil into the cooling flow, which may then cover the oil cooler fins resulting in reduced heat transfer and the possibility of an APU automatic shutdown because of excessive oil temperature.  
         [0005]     An alternative to fans is a simple exhaust eductor system having a primary nozzle and an exhaust mixing tailpipe. This eductor uses the kinetic energy of the APU exhaust gas to entrain ambient cooling flow through the compartment and over an oil cooler  
         [0006]     The APU&#39;s shaft power can be delivered to the gearbox and load compressor in one of two engine architectures. In a single shaft direct drive arrangement, the core engine, the load compressor, and the gearbox are all connected to the same shaft and rotate at the same speed. In another arrangement, the core engine compressor and turbine are connected via one shaft while the gearbox and the load compressor are driven by a free turbine via another shaft.  
         [0007]     Each of these engine architectures has their advantages and disadvantages. The eductor performance in a free turbine APU can be reduced during no pneumatic condition. The eductor&#39;s cooling flow pumping capacity is directly related to the primary flow rate.  
         [0008]     In ground servicing of commercial aircraft, where ground crew fuel and provision the aircraft, and the like, certain noise level limits must be maintained to ensure the health and safety of the ground crew. Therefore, the propulsion engines of the aircraft are typically shut down and only an APU remains in use. The APU may be used in ground service to maintain aircraft interior cooling, oil cooling, engine cooling, to generate electricity for interior lighting, and other necessary operations.  
         [0009]      FIG. 1  shows a cross-sectional view of a prior art free turbine auxiliary power unit. A core engine turbine  160  may be coaxial with a free turbine  150 . The core engine turbine  160  may include a core engine combustor  140  and a core engine shaft  142  that may drive a core engine compressor  162 . Inside the core engine turbine may be located a turbine shaft  144  for delivering shaft power from the free turbine  150  to the load compressor  110  and the gearbox  120  driving generator  130 . Turbine exhaust may exit through the primary nozzle  30  and the mixing duct  90 . The primary nozzle  30  and the mixing duct  90  together function as the eductor. The turbine exhaust exiting the primary nozzle  30  into the mixing duct  90  entrains ambient air through the oil cooler  164 . When needed, the free turbine may be burdened by a generator  130 , a gearbox  120 , and/or a load compressor  110 .  
         [0010]     In a free turbine engine, the exhaust flow can vary depending on the load demand on the engine. In a free turbine engine at low pneumatic load, but high generator load, the exhaust flow could be considerably lower than when the free turbine engine is at high pneumatic load and high generator load, which would reduce cooling flow pumping while cooling flow demand for the generator and gearbox oil cooling would still be high.  
         [0011]     In certain APU operating conditions when the operator shuts off the demand for the pneumatic load (for example after the main engine start completion, the demand for the high pressure air to drive the starter turbine is shut off) the high pressure airflow must be dumped overboard to prevent the load compressor from surging. This air is often dumped in the exhaust tailpipe.  
         [0012]      FIG. 2  shows the prior art approach to mixing exhaust flow with surge bleed air and cooling airflow. Exhaust flow  170  from an upstream gas turbine engine (not shown) flows, as a primary driving flow, past turbine  50  and around center body  40  toward mixing plane  100 . Cooling air  174  flows from the external environment through oil cooler  60 , into cooling flow plenum  80 , and downstream through mixing plane  100 , entrained by eduction action of the exhaust flow  170 . Continuing downstream from the mixing plane  100 , surge bleed air  20  flows into mixing duct  90 , wherein the surge bleed air  20  may be entrained into the mixture of cooling airflow  174  and exhaust flow  170 . During periods of reduced pneumatic load, such as after a main engine start completion when the operator shuts off the demand for the pneumatic load, the primary driving flow from the gas turbine engine (not shown) may be diminished, while the generator load and need for gear box oil cooling may remain high. In this situation, the primary flow may not be sufficient to entrain sufficient cooling airflow  174  within the mixing duct  90  and provide adequate oil cooling. Dumping the surge bleed air  20  downstream of the primary nozzle  30  into the mixing duct  90  can further aggravate the low eductor pumping.  
         [0013]     As can be seen, there is a need for an improved apparatus and method for dumping surge bleed air into the primary flow to compensate for diminished pneumatic load and increased need for component cooling while maintaining a correspondingly high cooling flow rate and eductor performance.  
       SUMMARY OF THE INVENTION  
       [0014]     In one aspect of the present invention, a cooling apparatus for a gas turbine engine comprises a primary nozzle; a cooling flow plenum in fluid communication with the external environment and the cooling flow plenum configured to be larger in radius than the primary nozzle; and a surge air dump nozzle disposed adjacent to the cooling flow plenum, wherein the primary nozzle, the cooling flow plenum, and the surge air dump nozzle are configured so as to direct the surge bleed air into the primary nozzle.  
         [0015]     In a further aspect of the present invention, an auxiliary power unit comprises an oil cooler; a primary nozzle; a cooling flow plenum in air communication with the oil cooler and downstream of the oil cooler; and a surge air dump nozzle disposed adjacent to the primary nozzle, the surge air dump nozzle comprising mixing lobes, wherein the primary nozzle, the cooling flow plenum, and the surge air dump nozzle are configured so as to direct an exhaust flow and the surge bleed air into the primary nozzle and mix the surge bleed air with the exhaust flow, within the primary nozzle, and entrain the cooling air with the mixed surge bleed air and the exhaust flow.  
         [0016]     In another aspect of the present invention, a cooling apparatus for an auxiliary power unit comprises an oil cooler; a primary nozzle; a cooling flow plenum in air communication with the oil cooler and downstream of the oil cooler; and an annular surge air dump nozzle disposed about a center body, wherein the primary nozzle is disposed about the center body; and wherein the primary nozzle, the cooling flow plenum, and the surge air dump nozzle are formed so as to direct an exhaust flow and the surge bleed air into the primary nozzle and mix the surge bleed air with the exhaust flow, within the primary nozzle, and entrain the cooling air with the mixed surge bleed air and exhaust flow.  
         [0017]     In yet another aspect of the present invention, a cooling apparatus comprises an auxiliary power unit; an oil cooler; a primary nozzle; a cooling flow plenum in air communication with the oil cooler and downstream of the oil cooler; and a surge air plenum disposed about the primary nozzle; wherein the surge air plenum is in fluid communication with a hollow strut such that surge bleed air is directed to flow through the hollow strut and the surge bleed air mixes with an exhaust flow.  
         [0018]     In yet a further aspect of the present invention, an aircraft comprises a compartment; an auxiliary power unit housed within the compartment; the auxiliary power unit comprising a turbine; an oil cooler within the compartment; a primary nozzle downstream of the turbine; a cooling flow plenum in air communication with the oil cooler and the cooling flow plenum disposed downstream of the oil cooler; a surge air plenum in air communication with a surge air duct; a surge air dump nozzle disposed about the primary nozzle and downstream of the surge air plenum; and a mixing duct, wherein the primary nozzle, the cooling flow plenum, and the surge air plenum are formed so as to direct an exhaust flow from the auxiliary power unit and the surge bleed air into the primary nozzle and mix the surge bleed air with the exhaust flow, within the primary nozzle, and entrain the cooling air with the mixed surge bleed air and exhaust flow.  
         [0019]     In still a further aspect of the present invention, a method for cooling a gas turbine engine comprises directing a cooling airflow into a cooling flow plenum; directing a surge bleed air into a surge air plenum; drawing the surge bleed air into a primary nozzle of the gas turbine engine; mixing the surge bleed air with an exhaust flow from the gas turbine engine, wherein the mixing is performed within the primary nozzle; creating a mixed exhaust flow comprising the surge bleed air and the exhaust flow; entraining the cooling airflow through the cooling flow plenum and into the mixed exhaust flow; drawing the cooling airflow across an oil cooler; and directing the mixed exhaust flow and the cooling airflow into an eductor mixing duct.  
         [0020]     These and other aspects, objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a schematic representation of a free shaft auxiliary power unit of the prior art;  
         [0022]      FIG. 2  is a cross-sectional view of a prior art auxiliary power unit wherein surge bleed air is introduced downstream from the primary nozzle;  
         [0023]      FIG. 3A  is a cross-sectional view of an auxiliary power unit in an aircraft compartment, according to an embodiment of the present invention;  
         [0024]      FIG. 3B  is a cross-sectional view of the auxiliary power unit of  FIG. 3A , illustrating the mixing effect involving the surge bleed air, the primary flow, and the cooling airflow, according to an embodiment of the present invention;  
         [0025]      FIG. 3C  is an end view of the auxiliary power unit primary nozzle of  FIG. 3A , as viewed along the line  3 C- 3 C;  
         [0026]      FIG. 4  is a cross-sectional view of an auxiliary power unit, according to another embodiment of the present invention;  
         [0027]      FIG. 5  is an end view of the auxiliary power unit primary nozzle of  FIG. 4 , as viewed along the line  5 - 5 ;  
         [0028]      FIG. 6  is a cross-sectional view of an auxiliary power unit, according to yet another embodiment of the present invention;  
         [0029]      FIG. 7  is an end view of the auxiliary power unit primary nozzle of  FIG. 6 , as viewed along line  7 - 7 ;  
         [0030]      FIG. 8  is a cross-sectional view of an auxiliary power unit, according to still another embodiment of the present invention;  
         [0031]      FIG. 9  is an end view of the primary nozzle of  FIG. 8 , as viewed along line  9 - 9 ;  
         [0032]      FIG. 10  is a cross-sectional view of an auxiliary power unit, according to a further embodiment of the present invention;  
         [0033]      FIG. 11  is an end view of the primary nozzle of  FIG. 10 , as viewed along line  11 - 11 ; and  
         [0034]      FIG. 12  schematically represents a series of steps involved in a method of cooling a gas turbine engine, according to a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     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.  
         [0036]     Broadly, the present invention provides a low back pressure turbine eductor cooling system and a method of cooling a gas turbine engine. This system can include an eductor subsystem for entraining airflow through a compartment to provide all necessary cooling of a gas turbine engine, for example, by cooling the oil of the engine. This system may also provide an apparatus for dumping surge bleed air into the primary nozzle of a gas turbine engine. Additionally, the present invention may provide a method for compensating for reduced primary exhaust airflow to maximize eduction pumping. Because it provides these functions at minimal weight and within stringent space limitations, the gas turbine eductor cooling system of the present invention is suitable for use on aircraft, and particularly on an auxiliary power unit (APU), which may be mounted, for example, in the tail structure or tail cone of a commercial aircraft. Although the following description will describe the present invention as being used in aircraft, the following description should be understood to be applicable to other suitable uses, such as ground vehicles and stationary installations to provide continuous shaft and/or pneumatic power. The present invention may be used in systems that drive electric generators, hydraulic pumps, propulsion gas turbines, or other equipment.  
         [0037]     Because the present invention dumps surge bleed air into the primary nozzle, it differs from, and has advantages over, prior art turbine eduction cooling systems, which dump surge bleed air outside of the primary nozzle (for example, downstream of the primary nozzle). Dumping surge bleed air outside of the primary nozzle may cause eductor pumping to be low when the turbine flow rate in the primary nozzle is lowered, resulting in lowered cooling efficiency. Thus, in contrast to the prior art, the present invention, for example, compensates for lowered turbine flow rate in the primary nozzle during some engine operating conditions such as when generator load is being demanded but low or no pneumatic load is demanded; restores eductor pumping by maintaining primary flow, improves cooling efficiency; and ensures a high cooling flow eduction rate.  
         [0038]     In more specifically describing the present invention, and as can be appreciated from  FIG. 3A , the present invention provides an APU  18  which may be located inside a tail cone  10  of an aircraft. A compartment  14  may be confined within an aircraft skin  12  of the aircraft. Turbine exhaust flow  170  may flow through the APU  18 , past a turbine  50 , around center body  40 , and through a primary nozzle  30 . External ambient air may enter the compartment  14  through a compartment inlet  16 , through an oil cooler  60 , and into a cooling flow plenum  80 . The oil cooler  60  may be an air-to-oil heat exchanger. Surge bleed air  20 , which may be engine bleed air or another surplus compressed air produced by an upstream compressor (not shown), may flow through surge duct  22 , which may lead towards a surge air plenum  70 , and that may end in a surge air dump nozzle  32 .  
         [0039]     With reference to  FIG. 3B , the relationships between the various gas and airflows within the APU  18  may be understood. While the turbine exhaust flow  170  flows through the primary nozzle  30  and toward a mixing plane  100 , the surge bleed air  20  may traverse the surge duct  22  through the surge air plenum  70  and exit the surge air dump nozzle  32  upstream from the mixing plane  100 . At or about the location of the mixing plane  100 , the surge bleed air  20  may mix with the turbine exhaust flow  170  to produce the mixed exhaust flow  172 . At about the location of the mixing plane  100 , or downstream thereof, the lowered pressure of the adjacent mixed exhaust flow  172  may promote the flow of the cooling air  174  from the cooling flow plenum  80  and into the mixing duct  90 , wherein the cooling air  174  may be entrained within the mixed exhaust flow  172 . The primary nozzle  30  may be in fluid communication with the mixing duct  90 . The cooling flow plenum  80  may circumscribe the primary nozzle  30 .  
         [0040]     In  FIG. 3C , an end view, along line  3 C- 3 C of  FIG. 3A , is shown. The surge air plenum  70  and the surge air dump nozzle  32  may be configured such that the surge bleed air  20  may enter the primary nozzle  30  through at least a portion of the circumference of the cross-sectional area of the primary nozzle  30 . The surge air dump nozzle  32  and/or the surge air plenum  70  may not necessarily circumscribe the primary nozzle  30 . Instead, the surge air dump nozzle  32  and/or the surge air plenum  70  may only traverse a portion of the circumference of the primary nozzle  30 , as shown in  FIG. 3C .  
         [0041]     Another embodiment of the present invention is shown in  FIG. 4 . APU  18  may contain a surge air dump nozzle  32  with mixing lobes  42 , which may promote mixing of surge bleed air  20  with turbine exhaust flow  170 . The APU  18  may function similarly to the APU  18  as shown in FIGS.  3 A-C and as described above.  
         [0042]     As shown in  FIG. 5 , the surge air dump nozzle  32  may circumscribe the primary nozzle  30 , and is formed between the primary nozzle  30  and mixing lobe  42  and is connected to surge air plenum  70 .  
         [0043]     In  FIG. 6 , a further embodiment of the present invention is shown, using an annular surge dump nozzle  34 , which may circumscribe the primary nozzle  30 , as further shown in  FIG. 7 . The surge dump nozzle annulus  34  is formed between the primary nozzle  30  and surge air nozzle wall  36 , as shown in  FIGS. 6 and 7 . The surge air dump nozzle  34  is in fluid communication with the surge air plenum  70 .  
         [0044]     A still further embodiment of the present invention is shown in  FIG. 8 . In the embodiment as shown, center body  40  may be open, such that surge bleed air  20  may flow from the surge duct  22  and enter the mixing duct  42  after traversing the center body  40 . This arrangement may serve to use the formerly unused space occupied previously by the solid center body  40  (for example, as shown in  FIG. 4 ).  
         [0045]     As shown in the end view in  FIG. 9 , together with  FIG. 8 , the surge air duct  22  may be in fluid communication with the surge air plenum  70 , a surge flow scroll  72 , a hollow strut  74 , and the center body  40 . The center body  40  may include mixing lobes  42 .  
         [0046]     Yet another embodiment of the present invention is shown in  FIG. 10 . The APU  18  shown in  FIG. 10  may be similar to the APU  18  described hereinabove with reference to  FIG. 8 , except that mixing lobes  42  may be omitted from the center body  40 . As can be seen in  FIG. 10 , the surge bleed air  20  may mix with the turbine exhaust flow  170  upstream from the location of where the cooling airflow  174  is entrained into the mixed exhaust flow.  FIG. 10  also shows an arrangement of the eductor system by removing eductor cooling air plenum  80  where the aircraft skin  12  forms the cooling flow plenum  14 . In this arrangement the cool ambient air is first drawn through the oil cooler  60  placed in the aircraft compartment inlet  16  and then this air may pass through the APU compartment  14  for compartment cooling. All surge dump and primary nozzle  30  arrangements may work with either eductor configuration.  
         [0047]      FIG. 11  is an end view of the primary nozzle  30  and surge duct  22  and surge flow scroll  72  of  FIG. 10 , including the flow paths of the surge bleed air  20  through the surge flow scroll  72  and through the hollow struts  74 , into the center body  40 . The surge bleed air  20  may exit the surge air duct  22  and enter the surge flow scroll  72 , which may circumscribe the primary nozzle  30 . The surge air duct  22  may be in fluid communication with the surge flow scroll  72 . The hollow struts  74  may be in fluid communication with the surge flow scroll  72  and the center body  40 . As described above, the surge bleed air  20  may exit through the center body  40  and the surge bleed air  20  may mix with the turbine exhaust flow  170 , at or about the mixing plane  100 .  
         [0048]     With reference to  FIG. 12 , a method for cooling a gas turbine engine, by directing airflow into a nozzle for entrainment with primary gas flow is described. Method  200  may comprise a step  210  of drawing cooling airflow  174  into a cooling flow plenum  80 . Thereafter, step  220  may involve directing a surge bleed air  20  into a surge air plenum  70 . Another step  230  may comprise directing the surge bleed air  20  into the primary nozzle  30  of a gas turbine engine, for example, an APU  18 . A further step  240  may involve mixing the surge bleed air  20  with an exhaust flow  170  from the gas turbine engine, for example, the APU  18 , wherein the mixing is performed within the primary nozzle  30 . Thereafter, step  250  may comprise creating a mixed exhaust flow  172  comprising the surge bleed air  20  and the exhaust flow  170 . Another step  260  may involve entraining the cooling airflow  174  through the cooling flow plenum  80  and into the mixed exhaust flow  172 . Thereafter, step  270  may comprise drawing the cooling airflow  174  across an oil cooler  60 ; and step  280  may involve directing the mixed exhaust flow  172  and the cooling airflow  174  into an eductor mixing duct  90 .  
         [0049]     The cooling air  174  also may cool the APU compartment  14  air as it traverses the compartment  14  before or after the oil cooler  60  (depending on the  FIG. 3A  or the  FIG. 10  arrangement). In the arrangement in  FIG. 3A  the compartment  14  cooling may raise the ambient air temperature about 10 to 30 degrees Fahrenheit before traversing the oil cooler  60 . In the arrangement in  FIG. 10  the oil cooler  60  receives ambient air temperature and may raise the temperature of the cooling air by about 60 to 70 degrees Fahrenheit before passing through the compartment  14  for cooling. In both cases the compartment  14  and oil may be maintained below required temperatures. In the arrangement in  FIG. 10  an expensive cooling flow plenum  80  may be eliminated.  
         [0050]     Continuing with  FIG. 12 , the oil cooler  60  may cool oil from the auxiliary power unit  18 . The oil cooler  60  may also cool oil from an accessory driven by the auxiliary power unit  18 , such as a generator, a gear box, or any other accessory. The surge bleed air plenum  70  may circumscribe the primary nozzle  30 . The surge air plenum  70  may also be in fluid communication with a hollow strut  74  or a surge flow scroll  72 .  
         [0051]     It should be understood, of course, that the foregoing relates to exemplary 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.