Patent Publication Number: US-2012023896-A1

Title: Auxiliary power unit fire enclosure drain

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-06-0081 and Sub-Contract No. 4500019224 awarded by the United States Navy. 
    
    
     BACKGROUND 
     The present invention is directed to auxiliary power units having fire enclosures. More particularly, the invention relates to connecting joints for mounting drains to fire enclosures in the auxiliary power unit. 
     Auxiliary power units (APUs) comprise gas turbine engines that operate to provide various power inputs to aircraft, such as helicopters, when the main propulsion engines are not operating, such as during ground operations or during the event of an outage during flight. APUs can additionally provide supplemental power to that generated during main engine operations. APUs typically comprise gas turbine engines having a compressor and a turbine, between which a combustor burns fuel. Through a gearbox, the turbine provides mechanical input to an electrical generator, while compressed air bled from the compressor is used to supply various environmental control systems. 
     APUs are typically located within the outer skin of the fuselage of the aircraft. Thus, it is desirable to encapsulate hot sections of the APU to provide a fire break where fuel is present. Conventional practice, such as is described in U.S. Pat. No. 7,526,921 to Williams et al., involves rigidly bolting a fire enclosure to various fixed positions on the APU. Temperature variations that arise during different operating cycles of the APU produce thermal expansions of various APU components that alter the distances between the fixed positions. Thermal growth of the APU thus induces strain into the fire enclosure. The ability of the fire enclosure to tolerate deflection or bending is limited because the fire enclosure is not a structural component designed to absorb loading. 
     Furthermore, it is desirable to be able to drain fuel from the combustor out of the fire enclosure in the event of unburned fuel being present in the combustor, such as from a failed start or some other occurrence. Previous attempts at providing drains on APU fire enclosures involved using check valves that needed to be actively closed to prevent combustor air from escaping during operation of the APU and then opened to drain fuel. These valves, however, typically failed to a closed position, which could lead to fuel pooling within the combustor causing a fire hazard. Other fire enclosures, such as described in the aforementioned patent to Williams et al., involve bleed air ports having bulb seals that require precise alignment of parts. There is, therefore, a need for a fire enclosure drain that can accommodate thermal growths and misalignments within APU fire enclosures. 
     SUMMARY 
     The present invention is directed to a drain assembly for an auxiliary power unit having a hot zone formed by a combustor case. The drain assembly comprises a fire enclosure, a drain fitting and an orifice. The fire enclosure encapsulates the hot zone. The drain fitting connects to the fire enclosure. The orifice extends from the combustor case into the drain fitting to form an expansion joint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a fire enclosure of an auxiliary power unit having a drain assembly with an expansion joint. 
         FIG. 2  shows the expansion joint of  FIG. 1  connecting the fire enclosure to the combustor case through the drain assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a partial cross-sectional view of an example auxiliary power unit (APU)  10  having fire enclosure drain assembly  12  of the present invention. APU  10  includes compressor section  14 , turbine section  16  and combustor section  18 . Compressor section  14 , turbine section  16  and combustor section  18  comprise a gas turbine engine that may operate to provide mechanical input via shaft  22  to various components, such as an electrical generator (not depicted). Shaft  22  passes through compressor section  14 , which is supported by bearings  26 , and connects to turbine section  16 . The gas turbine engine of compressor section  14 , turbine section  16  and combustor section  18  is disposed within compressor case  28 , combustor case  30 , turbine case  31  and exhaust case  32 . Compressor case  28 , combustor case  30 , turbine case  31  and exhaust case  32  form a serpentine flow path for air and gas that passes through the gas turbine engine. Fire enclosure  33  provides a containment shield for containing heat generated by combustor section  18  of the gas turbine engine and for containing flames generated by fuel vapors which come into contact with the outside of the combustor case  30 . Drain assembly  12  permits fluid from inside combustor case  28  to drain outside of fire enclosure  33 . 
     Fire enclosure  33  and the other engine casing components are connected by a plurality of joints that provide structural integrity to APU  10 , while also permitting some of the cases to translate to absorb stresses generated during operation of APU  10 . Exhaust case  32  connects with turbine case  31 . Combustor case  30  connects with exhaust case  32 . Fire enclosure  33  connects with compressor case  28 . Exhaust case  32  connects with fire enclosure  33  through aft support ring  44  and joint  48 . Fire enclosure  33  provides a fire wall or fire break between the hot section of APU  10  and the surrounding environment. Drain assembly  12  also includes an expansion joint that permits relative radial and axial displacement between fire enclosure  33  and combustor case  30 . 
     Compressor case  28  comprises an annular body for housing compressor wheel  52  and compressor blades  54 . Compressor case  28  has a converging inlet between outer and inner walls that comprise a passageway for conducting inlet air A I  through compressor section  14 . Combustor case  30  comprises a single walled annular body having a generally cylindrical side-wall portion and a radially converging end portion that houses combustor liner  56 . The side-wall portion generally traverses the axial length of combustor liner  56  while the radially converging portion generally traverses the radial extent of combustor liner  56 . Combustor case  30  directs airflow from compressor case  28  into combustor liner  56 . 
     Turbine case  31  comprises an annular body for housing turbine wheel  62 . Turbine case  31  has a diverging inlet between outer and inner walls that comprise a passageway for conducting compressed inlet air A I  through turbine section  16 . Turbine case  31  connects to the outlet of combustor liner  56  such that combustor liner  56  is encapsulated between compressor case  28 , combustor case  30 , exhaust case  32  and turbine case  31 . Turbine case  31  directs airflow from combustor section  18  to exhaust case  32 . Exhaust case  32  comprises a cylindrical body having a generally straight upstream section and a slightly diverging downstream section. Exhaust case  32  extends into combustor section  18  and traverses the axial length of combustor liner  56 . Exhaust case  32  directs exhaust air A E  from turbine section  16  out of APU  10 . 
     Fire enclosure  33  is jointed to APU  10  radially outward of combustor case  30 . Fire enclosure  33  comprises a generally annular or cylindrical body that includes various shapes to accommodate incorporation of features such as drain assembly  12 . Drain assembly  12  includes fitting  70 , connector  72  and scupper  74 . Scupper  74  is joined to combustor case  30  and comprises trough  75  for collecting fluid from combustor section  18 . Fitting  70  connects to fire enclosure  33  and provides a receptacle for the trough of scupper  74 . Connector  72  provides a means for joining a hose or some other containment or flow conducting means to fitting  70 . As is discussed in greater detail with respect to  FIG. 2 , fitting  70  and scupper  74  form joint  76 , which comprises a radial and axial expansion joint that permits combustor case  30  to move relative to fire enclosure  33  during operation of APU  10 , while also limiting the amount of compressed air leaked from combustor case  30 . 
     In various embodiments, compressor case  28  and turbine case  31  comprise bodies that have been manufactured, i.e. cast and machined. In various embodiments, combustor case  30  and exhaust case  32  comprise thin sheet-like bodies that have been shaped and formed. Compressor case  28 , turbine case  31 , combustor case  30  and exhaust case  32  are formed of various metal alloys, such as stainless steel, aluminum or titanium. Fire enclosure  33  comprises a thin sheet-like structure that is shaped and formed. In various embodiments, fire enclosure  33  is also made from various metal alloys as previously listed. However, in order to reduce the weight of APU  10 , fire enclosure  33  is made from composite materials, such as a carbon fiber or fiber reinforced plastic composite, in other embodiments. 
     Inlet air A I  is drawn into APU  10  by operation of compressor section  14 . Inlet air A I  continues through compressor section  14  to combustor case  30 . Inside combustor case  30 , compressed inlet air A I  enters combustor liner  56 , which is connected to the inlet of turbine case  31 . Fuel is injected into liner  56  through fuel nozzles (not shown) and ignited by an igniter (not shown) to carry out a combustion process to generate high energy gases for turbine section  16 . The high energy gases flow to turbine section  16  where they are expanded and useful work is extracted by turbine section  16 . 
     Compressor section  14  and turbine section  16  are co-axially connected by shaft  22 . As exhaust air A E  passes through turbine section  16 , turbine wheel  62  rotates shaft  22  through blades  64 . Compressor wheel  52  is also coupled to turbine wheel  62  such that compressor blades  54  rotate to provide compressed air to combustor section  18  for carrying out the combustion process in combination with the fuel provided by the fuel nozzles. Shaft  22  extends from turbine wheel  62 , through compressor wheel  52  and bearings  26 . In other embodiments of the invention, drain assembly  12  can be used in other types of gas turbine engines having hot sections, such as industrial gas turbines, axial flow turbines and the like. 
     Operation of combustor section  18  produces heat. Temperatures within combustor section  18  can far exceed approximately 1,000 degrees Fahrenheit (˜538° Celsius). Temperatures outside of combustor liner  56  reach well above approximately 400 degrees Fahrenheit (˜204° Celsius) due to combustor section  18  and temperatures generated by compression of air in compressor section  14 . Combustor section  18 , therefore, comprises a hot zone within APU  10  where temperatures are above the flashpoint of fuel used in combustor section  18 . In order to reduce the potential for hazard, the hot zone is encapsulated within fire enclosure  33  and other ducts of APU  10 . Fitting  70 , connector  72  and scupper  74  of drain assembly  12  prevent flames from escaping fire enclosure  33 , and limit the amount of compressed inlet air A I  that escapes combustor case  30  from entering fire enclosure  33 . Drain assembly  12  also permits expansion and contraction of fire enclosure  33  and combustor case  30 . 
     Fire enclosure  33 , compressor case  28 , combustor case  30 , exhaust case  32  are joined. Compressor case  28 , combustor case  30  and exhaust case  32  form a flow path for inlet air A I  and exhaust air A E . A flow path is provided into which ambient air A A  flows. Fire enclosure  33  also includes drain assembly  12 , which includes drain fitting  70 , drain connector  72  and drain scupper  74 . 
     The combustion of air and fuel within combustor liner  56  and compression of air within compressor section  14  builds up heat and produces flames within APU  10 . The flames are contained by combustor case  30  and exhaust case  32  while the heat can conduct through combustor case  30  and exhaust case  32 . Due to the close proximity to liner  56  and heat generated by compressed air flow, combustor case  30  does not keep the temperatures outside of APU  10  below the flashpoint of fuel used in combustor section  18 . Fire enclosure  33  provides a layer of containment to flames generated by fuel vapors which contact the exterior surface of combustor case  30  or exhaust case  32  and heat to ensure safe operation of APU  10  under all conditions. In particular, fire enclosure  33  provides a flame-proof heat zone within APU  10  to prevent the spread of heat and flames. Fire enclosure  33  is supported within APU  10  by connection to compressor case  28 , combustor case  30  and exhaust case  32 . 
     An E-seal provided on an eductor inlet connects to a duct within the aircraft to which APU  10  is mounted. Ambient air A A  is allowed into fire enclosure  33 . Exhaust air A E  draws ambient air A A  through fire enclosure  33 . Within APU  10 , ambient air A A  cools combustor case  30 . Ambient air A A  also cools the exhaust plume produced by exhaust air A E  outside of APU  10 . Drain fitting  72  and drain scupper  74  of drain assembly  70  allow fuel from combustor  30  to drain out of APU  10  without entering fire enclosure  33 . This eliminates mixing of fuel with water that collects in fire enclosure  33 , which is drained separately form APU  10  via drain  77 . 
     Joint  76  provides degrees of freedom for movement of fire enclosure  33 . Joint  76  comprises a radial and axial expansion joint to provide freedom of movement to fire enclosure  33  in the radial direction. The ability of joint  76 , as well as other joints, to absorb displacement of combustor case  30  and exhaust case  32  reduces the strain induced in fire enclosure  33  and preserves the stability of fire enclosure  33 . This helps permit fire enclosure  33  to be made from lighter and more brittle material. 
       FIG. 2  shows expansion joint  76  connecting fire enclosure  33  to combustor case  30  within drain assembly  12 . Drain assembly  12  includes fitting  70 , connector  72  and scupper  74 . Fitting  70  comprises first collar  78 , second collar  80 , base  82 , passageway  84  and fastener  86 . Connector  72  comprises first stem  88 , second stem  90 , flange  92  and through-bore  94 . Scupper  74  includes first leg  96 A, second leg  96 B, cylindrical extension  98  and orifice  100 . In one embodiment, fitting  70 , connector  72  and scupper  74  are composed of a metal material, such as a stainless steel, aluminum or titanium alloy. 
     As illustrated in  FIG. 2 , drain assembly  12  is positioned between combustor case  30  and fire enclosure  33  at a position where combustor case  30  and fire enclosure  33  comprise generally parallel running annuluses; however, it will be understood that other configurations are contemplated within the scope of the invention. Fire enclosure  33  includes opening  102  into which drain fitting  70  is positioned. Base  82  is positioned around the exterior of opening  102  such that first collar  78  extends into fire enclosure  33 . Fastener  86  is inserted through opposing bores in base  82  and fire enclosure  33  and secured with a bushing or nut to rigidly join fitting  70  to enclosure  33 . Fastener  86  comprises one of several fasteners and in one embodiment three fasteners are used. Second collar extends from base  82  away from fire enclosure  33 . Passageway  84  extends through first collar  78 , second collar  80  and base  82  to link the interior and exterior of fire enclosure  33 . 
     Drain scupper  74  is joined to a radially outward surface portion of combustor case  30  that includes perforations  104 . First leg  96 A and second leg  96 B are joined to combustor case  30  via welding, brazing or some other such suitable fastening means at positions  106 A and  106 B, respectively. First leg  96 A and second leg  96 B slope away from combustor case  30  to join with cylindrical extension  98  at a trough, or low point, of scupper  74 . First leg  96 A is longer than second leg  96 B such that angle of each leg with respect to combustor case  30  is different. The length of legs  96 A and  96 B are determined to position the trough at a low point within APU  10  based on the orientation of APU  10  when mounted in the aircraft in which it is used. Thus, in other embodiments, second leg  96 B can be longer than first leg  96 A. First leg  96 A and second leg  96 B terminate at a position radially outward of opening  102  in fire enclosure  33  and fitting  70 . Cylindrical extension  98  extends from the trough into first collar  78  of fitting  70 . Orifice  100  is positioned at the distal end of cylindrical extension  98  and aligns generally co-axially with passageway  84  of fitting  70  to feed connector  72 . 
     Drain connector  72  is joined with fitting  70  to provide a means for removing fluid from combustor section  18 . In the embodiment shown, connector  72  is configured to link with a hose to collect fluid from drain assembly  12 . First stem  88  is inserted into second collar  80  of fitting  70 . The outer diameter of first stem  88  is configured to form a force fit or interference fit with the inner diameter of second collar  80 . First stem  88  includes ribs or other friction-increasing features for improving gripping with second collar  80 . First stem  88  is inserted into second collar  80  until flange  92  engages collar  80 . Second stem  90  extends from flange  92  to form a nipple, or fitting, around which a hose can be positioned. Second stem  90  includes friction-increasing means to enhance connection with the hose. The ends of first stem  88  and second stem  90  include tapered tips to facilitate insertion into collar  80  or a hose, respectively. 
     In the event un-burned fuel is present within combustor section  18 , drain assembly  12  permits the fuel, or any other liquid, to pass through combustor case  30 , fire enclosure  33  and out of APU  10 . Fuel passes through perforations  104  and is funneled by first leg  96 A and second leg  96 B to cylindrical extension  98 . Cylindrical extension  98  extends into first collar  78  to position orifice  100  near opening  102  in fire enclosure  33 . As such, fuel drains into second collar  80 . First collar  78  extends across a majority of the length of cylindrical extension  98  to prevent fuel from splashing or otherwise escaping fitting  70  inside fire enclosure  33 , and to extend the length over which joint  76  is able to radially expand. The outer diameter of cylindrical extension  98  is smaller than the inner diameter of first collar  78  such axial displacement can be accommodated. Thus, if combustor case  30  grows or contracts during operation of APU  10 , cylindrical extension  98  remains within collar  78  such that orifice  100  remains generally aligned with through-bore  94 . After passing through orifice  100 , fuel enters collar  80  and through-bore  94  of connector  72 , whereby the fuel is permitted to drain out of APU  10 . The fuel is then collected and disposed of as appropriate. 
     Expansion joint  76  maintain the fire-proof integrity of fire enclosure  33 . Base  82  is flush-mounted to fire enclosure  33  to provide metal-to-metal contact that prevents flames from traveling through opening  102 . Also, the magnitudes of the inner diameter of cylindrical extension  98 , the inner diameter of through-bore  94  or the distance between the outer diameter of cylindrical extension  98  and the inner diameter of first collar  78  can be sized to provide a flame-quenching or flame-arresting path that smothers or otherwise deprives flames emitting from fire enclosure  33  of oxygen such that they are extinguished before exiting fire enclosure  33 . 
     Furthermore, joint  76  preserves the efficiency of the gas turbine of APU  10 . Combustor section  18  is pressurized by compressor section  14  during operation of APU  10 . Turbine section  16  operates more efficiently as the pressure is maintained throughout the combustion process. Thus, any leakage of compressed inlet air A I  from combustor case  30  decreases the efficiency of APU  10 . Orifice  100  comprises a small-diameter hole that is sized to permit liquid to pass through scupper  74 , but to limit the amount of combustor air that escapes from combustor case  30 . Due to the pressurization of combustor section  18 , any amount of air leaked out of orifice  100  tends to travel back into fire enclosure  33 . The presence of heated combustor air within fire enclosure  33  is undesirable as it is advantageous to maintain temperatures within enclosure  33  below the flashpoint of fuel. Air leaked into enclosure  33  from combustor case  30  is flushed from enclosure  33  by ambient air A A  introduced into enclosure  33 . 
     Expansion joint  76  provided by drain assembly  12  prevents flames from escaping fire enclosure  12 , limits the amount of compressed inlet air A I  that escapes combustor case  30 , and permits expansion and contraction of fire enclosure  33  and combustor case  30 . The evacuation of fire enclosure  33  by ambient air A A  allows fire enclosure  33  to be comprised of lightweight materials, such as composites, that reduce the overall weight of APU  10 . Furthermore, the presence of flowing ambient air A A  in enclosure  33  eliminates the need for a seal between cylindrical extension  98  and first collar  78 , further reducing the weight of APU  10 . The elimination of a seal also facilitates manufacturing of drain assembly  12  as the tolerances of fitting  70  and scupper  74  can be increased. 
     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.