Abstract:
A rotary aircraft exhaust system having an engine, an exhaust member in gaseous communication with the engine, and a longitudinally compressible bellow seal in sealing contact with both the engine and the exhaust member. The bellow seal is configured to provide a gaseous seal between the engine and the exhaust member while also allowing the exhaust member to move in the transverse, longitudinal, and pivoting directions relative to the engine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/662,829, filed 14 Mar. 2007, titled “Free-Floating Gas Seal,” which claims the benefit of International PCT Application No. PCT/US04/32606, filed 1 Oct. 2004, titled “Free-Floating Gas Seal,” both of which are both hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     1. Field of the Present Description 
     The present invention relates to seals. In particular, the present invention relates to seals useful in controlling the flow of exhaust gas exiting from a jet engine. 
     2. Description of Related Art 
     Many types of aircraft use turbines to provide the power necessary for flight. One example would be a rotary wing aircraft with a turbine driving the rotating wing. In such aircraft the turbine engine is securely fastened to the airframe of the rotary aircraft and an exhaust system may be attached to the exhaust end of the engine to redirect exhaust gases as desired. So long as the exhaust system is relatively lightweight, the engine can support the extra load. Because the exhaust system is attached directly to the engine the seal between the exhaust system and the engine is relatively simple. The main concern at this joint is the support of the exhaust system. 
     Recent advancements in exhaust systems have led to heavier exhaust systems that reduce the heat signature of the aircraft as viewed through infrared equipment, among other advantages. Such exhaust systems make the aircraft more difficult to spot and follow with infrared equipment, which is very important in military applications. 
     Due to the added weight of the infrared suppressing exhaust system, the exhaust system is no longer light enough to attach to the engine for support. Instead, the exhaust system must be mounted directly to the airframe. Because the engine and the exhaust are mounted to different parts of the airframe, and because airframes flex during use, the exhaust and the engine are no longer relatively static. The exhaust system may move in three dimensions relative to the output end of the engine. Therefore, a rigid connection between the engine and the exhaust system would put stresses on the engine and the exhaust system. 
     Several problems arise when trying to mate the exhaust system to the engine and provide for both axial and radial movement in the joint. The problems stem from the relative motion that must be accommodated, the high temperatures of the environment, and the need for an adequate seal. A first problem is leakage from seals such as a finger seal, which do not adequately seal the exhaust gases. A second problem is the large diameter of the seal when trying to use a labyrinth joint or rope seal that provides for sufficient radial movement. A third problem is the weight of the seal if a complex arrangement is used to accommodate the movement, but still provide adequate sealing. A fourth problem is the maintenance of the seal; longer service periods are needed and a passive failure is desired. 
     Although there have been significant developments in the area of sealing exhaust systems to turbine engines, considerable shortcomings remain. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a turbine engine powered rotary wing aircraft with an infrared reducing exhaust system; 
         FIG. 2  is a cross-sectional view of the engine and exhaust system of the aircraft of  FIG. 1 ; 
         FIG. 3  is a sectional close up of the exhaust seal shown in  FIG. 2 ; 
         FIG. 4  is an axial view of the exhaust seal shown in  FIG. 2 ; and 
         FIG. 5  is an axially exploded view of the components of the exhaust seal shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention represents the discovery that a free-floating seal comprising a bellow and face seals can provide for low leakage rates between components in a gas flow system while allowing significant longitudinal and transverse movement of the components relative to each other. The seal is particularly suited for use in a high temperature environment, such as an exhaust seal between a turbine engine and a separately supported exhaust system that experience axial and radial movement relative to each other. 
     Referring to  FIG. 1  in the drawings, a rotary wing aircraft  11  with a turbine engine  13  and exhaust system  15  is illustrated. Aircraft  11  has an airframe  17 . Engine  13  and exhaust system  15  are each attached to airframe  17  at separate points for structural support. Aircraft  11  is not limited to rotary wing aircraft, as turbines are widely used on other types of aircraft, such as fixed-wing and tiltrotor aircraft. Additionally, the seal disclosed below may be used wherever exhaust system  15  and engine  13  may experience significant relative axial and radial movement. 
     Referring now  FIGS. 2 and 3  in the drawings, a preferred embodiment of the invention is shown. Engine  13  is shown attached to exhaust system  15  in a sectional view. Engine  13  has an outer heat shield  19  which is attached to aft firewall  21 . Within heat shield  19  the engine terminates with a deswirl duct  25 . A circumferential member, such as split ring  23 , is attached to deswirl duct  25 . As shown in  FIG. 3 , split ring  23  has an inner circumference  27  and a preferably circumferential axial face  29 . An optional flow-directing means may be located near split ring  23 . For example, liner  31  is a cylindrical sleeve that extends axially along the inner circumference  27  toward exhaust system  15 . 
     Continuing with  FIG. 2  in the drawings, exhaust system  15  has an outer liner  33  and an inner liner  35 . An adapter can  37  attaches to heat shield  19  and abuts outer liner  33 . An aft seal ring  39  is attached to inner liner  35 . Aft seal ring  39  has a preferably circumferential axial face  41 . 
     Referring now to  FIG. 3 , a corrugated bellows  43  is positioned between axial face  41  of aft seal ring  39  and axial face  29  of split ring  23 . Bellows  43  is preferably a free-floating, circumferential unit, though one end of bellows  43  may be fixedly attached relative to engine  13  or to exhaust system  15 . A lip  45  is formed on each axial ends of bellows  43 , with lips  45  being formed to be parallel to axial faces  29 ,  39 . Bellows  43  is compressed slightly between axial faces  29 ,  41  to provide axial pressure between each lip  45  and the corresponding axial face  29 ,  41 . Lips  45  and axial faces  29 ,  41  cooperate to form face seals for preventing the escape of exhaust gases at the junction of engine  13  and exhaust system  15 . 
     Continuing with  FIG. 3 , a close-up sectional view of bellows  43  shows how it relates to the nearby parts. Bellows  43  has corrugations  47  between lips  45  that may be compressed axially and allow for some radial movement of lips  45  relative to each other. Axial face  29  has a radial thickness that allows for radial movement of corresponding lip  45 . A stop means, such as stop  49 , is located on an inner portion of face  29  to limit the radial movement of lip  45 . Additionally, axial face  41  has a radial thickness that allows for radial movement of corresponding lip  45  and a corresponding stop  51  to limit radial movement of corresponding lip  45 . Because all radial movement is relative between axial face  29  and axial face  41 , the radial thickness may be split evenly between axial faces  29 ,  41  or one of the axial faces  29 ,  41  may have more radial thickness than the other. As shown, axial face  29  has a slightly more radial thickness than axial face  41 . 
     Also apparent from  FIG. 3  is the function of liner  31  in directing exhaust gases past bellows  43 . As the exhaust gases flow from engine  13  to exhaust system  15 , the flow travels along the inner face of liner  31 , which extends for at least a portion of the length of bellows  43 , preventing the flow from directly impinging on bellows  43 . This reduces the pressure on bellows  43  and thereby reduces the overall leakage rate around bellows  43 . Although shown in the drawings as a cylindrical liner  31 , various types of flow-directing means may be substituted for liner  31  to limit the amount of flow pressure on bellows  43 . 
     One important aspect of bellows  43 , as shown, is that if lips  45  wear completely away, corrugations  47  will contact axial faces  29 ,  41  and provide some degree of sealing. This is known as a passive failure because the sealing effectiveness is reduced gradually, instead of an instantaneous complete failure of the seal. 
     Referring now to  FIG. 4  in the drawings, an axial view of bellows  43  and split ring  23  shows the use of centering bumpers  53  attached to liner  31 . While stops  49 ,  51  limit the radial movement of lips  45 , centering bumpers  53  are positioned to limit the radial movement of corrugations  47  between lips  45 . Bumpers  53  urge bellows  43  toward the center of the limits of travel and are particularly useful to prevent sagging of bellows  43  when engine  13  is positioned horizontally. 
     Referring now to  FIG. 5  in the drawings, a partially exploded view of the parts surrounding bellows  43  shows how the parts fit together. As shown, split ring  23  may be formed of multiple parts bolted together to allow ease of assembly and disassembly for maintenance purposes. Additionally, liner  31  may be bolted to inner circumference  27  of split ring  23  for ease of replacement. Adapter can  37  is shown as clearly larger in diameter than split ring  23 , bellows  43  and aft seal ring  39 , thus creating a space as shown in  FIG. 3 . 
     Because of the heat generated by engine  13  a heat resistant material is preferred when constructing bellows  43 . One example is Inconel®, which may be rolled from a sheet into a cylinder which may then be corrugated. Finally, lips  45  may be formed. Inconel® is well known for having high temperature resistance and high strength. Other similar materials may be used in this application. Additionally, a coating, such as chromium carbide, on the adjacent surfaces lips  45  and axial faces  29 ,  41 , may improve both the sealing characteristics and the wear characteristics of the system. 
     It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.