Patent Publication Number: US-6217277-B1

Title: Turbofan engine including improved fan blade lining

Description:
FIELD OF THE INVENTION 
     The present invention relates to rotary engines, and more particularly to turbofan engines having a fan blade lining including a hidden brush seal. 
     BACKGROUND OF THE INVENTION 
     In most turbofan engines, a lining is mounted between the engine casing and the first compressor stage or fan blade. The lining provides a tight clearance between the tip of the fan blade and the casing. In order to minimize the consequence of inadvertent contact between the rotating fan blade and the lining, the lining is formed from a material that may be abraded by the blade, and is often referred to as an abradable. An example abradable is disclosed in U.S. Pat. No. 5,655,701. 
     In the event the blade is struck by a foreign object, such as a bird entering the air intake of the engine, the fan blade may make a radial excursion coming into contact with the lining. As a result of the radial excursion, the lining is shorn by the fan blade tip. For a foreign object of significant size, up to 0.3″ (0.8 cm) of the abradable may be shorn. This, in turn, significantly increases the blade tip clearance, and may cause air recirculation at the blade tip. As a result the fan blade may stall at its outer span, causing serious consequences to the engine, such as engine surges. 
     Often, engine casings include slots extending into the compressor section near the fan blan. These slots increase the clearance margin before the tip of the blade stalls (referred to as stall margin). However, these slots also reduce overall engine performance. 
     Accordingly, an improved lining, reducing susceptibility of the engine to consequences of radial excursions of a fan blade is desirable. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved rotary engine and an improved fan blade lining for the casing of a rotary engine. 
     In accordance with an embodiment of the present invention, an engine includes a fan blade, rotatably mounted within a generally cylindrical casing for rotation about a lengthwise extending central axis of the engine. An annular lining is mounted within the casing between a tip of the fan blade and the casing. The lining includes a brush seal that extends around an inner circumference of the casing, and has a plurality of bristles that extend radially inward from the seal. A retaining membrane extends around the brush seal, and prevents the bristles from extending substantially in a radial direction from the brush seal. The retaining membrane is adapted to release the bristles to occupy a radial region between the tip of the fan blade and the casing upon a radial excursion of the fan blade. The bristles, once released, at least partially seal the tip of the fan blade. The invention may be embodied in a rotary engine; and engine lining; or the combination of an engine casing and lining. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In figures, which illustrate by way of example only, embodiments of the present invention: 
     FIG. 1 is a partial cross-sectional view of a rotary engine, exemplary of an embodiment of the present invention; 
     FIG. 2A is a further enlarged view of a portion of FIG. 1; 
     FIG. 2B is a front view of a portion the engine of FIG. 1, in cross section; 
     FIG. 3A illustrates the view of FIG. 2A after a radial excursion of a fan of the engine of FIG. 1; 
     FIG. 3B illustrates the front view of FIG. 2B after the radial excursion of a fan blade of the engine of FIG. 1; 
     FIG. 4A illustrates an enlarged view of a portion of a conventional turbofan engine, similar to the view of FIG. 2A; 
     FIG. 4B illustrates an enlarged view of a portion of the conventional turbofan engine of FIG. 4A, similar to the view of FIG. 3A; and 
     FIG. 5 is an enlarged cut-away view of another engine, exemplary of a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a turbofan engine  10 , exemplary of an embodiment of the present invention. Engine  10  includes, from front to rear a conventional fan section  12 ; conventional core engine section  14 , including at least one axial compressor, combustion section, and at least one turbine; and a conventional exhaust section  16 , all mounted within a generally cylindrical casing  18 . A by-pass duct  20 , extends about core engine section  14 , within casing  18 . 
     As illustrated, fan section  12  includes a rotatable fan blade  22 , mounted for axial rotation about a main central axis of engine  10 . A lining  24  including a conventional abradable  26  extends circumferentially about the interior of casing  18 , between casing  18  and the tip of fan blade  22 . Abradable  26  is made of a conventional material, such as an epoxy potting compound and may be bonded to the interior of casing  18 . The tip of fan blade  22  extends in close proximity to abradable  26 . Abradable  26  thus seals the tip of fan blade  22  within casing  18 . 
     FIG. 2A illustrates an enlarged view of a portion of FIG. 1, more particularly illustrating lining  24 . As illustrated, the region of liner  24  occupied by abradable  26  is made up of two portions, a front and aft portion  30  and  32 . Mounted between front and aft abradable portions  30  and  32  is a hidden brush seal  34 , retained between portions  30  and  32  by a retaining membrane  36 . Brush seal  34  includes a plurality bristles  38 , hidden by membrane  36 . These are compressed in a direction generally tangent to the outer circumference of casing  18  by membrane  36  as best illustrated in FIG.  2 B. For reasons that will become apparent, the bristles  38  of brush seal  34  are cocked in a direction, generally tangent to the rotation of fan blade  22 , at a relatively large angle. Preferably brush seal  34 , and in particular bristles  38  may be made of a cobalt based alloy, such as HAYNES-25. Membrane  36  is preferably a ring formed of an easily breakable material, such as plastic, and may be partially embedded in abradable portions  30  and  32 . Lining  24  including brush seal  34  and abradable  26  may be affixed to the casing  18  by bonding, bolting, brazing or in any other suitable manner known to those of ordinary skill in the art. 
     In normal, steady-state, operation fan blade  22  draws air into a compressor section of core engine section  14 , of engine  10  (FIG.  1 ). Similarly, blade  22  draws air through by-pass duct  20 , about the main engine section  14 . Compressed air exits the compressor section and enters the combustion chamber (not shown) where it is admixed with fuel. The fuel and air mixture is combusted, and exits the rear of the combustion chamber to at least one turbine, coupled to cause fan blade  22  to rotate. Exhaust gases are discharged through exhaust section  16 . 
     In normal operation, abradable  26  seals the tip of fan blade  22  within casing  18 , thereby preventing recirculation of air at its tip. 
     Now, if fan blade  22  is struck by a foreign object, such as for example a bird, fan blade  22  may undergo a radial excursion. Of course, this will depend on the relative size of the foreign object to fan blade  22  and engine  10 . Two one (1) pound birds, for example, may cause as much as a 0.3″ (0.8 cm) radial excursion for a typical fan blade. This radial excursion causes fan blade  22  to contact abradable  26  and shear or tear abradable  26 , as illustrated in FIG.  3 A. Similarly, membrane  36  is at least partially torn by fan blade  22 , undergoing its radial excursion. Once membrane  36  is torn, some or all of the hidden bristles  38  of brush seal  34  become liberated. Advantageously, membrane  36  and abradable  26  buffer the impact of fan blade  22 , limiting damage caused by fan blade  22  to brush seal  34 . As membrane  36  cocks bristles  38  in the direction of rotation of fan  20 , and bristles  38  are flexible, they are not immediately cut by rotating fan  20 . Instead, the bristles  38  of brush seal  34  extend radially inward gradually, and particularly once fan blade  22  has completed its radial excursion and is again centered about its axis of rotation, as best illustrated in FIG.  3 B. The liberated bristles  38  of brush seal  34  now occupy much of the radial gap formerly occupied by abradable  26 . As will be appreciated, depending on the nature of the radial excursion, not all areas of lining  24  need be contacted by fan blade  22 . Instead only, a portion of lining  24  and membrane  36  may be shorn, and only some of the bristles  38  may be liberated. In any event, as a result of the liberated bristles  38 , recirculation at the fan tip is reduced or eliminated. Similarly, any associated stalling of the fan at its outer span and any resulting engine surge is reduced or eliminated, so that an aircraft can land safely after the foreign object has struck. 
     As should be appreciated, once membrane  36  is broken, engine  10  should be serviced to replace or repair lining  24 . As will further be appreciated, constant contact between the tip of fan blade  22  and bristles  38  will cause bristles  38  to wear. 
     The operation of exemplary engine  10 , and recirculation of air at the tip of its fan blade  22  may be better appreciated with reference to FIGS. 4A and 4B illustrating a portion of a conventional turbofan engine  100 , similar to the portion of turbofan engine  10  illustrated in FIGS. 2A and 3A. As illustrated, the conventional turbofan includes fan blade  122 , and an abradable  126  mounted within casing  118 . Engine  100 , however, does not include lining including a membrane and hidden brush seal. In normal operation, air is drawn into engine  100 , as illustrated in FIG.  4 A. Upon a radial excursion of blade  122 , as illustrated in FIG. 4B, abradable  118  is shorn, causing air to recirculate at the tip of blade  122 . This, recirculating air causes a blockage region near the tip of blade  122 , as illustrated. In this region, air cannot be adequately drawn into the engine  100 , thereby potentially causing the engine to stall. In exemplary engine  10 , on the other hand the extension of brush seal  38  reduces recirculation at the tip of fan blade  22  thereby reducing the size of the blocked region and reducing the likelihood of stall, as illustrated in FIG.  3 A. 
     In a second embodiment illustrated in FIG. 5, several brush seals  34 ′ may be combined in a single lining  24 ′. As illustrated two or more brush seals mounted  34 ′ including bristles  38 ′ form part of lining  24 ′ and are mounted beside each other, also circumferentially about casing  18 ′ at the tip of fan blade  22 ′. Abradable  26 ′ made of three regions also form part of lining  24 ′. The bristles  38 ′ of the multiple brush seals  34 ′ may all be retained and released by membrane, formed of membrane portions  36 ′ a  and  36 ′ b  in a manner analogous to bristles  38  of membrane  36  (as illustrated in FIGS. 2A,  2 B,  3 A and  3 B). 
     As will be appreciated lining  24  (or  24 ′) and casing  18  may be combined in an article of manufacturer produced by a casing supplier, into which the remainder of engine  10  may be inserted. 
     The above described embodiments are intended to be illustrative only, and in no way limiting. The embodiments are susceptible to many modifications of form, size, arrangement of parts and details of operation. For example, while retaining membranes  36  and  36 ′ have been described as breaking upon radial excursion of fan blade  22  and  22 ′, these membranes could be otherwise adapted to release bristles  38  and  38 ′. For example, membrane  38  could be retractable. 
     The invention, rather, is intended to encompass all such modification within its scope as defined by the claims.