Patent Publication Number: US-7591631-B2

Title: Flow delivery system for seals

Description:
This invention was conceived in performance of U.S. Air Force Contract No. F33657-91-C-0007. The government may have certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a flow delivery system used for pressurizing seals in a turbo machine. 
     Turbo machines, such as a turbofan engine used in aircraft, incorporate carbon seals to separate a bearing compartment from a buffer compartment. The bearing compartment includes bearings supporting, for example, a turbine for rotation relative to a housing of the engine. The bearing compartment contains a lubricant that lubricates the bearings. The buffer compartment contains pressurized air that leaks past the seals which prevents the lubricant from weeping past the seals. 
     The carbon seals require a predetermined differential pressure across the seal in order to prevent leakage of lubricant past the seal. One problem is that lubrication has been known to leak past the carbon seals at idle conditions, because of an inadequate pressure differential across the seals. 
     The buffer compartment consists of a body which is generally cylindrical. Compressor bleed air flows into the body in a direction normal to a plane that is tangential to the body. As a result, a stagnation area forms within the body directly across from where the flow enters the body. This causes an uneven pressure distribution along the cylindrical wall of the body, and if one of the carbon seals is arranged near the cylindrical wall, the uneven pressure on the seal may result in leaks. Notwithstanding the position of the seal, the pressure in the buffer compartment is inadequate at idle. 
     Increased pressure is required within the buffer compartment in the vicinity of the carbon seals for the seals to be effective. 
     SUMMARY OF THE INVENTION 
     The present invention provides a turbo machine that includes a housing having a bearing compartment for receiving lubrication. The housing also provides a buffer compartment for receiving air, for example, compressor bleed air. A turbine shaft is supported within the housing on a bearing for rotation relative to the housing. The bearing is arranged within the bearing compartment. A seal is arranged between the turbine shaft and the housing and separates the bearing and buffer compartments. The seal includes opposing lubrication and air sides that are respectively exposed to the bearing and buffer compartments. A buffer tube is fluidly connected to a body of the buffer compartment. The buffer tube introduces flow generally tangential to an inner surface of the body for generating a swirl within the buffer compartment. 
     The buffer tube includes a velocity control device such as a venturi arranged at an exit of the tube to control the velocity of the flow entering the body. A flow control device such as an orifice plate is arranged upstream of the venturi to control the flow to a desired flow rate. The swirling flow within the body at the desired flow rate and velocity generates a uniform radial pressure gradient. At idle the radial pressure gradient results in a large enough pressure magnitude at the periphery of the buffer compartment to create the desired pressure differential across the seal. The increased pressure at the periphery prevents leakage of lubricant past the seal at idle. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial, cross-sectional view of a portion of a turbofan engine. 
         FIG. 2  is an enlarged view of a portion of the turbofan engine shown in  FIG. 1 . 
         FIG. 3  is a further enlarged view of a portion of the buffer compartment in the turbofan engine shown in  FIG. 2 . 
         FIG. 4  is a schematic view of a tube introducing flow into a body of a buffer compartment looking parallel to an engine axis. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A portion of a turbofan engine  10  is shown in  FIG. 1 . The engine  10  includes a housing  12  that is constructed from multiple pieces secured to one another. The housing  12  supports a turbine shaft  16  for rotation relative to the housing  12  by bearings  21 , best shown in  FIG. 2 . The turbine shaft  16  supports a hub  15 . Multiple turbine blades  18  are secured to the hub  15  by fastener  17 . 
     The bearings  21  are arranged within a bearing compartment  20 . First and second seals  26  and  28  contain the lubricant within the bearing compartment  20 . An insufficient differential pressure exists across the seals, which are carbon seals in one example, so that oil can leak out of the bearing compartment  20  and collect in the housing  12  and at the bottom of the turbine flow path  19 . 
     Referring to  FIGS. 2 and 3 , the housing  12  includes a cylindrical wall  23  and dome  25  that partially define a buffer compartment  33 . The buffer compartment  33  provides pressurized air to an air side  30  of the first and second seals  26  and  28 . The seals are effective once a predetermined differential pressure has been achieved. Inadequate pressure in the buffer compartment  33  may result in leakage past the first and second seals  26  and  28  under idle conditions. 
     A tube  34 , schematically shown in  FIG. 2 , is connected to the body  22  by an inlet  36 . The tube  34  carries pressurized air to a chamber on the air side  30  of the second seal  28 . A buffer tube  38  supplies air to the body  22  from a compressor bleed source  40 . Of course, air can be provided to the air side  30  in any suitable manner using any suitable air source. A vent  60  is shown schematically in  FIG. 2  and is used to release pressure from the bearing compartment  20 . 
     The present invention introduces flow  54  from an exit of the buffer tube  38  in a generally tangential plane T to an adjoining inner surface  24  of the cylindrical wall  23 , as shown in  FIG. 4 . Introducing the flow  54  in this manner generates a swirl that promotes even pressure, as opposed to the stagnant area that would result from a flow introduced normal to the cylindrical wall  23 . The normal plane N is also shown in  FIG. 4 . The velocity and flow rate of air from the buffer tube  38  are controlled by a velocity control device  46  and a flow control device  48 . In the example shown, the velocity control device is a venturi  50  having a throat  56  arranged near where the flow from the buffer tube  38  exits into the body  22 . The flow control device  48  is an orifice plate  52  arranged upstream from the venturi  50 , in the example shown. The orifice plate  52  includes an orifice  58  that is sized to control the flow and, as a result, limit the velocity of flow  54  exiting the venturi  50 . The relationship of the change in pressure within the body relative to the change in radial position within the body can be expressed by the following equation: 
                         ⅆ   P       ⅆ   r       =       ρ   ⁢           ⁢     w   2       r       ,           (     Equation   ⁢           ⁢   1     )               
where ρ is the density of the air, w is the velocity of the air exiting the venturi, and r is the radial position for which the pressure is calculated. The pressure at the seal  26  can be adjusted to a desirable magnitude by changing the velocity at which the air is introduced into the buffer compartment, or the radius at which the air is introduced. In addition, the pressure at the seal  28  can be adjusted by changing the radial position at which the supply air is extracted from the buffer compartment.
 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.