Abstract:
A diffuser pipe for a gas turbine engine comprises a hollow pipe body including a first end, a second end fluidly connected to the first end, and at least one flattened area proximate to the second end. A ring is connected to the second end. The ring is an outlet of the diffuser pipe. At least one stiffener is disposed on the at least one flattened area. The ring and the at least one stiffener reduce vibratory stresses at the second end of the pipe body. A method of manufacturing a diffuser pipe of a gas turbine engine is also presented.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority on U.S. provisional patent application No. 61/835,701 filed on Jun. 17, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    The application relates generally to gas turbine engines and, more particularly, to diffuser pipes. 
       BACKGROUND OF THE ART 
       [0003]    Diffuser pipes are provided in gas turbine engines for directing flow of compressed air from the centrifugal compressor impeller to an annular chamber containing the combustor. Diffuser pipes are typically made from sheet metal and may be sensitive to vibratory stresses as a result of the engine operation. 
       SUMMARY 
       [0004]    In one aspect, there is provided a diffuser pipe for a gas turbine engine, the diffuser pipe comprising: a hollow pipe body including: a first end; a second end fluidly connected to the first end; and at least one flattened area proximate to the second end; a ring connected to the second end, the ring being an outlet of the diffuser pipe; and at least one stiffener disposed on the at least one flattened area, the ring and the at least one stiffener reducing vibratory stresses at the second end of the pipe body. 
         [0005]    In another aspect, there is provided a method of manufacturing a diffuser pipe of a gas turbine engine, the method comprising: forming a hollow diffuser pipe body from at least one sheet metal; adding a raised structure on a flat portion of the diffuser pipe body near an end of the diffuser pipe body; and connecting a unitary formed ring to the end of the diffuser pipe body. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures in which: 
           [0007]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0008]      FIG. 2  is a front perspective view of a diffuser pipe according to one embodiment for the gas turbine engine of  FIG. 1 ; 
           [0009]      FIG. 3  is a top perspective view of a front portion of a diffuser pipe according to another embodiment for the gas turbine engine of  FIG. 1 ; 
           [0010]      FIG. 4  is a top perspective view of a front portion of a diffuser pipe according to yet another embodiment for the gas turbine engine of  FIG. 1 ; and 
           [0011]      FIGS. 5 to 10  are schematics of different shapes of stiffeners for a diffuser pipe such as the ones of  FIG. 3  or  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally extending along a longitudinal axis  18 . The engine  10  includes in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. A number of diffuser pipes  20  are provided for directing flow of compressed air from the centrifugal compressor impeller of the compressor section  14  to an annular chamber or plenum containing the combustor  16 . The diffuser pipes  20  are connected of a common diffuser case (not shown). 
         [0013]    Referring to  FIG. 2 , each diffuser pipe  20  has a body  22  made of two formed sheet metals. In the embodiment shown herein, the sheet metals are welded to each other. A weld line  13  is best shown in  FIG. 3 . It is contemplated that the diffuser pipes could be stamped, hydroformed, cast or machined. The sheet metals have each a thickness t 1  (not shown). In the embodiment shown herein, t 1  is 0.035 inches. 
         [0014]    The body  22  of the diffuser pipe  20  includes a first end  24 , a second end  28  fluidly connected to the first end  24 , and a curvature  26  disposed between the first end  24  and the second end  28 . The first end  24  is welded to a ferrule  25 , which connects the diffuser pipe  20  to the diffuser case by bolting. It is contemplated that the diffuser pipe  20  could be connected to the diffuser case by brazing as well. The first end  24  is an inlet of the diffuser pipe  20 . The second end  28  is an outlet of the diffuser pipe  20  and also known as the “lip” of the diffuser pipe  20 . The second end  28  of the diffuser pipe  20  discharges the compressed air in a direction of the longitudinal axis  18  of the engine  10  (see arrow  27 ). For orientation purposes, an axis perpendicular to the second end  28  at the lip  28  will be referred to as a first axis  21 , and an axis in the direction of the second end  28  at the lip  28  will be referred to as a second axis  11 . In the embodiment shown herein, the second axis  11  is parallel to the longitudinal axis  18  of the engine  10 . The first and second axes  21  and  11  form a plane P, a perpendicular axis/line to the plane P will herein be referred as a vertical V. 
         [0015]    A ring  40  is connected to the lip  28  and forms a free end of the diffuser pipe  20 . The ring  40  is shown herein to be connected to an outside  15  of the diffuser pipe  20  but could be connected to an inside  17  of the diffuser pipe  20 . The ring  40  acts as a stiffener to the diffuser pipe  20  which may be vulnerable to vibratory stresses as a result of the engine  10  operation. The diffuser pipe  20  has one or more natural frequencies that may be in the range of the vibration frequencies of the engine  10  (generally high frequencies). The ring  40  stiffens the diffuser pipe  20  and reduces the vibratory stresses of higher natural modes of the diffuser pipe  20  at the lip  28  (i.e. lip modes) during engine  10  operation. In turn, the diffuser pipe  20  may be less prone to early fractures (a.k.a. lip modes failure). 
         [0016]    The ring  40  is unitary formed (e.g. through machining or casting). By being unitary formed, the ring  40  reduces vibratory stresses compared to non-unitary formed rings (e.g. welded). In the embodiment shown herein, the ring  40  is unitary formed by machining. It is contemplated that other fabrication methods could be used to form the unitary ring  40 . For example, the ring  40  could be cast. 
         [0017]    The ring  40  has a width I 1  (shown in  FIG. 3 ) in a direction of the second axis  11 , and a thickness t 2  (shown in  FIG. 3 ). In the embodiment shown herein, t 2  is 0.070 inches and I 1  is 0.500 inches. The thickness t 2  is larger than the thickness t 1  of the diffuser pipe  20 . However, it is contemplated that the ring  40  could have the thickness t 2  smaller than the thickness t 1  of the diffuser pipe  20 . While various choices of t 2  and t 1  can provide stiffening of the diffuser pipe  20 , a ratio t 2 /t 1  is preferably comprised between 0.25 and 3 to provide vibratory stress reduction. There are several factors that contribute to a reduction of stresses at the lip  28  due the presence of a ring such as the ring  40 . One factor is the thickness t 1  of the ring  40  relative to the thickness t 2  of the diffuser pipe  20 . A ring with a greater thickness than the diffuser pipe reduces stresses at the lip. Another factor is the presence of connecting points such as welds to form the ring. A unitary ring such as the ring  40  reduces stresses experienced by the ring itself during vibration, as welds are sources of potential local high stresses. In view of the above, there could be cases where a unitary ring having a thickness smaller than a thickness of the diffuser pipe would act as a stiffener to the diffuser pipe despite its relative thinness. There could also be cases where a welded ring having a thickness greater than a thickness of the diffuser pipe would act as a stiffener to the diffuser pipe. But the vibratory stress reduction of such ring may be less than the one of the thinner welded ring. However, a welded ring having a thickness smaller than a thickness of the diffuser pipe may not reduce vibratory stresses to the desired levels. 
         [0018]    The body  22  has two flattened areas  30  facing each other (only one flattened area  30  being shown in the Figures). As a result, a cross-section of the lip  28  is a generally flattened elliptical cross-section E. A longer diameter of the flattened ellipse E is d 2 , and a smaller diameter of the flattened ellipse E is d 1  (both shown in  FIG. 3 ). In the embodiment shown herein, the longer diameter d 2  is in the direction of the first axis  21 , while the shorter diameter d 1  is in a direction of the vertical V. In the embodiment shown herein, d 1  is 1.181 inches and d 2  is 2.102 inches. It is contemplated that the flattened areas  30  could not be totally flat but could have some curvature. 
         [0019]    Each flattened area  30  includes a stiffener  50 . The stiffener  50 , which may have various shapes as described below, is a raised portion of the flattened area  30  (when seen from an outside  15  of the diffuser pipe  20 ). The stiffener  50  may sometimes be known as “dimples” although when seen from the outside  15  of the diffuser pipe  20 , they are raised. However, when seen from the inside  17  of the diffuser pipe  20 , the stiffener  50  is a local depression. The stiffener  50  is raised at a distance t 3  vertically from a rest of the flattened area  30 . In the embodiment shown herein, the raised distance t 3  is 0.060 inches. While various choices of t 3  and t 1  can provide stiffening of the diffuser pipe  20 , a ratio t 3 /t 1  is preferably comprised between 0.25 and 3 to provide vibratory stress reduction. The stiffener  50  is made by stamping the flattened area  30 . It is contemplated, however, that the stiffener  50  could be added to the diffuser pipe  20 , and as such be full. It would then remain a raise when seen from the outside  15  of the diffuser pipe  20 , and would be flat when seen from the inside  17  of the diffuser pipe  20 . It is contemplated that the stiffener  50  could be a depression portion of the flattened area  30  instead of being a raise. Although the stiffener  50  is described herein to be on both flattened areas  30  of the diffuser pipe  20 , it is contemplated that the stiffener  50  could be on only one of the two flattened areas  30 . 
         [0020]    The presence of the stiffener  50  on the flattened area  30  of the diffuser pipe  20  reduces vibratory stresses for high dynamic modes of vibration of the diffuser pipe  20  during the engine  10  operation, similarly to what has been discussed above for the ring  40 . While the ring  40  reduces stresses at the lip  28 , the stiffener  50  reduces stresses upstream of the ring  40  in the flattened area  30 . The combined use of the stiffener  50  and the ring  40  ensures a vibratory stress reduction of the diffuser pipe  20  greater than the individual contribution of the stiffener  50  and the ring  40 . 
         [0021]    Turning now to  FIG. 3 , the stiffener  50  will now be described in details. The stiffener  50  is one example of stiffener that could be applied to a diffuser pipe. Other examples of stiffeners are given below. 
         [0022]    The stiffener  50  is D-shaped, with a straight portion  52  of the D parallel to the ring  40 . Although the straight portion  52  is shown herein to be parallel to the ring  40 , it is contemplated that the straight portion  52  could be at an angle with the ring  40 . For example, the straight portion  52  could make an angle of 10 degrees with the ring  40 . A shape, size and orientation of the straight portion  52  is linked to the stiffening properties of the stiffener  50 . For example, stiffening may be reduced when the straight portion  52  is at an angle with the ring  40 . A distance I 2  of the straight portion  52  to the ring  40  in a direction of the second axis  11  influences a stiffening of the diffuser pipe  20 . A shorter distance I 2  was found to increase the stiffening of the diffuser pipe  20 . Although the distance I 2  is desired to be short, it is not zero, i.e. the stiffener  50  does not abut the ring  40 . In the embodiment shown in  FIG. 3 , I 2  is 0.142 inches. While various choices of I 1  and I 2  can provide stiffening of the diffuser pipe  20 , a ratio I 1 /I 2  is preferably comprised between 1.2 and 3.5 to provide vibratory stress reduction. 
         [0023]    The straight portion  52  has a span s 1  in the direction  21 . In the embodiment shown in  FIG. 3 , s 1  is 1.368 inches. It is been observed that a larger span s 1  increases stiffening of the diffuser pipe  20 . While various choices of d 2  and s 1  can provide stiffening of the diffuser pipe  20 , a ratio d 2 /s 1  is preferably comprised between 1.2 and 1.6 to provide vibratory stress reduction. It is contemplated that a portion of the stiffener  50 , closest to the ring  40  could not be straight. For example, it could be an O-shaped stiffener such as the one shown in  FIG. 7 . In such cases, vibratory stresses may not be reduced. 
         [0024]    A thickness of the stiffener  50  is determined by parameter I 3 , defined as a thickness of the straight portion  52  in a direction of the second axis  11 . In the embodiment shown in  FIG. 3 , I 3  is 0.270 inches. While various choices of I 3  and t 3  can provide stiffening of the diffuser pipe  20 , a ratio I 3 /t 3  is preferably comprised between 4.5 and 5 to provide vibratory stress reduction. It is contemplated that the ratio I 3 /t 3  could have other values, yet not zero. 
         [0025]    A width of the stiffener  50  is determined by parameter I 4 , defined as a span of the stiffener  50  in the direction of the second axis  11 . In the embodiment shown in  FIG. 3 , I 4  is 1.190 inches. While various choices of I 4  and d 1  can provide stiffening of the diffuser pipe  20 , a ratio I 4 /d 1  is preferably comprised between 1 and 1.25 to provide vibratory stress reduction. 
         [0026]    Turning now to  FIG. 4 , a second embodiment of a stiffener  50   b  will now be described on a diffuser pipe  20   b  having a ring  40   b.  The diffuser pipe  20   b  and ring  40   b  are similar to the diffuser pipe  20  and ring  40  but have different dimensions: d 1 ′ is 1.033 inches, d 2 ′ is 1.625 inches, and I 1 ′ of 0.400 inches. It is contemplated that the diffuser pipe  20   b  and ring  40   b  could have the same dimensions as the diffuser pipe  20  and ring  40 . 
         [0027]    The stiffener  50   b  is similar to the stiffener  50 , but has a T-shape instead of a D-shape. As such, the stiffener  50   b  will not be described in details herein again. The stiffener  50   b  includes a straight portion  52   b  parallel to the ring  40   b.  This straight portion  52   b  is similar to the straight portion  52 , and achieves similar vibratory stress reduction properties as the straight portion  52  does. Designs parameters t 1 ′, t 2 ′, t 3 ′, d 1 ′, d 2 ′, I 1 ′, I 2 ′, I 3 ′, I 4 ′, s 1 ′ are defined similarly as the designs parameters t 1 , t 2 , t 3 , d 1 , d 2 , I 1 , I 2 , I 3 , I 4 , s 1  of the stiffener  50 . In the embodiment of the diffuser pipe  20   b  shown in  FIG. 4 , I 2 ′ is 0.149 inches, I 3 ′ is 0.281 inches, I 4 ′ is 1.166 inches, and s 1 ′ is 1.272 inches. The designs parameters t 1 ′, t 2 ′, t 3 ′, d 1 , d 2 ′, I 1 ′, I 2 ′, I 3 ′, I 4 ′, s 1 ′ may have the same values as the designs parameters t 1 , t 2 , t 3 , d 1 , d 2 , I 1 , I 2 , I 3 , I 4 , s 1 , or may have different values as long as they are kept within the ranges for the ratios described above. 
         [0028]      FIGS. 5 to 10  show yet other shapes of stiffeners to be used with the diffuser pipes  20 ,  20   b,  or any other diffuser pipes for gas turbine engines. The stiffener  50   c  is a straight line and preferably disposed parallel to the ring. The stiffener  50   d  is Pi-shaped and has a straight portion  52   d  preferably disposed parallel to the ring  40 . The stiffener  50   e  is O-shaped. The stiffener  50   f  is H-shaped and has a straight portion  52   f  preferably disposed parallel to the ring. The stiffener  50   g  is I-shaped and has a straight portion  52   g  preferably disposed parallel to the ring. The stiffener  50   h  is X-shaped. The X-shape stiffener  50   h  is preferably oriented to have a top of one of the V forming the X parallel to the ring. Designs parameters of the stiffeners  50   c,    50   d,    50   e,    50   f,    50   g,    50   h  are similar to and may have same values as the designs parameters t 1 , t 2 , t 3 , d 1 , d 2 , I 1 , I 2 , I 3 , I 4 , s 1  of the stiffeners  50  or  50   b . All the stiffeners  50   c  to  50   h  shown on  FIGS. 5 to 10  are schematics. Corners between the different components of each of the stiffeners  50   c  to  50   h  are smoothen out to avoid high local stresses and for manufacturability requirements. The same holds for stiffeners  50  and  50   b.    
         [0029]    Using a stiffener or a ring on the flat portion of the diffuser pipe as described above, may reduce vibratory stress compared to diffuser pipes having no such stiffener or ring. In addition, the diffuser pipes having the stiffener and the ring were found to be undergoing less vibratory stresses than the diffuser pipes having only the stiffener and only the ring, or those having no ring and no stiffener. The ring and stiffener work in combination to reduce vibratory stresses, especially when designed using the ratios described above. Shapes and positions of the stiffener and ring are determined analytically so as to reduce vibratory stresses on the diffuser pipe by calculating the stresses for the lip mode(s). For example, diffuser pipes having the ring and a D-shaped stiffener such as the stiffener  50  underwent a reduction of 36% of vibratory stresses compared to same diffuser pipes having no ring and a T-shaped stiffener such as the stiffener  50   b.  The above described stiffeners can be added to existing diffuser pipes without the need to replace the diffuser pipe. The formation of the stiffener and the welding of the ring can be performed without undue burden. 
         [0030]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. The diffuser pipes described herein have been shown for a gas turbine engine for use in subsonic flight. It is however contemplated that the diffuser pipe could be used in other types of engines and in supersonic flights. Examples of such engines include: auxiliary power unit, turbofan engines, turboshaft engines and turbo prop engines. Any of the described stiffeners may be oriented relative to the ring differently from described herein, with a repercussion on the vibratory stress reduction of the diffuser pipe. Vibratory stress reduction properties of those stiffeners that have their straight portion at an angle relative to the ring may be reduced compared to those stiffeners that have their straight portion parallel to the ring. Any of the described stiffeners may be disposed more or less away relative to the ring. Vibratory stress reduction properties those stiffeners that are away from the ring may also be reduced compared to those stiffeners that are close to the ring. The diffuser pipe may have more than one stiffener on each flattened area. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.