Abstract:
A diffuser for diffusing the exhaust gas produced by an engine is provided. The diffuser includes a wall or walls defining sections of the diffuser, the diffuser including a first section and a second section following the first section, wherein the rate of increase of the cross-sectional area of the diffuser, which includes using an outer annular wall as an outer boundary and the inner annular wall as an inner boundary, in a direction of fluid flow is greater in the first section than it is in the second section. An upstream end of the inner annular wall or an upstream end of the outer annular is bent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation in part of U.S. Ser. No. 11/631,052 filed on Dec. 28, 2006 which is the US National Stage of International Application No. PCT/EP2005/052673, filed Jun. 9, 2005 and claims the benefit thereof. The International Application claims the benefits of British Patent application No. 0414846.6 filed Jul. 2, 2004. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates to a diffuser for diffusing the exhaust gas produced by an engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention finds particular application in an annular diffuser as may be used for an exhaust duct of a gas turbine engine. 
         [0004]    A diffuser is a device used to increase static pressure of a working fluid and it does so by decreasing the fluid kinetic energy. The amount of energy thus saved is, through the increase in pressure, able to do work elsewhere. For example, in a gas turbine an increase in pressure ratio across a turbine section will result in more work done by the turbine. 
         [0005]    An increase in static pressure in an exhaust duct may be achieved simply by progressively increasing the cross-sectional area of the duct in the fluid flow direction, expanding the fluid. However, the flow behaviour of such expanded fluid is dependent on several factors and not easily predicted to the extent there is no recognised best method of establishing an optimum diffuser profile for a given situation. For a given turbine, different engineers are likely to come up with different diffuser profile designs, each having different performance characteristics. 
         [0006]    One factor generally regarded of importance is boundary layer separation. When the fluid next to a diffuser wall (the boundary layer) becomes turbulent and separates from the wall there is a loss in diffusing area and pressure recovery is reduced, i.e. the diffuser performance is degraded. Effectively, wherever the flow separates in a diffuser the static pressure (and hence recovery) is fixed at that point, i.e. diffuser exhaust pressure equals static pressure at the separation point. It is well known that wider angles of divergence in a diffuser encourage flow separation whereas smaller angles do not. 
         [0007]    A conservative approach to the flow separation problem will generally result in a diffuser of small divergence angle and relatively long length. However, a long diffuser has disadvantage in that where it is part of a gas turbine engine it makes the engine design more complicated and expensive. Also, the length of an engine may be of significant importance for example on an oil platform at sea where available space is limited. 
         [0008]    The opposite, aggressive approach, to diffuser design may provide a greater divergence angle and be shorter in length but may suffer in terms of best performance. 
       SUMMARY OF INVENTION 
       [0009]    It is an objective of the present invention to provide a diffuser profile that optimises performance and length. 
         [0010]    According to a first aspect of the present invention there is provided a diffuser for diffusing the exhaust gas produced by an engine, said diffuser comprising a wall or walls defining sections of the diffuser, said diffuser comprising: an initial section which extends for substantially one third the length of the diffuser, at the entry to said initial section the shape factor of the boundary layer flow at the or each wall of the section being in the range 1.3 to 1.6, said initial section being configured so as to substantially linearly increase said shape factor so that at the exit of said initial section said shape factor is in the range 1.6 to 2.2; and a main section following the initial section and extending for the remaining length of the diffuser, said main section being configured so as to maintain said shape factor in the range 1.6 to 2.2 over the length of the main section. 
         [0011]    According to a second aspect of the present invention there is provided a diffuser suitable for diffusing the exhaust gas produced by a gas turbine engine, said diffuser being of annular form and comprising inner and outer annular walls defining sections of the diffuser, said diffuser comprising a first section and a second section following the first section, wherein the rate of increase of the cross-sectional area of the diffuser is greater in said first section than it is in said second section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0012]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a table and associated graph defining a gas turbine engine annular diffuser in accordance with the present invention; 
           [0014]      FIG. 2  is a schematic illustration of the diffuser of  FIG. 1  and shows the geometrical dimensions used in the table/graph of  FIG. 1 ; 
           [0015]      FIG. 3  is a graph of shape factor range and distribution for an annular diffuser in accordance with the present invention; 
           [0016]      FIG. 4  is a diffuser with a concave outer wall; and 
           [0017]      FIG. 5  is a diffuser with a concave inner wall. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0018]    The invention focuses on a particular factor that characterises fluid boundary layer condition and which may be used to indicate the point of separation of a fluid from a surface. This factor is known as Shape Factor (H) and is defined as boundary layer displacement thickness divided by momentum thickness. 
         [0019]    There is no prior art agreed accurate figure for H to say when boundary layer separation actually occurs, merely a broad range is indicated. For example, a thesis by Kristian Angele ‘Experimental studies of turbulent boundary layer separation and control’ KTH Mechanics, S-100 44 Stockholm, Sweden offers in chapter 6, variously H=3.3, H=2.35 and H=2.85. 
         [0020]    It has been appreciated that broadly there is an advantage in a shape factor range and distribution for a diffuser whereby: 
         [0021]    (i) At the diffuser entry H is in the range 1.3 to 1.6. 
         [0022]    (ii) There is an approximately linear increase in H from the entry until a point approximately one third length of the diffuser  500  from the entry (the initial section)  503  at which point H is in the range 1.6 to 2.2. 
         [0023]    (iii) From the end of the initial section  504  and for the remaining effective length of the diffuser (the main section)  500  H is in the range 1.6 to 2.2. 
         [0024]    More specifically it has been appreciated that there is particular advantage in a shape factor range and distribution whereby: 
         [0025]    (i) At the diffuser entry  503  H is in the range 1.3 to 1.6. 
         [0026]    (ii) There is an approximately linear increase in H from the entry until a point approximately one third length of the diffuser  500  from the entry (the initial section)  503  at which point H is in the range 1.6 to 2.1. 
         [0027]    (iii) From the end of the initial section  504  and for a distance of one half the remaining effective length of the diffuser  500  (the second section) H is in the range 1.6 to 2.1. 
         [0028]    (iv) From the end of the second section and for the remaining effective length of the diffuser  500  (the third section) H is in the range 1.6 to 2.2. 
         [0029]    The lower value of H described at diffuser entry is found desirable in order that a stable boundary layer may become quickly established. 
         [0030]    As previously stated, there are many methods to choose from when designing a diffuser  500 . In the case of the present invention it is preferred to assess the fluid flow characteristics of the diffuser using an axisymmetric streamline curvature throughflow method coupled to an axisymmetric boundary layer method, the results of which are checked in a three dimensional (3D) Navier Stokes calculation. The method is run iteratively until the desired H values are obtained over the whole of the diffuser  500  thus presenting a diffuser wall profile which gives superior performance for any given diffuser length. Naturally for an annular diffuser the values for H must be achieved at both the inner and outer annular diffuser walls  507 , 508 . 
         [0031]    The diffuser  500  of  FIGS. 1 and 2  and shown in  FIG. 4 , has been designed using the aforesaid preferred method. In  FIGS. 1 and 2  “hub” refers to the inner annular wall  507  and “tip” refers to the outer annular wall  508 . The first and third columns of the table of  FIG. 1  contain axial x and radial r coordinate pairs defining the position of the inner annular wall  507 . As can be seen from  FIG. 2 : the axial distance x is measured from the tip of the trailing edge of the last low pressure (LP) rotor blade of the gas turbine engine; and the radial distance r is measured from the axis of the annular diffuser (also the centre line of the engine). The second and fourth columns of the table contain axial x and radial r coordinate pairs defining the position of the outer annular wall. The fifth column of the table headed xhub_mod contains the values of the third column headed xhub after subtraction from each of these values of the first value 156.5 of the fourth column xtip. Similarly, the sixth column headed xtip_mod contains the values of the fourth column headed xtip after subtraction from each of these values of the first value 156.5 of the fourth column xtip. The seventh to tenth columns contain the values of the first, second, fifth and sixth columns respectively after division of each of these values by the height H of the last LP rotor blade as shown in  FIG. 2  (H equals 137 mm). The graph of  FIG. 1  is plotted using the coordinates of the seventh to tenth columns of the table. 
         [0032]    The graph of  FIG. 3  contains four plots. The “Lower limit for H” and “Upper limit for H” plots correspond to the more specific diffuser shape factor range and distribution given above. The optimised diffusion “OD hub” and “OD tip” plots illustrate shape factor behaviour of the inner and outer annular walls respectively of an annular diffuser in accordance with the present invention. It is to be noted that this behaviour is within the ranges of the aforesaid more specific diffuser shape factor range and distribution. 
         [0033]    The present invention achieves a relatively low exit velocity (high pressure recovery) over a relatively short length. It does this by making use of a relatively low shape factor H present at diffuser entry. As H is low initial diffusion may be more aggressive without risk of fluid separation. Thus, the rate of increase of the cross-sectional area of the diffuser (and hence expansion of the fluid) may initially be relatively rapid. As H increases this rate of increase of cross-sectional area is reduced to avoid separation. The diffuser can therefore be considered to comprise two general sections: a first section  501  over which the rate of increase of cross-sectional area is relatively rapid (and hence diffusion more aggressive); and a second section  502  over which the rate of increase of cross-sectional area is relatively slow (and hence diffusion less aggressive). 
         [0034]    In the diffuser  500  of  FIG. 1 , as shown in  FIG. 4 , the relatively fast followed by relatively slow increase in cross-sectional area is achieved by the form of the outer annular wall  508 . The outer annular wall  508  is generally concave and curves towards the inner annular wall  507 . The inner annular wall  507  is conical in form. Particularly, in downstream direction, the outer annular wall  508  increases the distance to an axis of symmetry substantially rapidly at an upstream end of the diffuser  500 —at an upstream end of the first section  501 —and having an inward curvature such that the outer annular wall  508  smoothly merges to a region of the outer annular wall  508  with a substantially conical form. In the second section  502 , the outer annular wall  508  then follows this substantially conical form, still expanding in downstream direction. Please note that the curvature in the first section  501  as shown in  FIG. 4  may be exaggerated in the drawing and may hardly be visible in a preferred embodiment. The conical form of inner annular wall  507  may particularly be homogenous over both sections  501 ,  502 . Particularly, in the second section  502 , the outer annular wall  508  and the inner annular wall  507  may have substantially parallel walls. It is to be understood that if both the inner and outer annular walls  507 ,  508  were conical in form then the rate of increase in cross-sectional area would not vary from diffuser entry to diffuser exit. By the concave form of the outer wall  508  the relatively fast followed by relatively slow increase in cross-sectional area is achieved. It is of course the case that the same effect could be achieved by an outer annular wall that is conical in form and an inner annular wall that is generally concave and curves towards the outer wall, as shown in  FIG. 5 . In this case, in downstream direction, the inner annular wall  507  increases the distance to an axis of symmetry substantially slowly at an upstream end of the diffuser  500 —at an upstream end of the first section  501 —and having an outward curvature such that the inner annular wall  507  smoothly merges to a region of the inner annular wall  507  with a substantially conical form. In the second section  502 , the inner annular wall  507  then follows this substantially conical form, still expanding in downstream direction. The conical form of outer annular wall  508  may particularly be homogenous over both sections  501 ,  502 . Particularly, in the second section  502 , the outer annular wall  508  and the inner annular wall  507  may have substantially parallel walls. 
         [0035]    In the diffuser  500  of  FIG. 1 , shown in  FIG. 4 , at the entry to the diffuser the ratio of the radius of the outer annular wall  508  to the radius of the inner annular wall  507  is 1.6. Use of the above mentioned preferred diffuser design method suggests that this ratio may be varied within the range 1.4 to 1.8. Similarly, the L/H of the diffuser  500  of  FIG. 1  is 10, where L is the total axial length of the diffuser, and H is as above as shown in  FIG. 2 . Use of the design method suggests that the L/H may be varied within the range 7 to 12. Further, in the diffuser  500  of  FIG. 1  the angle between the conical inner annular wall  507  and the axis of the annular diffuser is 6.5 degrees. Use of the design method suggests that this angle may be varied within the range 5 to 15 degrees. In the diffuser  500  of  FIG. 1  the ratio of the cross-sectional area of the diffuser at diffuser entry to diffuser exit is 3.3. Use of the design method suggests that this ratio may range up to 3.5.