Patent Publication Number: US-9890792-B2

Title: Radial or mixed-flow compressor diffuser having vanes

Description:
FIELD OF THE INVENTION 
     The invention relates generally to gas turbine engines, and more particularly to a diffusion stage of a radial or mixed-flow compressor of a gas turbine, as well as to an associated compressor. 
     TECHNOLOGICAL BACKGROUND 
     A compressor includes one or more rotating discs (rotor or wheel), bladed or not, and one or more wheels with fixed vanes (straightener stages). 
     A radial (or centrifugal) compressor has at least one radial compressor stage, that is capable of achieving a flow of gas perpendicular to the central axis of the compressor. It includes at least one wheel with radial blades which aspirate the air axially which, under the influence of the radial force, is accelerated, compressed and driven out radially. This air is then straightened in a diffuser (fixed vanes) which transforms part of its speed into static pressure by slowing the gases leaving the wheel. The operation must take place with a minimum of total pressure loss while still maintaining a satisfactory level of stability of the compressor so as to maintain an acceptable surge margin for operation of the turbomachine. 
     The gases are then guided toward the combustion chamber. 
     A mixed-flow (or axial-radial) compressor has at least one compression stage diagonal with respect to said central axis, so that the fluid leaves the wheel of the compressor forming a nonzero angle with the radial direction. 
     A diffuser of a radial compressor consists of a wheel made up of two flanges between which the gases flow radially or diagonally from the center toward the periphery. Vanes are distributed between the flanges along the entire wheel. These vanes form a flow cascade between the leading edges of these vanes and the trailing edges. 
     However, deflection of the air flow leaving the wheel by the vanes of the diffuser can cause separation of the fluid on the intrados or the extrados of the vanes, which separation, if it is considerable, can lead to stalling of the fluid and, as a result, of pumping. It is known that this pumping phenomenon is harmful to the elements constituting the compressor, so that one seeks to avoid it to the extent possible. 
     Usually, the vanes of the diffuser consist of an intrados wall and an extrados wall with a circular arc shape, and include a quasi-linear right angle. An example of such a vane is illustrated in  FIG. 1 . However, these vanes have a limitation in terms of diffusing capacity. Indeed, an increase in diffusion by these vanes causes a drop in isotropic efficiency and an increase in the instability of the compressor. 
     A diffuser for a radial compressor including vanes conforming to the preamble of claim  1  was proposed in document WO 2012/019650. In particular, this document describes vanes the profile whereof has a camber line defined by a function having an inflection point. For this reason, the camber line has an “S” shape and makes it possible to distribute loads along the profile of the vane, with a low loading in the area of the leading edge, which increases progressively up to the inflection point of the vane, where it is a maximum. However, the use of such a vane showing such an “S” profile requires limiting the cross-section at the diffuser throat (that is at the fluid entry cross-section). This has the effect of shifting the Flow rate/Rate characteristic toward lower flow rates, and reduces the aerodynamic choking flow of the diffuser. 
     Vanes with a profile curve similar in shape to an “S” have also been described in document JP 2011-252424. In particular, the vanes of this document are configured so that an angle formed between the line of curvature and the circumferential profile curve rises, then drops, then rises again between the leading edge and the trailing edge of the vane. Here too, the cross-section at the diffuser&#39;s throat must therefore be limited, which has the effect of reducing the stability of the diffuser. 
     SUMMARY OF THE INVENTION 
     One objective of the invention is to improve the performance and the surge margin of the diffusers of radial and mixed-flow compressors of the prior art. 
     In particular, the invention has the objective of proposing a diffuser for a radial or mixed-flow compressor capable of limiting the isentropic efficiency drop of the compressor and of improving the capacity to slow down and to straighten the flow delivered by the wheel of the compressor while still maintaining the stability of the flow. 
     For this purpose, the invention proposes a vane for a diffuser for a radial or mixed-flow compressor, including a leading edge positioned facing a gas flow, a trailing edge positioned opposite to the leading edge, an intrados side wall and an extrados side wall which connect the leading edge and the trailing edge. The profile of the vane includes a camber line having at least two inflection points between the leading edge and the trailing edge. Furthermore, the curve of the intrados wall and the curve of the extrados wall substantially follow the curve of the camber line, so that: 
     the intrados wall includes, between the leading edge and the trailing edge, at least two convex parts separated by a concave part, and 
     the extrados wall includes, between the leading edge and the trailing edge, at least two concave parts separated by a convex part, and the profile defines a chord line which extends between the leading edge and the trailing edge, and the convex parts of the intrados wall and the concave parts of the extrados wall extend at least partially on the same side of said chord line. 
     The invention also proposes a diffuser including at least one vane as described above, as well as a radial or mixed-flow compressor including such a diffuser, and an engine including such a compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, aims and advantages of the present invention will be more apparent upon reading the detailed description hereafter, and with regard to the appended drawings given by way of non-restricting example and wherein: 
         FIG. 1  illustrates an example of a vane profile conforming to the prior art, 
         FIG. 2  illustrates an example of a vane profile of a diffuser conforming to the invention, 
         FIG. 3  is a detail view of a vane of  FIG. 2 , on which is shown a chord line and a vane median line, and 
         FIG. 4  illustrates an example of an engine which can include a diffuser conforming to the invention. 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT 
     A radial diffuser according to the present invention is in particular designed to be used with a radial or mixed-flow compressor  2 . 
       FIG. 4  is a partial section of an engine  1  comprising a radial compressor  2 . A gas flow F is first aspirated into an air entry channel, then compressed between the blades  3   a  of a wheel  3  of the radial compressor  2  and its casing. The compressor  2  has axial symmetry about an axis X. The compressed gas flow F then exits radially from the wheel  3 . If the compressor  2  were of mixed-flow type, the gas flow would exit inclined at a nonzero angle with respect to a direction radial to axis X. 
     The compressed air exits the wheel  3  radially while still having angular momentum, and passes into diffuser  5 . The role of the diffuser is to convert part of the kinetic energy of the gases coming from the compressor  2  into static pressure by slowing the speed of the gases, and to straighten the flow exiting the wheel  3 . It includes for this purpose a plurality of vanes  10  positioned along its circumference, which extend between a forward flange  5   a  and a rear flange  5   b . Each of the vanes  10  has, in known fashion, a leading edge  11  positioned facing a flow of gas, a trailing edge  12  opposite to the leading edge  11 , an intrados side wall  13  and an extrados side wall  14  which connect the leading edge  11  to the trailing edge  12 . 
     The front  5   a  and rear  5   b  flanges can be planar. As a variant, one at least of the flanges  5   a ,  5   b  can include, in the space that they define between them, at least one area with alternating curvature between two vanes  10 , so that the air stream can include adaptive tip and root meridians. Reference can be made to document FR 2 976 633 by the Applicant for more detail about the front  5   a  and/or rear  5   b  flanges having such an alternating curvature. 
     According to yet another variant, flanges  5   a ,  5   b  can have adaptive axisymmetric shapes. 
     Moreover, the front  5   a  and rear  5   b  flanges can be adapted to as to allow aspiration and blowing in the diffuser  5 . 
     At least one of the vanes  10  of the diffuser  5 , preferably all the vanes  10 , includes, from upstream to downstream in the gas flow direction: 
     a first area, called a collection area, the shape whereof is configured to adapt to the upstream flow, and 
     a second area, called the diffusion area, the shape whereof is configured to more strongly straighten the flow coming from the collecting area, so as to obtain a greater static pressure leaving the diffuser  5  and facilitate the feeding of the downstream part, generally an axial diffuser  5 . 
     The vane  10  includes a profile, of which the camber line  15  has at least two inflection points I 1 , I 2  between its leading edge  11  and its trailing edge  12 , that is at least two changes in concavity. 
     Hereafter, the term “inflection point” will be understood to mean a point in a curve at which the curve crosses over its tangent. Moreover, the profile of the vane  10  is understood to mean a cross-section of the vane  10 , that is a section of the vane  10  in a plane generally perpendicular to the extrados  13  and the intrados  14  of the vane  10 . Finally, the “camber line  15 ” of the profile corresponds to the fictitious line which includes all the points equidistant from the extrados  13  and the intrados  14  of the vane  10 , while the “chord  16 ” corresponds to the segment which has as its ends the leading edge  11  and the trailing edge  12 . 
     The inflection points I 1 , I 2  together delimit the collection area, which includes the part of the vane  10  extending upstream of the first inflection point I 1 , and the diffusion area, which includes the part of the vane  10  extending downstream of the inflection point I 2 . 
     Preferably, so as to optimize the stability of the diffuser  5  and the static pressure at the output of the diffuser  5 , the inflection points I 1 , I 2  are located between 10% and 90% of the chord  16 , preferably between 30% and 70%. For example, the first I 1  of the inflection points can be located between 35% and 55% of the chord  16 , while the second inflection point I 2  is located between 55% and 65% of the cord  16 . The inflection points I 1 , I 2  can in particular be disposed symmetrically with respect to the center of the chord  16 . 
     As a variant, the profile of the vane  10  can include additional inflection points I 1 , I 2 . 
     Thus, the camber line  15  has successively, between the leading edge  11  and the trailing edge  12 , at least a first concavity, a second concavity different from the first concavity, then a third concavity. When the inflection points I 1 , I 2  are symmetrical with respect to the center of the chord  16 , the second concavity is then centered in the vane  10 . 
     According to one embodiment, the intrados wall  14  and the extrados wall  13  substantially follow the curve of the camber line  15 , and therefore have as many inflection points I 1 , I 2 . 
     In the embodiment shown in  FIGS. 2 and 3 , the intrados wall  14  and the extrados wall  13  thus include two inflection points I 1 , I 2 . The intrados wall  14  includes a convex part  14   a  between the leading edge  11  and the first inflection point, then a concave part  14   b  between the two inflection points I 1 , I 2 , then a convex part  14   c  between the second inflection point and the trailing edge  12 . The extrados wall  13 , for its part, includes a concave part  13   a  between the leading edge  11  and the first inflection point, then a convex part  13   b  between the two inflection points I 1 , I 2 , then a concave part  13   c  between the second inflection point and the trailing edge  12 . 
     Moreover, the camber line  15  extends between the intrados wall  14  and the chord  16 . In other words, at all points between the leading edge  11  and the trailing edge  12 , the camber line  15  and the intrados wall  14  extend remotely from the chord  16 . In addition, the concave areas of the extrados wall  13  cross the chord  16  and are therefore found at least in part on the same side of the camber line  15  as said chord  16 . 
     Thanks to this configuration, made possible by the two inflection points I 1 , I 2  of the camber line  15 , the leading edge  11  and the trailing edge  12  are oriented in the same general direction with respect to the gas flow as that usually encountered in conventional diffusers  5 , which makes it possible to preserve the cross-section of the throat, that is the fluid entry cross-section between two adjoining vanes  10 . In this manner, the stability of the diffuser  5  is maintained while still improving diffusion of the flow. 
     The angle of attack α (which corresponds to the angle between the tangent to the camber line  15  at the leading edge  11  and the chord  16 ) can be substantially identical to that of conventional vanes  10 . For example, the angle of attack α can be comprised between about 0° and about 45°. In this manner, it is possible to retain substantially the shape of the vanes  10  of conventional diffusers  5  in their collection area, which makes it possible to preserve the stability of the flow. Moreover, the presence of the second inflection point I 2  makes it possible to modify the shape of the vanes  10  in their diffusion area to increase the efficiency of the diffuser  5 , without thereby changing the shape of the collection area. Indeed, it is at present possible to increase the angle between the camber line  15  at the trailing edge  12  and the chord  16 , independently of the shape of the collection area, which makes it possible to more strongly straighten the gas flow and thereby increase the static pressure and the total pressure rate at constant temperature rise at the output of the diffuser  5 , and to therefore improve the isotropic efficiency of the diffuser  5  while still maintaining the surge margin and thus the stability of the compressor  2 . 
     As stated previously, the camber line  15  of the profile of the vane  10  includes at least two inflection points I 1 , I 2 . Preferably, the number of inflection points I 1 , I 2  can be even so as to maintain the general orientation of the leading edge  11  and of the trailing edge  12  with respect to the flow, and therefore to preserve the cross-section at the throat. Moreover, according to one embodiment, the corresponding camber line  15  extends here between the intrados wall  14  and the chord  16 , so that, at every point between the leading edge  11  and the trailing edge  12 , the camber line  15  and the intrados wall  14  extend remotely from the chord  16 , and the concave areas of the extrados wall  13  cross the chord  16 .