Abstract:
A deswirler system for a centrifugal compressor of a gas turbine engine that improves overall engine performance as a result of exhibiting significantly reduced friction losses. The deswirler system generally entails an annular-shaped manifold having an inlet configured to receive radially-outward flowing gas from a diffuser of the compressor, an outlet configured to discharge the gas in an axial downstream direction, and an arcuate passage therebetween. The deswirler system further includes a plurality of deswirler vanes directly within the arcuate passage and closely coupled to the diffuser.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the components of a gas turbine engine that receive radial high-velocity airflow from a centrifugal compressor, and then deliver the air to an annular-shaped combustor of the engine. More particularly, this invention relates to a compact deswirler system closely coupled to a diffuser and composed of deswirler vanes located within a bend that redirects the airflow from a radially outward direction to a generally axial direction. 
     BACKGROUND OF THE INVENTION 
     Shown in FIG. 1 are portions of a centrifugal compressor  10  and annular-shaped combustor  12  of a gas turbine engine. The compressor  10  generally includes a rotating impeller  14  configured to accelerate and thereby increase the kinetic energy of the gas flowing therethrough. A stationary annular-shaped diffuser  16  circumscribes the impeller  14 , and serves to decrease the velocity of fluid flow leaving the impeller  14  and thereby increase its static pressure. Diffusers are typically composed of either vanes or pipes that define a plurality of circumferentially-spaced passages  18 . The cross-sectional area of each passage  18  typically increases downstream of the impeller  14  in order to diffuse the flow exiting the impeller  14 . 
     Both vane and pipe-type diffusers generally include a transition region  20  downstream of the diffuser passages  18  to match the diffuser flowpath to the geometry of the combustor  12 . As shown in FIG. 1, the transition region  20  includes an annular manifold  22  that receives the radially-outward air flow from the diffuser  16 , and redirects this airflow aft and often radially inward (as shown) toward the annular-shaped entrance of the combustor  12 . The manifold  22  terminates with a generally straight section  24  in which a number of deswirler vanes  26  are positioned immediately upstream of the entrance to the combustor  12 . The vanes  26  serve to remove the residual circumferential swirl from the flow exiting the diffuser  16  by converting the high tangential velocity component of the flow exiting the diffuser passages  18  to a more useful static pressure. As a result, the flow exiting the deswirler vanes  26  and directed into the combustor  12  is characterized by relatively low swirl and Mach number and a particular meridional (“spouting”) angle that together achieve more stable and efficient combustor performance. In a multistage centrifugal compressor, a diffuser and transition region may be used between each consecutive pair of stages to decelerate and deswirl the air flow exiting the leading stage to a level appropriate for the trailing stage. 
     The manifold  22  shown in FIG. 1 generally defines an axi-symmetric free bend that is bounded by one (outer) surface, though bends bounded by two (inner and outer) surfaces are also known. The deswirler vanes  26  within the straight section  24  that follows the bend within the manifold  22  are generally arranged on a conical axi-symmetric flow path. Though a single row of vanes  26  is shown, double-row configurations are known. As a rule, the vanes  26  have been placed downstream of the bend and immediately upstream or at the entrance of the combustor  12 . 
     While diffuser and deswirler systems of the type shown in FIG. 1 perform well in a number of successful gas turbine engines, further improvements in the performance are continuously being sought. Of primary interest is achieving reductions in pressure losses that reduce engine performance. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a deswirler system for a centrifugal compressor of a gas turbine engine that improves overall engine performance as a result of exhibiting significantly reduced diffusion (secondary flow) and friction losses. According to this invention, the deswirler system generally entails an annular-shaped manifold having an inlet configured to receive radially-outward flowing gas from a diffuser, an outlet configured to discharge the gas in an axial downstream direction, and an arcuate passage therebetween. In contrast to prior art practices, the deswirler system of this invention provides a plurality of deswirler vanes directly within the arcuate passage and closely coupled to the diffuser, instead of being limited to being within a straight section downstream of the arcuate passage. 
     A significant advantage of the deswirler system of this invention is the reduction in pressure losses that reduce engine performance. Though not wishing to be held to any particular theory, it is believed that placing the deswirler vanes within the bend that turns the air/gas flow from the radial flow direction of the diffuser to the generally axial flow direction required by the compressor, reduces the amplification of the secondary flow as the air/gas leaves the diffuser. Consequently, the deswirler system of this invention is believed to eliminate bend losses and reduces secondary flow losses attributable to a tangentially unguided bend. 
     Another significant advantage of this invention is that the total length over which the air/gas travels from the diffuser exit to the combustor plenum is reduced, resulting in less total surface area wetted by the air/gas and, therefore, reduced skin friction losses. The diffuser/deswirler system is also more compact than prior art systems, and enables the weight of the engine to be significantly reduced. 
     Yet another important aspect of this invention is the determination that placement of the deswirler vanes within the arcuate passage immediately adjacent the diffuser allows for aerodynamic advantages through close coupling the deswirler vanes to the diffuser. For example, improved efficiencies can be realized through appropriate relative circumferential positioning of the deswirler vanes relative to the diffuser passages. As a result, the invention provides greater design flexibility in terms of optimizing the diffuser-deswirler system match to further minimize losses attributable to the diffuser-deswirler interface. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of a diffuser and deswirler system for a centrifugal compressor of a gas turbine engine of the prior art. 
     FIGS. 2 and 3 represent cross-sectional and perspective views, respectively, of a diffuser and deswirler system in accordance with this invention. 
     FIG. 4 represents an isolated perspective view of the deswirler vanes shown in FIGS. 2 and 3. 
     FIGS. 5 through 7 represent isolated perspective views of alternative embodiments for the deswirler vanes shown in FIGS. 2 through 4. 
     FIG. 8 represents an aft-looking-forward view of the diffuser and deswirler vanes shown in FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 represents in cross-section a closely-coupled diffuser and deswirler system in accordance with a preferred embodiment of this invention, while FIG. 3 is an isolated perspective view of the system shown in FIG.  2 . Common to the system shown in FIG. 1, the deswirler system of this invention is employed with a stationary diffuser  116  equipped with vanes  118  that direct the swirling air or gas that flows generally radially from the impeller of a centrifugal compressor (not shown) to the annular-shaped inlet  112  of a gas turbine engine combustor (not shown). The deswirler system of this invention also includes a transition region  120  immediately downstream of the diffuser  116 . As with the system shown in FIG. 1, the transition region  120  includes an annular manifold  122  that receives the radially-outward air flow from the diffuser  116 , and redirects this airflow aft and radially inward toward the entrance  112  of the combustor. It is within the scope of this invention that the manifold  122  could turn the flow from the diffuser  116  by as little as about 90 degrees, and as much as about 180 degrees, though it is believed that a turn angle of about 130 to about 140 degrees would be more typical. While the diffuser  116  will be described in terms of having a vane-type configuration, the teachings of this invention are also applicable to pipe-type diffusers. 
     The manifold  122  shown in FIGS. 2 and 3 defines an axi-symmetric bend  124  bounded by a pair of radially inner and outer surfaces  128  and  130 , respectively, that are typically defined by the compressor hub and casing. The manifold  122  causes the flow entering the combustor to be characterized by a relatively low Mach number and a particular meridional (“spouting”) angle that together achieve more stable and efficient combustor performance. 
     Disposed within the axi-symmetric bend  124  of the manifold  122  are a number of deswirler vanes  126 . As such, the deswirler vanes  126  of this invention are not limited to being located within a straight section downstream of the bend  124 , such as within the conical axi-symmetric flow path shown for the prior art in FIG.  1 . The vanes  126  serve the traditional role of removing the residual circumferential swirl from the flow exiting the diffuser  116  by converting the high tangential velocity component of the flow exiting the diffuser  116  to a more useful static pressure. However, the placement of the vanes  126  within the bend  124  also enables the vanes  126  to be closely coupled to the diffuser  116 , in addition to being closely coupled to the combustor inlet  112 . As used herein, the term “closely coupled” is used to denote that clearances are reduced to those necessary for component assembly and operation without interference. Accordingly, the vanes  126  shown in FIGS. 2 and 3 are closely coupled to the diffuser  116 , while the deswirler vanes  26  of FIG. 1 are not closely coupled to the diffuser  16 . 
     In a preferred embodiment, the deswirler vanes  126  are equally circumferentially spaced within the manifold  122 . The radially inward and outward edges of each vane  126  are shown as being delimited by the two axi-symmetric curved surfaces  128  and  130  of the manifold  122 . The shape of each vane  126  is determined aerodynamically so that the air or gas is simultaneously but gradually turned from the outward radial direction with substantial swirl angle (when it leaves the diffuser  116 ) to the meridional spouting direction with approximately zero swirl (as it enters the combustor inlet  112 ). For this purpose, and as best seen in FIG. 4, each vane  126  is also circumferentially-arcuate (i.e., arcuate relative to a longitudinal line parallel to the centerline of the engine), so as to provide arcuate gas flow path surfaces within the manifold  122  that promote the elimination of swirl. The radial height of each vane  126  will typically be dependent on the particular arcuate shape of the vane  126 , as understood by those skilled in the art. 
     As shown in FIGS. 2 through 4, the leading edge  132  of each vane  126  is closely coupled to the diffuser  116 , and the trailing edge  134  of each vane  126  is closely coupled to the combustor inlet  112 . As such, each of the vanes  126  extends the entire length of the bend  124  between the inlet and outlet of the manifold  122 . In FIG. 5, an alternative embodiment is shown in which alternate deswirler vanes  126  extend the entire length of the bend  124  between the inlet and outlet of the manifold  122 , but those vanes  136  between the alternate vanes  126  do not. As shown in FIG. 5, the leading edge  138  of the shorter vane  136  is decoupled from the diffuser  116 , while the trailing edge  140  remains closely coupled to the inlet  112  of the combustor. A benefit of this embodiment of the invention is a further reduction of engine axial length and reduced weight while maintaining performance improvements. 
     Shown in FIGS. 6 and 7 are two additional embodiments for deswirler vanes of this invention. In FIG. 6, deswirler vanes  142  are shown having a thicker trailing edge  146  as compared to their leading edges  144 . In addition, a hole  148  is formed in one of the vanes  142  to accommodate the passage of a cooling or lubrication tube (not shown) through the vane  142 , which may be necessary or advantageous in view of the compactness of the deswirl system of this invention. FIG. 7 also shows deswirler vanes  150  with thicker trailing edges  154  as compared to their leading edges  152 . In contrast to the embodiment of FIG. 6, one of the vanes  150  is equipped with a slot  156  to accommodate a cooling or lubrication tube. By incorporating cooling and lubrication tubes within the vanes  142  and  150 , a more uniform exit condition can be achieved, further reducing the risk of affecting the compressor stall margin. 
     An important aspect of the present invention is the potential for aerodynamic advantages realized through close coupling the deswirler vanes  126 ,  142  and  150  to the diffuser  116 . At least one benefit arising from this feature of the invention is the determination that improved efficiencies can be achieved through appropriate relative circumferential positioning of the deswirler vanes  126 ,  142  and  150  relative to the passages between adjacent diffuser vanes  118 . The benefits of this aspect of the invention are believed to be possible if the number of full-length deswirler vanes  126 ,  142  and/or  150  is an integer multiple of the number of diffuser passages, and more preferably equal to the number of diffuser passages. Testing has confirmed that enhanced engine performance occurs if each of the full-length deswirler vanes  126 ,  142  and/or  150  is circumferentially offset from one of the diffuser vanes. 
     In FIG. 8, this offset is schematically illustrated by an aft-looking-forward view of the diffuser vanes  118  and deswirler vanes  126 , with the centerline of the engine indicated at “C.” Tick marks are shown at intervals of one-quarter of the pitch “P” along the interface between the outer diameter of the diffuser vanes  118  and the inner diameter of the deswirler vanes  126 . While offsets of between one-quarter and three-quarters have been evaluated, optimum results for the engine tested have been achieved where the offset between deswirler and diffuser vanes was between one-quarter and one-half pitch, approximately at about three-eighths pitch. The optimum offset for a given engine may vary for different compressor and combustor designs. However, the unconventional capability with this invention to optimize the diffuser-deswirler system match provides greater design flexibility in terms of minimizing losses attributable to the diffuser-deswirler interface. 
     While the invention has been described in terms of preferred and alternative embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the deswirler system of this invention could be employed within a multistage centrifugal compressor and placed between each consecutive pair of stages. Therefore, the scope of the invention is to be limited only by the following claims.