Abstract:
The diffuser of the present invention comprises a plurality of passages which intersect at radially inner ends thereof to define a quasi-vaneless annular inlet for receiving accelerated gases from an impeller of a centrifugal compressor. Each of the passages includes a throat portion having a quadrilateral cross section, including two subtantially parallel linear sidewalls and two substantially arcuate opposing sidewalls, effective for reducing the length of and thereby pressure losses from the annular inlet. The linearity and regularity of the diffuser passages enables the diffuser to be manufactured to close tolerances by electric discharge milling an annular plate utilizing a single tool. This assures uniformity and consistency between diffusers.

Description:
The U.S. government has rights in this invention pursuant to Contract No. DAAK51-83-C-0014 awarded by the Department of the Army. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a diffuser and, more particularly, to a diffuser for a centrifugal compressor which is configured to optimize flow distribution to a combustion chamber and which may be manufactured to close tolerances in a manner which assures uniformity between diffusers. 
     A centrifugal compressor includes a rotating impeller arranged to accelerate and thereby increase the kinetic energy of a gas flowing therethrough. The diffuser is generally characterized by a quasi-vaneless annular space surrounding the impeller. The diffuser acts to decrease the velocity of the gas flow leaving the impeller to transform the energy thereof to an increase in static pressure, thus generating pressurized gas. 
     Prior art diffusers have generally included a plurality of circumferentially spaced passages which converge to the annular space surrounding the impeller. These passages expand in area downstream of the impeller in order to diffuse the flow exiting the impeller. It has been found for prior art diffusers of this type which are to be utilized with gas turbine engines that it is preferable to have the diffuser passages assume an initial circular cross section so as to accommodate with minimal losses the relatively high-flow velocities of the gases exiting the impeller and thereafter gradually merge into a near-rectangular outlet to minimize losses. 
     One diffuser of this type is disclosed in U.S. Pat. No. 4,027,997 issued to A. C. Bryans on June 7, 1977, and assigned to the assignee of this invention. The Bryans diffuser comprises a plurality of linear passages in flow communication with a quasi-vaneless annular inlet surrounding the impeller of a centrifugal compressor. Each passage gradually merges from a circular cross section at a throat portion near its inlet end, to a near rectangular cross section at its outlet end defined by two flat opposing parallel sides and two flat opposing curved sides which produce a razor sharp trailing edge at the diffuser outlet. This near rectangular shape of the diffuser outlet optimizes the flow distribution to an annular combustion chamber in flow communication with the diffuser outlet. 
     Diffusers constructed in accordance with the teachings of the Bryans patent have demonstrated significant improvements in the performance of centrifugal compressors for gas turbine engines. However, inasmuch as the quasi-vaneless inlet of the diffuser receives accelerated gases directly from the impeller, it is subjet to relatively high viscous drag which results in undesirable pressure losses. 
     Accordingly, it is an object of the present invention to provide a new and improved diffuser for a centrifugal compressor. 
     Another object of the present invention is to provide a diffuser wherein the length of the quasi-vaneless inlet is reduced for reducing the total of viscous drag thereover. 
     Another object of the present invention is to provide a diffuser for a centrifugal compressor which is configured to optimize flow distribution to a combustion chamber and which may be easily manufactured to close tolerances in a manner which assures uniformity between diffusers. 
     SUMMARY OF THE INVENTION 
     The diffuser of the present invention comprises a plurality of passages which intersect at radially inner ends thereof to define a quasi-vaneless annular inlet for receiving accelerated gases from an impeller of a centrifugal compressor. Each of the passages includes a throat portion having a quadrilateral cross section, including two substantially parallel linear sidewalls and two substantially arcuate opposing sidewalls, effective for reducing the length of and thereby pressure losses from the annular inlet. The linearity and regularity of the diffuser passages enables the diffuser to be manufactured to close tolerances by electric discharge milling an annular plate utilizing a single tool. This assures uniformity and consistency between diffusers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a fragmentary sectional view of a compressor including a diffuser in accordance with the present invention. 
     FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1. 
     FIG. 3 is a diagrammatic view illustrating and comparing the intersection of a passage having a throat portion in accordance with this invention and a passage of equal area having a circular cross section. 
     FIG. 4 is a sectional view taken along line 4--4 of FIG. 1. 
     FIG. 5 is a sectional view taken along line 5--5 of FIG. 1. 
     FIG. 6 is a sectional view taken along line 6--6 of FIG. 1. 
     FIG. 7 is a perspective view of an exemplary EDM electrode suitable for machining a passage similar to the ones illustrated in FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Illustrated in FIG. 1 is a fragmentary sectional view of a centrifugal compressor 10 which is effective for pressuring air to a combustion chamber of a conventional gas turbine engine (not shown). The compressor 10 includes an annular impeller 12 in flow communication with an improved annular diffuser 14 according to one embodiment of the present invention disposed radially outwardly thereof. Centrifugal compressors are well known in the art for converting the relatively high kinetic energy of gases accelerated by the rotating impeller 12 into static-pressure energy. The diffuser 14 according to the present invention, however, represents an improvement over conventional diffusers, and, in particular, over the above-described diffuser disclosed in U.S. Pat. No. 4,027,997--A. C. Bryans, incorporated herein by reference. 
     The conventional impeller 12 includes a plurality of circumferentially spaced impeller blades 16 supported by an annular web 18. The diffuser 14 includes an annular diffuser housing 20 having a plurality of tangentially disposed flow passages 22, which are disposed along linear centerlines, spaced about the circumference of the housing 20 and extending therethrough. The passages 22 are partly defined and bounded by a plurality of spaced, generally convex vanes 23. Adjacent ones of the passages 22 intersect with each other at radially inner, inlet portions 24 that define a quasi-vaneless annular inlet 26 of the diffuser 14. Each passage 22 further includes a throat portion 28 which is integral with the inlet portion 24 and has a first quadrilateral cross section 30, which defines the flow passage thereof, including: two substantially parallel linear opposing sidewalls 32 and 34 and two substantially arcuate opposing sidewalls 36 and 38 (see FIG. 2). 
     As illustrated in FIGS. 1, 4 and 5, the inlet portion 24 is a partially bounded passage having a generally semi-circular cross section, open at an apex thereof, at its upstream end which tapers to a generally flat, linear-sided section at its downstream end where it intersects the throat portion 28. The throat portion 28 represents the first fully bounded flow portion of the passage 22. The annular inlet 26 is defined as quasi-vaneless because the vanes 23 primarily end at the upstream end of the throat portion 28 with only relatively small lip or step portions 23a and 23b extending and tapering from the throat portion 28 to the upstrean end of the inlet portion 24. 
     A significant feature of the present invention is the introduction of the step portions 23a and 23b in the inlet portion 24, as illustrated in FIG. 5, which include flat, radially outwardly facing surfaces represented by portions of the sidewall 32 and arcuate, radially inwardly facing surfaces represented by portions of the sidewalls 36 and 38. The flat surfaces of step portions 23a and 23b act as walls to help confine airflow in the quasi-vaneless inlet 26 for reducing distortion thereof and the possibility of stall therein. 
     More specifically, it will be appreciated that pressure of the airflow in the inlet 26 increases in a radially outward direction. Higher pressure at radially outer portions will tend to cause the boundary layer found along the walls of the inlet 26 to be forced radially inwardly, which can possibly lead to stall. Accordingly, the flat surfaces of the step portions 23a and 23b will assist in preventing the boundary layer from being driven toward the impeller, thusly reducing the likelihood of stall, thereby increasing stall margin, and allowing for increased performance of the diffuser 14. 
     The throat portion 28 according to the present invention represents a significant improvement in the aerodynamic efficiency of the diffuser 14. More specifically, it is conventional to design a compressor considering engine performance, pressure ratio and flow volumes, for example, for determining the required quantity of the plurality of tangentially disposed diffuser passages 22, the required flow area A of the throat portions 28 and the width 2b of radially outer tip ends of the impeller blades 16 (see FIG. 5). With the throat flow area A being known, a particular shape or cross section of the throat portion 28 is then determined. In conventional high-performance diffusers, a circular profile of the throat portion 28 is preferred. However, in accordance with the present invention, it has been determined that the throat portion 28 having the quadrilaterial cross section 30, wherein the spacing between the sidewalls 32 and 34 is less than the diameter of an equal area circle, provides for improved aerodynamic performance of the diffuser 14. 
     More specifically, and referring to FIGS. 1, 4, and 5, the inlet portion 24 of the passages 22 is illustrated and has a length L l . It will be appreciated that the inlet portion 24 receives gases from the impeller 12 at relatively high velocities and relatively low pressures, and is therefore subject to relatively high viscous drag. Accordingly, it has been discovered that any reduction in the length L 1  of the inlet portion 24 will decrease the surface area subject to relatively high viscous drag and therefore result in a decrease of the total viscous drag. 
     FIG. 3 represents a diagram which illustrates more clearly how the first quadrilateral cross section 30 of the throat portion 28 is effective for obtaining a reduced length L 1  of the inlet 24. The circle of tangency of the centerlines of the plurality of passages 22 is shown and has a radius r, which radius r is substantially equal to the radius of the impeller 12. Also illustrated are two adjacent intersecting passages having superimposed cross sections: the first quadrilateral cross section 30 and a reference, circular cross section 40 sharing common tangential centerlines, both of which have a cross-sectional area equal to A. 
     It will be appreciated that passages having a circular cross section 40 will intersect with each other at a distance L 2  measured perpendicularly with respect to the radius r at the point of tangency of the centerline of the top passage. In contrast, passages having the first quadrilateral cross section 30 would intersect with each other at a distance L 1 , wherein L 1  is substantially less than L 2 . Accordingly, for a given cross-sectional flow area A, a throat portion 28 having the first quadrilateral cross section 30 instead of a circular cross section 40 will result in an inlet portion 24 having a reduced length L 1  for reducing viscous drag forces. 
     The preferred dimensions of the first quadrilateral cross section 30 have been selected for not only reducing the length L 1  of the inlet portions 24 but also for retaining part of the circular cross section, which circular sections have proven effective for reducing losses due to flow separation. 
     It will be appreciated that circular cross sections have been conventionally preferred because for a given cross-sectional area they represent the least wetted surface, i.e. the circumferential length, subject to drag forces. In contrast, a purely rectangular cross section of the same area has a wetted surface, i.e. the perimeter length, which is greater, and, which, therefore, results in greater drag losses. The quadrilateral cross section 30 retains benefits of the circular cross section in the arcuate sidewalls 36 and 38, yet is also effective to reduce the length L 1  as above described. 
     More specifically, and referring to FIGS. 2 and 5, the linear sidewalls 32 and 34 are spaced from each other a distance 2b. The arcuate sidewalls 36 and 38 of the first quadrilateral cross section 30 are defined by a radius R which is determined from the solution of the integral equation: ##EQU1## 
     The solution of this integral equation is: ##EQU2## and applying the bounds of integration results in: ##EQU3## 
     The solution of this integral equation is obtained using conventional techniques. In the integral equation, &#34;A&#34; represents the designed-for flow area of the throat portion 28, which is conventionally determined. &#34;x&#34; represents the distance measured outwardly from the center of the first quadrilateral cross section 30 between the two linear sidewalls 32 and 34, and &#34;b&#34; represents one half the spacing between sidewalls 32 and 34. 
     The distance between the two linear sidewalls 32 and 34 has a value equal to 2b, which is preferably equal to the tip width of the impeller blades 16, and with the radius R of the two arcuate sidewalls 36 and 38 determined as above described, the first quadrilateral cross section 30 is thereby defined completely. 
     As illustrated in FIGS. 1 and 4, the throat portion 28, which represents the first fully enclosed portion of the passage 22 which receives gases accelerated by the impeller 12, extends in a tangential direction, a finite length L 3 . The length L 3  is chosen so that as the throat portion 28 erodes from wear, the preferred first quadrilateral cross section 30 is maintained for the designed-for life. Accordingly, the length L 3  may be made generally equal to the diameter of a circle having an area equal to the area A of the first quadrilateral cross section 30 of the throat portion 28. 
     Referring again to FIG. 1, it will be seen that each of the passages 22 further includes a diffuser portion 42 which is integral with the throat portion 28. At a downstream end, the diffuser portion 42 has a second quadrilateral cross section 44 including two substantially parallel opposing linear sidewalls 46 and 48 and two substantially arcuate opposing sidewalls 50 and 52 (see FIG. 6). The diffuser portion 42 has an upstream end integral with the throat portion 28 which includes a third quadrilateral cross section 54 which is substantially identical to the first quadrilateral cross section 30 of the throat portion 28. The diffuser portion 42 is gradually tapered between its upstream and downstream ends. 
     As illustrated in FIGS. 2 and 6, the second quadrilateral cross section 44 of the diffuser section 42 is oriented substantially 90 degrees with respect to the first quadrilateral cross section 30. As illustrated in FIG. 5, the linear sidewalls 32 and 34 of the first quadrilateral cross section 30 of the throat portion 28 are disposed substantially parallel to the tips of impeller blades 16 and substantially normal to a radial axis of the diffuser 14. 
     The improved diffuser 14 according to this invention lends itself to relatively inexpensive manufacturing techniques which can maintain close tolerances and uniformity between diffusers. Inasmuch as the centerline as well as the walls of the diffuser passages 22 may be linear and make gradual and smooth transitions, the diffuser 14 may be easily manufactured by known electric discharge milling (EDM) techniques. 
     More specifically, an exemplary EDM electrode 56 suitable for manufacturing passages generally similar to the diffuser passages 22 is illustrated in FIG. 7 and can be very accurately machined firstly on a lathe with appropriate cylindrical, conical and curved sections. Portions of the electrode 56 which create the features of the passages 22 are identified by using the corresponding reference numerals of the passages 22. To easily and accurately obtain the first and second quadrilateral cross sections 30 and 44, the linear sidewalls 32, 34, 44 and 46 may be simply and accurately machined or planed off in a tapered manner for obtaining relatively smooth transitions. 
     While a preferred embodiment of the present invention has been disclosed, it will be understood that many modifications and changes may be made thereto without departing from the scope of the invention as defined in the intended claims. 
     Having thus described the invention, what is desired to be secured by Letters Patent of the United States is: