Abstract:
Method and apparatus for compression of elastic fluids such as air in a rotary continuous flow process particularly of interest in the turbomachinery field. Resulting structure has an envelope or outer diameter favorably comparable with axial flow compressors and a static pressure ratio, cost, and resistance to FOD comparable to centrifugal compressors. Jet propulsion engine apparatus incorporating such a compressor is also presented.

Description:
The U.S. Government has rights in this invention pursuant to Contract No. F33615-79-C-2028 awarded by the U.S. Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the present invention is compression or pressurization method and apparatus of rotary continuous-flow type for use with elastic fluids such as air. 
     More particularly, the present invention is concerned with turbomachinery compressor method and apparatus of a type having characteristics both of known axial-flow and known centrifugal-flow types, but differing quite remarkably in structure and method of operation from either of these known turbomachinery types. Consequently, the present invention is related in a general way to known turbomachinery compressor method and apparatus commonly grouped under the genus of mixed-flow axial-centrifugal type. 
     The present invention is also related to a combustion turbine engine employing turbomachinery compressor method and apparatus of the type described above. 
     The cost and reliability of modern combustion turbine engines are both strongly affected by the number of compression stages, blade rows, or acceleration/diffusion operations in the compressor sections of these engines. Accordingly, reducing the number of compressor stages has been a long-recognized objective in the field of turbomachinery design, and particularly in the jet propulsion field. 
     The conventional way to achieve a reduction in compressor stages has been to use one or more centrifugal-flow compressor stages in place of a greater number of axial-flow compressor stages. Centrifugal compressor stages in comparison with axial-flow compressor stages are recognized as offering a lower cost and higher static pressure ratio. They have also been recognized as offering superior resistance to damage from ingestion of foreign objects (hereinafter, foreign object damage, FOD) and superior tolerance to distortion or nonuniformity of inlet air flow distribution. However, centrifugal compressors are in general slightly less efficient and have a larger outer diameter than comparable axial flow compressor. 
     Balancing all these factors, early developments of jet engines for aircraft uses concentrated on axial-flow compressor stages and avoided centrifugal compressor designs primarily because of the adverse engine envelope or increased frontal area which would have resulted from the use of centrifugal compressor stages. Such increased envelope of centrifugal compressors is attributable primarily to the substantial radius change in the rotor of the centrifugal compressor stage. This radius change results in an outlet air flow having, in addition to a substantial tangential velocity component, a high radially outward velocity component. Conventionally, this high radially outward air flow velocity component dictated a stationary diffuser disposed annularly around and radially outwardly of the compressor rotor. It is this diffuser structure primarily which results in the comparatively large outer diameter of centrifugal compressors. 
     The theoretical possibility of structuring the rotor of a centrifugal compressor with an outlet portion turning the outlet flow toward an axial direction has been recognized in the pertinent art for many years. Such a rotor construction would allow the diffuser structure to be disposed axially of the rotor rather than radially outwardly thereof and would result in a decreased overall outer diameter. Such compressors are depicted by the U.S. Pat. Nos. 2,570,081; to B. Szczeniowski; and 2,648,492; 2,648,493; to E.A. Stalker. However, it has been learned from practical experience that substantial turning of a centrifugal compressor flow from radially outwardly toward the axial direction within the rotor itself as taught by these patents occasions such large aerodynamic losses that these designs are unattractive by contemporary performance standards. 
     Another alternative proposal has been to structure a compressor rotor according to centrifugal-flow teachings, but with the air flow through the rotor turning only partially toward the true radial direction despite enjoying a significant increase of radial dimension in traversing the rotor. The flow from such a mixed axial-centrifugal rotor is then received by a modified channel or pipe diffuser which initially turns the flow from axially and radially outward to, or past, the axial direction to flow axially, and perhaps radially inwardly, all substantially without diffusion. The diffuser also includes divergent pipe diffuser channels which extend a considerable distance in the downstream axial direction, and which thus contribute to an undesirably long axial dimension for such a compressor stage. U.S. Pat. No. 2,609,141, of G. Aue proposes a mixed-flow compressor of the above-described type wherein it is proposed the modified channel pipe diffuser may relieve only the radially outward, or both radially outward and tangential components of air flow velocity exiting from the rotor. However, practical experience has again shown that the radially outward component of air flow leaving such a proposed rotor is of sufficient magnitude that when the modified channel pipe diffuser is configured to relieve only this radially outward component, performance of the compressor is unacceptably low by contemporary standards. Configuration of the channel pipe diffuser to relieve both radial and tangential velocity components of air flow from the compressor rotor further increases the performance shortfall of such a compressor by current standards. 
     Yet another theoretical proposal has been to structure a compressor with what is essentially an axial-flow rotor having an increase of radial dimension from inlet to outlet, at least with respect to the mean radius of bulk flow through the rotor. In theory, such a compressor rotor enjoys, at least to some small degree, the advantages which centrifugal compressor rotors derive from their increase of radial dimension from inlet to outlet. Such a compressor is proposed by the U.S. Pat. No. 2,806,645, to E.A. Stalker. Again, practical experience has shown such a proposed compressor to be theoretically unsound and to offer performance far short of contemporary standards. 
     SUMMARY OF THE INVENTION 
     In view of the above, the objects broadly stated for a compressor according to this invention are to achieve a compressor envelope or outer diameter the same as, or only slightly larger than, that offered by the best conventional axial-flow compressor technology; to achieve a static pressure ratio, cost, inlet distortion tolerance and resistance to damage from ingestion of foreign objects (FOD) substantially the same as that offered by the best conventional centrifugal flow compressor technology; and to achieve a compressor efficiency at least equal to that of the conventional centrifugal compressors, and preferably approaching the efficiency of conventional axial-flow compressors. 
     With greater particularity, the present invention contemplates a transonic mixed-flow compressor comprising a housing defining an axially and circumferentially extending annular wall defining at an inlet portion thereof an inlet passage of right circular cylindrical shape in transverse section, said annular wall further defining at an outlet portion thereof spaced axially downstream from said inlet portion an outlet passage of right circular conical shape in transverse section which diverges downstream relative to said inlet portion, intermediate of said inlet portion and said outlet portion said annular wall transitioning from said right circular cylindrical shape to said right circular conical shape to define an intermediate passage; and an axially extending rotor journaled for rotation about said axis within said inlet passage, said intermediate passage, and said outlet passage; said rotor including a substantially cone-shaped hub portion and a plurality of axially and circumferentially extending vanes extending substantially radially outwardly toward but short of said annular wall to closely conform thereto at respective axial locations throughout said inlet portion, said intermediate portion and said outlet portion. 
     Still further, the present invention presents a transonic mixed flow compressor as set out immediately above and wherein said rotor and said housing define cooperating means for receiving at said inlet passage a flow of elastic fluid having a first relative velocity vector sum of tangential and meridional velocities of at least Mach 1.2 with respect to a selected reference, and for diffusing said received fluid flow to a second subsonic relative velocity less than said first relative velocity while maintaining radially outer local relative velocity vectors within 10° of said first relative velocity vector. 
     According to another aspect of the present invention, a transonic mixed flow compressor is presented comprising a housing defining an inlet portion, an outlet portion, and an axially extending flow path extending therebetween for flow of said elastic fluid; a rotor journaled in said flow path for rotation about said axis and having a respective inlet end and outlet end, said housing and rotor defining cooperating means for defining an annular stream tube extending axially from said inlet toward said outlet in said flow path and diverging downstream radially outwardly to define at a radially outer boundary thereof upstream of said rotor outlet end substantially a right circular conical section. 
     A method of compressing elastic fluid is also encompassed by the present invention comprising the steps of forming a rotational annulus of axially flowing fluid having an inner diameter, an outer diameter, and a first relative velocity vector sum of meridional and tangential velocities of less than Mach 1; diffusing said flowing fluid to a second relative velocity less than said first relative velocity while increasing progressively downstream said outer diameter and increasing the radially outward component of said meridional velocity; holding said increase of said outer diameter to a constant axial rate while further diffusing said flowing fluid to a third relative velocity less than that of said second relative velocity while decreasing the radially outward component of said meridional velocity to a value less than that of said second relative velocity. 
     This invention also presents a method of compressing an elastic fluid according to another aspect thereof comprising the steps of forming a tubular stream of said fluid having a radially inner diameter, a radially outer diameter, and a first relative velocity vector sum of meridional and tangential velocities of at least Mach 1.2 at said radially outer diameter; diffusing said fluid to a second supersonic relative velocity less than said first relative velocity while limiting deviation of radially outer local relative velocity vectors to no more than 10° with respect to said first relative velocity vector; passing said fluid through a normal shock to a third relative velocity of less than Mach 1; and further diffusing said fluid stream while increasing downstream both the radially inner and radially outer diameters thereof to impart a significant radially outward component of meridional velocity thereto. 
     This invention also presents a jet propulsion engine incorporating compressor method and apparatus in accordance with the above. In accordance with the above, it will be seen upon further consideration that the present invention substantially satisfies each of the objectives enumerated therefor hereinabove, and by so doing provides the highly desirable advantages resulting therefrom. Additionally, the applicants have found the present compressor, because of its diffuser structure presenting a diffuser flow path defined between coannular right circular cylindrical wall sections, affords a structure of greater strength for a particular weight than either conventional centrifugal or mixed-flow diffuser structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 schematically depicts a longitudinal partially cross sectional view through a jet propulsion turbofan engine according to the invention; 
     FIG. 2 depicts an enlarged fragmentary axial view taken at line 2--2 of FIG. 1; 
     FIG. 3 depicts an enlarged fragmentary view of an encircled portion of FIG. 1, partially in cross section and having portions of the structure omitted for clarity of illustration; 
     FIG. 4 is similar to FIG. 3, with details of construction omitted to more clearly present geometric aspects of the invention; and 
     FIG. 5 depicts a fragmentary view taken parallel with line 5--5 at the radially outer tip of the compressor rotor of FIG. 3 with the perspective being radially inward. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 schematically depicts a turbofan jet propulsion engine 10 which includes an elongate housing 12. Housing 12 defines an inlet opening 14 through which ambient air is inducted, and an outlet opening 16 through which a jet of heated air and combustion products is expelled to the atmosphere. Journaled within the housing 12 is a shaft 18 which is driven by a turbine section 20 of the engine 10. At its forward end the shaft 18 carries a mixed-flow compressor rotor 22 which draws ambient air through the inlet opening 14 and pressurizes the inducted air for use by the remainder of engine 10. Immediately downstream of the rotor 22, the housing 12 defines an annular flow path 24 wherein is disposed a diffuser structure generally referenced with numeral 26, and which in combination with rotor 22 composes the first compressor stage of the engine 10. 
     Downstream of the diffuser 26, the flowpath 24 is bifurcated into an outer annular flowpath passage 28, and an inner annular core engine flowpath 30. The flowpath 28 communicates directly downstream with a tailpipe portion 32 of the engine 10; which tailpipe portion communicates with outlet opening 16. Accordingly it will be appreciated that the compressor rotor 22 serves also in the capacity of a fan with respect to the turbofan nature of the engine 10. 
     The core engine flowpath 30 proceeds downstream through a two-stage axial flow compressor section referenced with numeral 34, the two axially spaced apart blade wheels of which are drivingly carried by shaft 18. Flow path 30 subsequently extends through an annular combustor 36, and through the turbine section 20. Turbine section 20 also communicates with tailpipe portion 32 and with outlet opening 16. 
     Turning now to FIGS. 2 and 3, a frontal axial view of the compressor rotor 22 is presented along with a fragmentary longitudinal view of compressor rotor 22 and diffuser 26. FIG. 2 illustrates that compressor rotor 22 includes a hub portion 38, which reference to FIGS. 1 and 3 will show to define an outer surface 40 of elongate conical shape. Disposed upon the hub 38 and extending radially outwardly thereon is a plurality of axially and circumferentially extending blades 42. According to the preferred embodiment of the invention as depicted, the blades 42 number 17 and are equiangularly circumferentially spaced apart. Each blade 42 defines a radially extending leading edge 44, a radially extending trailing edge 46, and a radially outer axially and radially extending tip edge 48. 
     With more particular attention to FIG. 3, it will be seen that the blades 42 extend radially outwardly toward a wall portion 50 of housing 12 to terminate in the radially outer tip edges 48 which are spaced slightly radially inwardly of and in shape matching conforming relationship with a radially inner surface 52 defined by wall portion 50. The wall portion 50 extends continuously axially and circumferentially from inlet opening 16 downstream past compressor rotor 22, flow path 24, and diffuser section 26. Beginning at inlet opening 16 and continuing downstream (rightwardly, viewing FIG. 3) a selected distance therefrom the wall portion 50 defines a radially inner surface subsection 52a thereof which defines a right circular cylindrical surface. The right circular cylindrical surface portion 52a of wall 50 extends downstream beyond the leading edges 44 of blades 42. 
     On the other hand, the wall portion 50 adjacent the trailing edges 46 and extending certain distances both upstream and downstream of the virtual intersection thereof with wall surface 52 (leftwardly, and rightwardly, respectively viewing FIGS. 3 and 4) defines a radially inner surface subsection 52b thereof which defines a truncated right circular conical surface. Intermediate of the right circular cylindrical subsection 52a of wall 50 and the right circular conical subsection 52b thereof, the wall portion 50 defines an axially curvilinear radially inner transition surface subsection 52c which is radially inwardly convex. In other words, intermediate of the leading edges 44 and trailing edges 46 of blades 42, the wall 50 defines a subsection 52c which is an axially curvilinear transition surface of revolution, and which avoids a defined cusp between the cylindrical and conical subsections 52a, 52b thereof. Importantly, the curvilinear transition subsection 52c does not extend to the trailing edges 46, and in fact joins subsection 52b some distance upstream of these trailing edges. 
     More particularly with reference to FIGS. 3 and 4, upstream of the virtual intersection of leading edges 44 with wall 50 (FIG. 4, point B) and extending downstream thereto, the right circular cylindrical surface 52a has a axial dimension of from about 10% to about 20% of the meridional dimension of blades 42 at tip edges 48 (FIG. 4, A-B dimension). Similarly, extending downstream from the the virtual intersection of leading edges 44 with wall 50 (FIG. 4, point B), the right circular cylindrical surface 52a has an axial dimension of from about 10% to about 30% of the meridional dimension of blades 42 at tip edges 48 (FIG. 4, B-C dimension). 
     Adjacent the virtual intersection of trailing edges 46 with wall 50 (FIG. 4, point E), the right conical surface portion 52b extends both upstream and downstream. The downstream meridional extension of surface 52b is from about 5% to about 15% of the meridional length of blades 42 at tip edges 48. The upstream meridional extension of surface 52b from point E is from about 10% to about 30% of the meridional dimension of blades 42 at edges 48 (FIG. 4, D-E dimension). Consequently the transition surface subsection 52c defines from about 40% to about 80% of the meridional dimension of the blades 42 at edges 48. 
     Viewing FIG. 4 in particular, it will be seen that in axial cross section the flow path coextensive with rotor 22 is radially outwardly bounded by surface 52a and 52b defining two relatively augulated axially extending straight line segments. the straight line segments of surfaces 52a and 52b are joined by a continuous, smooth, nonlinear curved surface section 52c tangent with both of the straight line surface sections. Preferably, the surfaces 52a and 52b define an acute angle referenced with numeral 54 of about 22° with respect to one another. However, the angle 54 may be from about 5° to about 45°. 
     Also viewing FIGS. 3 and 4, it will be seen that the leading edges 44 of the blades 42 are swept downstream radially outwardly with respect to a radially extending line 56 perpendicular to the rotational axis of rotor 22. Preferably, the leading edges 44 define an acute angle referenced with numeral 58 of about 7° . However, the angle 58 may be from about 0° to about 15°. Similarly, the trailing edges 46 are swept upstream radially outwardly with respect to a radially extending line 60 perpendicular to the rotational axis of rotor 22. Preferably, the trailing edges 46 define an acute angle referenced with numerical 62 of about 23°. The angle 62 may, however, be from 0° to about 35°. 
     Further, with respect to the hub 38 and blades 42 thereon, viewing FIG. 4 will show that a radius dimension RBi is defined at the intersection of leading edge 44 with the outer surface 40 of hub 38. Similarly, at the intersection of trailing edge 46 with surface 40 a radius dimension REi is defined. According to the invention, the ratio of REi to RBi is about 2.75. However this ratio may permissibly vary between about 1.5 and 3.5. 
     Importantly, the applicants have discovered that in combination with the other salient features herein described, a relatively small ratio of outer radius change of the rotor 22 from leading edge to trailing edge of blades 42 may be employed. In other words, at the virtual intersection of leading edge 44 with surface 52 a radius dimension RBo is defined. At the virtual intersection of trailing edge 46 with surface 52 a radius dimension REo is similarly defined. the ratio of REo to RBo is preferably 1.17. This ratio may however vary from about 1.05 to about 1.76 according to the invention. As will be seen, this relatively low ratio of radius increase from inlet to outlet of the rotor 22 contributes to a relatively small overall diameter for a compressor according to the invention in comparison to its inlet diameter. 
     A further geometric aspect of the rotor 22 which is considered of importance by the applicants is a dimensionless ratio termed Aspect Ratio (AR), defined below ##EQU1## 
     The average meridional blade length of blades 42 is depicted on FIG. 4 as line 64, which is generated by those points on the blade lying radially midway between surface 40 and tip edge 48. Preferably, the ration AR is 1.12. This ratio may, however, vary between about 0.75 and 1.30. 
     Downstream of the trailing edges 46, the housing 12 defines annular fluid flow path 24 by the cooperation of radially outer wall 50 with an annular radially inner wall 65 which is spaced radially inwardly of wall 50 and defines a radially outwardly disposed surface portion 66a. Viewing FIG. 4 once again, it will be seen that the surface 66a is a curvilinear surface of revolution having a radius referenced with numerical 68 originating from a center point 70. The radius 68 is related to the height of the blades 42 at the trailing edge 46. That is, the radial distance along the trailing edge 46 from its intersection with surface 40 of hub 38 to its virtual intersection with surface 52 is considered the blade height at the trailing edges of blades 46. The ratio of radius 68 to blade height at trailing edge 46 is preferably 2.0. However, this ration may vary from about 1.0 to about 4.0. 
     Immediately downstream of the trailing edges 46, the flow path 24 is circumferentially continuous and radially open between walls 50 and 65. The radially outer wall 50 defines a surface portion 52d which is a curvilinear surface of revolution tangent at it upstream end with the right circular conical surface portion 52c. The radius of wall surface 52d is matched to that of surface portion 66a so that a flow path portion 24a is defined which is of substantially constant area despite the radius change in the flow path with respect to the rotational axis of rotor 22. At its downstream end, the surface portion 52d is also tangent with a right circular cylindrical surface portion 52e defined by wall 50 (viewing FIG. 4). Similarly the wall 65 defines a radially outwardly disposed right circular cylindrical surface portion 66b which at its upstream end is tangent with surface portion 66a of wall 65. 
     Viewing FIGS. 3 and 4, it will be seen that an annular array of radially extending and circumferentially spaced apart diffuser vanes 72 extend between the walls 50 and 65 from surface portion&#39;s 52e to 66a thereof, respectively. The vanes 72 each define a leading edge 74, and a trailing edge 76 spaced downstream thereof. While it will be noted that at their radially inner ends, the vanes 72 are relatively close to the trailing edge 46 of compressor blades 42 and intersect the curvilinear surface portion 66a, the radially outer ends of the vanes 72 intersect with cylindrical surface portion 52e. That is, the diffuser vanes are swept downstream radially outwardly to intersect with the radially outer wall 50 at cylindrical portion 52e thereof. Importantly, the leading edge 74 of vanes 72 intersects with wall 50 downstream of the curvilinear surface portion 52d. That is, the vanes 74 at their radially outer ends intersect with the right circular cylindrical portion of wall 50 at surface 52e. It will be noted that the vanes 72 are swept downstream radially outwardly with respect to a radial perpendicular from the rotational axis of rotor 22. The physical sweep angle is in the range from zero degrees to twenty-five degrees. However, vanes 72 are swept to an even greater aerodynamic degree with respect to air flow from rotor 22. Such is the case because of the combination of axial and radially outward velocity components of this air flow, as will be more fully explained hereinafter. 
     Downstream of the trailing edges 76 of diffuser vanes 72 the flow path 24 is once again circumferentially continuous to define a vane-wake dissipation area 24b. Recalling that the surfaces 52e and 66b are right circular cylindrical surface section&#39;s it will be appreciated that the flow path portion 24b is of constant cross sectional flow area. As a result, substantially no flow diffusion occurs within portion 24b. 
     Downstream of the diffuser vanes 72, and spaced axially apart therefrom by approximately one-half a chord dimension of the later, diffuser 26 also includes a second annular array of radially extending and circumferentially spaced apart diffuser vanes 78. The diffuser vanes 78 include leading edges 80 and trailing edges 82 which are swept with respect to a radial perpendicular from the rotational axis of rotor 22. As will be more fully appreciated hereafter, the geometric sweep of the vanes 72 closely approximates their aerodynamic sweep angle because the air flow traversing these diffuser vanes has little or no radial velocity component. 
     Having observed the structure of the compressor stage composed of compressor rotor 22, walls 50 and 65 of housing 12, and diffuser 26 disposed in flow path 24, attention may now be directed to its operation according to aerodynamic theory. During operation of the engine 10, the turbine section 20 drives shaft 18 at a high speed of rotation. The shaft 18 rotational drives compressor rotor 22. Viewing FIGS. 3 and 4, it will be observed that the wall 50 defining surface portion 52a of housing 12 cooperates with an axially extending conical nose portion 38a of hub 38 to define an axially extending annular passage which is referenced with numeral 24&#39;. In response to rotation of rotor 22, ambient air is drawn through passage 24&#39;, as is represented by arrow 84. Consequently, it will be seen that the passage 24&#39; defines an axially extending annular flow stream of axially flowing air 84. 
     Upon encountering the leading edges 44 of blades 42, the annular flow stream of air 84 is subdivided into substreams which are circumferentially spaced apart by the blades 42, viewing FIG. 5. As FIG. 5 depicts, adjacent the tip edge 48 the leading edges 44 have a tangential velocity vector (Vt) referenced with numeral 86. Consequently air flow 84 approaching the blades 42 has a negative relative tangential velocity vector of -Vt, which is referenced by numeral 88. The air flow 84 adjacent the tip edges 48 also has a meridional component of relative velocity represented by vector (Vm) and referenced with numeral 90. Meridional velocity as used herein is the vector sum of axial and radial airflow velocity components. Consequently, air flow 84 has a relative velocity vector sum of Vt and Vm which is represented by vector Vr, and referenced with number 92. Meridional relative velocity vector Vm includes also any radial relative velocity component (VR) between blades 42 and air flow 84. However, adjacent the intersection of tip edges 48 and leading edges 44 the outward radial velocity of air flow 84 must be zero, or substantially zero because wall surface 52a  is of right circular cylindrical shape. Relative velocity vector 92 (Vr) has a magnitude of at least Mach 1.2, and preferably in the range of Mach 1.3 to 1.5, or higher. 
     As a consequence of the high relative velocity of vector Vr, the leading edge 44 of each blade 42 is believed to originate a Mach wave which is referenced with numeral 94, viewing FIG. 4. Also, according to the invention the surface of blades 42 extending downstream of leading edge 44 and facing in a tangential direction opposite to the rotational direction of rotor 22 is shaped according to known aerodynamic principles to originate multiple additional Mach waves, which as depicted are two in number and are referenced with numerals 96,98. It will be understood that the number of additional Mach waves may be other than two as depicted. However, it is an important aspect of the invention that at the selected operating condition for the compressor stage, one of the Mach waves (wave 98 as depicted) encounters the next circumferentially adjacent blade 42 opposite to the direction of rotation of rotor 22 at or downstream of the leading edge 44 thereof. As a result, the wave 98 becomes a captured Mach wave. At or adjacent the leading edge 44 which has captured the Mach wave 98, an oblique shock wave 100 is believed to originate and to extend toward the next circumferentially adjacent blade 42 in the direction of rotation of rotor 22. Subsequently, a normal shock 102 is believed to be formed downstream of oblique shock 100. Each of the Mach waves 94, 96, 98, oblique shock 100 and normal shock 102 is believed to effect a diffusion or slowing of the flow of air 84 relative to blades 42, and a concomitant increase of total pressure of the airflow. As an aid to the reader, notations have been placed upon FIG. 5 which are generally indicative of the relative velocity of the airflow field at the location of the particular notation. 
     Recapping the foregoing, the airflow 84 approaches rotor 22 as an axially flowing annular stream tube having a relative velocity of at least Mach 1.2 (FIG. 5, M&gt;&gt;1) represented by vector 92 (Vr). Upon encountering the blades 42, the airflow is weakly diffused through successive plural Mach waves 94-98, the last of which is a captured Mach wave preceding a stronger oblique shock 100. Upstream of the oblique shock 100, the airflow has a relative velocity vector 104 which is supersonic (FIG. 5, M&gt;&gt;1), but lesser in magnitude than velocity vector 92. Downstream of the oblique shock 100, the airflow has a relative velocity greater than Mach 1, (FIG. 5, M&lt;1), but less than the velocity upstream of the shock 100. Immediately downstream of the normal shock 102, the airflow has a relative velocity less than Mach 1 (FIG. 5, M&lt;1), and a direction substantially perpendicular to the shock 102, as is represented by vector 106. 
     Importantly, the vector 106 has a direction which deviates by no more than 10° with respect to vector 92. Large turning angles in the presence of diffusion of supersonic flow are extremely difficult to obtain without extremely high aerodynamic losses. The Applicants have discovered that a turning of 10° or less will allow diffusion of supersonic airflow preceding a normal shock with high efficiency. Such a limited supersonic flow turning in the range of about 10° or less enhances, the applicants believe, the probability of achieving a shock structure having only a single normal shock, and monotonically decelerating flow to a velocity of less than Mach 1. 
     Returning once again to FIG. 3, it will be seen that the hub 38 increases in radial dimension somewhat uniformly with axial dimension. However, the tangential velocity of points on the blades 42 increases with radius and is a product of rotor angular velocity and radius. Consequently, the relative velocity adjacent the hub 38 is much less than the level of Mach 1.2, or higher, which is experienced at the tip edges 48 adjacent leading edges 44. As a result, flow turning adjacent the intersections of leading edges 44 with the hub 38 may permissibly exceed 10°. 
     Nevertheless, adjacent the intersection of tip edges 48 and leading edge 44 where the relative velocity is Mach 1.2 or higher the flow turning cannot, the applicants believe, be allowed to exceed 10° without incurring undesirable aerodynamic losses. It follows that the cylindrical wall subsection 52a is of importance in the present invention because such subsection limits the radial component of local meridional velocity to substantially zero. Accordingly, the tangential velocity component Vt and rotational speed of rotor 22 is easily determined so that Vr at the intersection of leading edges 44 with tip edges 48 is Mach 1.2 or higher. While the cylindrical wall subsection 52a is considered an important and desirable feature of a transonic mixed flow compressor according to the invention, deviation from the cylindrical shape is permissible so long as the resulting radial component of Vm (vector 90) is taken into consideration. 
     Further considering FIG. 3, it will be recalled that wall subsection 52c curves outwardly to increase in radial dimension with downstream axial dimension. This outward flare of the wall subsection 52c lending with the upstream end of the right circular conical wall subsection 52b is accompanied by an increase of the radially outward component (VR) of relative meridional velocity and further diffusion to a relative velocity considerably below Mach 1. Such increase of the radially outward component of meridional velocity with increasing radius is expected and is the advantageous effect of centrifugal compressors which conventional mixed flow compressors have attempted to utilize. However, such radially outward velocity component continues to increase with radial dimension as air flows along the rotor in conventional centrifugal and mixed-flow compressors to such an extent that conventional downstream flow turning losses, diffuser structure difficulties, and excessive overall outer diameter limitations have heretofore persisted. 
     Surprisingly, the applicants have discovered that in a compressor according to the invention even though the annular flow stream is diverging downstream and increasing in radial dimension, if the radially outer flow stream boundary is limited to increase of radial dimension as a substantially constant linear function of axial dimension, then the growth of the radially outward velocity component will cease or reverse itself within the axial confines of the compressor rotor. In other words, the meridional velocity vector (summation of axial and radial velocity components) which begins to swing radially outwardly with increasing radial dimension of the annular flow stream downstream of wall subsection 52a (arrow 108, viewing FIG. 3) actually stops such outward swing and begins to swing back toward the true axial direction (arrow 110, viewing FIG. 3) upstream of the trailing edges 46 of rotor 22 despite the increasing radial dimension of the annular flow stream. 
     In view of the above, the importance of the right circular conical wall subsection 52b is easily appreciated. This wall subsection 52b forms a critically important radially outer boundary of the annular flow stream traversing the compressor rotor 22. Preferably, the right circular conical subsection 52b extends upstream of trailing edges 46 about 10% to 30% of the meridional dimension of the tip edges 48. However, a relatively shorter segment of conical wall section may be employed within the scope of the present invention provided that the conic section extends upstream of the trailing edges of the rotor blades sufficiently to limit the growth of, hold constant, or effect a decrease of the radially outward component of meridional airflow velocity. 
     The decrease of radially outward airflow velocity on rotor 22 described above makes possible the use of a diffuser structure 26 which is believed to be more akin to axial-flow technology and quite different than any conventional mixed-flow compressor stage. Considering diffuser 26, the wall 50 radially outwardly of flowpath subsection 24a is axially curvilinear to effect a limited turning of the air flow from rotor 22 which has tangential, axial and radially outward absolute velocity components. Turning of the airflow in response to curvature of wall 50 effects a reduction of the radially outward velocity component. Additionally, the vaneless space 24a is believed to effect an accommodation of local flow aberrations so that the flow is more fully homogenized before encountering diffuser vanes 72. 
     The diffuser vanes 72 have a leading edge 74 which is apparently swept in a physical sense, i.e. swept relative to a perpendicular from the rotational axis of rotor 22. The vanes 72 are swept with their radially outer leading edge farther downstream than their radially inner leading edge. What is not immediately apparent from viewing FIG. 3 is that the vanes 72 have an aerodynamic sweep angle exceeding their physical sweep angle. Such is the case because the air flow in diffuser space 24a has a substantial radially outward velocity component so that the airflow may be represented by a radially outwardly angulated vector like arrows 108 and 110, viewing FIG. 3. It will be appreciated that because the air flow is angled radially outwardly, the aerodynamic sweep angle of vanes 72 exceeds their physical sweep angle according to the angulation of the air flow in space 24a relative to the axis of rotor 22. 
     An additional aspect of the diffuser vanes 72 which is not observable from structure alone is that while the vanes extend substantially radially between the walls 50 and 65, they are leaned tangentially in an aerodynamic sense. In order to understand this aerodynamic lean it must be recalled that the airflow in diffuser space 24a has a significant tangential component of velocity. Further, this tangential velocity component increases radially outwardly from the wall 65 toward the wall 50. Consequently, even though the vanes 72 extend substantially radially between walls 50,65, they are aerodynamically leaned radially outwardly in a direction opposite to the direction of rotation of rotor 22, and opposite to the direction of tangential velocity of airflow in diffuser space 24a. In other words, the radially outer end of each vane 72 is aerodynamically displaced tangentially relative to the radially inner end of the vane in a direction opposite to the direction of rotation of the rotor 22. Stated still differently, an air flow element exiting rotor 22 near the tip edge 48 will, because of its higher tangential and axial velocity, encounter the leading edges 74 of diffuser vanes 72 in a shorter time than an airflow element exiting the rotor near the hub 38. Consequently, the diffuser vanes 72 are effective not only to reduce the tangential component of airflow velocity from chamber 24a while diffusing the airflow to a lower absolute velocity and higher static pressure, but also to reduce the remaining radially outward component of airflow velocity to substantially zero. 
     Downstream of the array of diffuser vanes 72, the diffuser section 26 includes another vaneless space 24b which extends axially and is open radially between the walls 50 and 65. Similarly to the vaneless space 24a, the space 24b is believed to allow &#34;adjustment&#34; or accommodation of local flow aberrations so that the airflow is more fully homogenized before encountering the diffuser vanes 78. The diffuser vanes 78 extend radially between the walls 50, 65. Vanes 78 diffuse the airflow from diffuser space 24b to a lower absolute velocity and increased static pressure while reducing the tangential velocity component thereof to substantially zero. Further, the radial component of airflow velocity which was reduced to substantially zero by the diffuser vanes 72 is maintained at the substantially zero level by vanes 78. Consequently, the diffuser vanes 78 deliver to flow path subsection 24c an airflow having substantially pure axial flow. 
     In order to complete this description of the inventive compressor stage it must be noted that the shape of blades 42 on rotor 22 is determined in accordance with conventional airflow streamline predictive techniques. In other words, with a view to the inventive compressor methods herein described, the airflow streamline directions at points in the annular flow stream are predicated based upon known aerodynamic principles. Appropriate blade surface segments are then established based upon accepted blade pressure loading, airflow velocity, and diffusion parameters. The resulting blade surface segments are stacked radially outwardly from the hub 38 and blended to determine the resulting shape of the blades 42. Accordingly, the precise shape, thickness, curvature and other characteristics of the blades 42 will vary dependent upon the design objectives of each particular compressor stage. 
     An actual reduction to practice of the present invention has been made substantially according to the preferred embodiment herein described. This actual reduction of the invention included a rotor having 17 blades, an overall axial dimension of 4 inches excluding the rotor nose portion, a hub/tip diameter ratio of 0.35 at the blade leading edges, and an operating speed of 33,216 R.P.M. With a compressor stage static pressure ratio of 3.0 and a corrected through flow of 20 pounds of air per second the rotor adiabatic efficiency was 89.5%, and the stage efficiency was 85.3%, with an airflow velocity of Mach 0.54 downstream of the stator vanes 78. The diffuser vanes 72 and 78 each numbered 44 vanes equiangularly disposed downstream of the rotor 22. The overall compressor stage length was 10.6 inches with a rotor outer diameter of 12.8 inches. As will be appreciated viewing FIG. 3, the overall outer diameter of the compressor stage, at about 105% of the rotor outer diameter, was only slightly larger than the outer diameter of the compressor rotor. 
     Accordingly it will be seen in view of the above that the present invention provides a transonic mixed-flow compressor which achieves a static pressure ratio favorably comparable to a centrifugal compressor while achieving an efficiency approaching the best contemporary axial-flow compressor technology. Testing has verified the tolerance to inlet airflow distortion of a compressor according to the invention. Resistance to damage from ingestion of foreign objects is believed to be substantially the same for a compressor according to the invention as for a centrifugal compressor. Further, it is believed that a compressor according to the present invention will have a cost comparable to contemporary centrifugal compressors. Finally, the outer diameter or envelope of a compressor stage according to the present invention is very favorably comparable to conventional axial flow compressors. 
     The present invention has been depicted described and defined with reference to a particular preferred embodiment thereof. Reference has also been made herein to an actual reduction to practice of the present invention. However, such references do not imply a limitation upon the invention, and no such limitation is to be inferred because of such references. The present invention is intended to be limited only by the spirit and scope of the appended claims which also provide a definition of the invention.